Session NP05: Poster Session (4:16pm - 5:16pm)

Configurations and dynamics of membrane-bound elastic filaments

Changes in the curvature and topology of cell membranes are responsible for numerous biological processes. Many of these changes seem to be driven by interactions with thin filament-like protein structures which form on the surface of membranes. While there are a number of proposed mechanisms, how exactly the filament-membrane interactions produce changes in curvature remains an open question. The feasibility of proposed mechanisms can be be investigated by modeling the filament as a thin elastic rod which is confined to the membrane surface. The interplay between the geometries of the the surface and the filament give rise to complex distributions of force and torque which are believed to play a crucial role in reshaping the membrane. We discuss the mechanics of surface-bound filaments and present a collection of analytical and numerical results.   

Dynamics of pulsing soft corals

Soft corals of the family Xeniidae have a pulsing motion that generates flow in their surrounding fluid. This flow brings new samples of fluid towards the coral, allows sufficiently slow mixing for removal of photosynthetic waste to occur, and then transports the fluid away from the polyp to reduce resampling (Samson et al. 2019). Generating this flow allows the pulsating corals to perform photosynthesis at much higher rates than non-pulsating soft corals (Kremien et al. 2013). Numerical simulations of the pulsations of the coral were conducted using the immersed boundary method. By quantifying flow characteristics such as velocity, vorticity, and Lagrangian coherent structures, we investigated how the flow changed as we varied the frequency-based Reynolds number and the length of the resting period between pulses. Further investigations into the efficiency of fluid transport by the polyps will also be presented. Key words: Immersed boundary method, pulsing soft corals, computational fluid dynamics, biomechanics   

Improving the Foundations for Large Eddy Simulation of Katabatic Flow

Large Eddy Simulations (LES) can pose an advantage over Reynolds Averaged Navier-Stokes in that only turbulence at and below the sub-grid scale (SGS) is dictated by a closure model. At the same time, LES can pose an advantage over Direct Numerical Simulations (DNS) in that it can be used to simulate flows at large Reynolds numbers. However, LES results can be sensitive to the choice of SGS turbulence model, especially in the near-surface region of stably stratified flows, making simulation accuracy highly dependent on the near-wall treatment and on grid resolution. For katabatic flow, it is surmised, based on the observed anisotropy and surface-normal flux divergence, that results are sensitive both to the choice of SGS model and to the wall-layer model. However, a mathematically consistent and physically grounded choice does not yet exist in either case, posing a bottleneck for the applicability of LES for katabatic flow simulation. Here, we propose improved models that explicitly account for the physics important to a statically stable fluid flowing under the influence of gravity over a sloped rough boundary. In this way, we contribute to increasing the utility of LES for studies on katabatic flows.   

Three-dimensional bubble mechanics in a corner subject to an acoustic wave

The development of cavitation bubbles in a corner subject to an acoustic wave has important applications in ultrasonic cleaning, and cavitation damage. Previous developments have been made on the effects of an acoustic wave on a microbubble near a rigid wall, (K. Manmi 2014), highlighting the effects of the high intensity ultrasound on the bubble's timespan, jet direction and overall shape of the bubble surface, regarding the initial radius of the bubble and the amplitude of the acoustic wave. Furthermore, numerical simulations have been made to accurately simulate the evolution of a cavitation bubble in a corner created by the intersection of two rigid walls for differing angles. In this presentation, I introduce the mathematical model used for a bubble in a corner subject to an acoustic wave along with the boundary integral method used to numerically model the development of the bubble surface with time. From this, numerical results are presented to highlight the effects of the corner and the acoustic wave for a range of angles, acoustic wave amplitude, and initial bubble radius. In doing so, the jet direction upon collapse can be found, along with the bubble migration towards the corner over time.   

Reliability of general purpose finite-volume solvers for wall-modeled large-eddy simulation of open channel flow at a moderate Reynolds number

Wall-modeled large-eddy simulation is increasingly being used in both industry and academia for the characterization of wall-bounded high Reynolds number flows. Numerical simulations are often carried out using general-purpose finite volume solvers, whose solution is known to be particularly sensitive to the nature of the discretization scheme. Such a sensitivity introduces a degree of uncertainty in model results that is yet to be fully quantified. We here assess the quality and reliability of a general-purpose finite volume solver in wall-modeled large-eddy simulation of a pressure-driven, open-channel flow at friction Reynolds number 2000. Simulations are performed using an algebraic, equilibrium, wall-layer model. Results are contrasted against corresponding DNS data and predictions from a single-domain pseudo-spectral solver. Model predictions will be discussed with a lens on the impact of the grid aspect-ratio, grid resolution, discretization schemes for the velocity and pressure, time stepping procedure, and velocity-pressure coupling scheme.   

Energy spectra for turbulent Rayleigh-B\'enard convection

We investigated the scaling behaviors for numerically simulated, turbulent Rayleigh-B\'enard convection by determining the kinetic and thermal energy spectra. The systems have aspect ratio $1$, Prandtl numbers $0.7$, $0.021$ and $0.005$, and various Rayleigh numbers ranging from $10^{5}$ to $10^{10}$. Whereas in previous studies the frequency spectra from a time series were considered, we calculated the energy spectra from spatial fields. In particular, we performed Fourier analysis on two-dimensional cross sections of the temperature and velocity fields. We also computed the conditional velocity and temperature structure functions on the same cross sections to verify our findings. Lastly, we tested whether Kolmogorov's 1941 (K41) scaling law or the Bolgiano-Obukhov (BO) scaling law applied to both the kinetic and thermal energy spectra for the various systems.   

Using reservoir computing to predict temperature fluctuations in turbulent Rayleigh-B\'enard convection

Turbulent flows are ubiquitous in the fundamental processes of nature---from the Earth's mantle to its atmosphere. Among their many complexities, turbulent flows are chaotic, which makes them computationally expensive to simulate and difficult to study analytically. Thus, prediction of turbulent systems is an ongoing challenge. Machine learning methods, specifically reservoir computing, have recently shown promise in predicting turbulent flows. Reservoir computing is a recurrent neural network model that uses a reservoir of randomly connected nodes to process the input data and predict the output for the next time step. As a proof-of-concept, here we test the effectiveness of reservoir computing on the prediction of temperature fluctuations in turbulent Rayleigh-B\'enard convection (RBC) because it has a relatively simple experimental setup and relevant applications, like weather prediction. We built a water-filled cylindrical convection apparatus, heated from below and cooled from above, and achieved Rayleigh numbers up to $4.5\times10^{11}$. We train the reservoir computer on our experiment’s temperature time-series data, collected with thermistors inserted at multiple heights in the flow, aiming to predict temperature fluctuations in turbulent RBC flow.   

The Hydrodynamic Analysis of Fluid Flow over a Hydrofoil beneath a Free Surface in Subcritical Froude Number.

In this work, a numerical simulation of two-dimensional unsteady incompressible viscous ?ow generated by a shallowly submerged hydrofoil under the free surface is presented. The computation is based on finite volume discretization incorporated with the interface capturing volume of fluid method to solve the fluid equation in the motion. The SST k-$\omega $ turbulence model is used to capture the turbulent flow for the wave around the hydrofoil. Submerged depths and different angles of attack will be two main variables in this numerical simulation. A comparison of the present numerical results with previous experimental and numerical results will be presented to show how accurate to use turbulence model to simulate the result. A comprehensive simulation of quantities like wave pro?le and forces is performed for angle of attack (AoA) ranging from -15 to 15, and h/c from 0.2 to 0.9 resulting in low Froude numbers ranging from 0.1 to 0.9. The results provide an understanding on the complex flow interactions between the free surface and the hydrofoil of varying AoA and submerged depths.   

Traveling-Wave Solutions of Two BiDirectional Whitham Equations

Following the work of Carter \& Rozman (2019), we study traveling wave solutions of two versions of the bidirectional Whitham equation. These equations are extensions of Boussinesq-type models that enable the phase velocities to match those of the Euler equations. We study the systems introduced by Hur \& Pandey (HP) (2018) and Aceves-S´anchez et al (2013). We compute families of periodic traveling wave solutions to the HP equation. Our goal is to compare the traveling-wave solutions of both equations to gain further understanding of the differences between them.   

A Control strategy for Radial Miscible Viscous Fingering: Non Linear Simulations.

The displacement of a more viscous fluid by a less viscous one in a porous medium results in a hydrodynamic instability called viscous fingering (VF). Chui \textit{et al., }Phys. Rev. E 92, 041003(R) (2015), experimentally reported the existence of a competition between advective and diffusive forces during the later stages of miscible radial VF. We numerically capture the competition between the aforementioned opposing forces from start to end of the instability. Many attempts have been reported in literature to control VF by utilising time-dependent strategies, modifying the geometry and altering the fluid properties. However, we utilize the competition to control the instability. Stable displacement is observed despite a less viscous fluid displacing a more viscous one. The \textit{M-Pe }parameter space is divided into stable and unstable zones by critical parameters following the relation $M_{c}=$\textit{Pe}$_{c}^{-0.55}. $The results are validated by diligently designed experiments.   

Investigating Nontrivial Time-Periodic Solutions to the Whitham Equation

We are interested in waves on shallow water and are studying the existence and evolution of nontrivial time-periodic solutions to the Whitham equation. These solutions can be described as a small-amplitude traveling-wave interacting with a carrier wave. Ambrose and Wilkening (2009) introduced a numerical method for computing such solutions to the Benjamin-Ono equation via minimization of a functional. They noted that their method can be applied to any partial differential equation that admits nontrivial time-periodic solutions. We are using their method to look for nontrivial time-periodic solutions of the Whitham equation.   

Large Eddy Simulation of Turbulent Coherent Structures in a Storm-forced Surface Ocean Mixed Layer

Submesoscale roll vortices in the oceanic mixed layer (OML) are important to understanding and constraining the flux of momentum from the atmosphere into the ocean. In this study, we use large eddy simulations (LES) to investigate the impact of the superposition of Ekman inflection-point (EIP) instabilities on the entrainment and deepening of a shallow coastal ocean pycnocline under the wind forcing of Hurricane Irene (2011). We used reanalysis of the hurricane winds to force LES domains 100m to 4km in the horizontal dimension to observe the development and influence of EIP rolls. The EIP instability has a domain-scale independent wavelength on the order of 500m to a kilometer and transfers momentum to the shear instabilities at the pycnocline, leading to a more rapid deepening and cooling of the OML during the hurricane. By understanding the physical processes driving the changes in the OML both ahead-of-eye and at eye-passage will aid in predictions of tropical cyclone intensity over stratified coastal oceans.   

Three-dimensional visualization of stratified turbulent wakes at varying Reynolds number

We present a series of three-dimensional visualizations of numerically simulated turbulent wakes in a stably stratified fluid, with a focus on the effects of the wake Reynolds number, $Re$, on the wake flow. The visualization of stratified wakes is complicated by the coexistence of regions of distinct dynamics in the flow, including the large-scale `pancake vortices', small-scale shear instabilities within layers of concentrated vertical shear, and internal waves emitted by the wake flow to the ambient fluid. We apply various techniques to separate the dynamically distinct regions within a given wake and visualize them respectively. The volume fractions occupied by each of these regions are also quantified. Through the visualizations, we observe several significant effects on the wake's evolution associated with increasing wake Reynolds number, which will be presented in the poster.   

Providing biomimetic passive pumping on an implantable biosensor

Microfluidic technologies have opened doors to rapid, inexpensive and compact medical diagnostic testing, with one important example being the glucose meter. Technology continues to evolve to continuous monitoring through mounting microfluidic devices onto the patient. A major hurdle in the development of on-body sensors is that they typically require flow of bodily fluids such as interstitial fluid or blood through the device. As a result, on-body sensors are typically bulky, requiring a pump and power source. We propose a solution for passive pumping on an implantable biosensor by mimicking nature's elegant solution to the problem -- lymphatic valves. The lymphatic system pumps fluid using unidirectional flexible valves which open and close with fluctuations in fluid pressure, driven by contraction and expansion of surrounding muscle and of the lymphatic vessels. The ability of easily manufacturable bioinspired microfluidic valves to provide passive pumping in a low Reynolds number environment is explored through use of a fully coupled three-dimensional fluid-solid solver. High-aspect-ratio polydimethylsiloxane (PDMS) bioinspired valves, anchored to the microchannel floor, mimic lymphatic valves, opening and closing to allow forward flow and prevent backflow, respectively. The design's size scale, biocompatible materials, and reliance on biologically driven pressure gradients allow implantability and defeat the need for active pumping from an external source.   

Multiplicity in Stable Orbits for Prolate Capsules in Simple Shear Flow

Artificial capsules have been applied in numerous fields such as bioengineering, pharmaceutics, and food industry. This work investigates the motion of a deformable prolate capsule in unbounded simple shear flow at zero Reynolds number using direct simulations. The deformability, bending stiffness, initial orientation, aspect ratio of the capsule and the viscosity ratio between the inner and outer fluids are varied over a wide parameter space. At a low viscosity ratio, a capsule with large bending stiffness always tends towards an in-plane stable orbit, either tumbling or swinging, depending on the deformability, i.e., capillary number (Ca). At a high viscosity ratio, however, a tumbling-to-rolling transition is observed for a capsule with large bending stiffness at increasing Ca. In the transition regime, the capsule is found to adopt multiple stable orbital modes including tumbling, precessing and rolling, depending on the initial orientation. This multiplicity regime becomes broader as the aspect ratio of the capsule increases, while showing an opposite dependency on the viscosity ratio. The multiplicity in stable orbits leads to a multiplicity in the rheological behavior for a dilute suspension of such capsules.   

Water Desalination with Two-dimensional Metal Organic Framework Membranes

Providing fresh and drinkable water is a grand challenge the world is facing today. Development in nano-materials can create possibilities of using energy-efficient nanoporous materials for water desalination. ~In this work, we demonstrated that ultrathin Metal Organic Framework (MOF) is capable of efficiently rejecting ions while giving access to high water flux. Through molecular dynamic simulation, we discovered perfect ion rejection rate by two-dimensional multi-layer MOF. The naturally porous structure of 2D MOF enables significantly 3 to 6 orders of magnitude higher water permeation compared to that of traditional membranes. Few layers MOF membranes show one order of magnitude higher water flux compared to single layer nanoporous graphene or molybdenum disulfide without the requirement of drilling pores. The excellent performance of 2D MOF membranes is supported by water permeation calculations, water density/velocity profiles at the pore and the water interfacial diffusion near the pore. Water desalination performance of MOF offers a potential solution for energy-efficient water desalination.   

Effects of Implant Separator Structure on Drug Delivery to the Posterior Eye.

The prevalence of visual impairment and blindness attracts attentions to develop new types of implant for drug delivery that can reduce drug administration doses and regulate drug release rates. We use Comsol Multiphysics to simulate drug transport from a hydrogel implant behind the sclera layer to the posterior eye. The focus is on the time evolutions of the drug concentration in the sclera, choroid, and retina layers respectively. The computational domain and dimensions are from a prior study (Kavousanakis \textit{et al}$.$ 2014). The governing equations are coupled between incompressible Darcy's law for flow and diffusion-advection for concentration. Drug delivery from the hydrogel implant directly agrees with open data. A new polymeric implant, containing several compartments defined by porous separators, is then introduced between the hydrogel and sclera to regulate the drug delivery rate. It is found that the peak concentration appears at approximately the same time but with much larger magnitude with no separator vs. with zero blockage separator. When the blockage widths in the separator is increased, the peak concentration appears in later time and smaller value. Such a new implant seems to be a plausible alternative to the drug delivery implants available to date in the market.   

Suspension flows in a pipe covered with permeable surfaces

The flow of particulate suspensions driven by a constant pressure gradient is examined in a pipe where its walls are replaced with a permeable surface. We explore non-colloidal suspensions of rigid and spherical particles in a Newtonian fluid over a wide range of bulk particle volume fraction (0.1$\le \phi_{b}\le $0.5) and the permeability of the porous medium. Direct numerical simulations (DNS) are performed to solve conservation equations of the flow coupled with the constitutive equation of suspensions (Diffusive Flux Model) and Darcy's law in a porous medium. We aim to elucidate the effect of the permeable surface on the suspension flows. The velocity and concentration profiles are presented for various control parameters and show that the velocity of suspensions enhances by the “slip” effect at the suspension-porous interface. We evaluate the rate of suspension flows and slip velocity at the suspension-porous interface induced by the permeable surface. It reveals that the rate of suspension flow decreases as the bulk volume fraction $\phi_{b}$ increases and it builds up as the permeability of the porous media increases. We also show that there are two different regimes characterizing the slip velocity normalized by both shear rate and penetration depth, at the suspension-porous interface, namely the strong permeability regime and the weak permeability regime.   

Developing Notebook-based Flow Visualization and Analysis Modules for Computational Fluid Dynamics

Visualization techniques are essential in identifying complex features and patterns in fluid flows. Three dimensional visualization has long been confined to high-end software. Recently, packages for Python have been developed to overcome this limitation. We seek to implement them in the realm of fluid mechanics using Jupyter notebooks. To this end, we make use of Python's K3D-jupyter package to generate 3D volume rendering from Large Eddy Simulation (LES) datasets. Using volume rendering, we were able to visualize velocity and concentration fields of oil droplets of varying sizes injected at the centerline of a turbulent jet influenced by a uniform crossflow. Alternately, uniformly random spheres modulated by the LES computed concentration field were generated to represent the Eulerian concentration field. Interactive features allowed the 3D structure of the jet to be probed and the turbulent structure's role in the oil's spatial distribution to be inferred. Additionally, we were able to implement the modules to visualize Direct Numerical Simulation (DNS) data obtained from the Johns Hopkins Turbulence Data Base (JHTDB). The capability of exporting the interactive plots as html files, which can be embedded into a website or online research article, facilitates distribution.   

Inferring an effective eddy viscosity from High Fidelity Data

Different ways to compute an inferred eddy viscosity from resolved turbulence data (from LES, DNS, experiments) based on optimization methods are investigated and assessed based on how well they reproduce the mean fields. The method is tested on wall bounded flows including a channel and a boundary layer. We further investigate the sensitivity of these methods to averaging error in the given Reynolds stress fields.   

Clustering of Vortex Wakes for Heaving-Pitching Foils Using Machine Learning

A heaving-pitching foil produces different vortex patterns that are associated with propulsive regimes of high efficiency or high thrust, however there are many intermediate modes not readily classified due to a chaotic wake structure. Hence, machine learning techniques can be applied to help cluster wake patterns and the associated performance. To evaluate the effectiveness of machine learning, a database of Reynolds-averaged Navier-Stokes simulations at Reynolds number of $10^6$ and high heave amplitudes is utilized. The data includes sweeps in flapping frequency and pitch amplitude to produce a wide range of kinematics and propulsive modes. A convolutional autoencoder with lasso regularization was used to extract a latent space of important features from vorticity images of the wakes. Applying different clustering algorithms to the latent space groups the data into distinct flow regimes, such as a low vorticity regime with minimal flow separation and a high vorticity regime with a reverse von K\'{a}rman wake. The results are validated against a pre-labeled subset of samples to explore ways of improving the clustering. To correlate wake clustering with performance, support vector regression was used to predict the values of efficiency, power and thrust based on the kinematics.   

Simulation of nonlinear ocean waves using volume-of-fluid method

The volume-of-fluid (VOF) method is widely used in the numerical simulation of multi-phase flows. In this work, we investigate the capability of VOF method to model nonlinear ocean waves, using several algebraic and geometric interface-capturing approaches. By considering a low viscosity and an initially irrotational flow field, the VOF solution can be validated against a fully nonlinear potential flow solution. We perform this study for both regular and irregular nonlinear waves, which benchmarks the capability of VOF to simulate the evolution of ocean waves. Furthermore, we develop a nonlinear wave inlet boundary based on the relaxation zone technique, enabling the simulation of prescribed incoming nonlinear waves into the computational domain. These developed capabilities are expected to be beneficial for different scenarios in ocean engineering.   

Numerical modeling of prostatic artery embolization: patient-specific blood flow and emboli transport

Benign prostatic hyperplasia (BPH) is the most common non-cancerous tumor found in men. Symptoms caused by BPH can be treated by a recently proposed minimally invasive procedure known as prostatic artery embolization (PAE), in which particles are injected through a catheter to limit blood supply to the enlarged prostate. The goal of this study is to characterize the complex blood flow patterns in common iliac and prostatic arteries and to model PAE with particle tracking in a patient-specific model. A computer model of common iliac arteries and the downstream vasculature was created from CT-angiography images using SimVascular. Image-based computational fluid dynamics (CFD) simulation using Oasis (a minimally dissipative open-source solver) was performed to obtain velocity data. Finally, the Maxey Riley equation was solved to model the PAE procedure and guide embolus injection. Our patient-specific computer model can provide valuable information for the PAE procedure.   

Wall shear stress manifolds and surface temperature patterns in heat transfer enhancement applications

Heat transfer enhancement has applications in many of the devices used by the public, directly or indirectly, on a daily basis. However, high-resolution heat transfer simulations are computationally costly, representing significant downtime in engineering industry and research. Meanwhile, wall shear stress (WSS) topology has been recently demonstrated as an effective estimator of surface concentration patterns in convective mass transport settings. Particularly, stable and unstable manifolds in WSS have been shown to control near-wall transport patterns in cardiovascular flows. Estimating surface concentration/temperature via WSS is desirable because of the significant reduction in computational time and the physical explanation of the results. In this study, it is investigated whether WSS topology proves an effective estimator of surface temperature patterns in heat transfer settings. The configuration used is the classical impinging jet, a widely used heat transfer enhancement application. WSS is obtained by computational fluid dynamics simulations and surface temperature is obtained by solving the advection-diffusion equation, both using the finite element method. Our results demonstrate a close connection between WSS manifolds and surface temperature patterns.   

2D CFD Analysis of NASA X-57 Maxwell High Lift Propeller Airfoil

The X-57 Maxwell is an experimental aircraft developed by NASA. This aircraft is designed to be quieter, lighter, and more efficient compared to the aircraft it is based on, the Tecnam P2006T. The aircraft has two large, outer propellers and 12 small, high lift, collapsible inner propellers. The high lift propellers are only active during low-speed flight which includes take-off and landing. During high-speed flights, the high lift propellers collapse into the wing of the aircraft and are no longer active. The goal of this aircraft is to demonstrate the benefits of distributed electric propulsion (DEP). Our research aims to calculate aerodynamic coefficients including lift, drag, and pitching moment on the airfoil of the high lift propellers. The coefficient tables generated will be used to obtain the aerodynamics for multi-body dynamics simulation testing of whirl flutter stability on the aircraft. To perform the computational fluid dynamics (CFD) calculations, we first conducted a validation study on a NACA 0012 airfoil. Using the same methodology, a CFD analysis was conducted on the high lift airfoil. The analysis on both the NACA 0012 and high lift airfoil were done using both Fluent and Genesis in order to validate the results obtained.   

Molecular Dynamics Analysis on the Dynamic Contact Angle Based on the Extraction of the Stress Distribution in Steady-State Non-Equilibrium Systems of a Single Lennard-Jones Fluid between Solid Walls under Shear

The Method of Plane (MoP) is a technique to calculate the stress distributions for systems in equilibrium molecular dynamics (EMD) simulations. In this study, we propose a method to extend the MoP to steady-state nonequilibrium molecular dynamics (NEMD) systems based on the velocity distribution function. Moreover, we examined the momentum balance exerted on a control volume set around the contact line for a single fluid between parallel walls under steady shear. By extending Bakker’s equation, which connects the stress distribution and solid-related interfacial tension, we showed that the force balance among the dynamic interfacial tensions around the contact line can be rewritten by a model equation equivalent to Young’s equation for equilibrium systems. We applied the model equation for steady state NEMD systems of single Lennard-Jones fluid by calculating the stress distribution, and showed that for the present system, the dynamic solid-liquid interfacial tension was constant sufficiently away from the contact line, and the constant value was almost equal to the static value irrespective of the shear rate tested. This result indicated that the dynamic apparent contact angle was not significantly different from the equilibrium apparent contact angle for the present system.   

Heat Transfer Analysis and Heat Sink Design for MELD Additive Manufacturing

MELD~manufacturing is a solid-state additive manufacturing process that provides an alternate path to fusion-based additive manufacturing. Understanding the thermal cycle and hardness of the weld zone for components made~using the~MELD manufacturing process are essential in predicting the quality of the manufactured components. To facilitate better understanding of the process, a simulation of the conjugate heat transfer problem was modeled in ANSYS Fluent. The model was then used to design a heat sink to better disperse thermal energy during material deposition. As a baseline for the heat sink design, a cold plate used in HVAC applications was modeled and tested experimentally. The computational solver was refined using grid size and time-step independence tests, and validated with experimental data. This computational solver was used to design a new heat sink plate to help better control deposited material temperatures. By controlling the temperature of the deposited material, better predictions for the thermal cycle and hardness are possible. This allows for improved prediction of the quality of solid-state additive manufacturing components.   

Understanding the dynamics of reservoir computing in chaotic dynamical systems

Data-driven prediction of chaotic and multiscale dynamical systems is a daunting task. Recently there has been a lot of interest in predicting chaotic and turbulent flow using deep learning methods which have shown mixed results. However, reservoir computing algorithms such as echo state networks (ESN) have shown promise in capturing this behavior both in short term direct prediction as well as the long-term statistics of chaotic systems better than the state-of-the-art in recurrent neural networks. ESNs are an appealing choice for data driven prediction since unlike deep learning algorithms that train the weights through an expensive backpropagation algorithm, the ESN only trains a layer of output weights with linear regression making it orders of magnitude cheaper to train. The memory of the network comes from updating a state vector in the reservoir, using the previous state and the input to the system. In this work we present a theoretical understanding of reservoir dynamics and how it learns the behavior of chaotic signals and how the signals split up as the ``echoes'' of the system, an analytical expression and bound for the ``dynamical memory'' in the reservoir and seek to understand the mechanism which brings about the effectiveness of ESNs in capturing chaotic dynamics.   

Non-negligible Flow Scale for the Vortex Generation in an Isotropic Homogeneous Turbulence

The present study investigates the flow scale that contributes to the vortex generation, in terms of the local flow topology. In order to specify the flow scale that leads to the flow transition into a vortex, we focus on the key flow that is expressed as the velocity gradient tensor component in specific coordinate system associated with predicted swirl plane, and the Fourier spectrum is applied to express the key flow as a function of wavenumber that is not the Fourier coefficient. This analysis is performed in an isotropic homogeneous decaying turbulence with low Reynolds number in Direct Numerical Simulation. Subjected nodes of the vortex generation are extracted by monitoring the swirlity that specifies the unidirectionality and intensity of the azimuthal flow. In the transition, middle and small flow scales, e.g., the flows composed of wavenumbers 20-40 and 60-120 in the total wavenumber less than 120, are indispensable for the creation of the key flow. Therefore, although much smaller flow scales have less kinetic energy and may be assumed not to affect greatly to turbulent flows in terms of flow dynamics, these may play non-negligible role for the vortex generation. This feature seems to be inherent in low Reynolds number of this turbulence.   

Analysis of Vortical Structure in Terms of Local Flow Topology in an Isotropic Homogeneous Turbulence

The present study investigates vortical structures in an isotropic homogeneous decaying turbulence in the Direct Numerical Simulation. We apply the swirl plane and physical quantities, the swirlity, sourcity and vortical flow symmetry quantity, associated with the invariant local topology (Nakayama, FDR 2014). The swirlity and sourcity specify the unidirectionality and intensity of the azimuthal flow and radial flow, respectively in an arbitrary plane, and the symmetry quantity specifies the direction and the degree of skewness of the vortical flow. The vortical structure is statistically analysed in terms of the distribution of azimuthal velocity and radial velocity which are extracted from the flow in the swirl plane around points where the swirlity has maximum value after respecting the direction of skewness of the vortical flow in that plane because vortical flow generally swirls with respective skewness. The swirling radius is estimated from the distribution of both azimuthal velocity and swirlity. The distributions of azimuthal velocity and radial velocity that are non-axisymmetrically and non-linearly, respectively, indicates that none of vortex models is similar to this vortical structure.   

Relationships between the eigen-vortical-axis line and the vortex stretching in an isotropic homogeneous turbulence

The present study investigates the relationships between the eigen-vortical-axis line and the vortex stretching in an isotropic homogeneous decaying turbulence in a low Taylor Reynolds number. This axis line has been proposed as a vortical axis based on the invariant local flow topology and shown that it has stronger characteristics to pierce intense vortical regions than the vorticity line. The vortex stretching can be classified into (1) ineffective stretching that increases vorticity parallel to the swirl plane and (2) effective stretching that increases the orthogonality of the vortical axis and develops a vorticity component associated with swirling. The characteristics of the vortex stretching in a vortical axis are analysed focusing on the effective stretching and an angle between the directions of the vortical stretching and the local axis. Furthermore, it is analysed that the influence of the vortex stretching to the local axis geometry that is defined by the eigenvalues in terms of the gradient of the local axis direction and shows the behavior of the axis line, e.g., twist and convergence. These analyses show that the eigen-vortical-axis line has higher correlation with respect to the effective stretching than the vorticity line.   

Qualitative Approaches to Understanding Coherent Structures in Turbulence

A review of several techniques is presented, with emphasis on recent developments in the mathematical treatment of the proper orthogonal decomposition (POD) as applied to experimental particle image velocimetry data. The experiment in question is the classical flow past a cylinder with three distinct shapes: round, square, and hexagonal at a Reynolds number of 16000. The purpose of the study is to analyze the identification of basic structures and estimation of energy content via filtering methods. Reynolds decomposition is the method of separating the flow field into its mean and fluctuating components. Galilean decomposition uses the bulk velocity to identify small-scale vortices within the flow field. LES decomposition is analogous to a low pass filter that reveals small length scales. POD is an advanced method that takes high dimensional nonlinear coherent structures and converts them into a finite dimensional linearized projection. Vorticity is used to describe the local spinning motion near an eddy and to recover a velocity field from a vorticity field. The accuracy of each method is compared and differences in structures produced by each geometry are discussed. Practical concerns include computational time, data compression, and spatial resolution are addressed.   

Modeling Systems of Drop Carrier Particles Through Energy Minimization

Drop Carrier Particles (DCPs) are solid microparticles designed to capture uniform microliter drops of a target solution without using costly microfluidic equipment and techniques. DCPs are useful for automated and high-throughput biological assays and reactions, as well as single cell analysis. However, little work has been done to understand the behavior of these particles in large scale systems, and researchers have had difficulty achieving uniform volume across the particles. Here we present a method for modeling a diverse range of geometric particle shapes using energy minimization techniques. Furthermore, interactions between two particles are modeled as a pairwise process of minimal energy splitting of the target fluid. We examined the effects of particle geometries on the expected number of interactions needed to achieve the desired uniform volume distribution. We then performed macro-scale experiments of two-particle interactions and compared the observed splitting behavior with our simulations. These comparisons indicate that the model accurately predicts particle splitting behavior and multi-particle volume distributions, thus providing engineers with insight regarding optimal particle geometries.   

Effects of Asymmetries on the Evolution of an Indirectly Driven ICF Capsule Outer Shell

Nuclear fusion offers clean power production in a compact form, and as such is a focus of many avenues of research. Inertial Confinement Fusion (ICF) is particularly promising, though many hurdles remain in the attainment of fusion experimentally. ICF methods involve imploding a spherical capsule composed of hydrogen fuel sheathed in layer(s) of specially selected materials. The implosion is driven indirectly by bathing the capsule in laser driven x-rays. The ionized outer shell is forced inwards by ablative force, compressing the inner layers as a piston, until the fuel reaches an energy state at which robust burn and fusion reactions occur. Fabrication of the shell is difficult, and the form is an imperfect spheroid. Imperfections lead to asymmetrical evolution as the outer shell implodes, introducing instabilities which reduce compressive efficiency. It is necessary to be able to characterize the shape of the outer shell, to predict with simulations the effect of surface features on evolution during implosion. In this presentation I discuss the development of a tool which reads manufacturer profile data of the capsule surface and returns orientation and spherical harmonics. This data was then used to determine the evolution of features during initial implosion stages using an ablative rocket model. Results from these calculations can be used to calculate growth factors for instabilities such as the ablative Rayleigh-Taylor.   

Frequency Downshift in the Ocean

Frequency downshift (FD) occurs when a measure of a wave’s frequency (typically its spectral peak or spectral mean) decreases monotonically. Carter \textit{et al.}~(2018) compared the efficacy of generalizations of the nonlinear Schr{\"o}dinger equation (NLS) at modeling waves with and without FD in wave tank experiments. Narrow-banded swell traveling across the Pacific Ocean can also undergo FD, as evidenced in the classical paper of Snodgrass \textit{et al.}~(1966). In this work, we compare (i) NLS, (ii) dissipative NLS, (iii) the Dysthe equation, (iv) the viscous Dysthe equation, (v) the dissipative Gramstad-Trulsen equation, and (vi) the Islas-Schober equation to see which model best describes the ocean data reported in Snodgrass \textit{et al.}, regardless of observed FD. We do so by comparing the Fourier amplitudes, spectral peak, spectral mean, and quantities representing mass and momentum between the ocean measurements and numerical simulations.   

Accuracy of Equations Modeling Higher Harmonics in Surface Water Waves

We study the evolution of the higher harmonics in surface water wave experiments. We compare numerical predictions from asymptotic reductions of the Euler equations and its dissipative generalizations with measurements from water wave experiments conducted at Penn State University. Our models include the (i) nonlinear Schr\"{o}dinger equation (NLS), (ii) dissipative NLS equation, (iii) Dysthe equation, (iv) viscous Dysthe equation, and (v) the dissipative Gramstad-Trulsen equation. We find the predictions from these models are not always consistent with the experimental data.   

Modeling Passive Drag-Based Fish Interactions and their Relation to Formation Behaviors

Recent work coupled high speed imagery-based fish schooling measurements with computational fluid dynamics to provide estimates of the fish's thrust and drag forces. Reconfigurations indicating reductions in body drag during periods with minimal changes in kinematic inputs suggest that passive drag-based mechanisms could play a role in school reconfiguration and reduce the on-board sensory feedback demands. We use simplified interaction models to study the effects of passive drag-based interaction mechanisms in biological formations. Incompressible hydrodynamic forces were modeled as 1-D incompressible functions of egomotion, position, and velocity. We find conditions for relative position stability within the formation and compare two cases: (a) egomotion and relative position sensitivity only, and (b) egomotion, relative position, and relative velocity. Model (a) shows the agents' relative positions are dynamically unstable theoretically and in simulation, while (b) is dynamically stable. This finding suggests that mechanisms to reconfigure individuals to lower drag states require relative velocity sensitivity, either through fluid interaction functions or active sensory feedback.   

Analysis of the flow of grains through a screw conveyor

Screw conveyors are widely employed in industry for the bulk transport of particulate materials. Several studies have attempted to correlate the discharge rate with the angular velocity of the screw and the pitch to diameter ratio via experiments and particle dynamics simulations. However, a detailed mechanical model that would assist in optimal design of screw conveyors, hasn't been attempted. In this study, we first construct a simple model that assumes the entire granular medium to move as a rigid body sliding along the surfaces of the screw and casing. By enforcing the balances of linear and angular momentum to a suitably chosen continuum element, we show that under certain limiting conditions, the discharge rate for a given angular velocity and screw geometry can be obtained. Further, the discharge can be maximized by setting the pitch to casing radius ratio to a particular value. We then study the detailed flow within the conveyor using the discrete element method and show that a significant fraction of the material exhibits solid body motion, in agreement with the simple model. We assess the effect of relaxing the limiting conditions employed in the model, thereby determining the connection between the friction at the walls and the discharge rate.   

A Yang-Mills Approach for Conformal Invariance in Turbulence

The relationship between conformal symmetry and stochastic processes has been a rapidly growing field over the last twenty years. Among other applications, this pairing was used to suggest the presence of conformal invariance in two-dimensional turbulence via numerical methods. We present a theoretical approach to realize this correlation for an incompressible fluid of the same dimension using techniques from Yang-Mills theory with a non-Abelian gauge group. In doing so, we also provide a new explanation for behavior near zero-vorticity lines.   

Asymmetric forcing of two coupled thermoacoustic oscillators

In many combustion systems, such as gas turbines and domestic boilers, the presence of self-excited thermoacoustic oscillations can reduce reliability and efficiency. Recent studies have shown that such oscillations can be eliminated by carefully tuning the dissipative and time-delayed coupling between adjacent combustors, exploiting a nonlinear phenomenon known as amplitude death (AD). However, although the coupling conditions required for AD are well known, they may still be beyond the reach of practical systems because of the inherent space and operational limitations that such systems face. To address this issue, we examine whether external forcing can be used to enlarge the AD boundaries of a coupled thermoacoustic system. The system consists of two self-excited thermoacoustic oscillators interacting with each other via tunable levels of dissipative and time-delayed coupling. By varying the strength of the forcing acting on each oscillator and the phase difference and detuning between the two forcing signals, we map out the forcing conditions required for AD under different internal coupling conditions, paving the way for an alternative method of suppressing harmful thermoacoustic oscillations in combustion systems.   

Closed-loop control of thermoacoustic oscillations using genetic programming

The use of genetic programming (GP) to discover model-free control laws for nonlinear flow systems has gained considerable traction recently, having been applied for the closed-loop control of recirculation zones behind backward-facing steps, flow separation over sharp edges and turbulent mixing layers. This unsupervised data-driven control strategy has been shown to outperform conventional open-loop forcing, by enabling successful individual control laws to spread their genetic traits from one generation to the next. In this experimental study, we use GP to discover model-free control laws for the suppression of self-excited thermoacoustic oscillations, which are detrimental to combustion systems. We evaluate every individual control law in a given generation on a real-time closed-loop control system equipped with a single sensor (a pressure transducer) and a single actuator (a loudspeaker). We rank the effectiveness of the control laws with a cost function and use a tournament process to breed subsequent generations of control laws. We then benchmark the performance of the final generation against that of open-loop forcing, providing improved control laws for the suppression of self-excited thermoacoustic oscillations.   

Aerodynamic characteristics and flight behavior of the turbo-jav in the javelic throw

In order to improve the record of the javelic throw in the junior Olympic games, it is important to elucidate the flight characteristics of the "turbo-jav" used in this throwing event. The turbo-jav has many geometrical features different from a spear for the javelin throw, including the presence of tail fins. In the present study, we performed wind tunnel tests in a low speed wind tunnel to measure the drag force, lift force and pitching moment exerted on the turbo-jav for various angles of attack. Using the aerodynamic coefficients obtained, we numerically calculated the flight orbit of the turbo-jav and the variation of its orientation, starting from the initial condition obtained at throwing experiments. The throwing experiments and numerical simulations showed that the turbo-jav flies with an oscillatory angle of attack around 0. The throwing distance predicted by the numerical simulation was found to be somewhat shorter than that obtained at the corresponding experiment. Since the amplitude of the oscillation in angle of attack during the flight was larger in the numerical simulation compared to that in the experiment, we proposed a new model in which the resistant term accompanying the unsteady pitching motion was taken into account.   

Using quadrotor IMU data to estimate wind velocity

Due to their hovering ability and light frame, Micro Aerial Vehicles (MAVs) have the potential to conduct cheaper wind measurements with greater spatial resolution than weather balloons. However, wind sensors increase MAV payload and therefore increase cost and decrease endurance. To avoid a mounted sensor, wind velocity estimation models have been constructed using on-board Inertial Measurement Unit (IMU) data collected during flight. Existing models are able to relate IMU data to wind velocity, but they rely on calibrations and physical assumptions that limit measurement accuracy. To improve the accuracy of velocity measurements, we used a quadrotor to collect IMU data while surveying the surrounding flow using Particle Image Velocimetry (PIV). We compared IMU and PIV data for a quadrotor in still air, in the wake of another quadrotor, and in a vortex. We then evaluated how well existing models could determine velocity and turbulence intensity based on IMU data alone. Using IMU data in place of a mounted anemometer could reduce payload and allow low-cost tracking of gas plumes, pollution, and weather patterns.   

Improving the Descent Performance of Small-Scale Rotorcraft through Added Geometries

The descent stage of all rotor vehicles---from helicopters to drones---results in a significant loss of thrust and increased fluctuations with respect to the system in hover condition. These losses are believed to derive from the reinjestion of the rotor flow that causes an accumulation of tip vortices at the rotor plane: often referred to as the vortex ring state (VRS). An approach of utilizing additional geometries within the proximity of the rotor plane was investigated by using enclosed shrouds and various props (with enhanced blade tip designs), in order to improve the stability and performance of rotorcraft in descent. These geometries were aimed to prevent the interaction between the blade tip vortices and the rotor disk. Results from single rotor thrust tests indicate that it is possible to reduce the thrust losses within the VRS by adding geometries in distinct locations relative to the rotor disk, while additional PIV analysis potentially outlines the underlying flow mechanism that causes these performance improvements.   

2D Particle Image Velocimetry and Computational Fluid Dynamics Study on Sidewall Brain aneurysm

Cerebral aneurysms are cerebrovascular abnormalities characterized by the weakening and dilation of a localized cerebral arterial wall. They are a prevalent vascular disorder affecting 2--5{\%} of the worldwide population. Aneurysm rupture can be fatal. To prevent aneurysm rupture, endovascular stents can be deployed to redirect blood flow away from the aneurysm, reducing blood flow velocity in the aneurysm sac and encouraging blood vessel remodeling. Since hemodynamics plays a key role in aneurysm progression, an in vitro experimental setup was developed to mimic the cerebral circulation with particle image velocimetry (PIV) testing different internal carotid artery sidewall aneurysm models. The Newtonian and the non-Newtonian working fluids with matching density and viscosity of human blood were used. In addition, a computational fluid dynamics (CFD) analysis was conducted in parallel. Computational model was first verified and validated against PIV measurements and showed good agreements. The study showed that the Newtonian model overestimated hemodynamic parameters, such as the blood velocity and wall shear stress in the cerebral aneurysm sac, comparing to the non-Newtonian model, suggesting the shear thinning effects might be more prominent in this region.   

Towards a better understanding of the flow mechanisms involved in blunt traumatic aortic rupture

Blunt traumatic aortic rupture is a heart injury that can occur in falls, automobile accidents, and sporting injuries involving impact to the thorax. Despite its severity and high morbidity rate, the research still does not provide a consistent description of the mechanism of rupture. In this study, a crash testing dummy with an~in-vitro~pumping heart, 3D printed ribcage, and ballistic gel damping layer was developed to reproduce a realistic response to thoracic impact. Testing was performed using a standardized pendulum used for calibration of crash test dummies, with the location of impact being the middle of the sternum. Different impact severities were tested by adjusting the kinetic energy at impact with the initial height of the pendulum. Measurements on the dummy include instantaneous aortic pressure waveforms during impact and accelerations at the spine and sternum. The results of this experiment show that aortic pressure experiences significant changes in magnitude during simulated impact. This work could help contribute towards a better understanding of the mechanisms leading to blunt traumatic aortic rupture and the development of preventative measures.~~~   

Modelling flow driven by travelling wave motion in the glymphatic system

The glymphatic system is a waste removal mechanism of the brain. The glymphatic pathway includes perivascular spaces (PVS), which are annular channels surrounding blood vessels in the brain. These channels are filled with cerebrospinal fluid (CSF). There is substantial experimental evidence that the pulsation of these blood vessels drives CSF flow through the PVS. Irregular or weak arterial pulsations may cause suboptimal flow and poor waste removal. A build-up of these wastes (e.g., amyloid-beta) is linked to the development of neurodegenerative disorders such as Alzheimer's disease. This research introduces a laboratory model of an artery and surrounding PVS. A viscous fluid fills the annular space between a small flexible tube and a rigid transparent cylinder. A travelling wave is generated by moving spherical beads through the flexible tube. The speed of the travelling wave is varied, and flow between the cylinders is quantified with a Lagrangian particle tracking algorithm. It was observed that increasing the speed of the travelling wave increased flow speed in a linear manner. This work provides an experimental foundation to investigate the effect of other parameters, such as frequency, amplitude, and wave shape.   

Hydrodynamics of Lemon shark's (\textit{Negaprion brevirostris}) dorsal fins

To improve understanding of the hydrodynamic functionality of dorsal fins of sharks, we focus on the Lemon shark (\textit{Negaprion brevirostris}), which features a second dorsal fin that is almost as large as its first dorsal fin; an uncommon feature of migratory shark species that is not well understood from both a physical and biological perspective. Laboratory experiments are performed in a flume using PIV to measure the near wake flow behind the first and second dorsal fins, and behind the tail of a deceased Lemon shark as well as a 1:1 ratio 3D printed flexible shark model. Vortex shedding in the wake is characterized through POD applied to vorticity fields estimated from the PIV data. Hydrodynamic forces are estimated using the velocity deficit in the wake to estimate drag, and a thrust model based on the characteristics of the vortex street in the wake. Drag and thrust coefficient results behind the first dorsal, second dorsal, and caudal fin are compared.   

Thrust generation of fish-fin geometries in continuous rotation

Fish are known to use flapping motions of their caudal fins to propel, while man-made propellers commonly use continuous motion to achieve thrust. It is not known if fish use the flapping due to its limited range of motion or if it is the most effective way to maneuver in the water. The objective of this work is to analyze the propulsive performance of propeller geometries similar to those of fish fins when performing continuous rotary motions. Because caudal fins possess distinctive morphologies and stiffnesses adapted to the fish’s specific modes of life, varying geometries and compliances have been considered. In order to evaluate the thrust generation properties of these fins, an underwater vehicle capable of generating continuous rotation and equipped with exchangeable propellers has been built and tested in a water tunnel. The thrust forces generated by the different fins are presented and compared.   

Vapor bubble condensation in Hele-Shaw cell submerged in subcooled pool

Cooling technologies based on boiling heat transfer have attracted attentions as the heat-generation density of electronic devices increases. It is known that the microbubble emission boiling (MEB) can overcome critical heat flux under the specific subcooled condition; thus, MEB regime is expected to be applied to next-generation cooling devices. However, the occurrence condition of MEB has been not yet understood. Since MEB has the remarkable feature that vapor bubbles abruptly condense and collapse, it is of great importance to understand the effect of the condensation on the MEB. In order to focus on the condensation process of the vapor bubble, we inject vapor into a narrow gap region between two glass plates (Hele-Shaw cell) submerged in subcooled pool of water. The Hele-Shaw cell allows us to access clear visualization inside the vapor bubble. Special attention has been paid on the correlation between the condensation process of vapor bubble and energy exchange through the bubble surface.   

Development of a nanoscale hot-wire probe for supersonic flow applications

A new sensor based on the nanoscale thermal anemometry probe (NSTAP) was designed and fabricated in Princeton University's clean room to obtain well-resolved mass flux measurements in supersonic flows. In order to withstand high forces, the sensing element was redesigned to be 400 nm thick, 2 $\mu $m wide and 30 $\mu $m long. The sensor was tested in the Trisonic Wind Tunnel Munich (TWM) at Bundeswehr University. The TWM is a two-throat blowdown tunnel with a unique capability of altering both the Mach number and the Reynolds number independently and in real-time. Freestream measurements were taken at M$=$2 to investigate the convective heat transfer characteristics of the sensor. A linear calibration between the Nusselt number and the Reynolds number appeared to best fit the data. This linear Nu-Re dependence has previously been observed when operating hot-wires in free-molecule flows. Given the relatively large Knudsen numbers experienced by the sensing element (Kn\textgreater 0.01 based on both width and thickness of sensing element), the sensor is believed to operate in slip flow conditions, exhibiting unique heat transfer characteristics. The importance of both the thickness and the width of the sensing element was also investigated theoretically for various Mach numbers.   

Three distinct liquid drop detachment dynamics on vibrating 1D rod structure

Along with physical impact of liquid drops, condensations can induce undesirable drop formations on solid surfaces. This outcome can possibly downgrade the heat transfer efficiency and can even contaminate sophisticated structures that may cause costs during microfabrication processes. This problem can be alleviated by vaporizing unwanted drops through controlling temperature or humidity. However, this method is cumbersome and cannot be applied in the case of viscous fluids. Thus here, we explore precise dynamics of overall fluid drop detachment on solid surfaces through damped harmonic oscillations. To model the complexity of the surfaces, we utilized 1D cantilever rods having high curvatures. The vibration of cantilever beams with small deflections (below 1 cm) were tested to see three different types of drop detachment behaviors depending on surface energy, fluid viscosities and volumes. Three dimensionless numbers (Weber, Capillary, and Bond numbers) were used to identify at which conditions the drops detach from the beam surface not affecting neighboring structures or other parts of the same body. We concluded that high gravitational and inertial forces (We$\ge $10 and Ca/Bo$\le $0.1) are favorable for clean drop detachment leaving no residue on solid surfaces.   

Generating Unsteady Pressure Gradients Using Rapidly Deforming Surface

This study demonstrates the capabilities of an experimental facility constructed to impose dynamic pressure gradients on the flow. A rapidly deforming ceiling causes a dynamically strengthening favourable and adverse pressure gradient in sequence. Curvature of the ceiling is characterised in an instantaneous manner by using a high-speed camera, along with the corresponding spatial variation of the strength of FPG and APG imposed. The maximum speed of deformation of the ceiling is 1 m/s, and the acceleration parameter, K, at the section at the point of maximum deformation ranges between $2.5 \times 10^{-6}$ and $8.5 \times 10^{-6}$. Time-resolved planar Particle Image Velocimetry is used to investigate the behaviour of the flow over and around the deforming ceiling. The different geometric and dynamic conditions tested will allow investigation of the effect of dynamic pressure gradients on the behaviour of turbulent boundary layers.   

Comparison of Complex Multi-Stream Supersonic Nozzle Geometry

Noise continues to be a concern with further developments of supersonic flow and the structural geometries surrounding it. A rectangular multi-stream supersonic nozzle with an aft deck is resolved utilizing stereo PIV along with near and far-field pressure and acoustic measurements. A focus is placed on optimizing for noise reduction while altering aft deck geometry through the development of a previously trained neural network. This is accompanied by incorporating a varied splitter plate which decomposes the flow field into two canonical flows, a core supersonic flow interacting with a subsonic wall-jet. The synergy of these two flows creates complex turbulent structures which feed into amplifying noise. Through the combined data from stereo PIV, near and far-field pressure and acoustic measurements, the goal is to design a low noise aft deck plate while furthering an understanding of how the splitter plate geometry effects the multi-stream flow interaction and relevant acoustic measurements.   

Analysis of an acoustic levitator flow field using Particle Shadow Velocimetry

High concentrations of Nitrogen Oxide (NOx) emissions from diesel engines pose a threat to the environment. However, selective catalytic reduction (SCR) offers a means to reduce these NOx emissions. Optimization of SCR technology requires study of urea-water-solution (UWS) evaporation behavior to tune droplet evaporation models. Validation data for this purpose may be obtained via observation of acoustically levitated droplets under controlled pressure, temperature, humidity, and flow field conditions. Establishing well-defined conditions prior to actual evaporation experiments is vital for model development. One driving factor behind the heat and mass transfer associated with droplet evaporation is the relative velocity imposed by the flow field of the gas phase near the droplet. To characterize the levitator flow field, Particle Shadow Velocimetry was employed. The optical setup features a double-frame camera equipped with a long-distance microscope that is maneuverable along three axes, allowing for targeted assessment of locations in the acoustic levitator without adjusting the illumination. This measurement technique facilitated incremental improvements which resulted in a symmetric flow field deemed suitable for investigation of UWS droplet evaporation.   

Drive an Object Using Photothermal Convection Around a Water Vapor Microbubble

By using the photothermal heating of the thin film under laser irradiation, a water vapor microbubble is formed in degassed water. The bubble involves rapid flow, which results from the Marangoni force and bubble oscillation caused by a steep temperature gradient. It is assumed that the flow direction is changed by giving an asymmetric temperature gradient. Therefore, we changed temperature distribution by laser irradiation with multiple spots. By focusing a laser spot on the thin film immersed in degassed water, a water vapor bubble with a diameter of approximately 10 $\mu $m was created. Simultaneously, a sub laser spot was focused next to the bubble to yield a temperature gradient in the direction parallel to the film surface. Consequently, the rapid flow was generated around the bubble, whose direction was dependent on the power and position of the sub. Then, we formed on a thin mica chip as a lighter substrate the bubble which can generate flow parallel to the film surface. Finally, we succeeded in moving the mica chip by using the reaction force of the photothermal convection. It is expected to be utilized as the technique for a driving force in microfluidics.   

Double-diffusive and Rayleigh-Taylor instabilities in particle-laden water stratified over salt water in a Hele-Shaw cell

An experimental and computational investigation is performed for double-diffusive (DD) and Rayleigh-Taylor (RT) instabilities in particle-laden fresh water initially stably-stratified over salt water in a Hele-Shaw cell. \textit{Computationally}, Darcy's Law coupled with an advection-diffusion equation for salt and an advection-diffusion equation for particle concentration that includes a settling velocity is solved for two-dimensional stratified fluids in the presence of particle-loading. The flows are parametrized in terms of a stability ratio, a gravity parameter, and a dimensionless settling velocity. Results are analyzed in terms of relative dimensions of concentration profiles of sediment and salt. \textit{Experimentally}, Schlieren imaging is used to image fresh water containing 3-6 $\mu $m glass microparticles layered above salt water both containing glycerol to slow dynamics. Dimensionless wavelength and time and distance until onset of instability are measured. Results are interpreted in terms of a ``nose'' region of increased density at the interface.   

Experimental analysis of dilute particle-laden liquids over and through patterned structures

Particle-laden liquids are encountered in various applications both in laminar and turbulent flows. However, the concentration and velocity profiles of dilute suspensions when they are flowing over and inside a patterned surface are not yet known. This experimental study is conducted to examine the interaction of the particles in various dilute suspensions over and through a patterned structure. The patterned surface consists of cylindrical rods arranged in a square array. Particle Tracking Velocimetry data provides velocity and shear rate at the interface between the free flow region and the surface for dilute suspension flows. We examined the velocity and concentration profiles through and above the structure for various dilute suspensions. We find that the shear rate and velocity profiles are strongly dependent on the suspension properties and geometry of the structure.   

Accumulation structure of low-Stokes-number particles in high-aspect ratio half-zone liquid bridge of high Prandtl number fluid

Various space experiments on the International Space Station (ISS) have been conducted to elucidate the Marangoni convection in liquid bridge formed under the micro-gravity environment. Since the cost of space experiments is apparently expensive, it is necessary to accumulate knowledge of the phenomena on ground as much as possible in term of preliminary experiments. In addition, ground experiments can allow us to clarify the effects of gravity. We focus on the particle accumulation structure (PAS) produced by the thermocapillary effect in a half-zone liquid bridge on the ground. To compare with the PAS realized in Japanese Experimental Module Kibo in the ISS, the PAS in high-aspect ratio half-zone liquid bridge of high Prandtl number fluid is studied. The role of gravity is discussed through the comparison of these experimental results also together with linear stability analysis.   

Water entry of hydrophilic spheres through fabric-fluid interfaces

The vertical impacts of solid projectiles with the free surface of a deep aqueous pool are traditionally investigated with respect to impactor shape, entry speed, and surface roughness. Free surface alteration in some cases, may be more readily achieved for the modulation of splashes. In this combined experimental and theoretical study, smooth, free-falling, hydrophilic steel spheres impact penetrable and non-penetrable fabrics resting atop the fluid surface for Weber numbers in the range of 430-2700. Penetrated fabrics remain near the free surface, suppressing the splash crown, but allowing passage of a Worthington jet whose height increases with the depth of the trailing cavity. Non-penetrable fabrics create deep seal cavities by veiling the descending impactor, generating higher Worthington jets, and pronounced splash crowns. Some fabrics, both penetrable and non-penetrable reduce drag with respect to clean surface impacts by providing the drag-reducing benefits of flow separation while not offering a high inertial penalty. Such observations augur well for military and industrial applications where splashes warrant control to mitigate damage to life and property.   

Plume Chamber studies to characterize Turbulent Buoyant Plumes using multiple sensors.

Turbulent axisymmetric buoyant plumes released into calm air are studied experimentally using multiple sensing techniques by a hot-wire anemometer, Schlieren imaging and a FLIR VUE pro thermal camera. Heated Carbon Dioxide is released into a plume chamber under controlled conditions at a nozzle exit Reynolds number ranging from 1300 to 2000. The buoyancy flux and momentum flux at the nozzle exit are varied to study the flow behavior as a function of initial conditions. Time-averaged statistics such as centerline velocity, temperature and plume half-width are calculated using hot-wire readings and image-processing of thermal camera and Schlieren recordings. To better understand the effect of buoyancy on turbulence and mean velocity, a reference case with no buoyancy flux at nozzle exit is studied.   

Interactions between bathtub vortices in a rotating experiment

The bathtub vortex is a flow pattern uniquely suited for laboratory study, as the flow through a drain hole provides a radial inflow and vortex stretching in a controlled location, thus stabilizing the vortex. While these radial flow patterns provide stability, the attraction they cause between any number of such vortices makes it difficult to achieve a stable state of multiple bathtub vortices, thus limiting their utility as models for experimental study of vortex interactions. Here we test the effect of global rotation on such vortices by spinning a cylindrical container with two drain holes in the bottom, which we find allows the formation of multiple bathtub vortices in a co-rotating stable state. The orbit period of these vortices is observed to change with the global rotation rate, and apparently spontaneous switching between states with different numbers of vortices is observed, with larger numbers of vortices possible at higher global rotation rates.   

Flow induced vibrations from two circular cylinders in close proximity

The present study is focused on investigating flow induced vibrations (FIV) of two circular cylinders (of diameter D and D/8) in proximity. The smaller diameter cylinder (referred to as the `control' cylinder) is controlled via a two degree of freedom (x,y) traverse system. The system uses a genetic algorithm in an effort to maximize the amplitude response of the FIV of the larger diameter cylinder (referred to as the `main' cylinder). Due to the difference in cylinder diameters and corresponding natural frequencies, at flow velocities where FIV begins to occur for the main cylinder, the control cylinder vibrates with a significant amplitude. To combat this issue, passive flow control methods are employed (i.e., helical strakes). The amplitude response of the main cylinder is investigated in detail in the present study using a combination of time-resolved planar PIV measurements and displacement measurements. The results show that the amplitude response and corresponding wake dynamics of the main cylinder are impacted significantly by the control rod and its positioning relative to the main cylinder.   

Noise Characterization of a Two Circular Cylinder Flow Induced Vibration Energy Harvester System

The development of flow induced vibration (FIV) energy harvesters is an active area of interest as an alternative to traditional hydropower systems. We have developed a mechatronic system to investigate FIV of two circular cylinders (of diameter D and D/8) in proximity. The smaller diameter cylinder (referred to as the `control' cylinder) is controlled via a two degree of freedom (x,y) traverse system. The system uses a genetic algorithm to find the optimal parameters describing a sinusoidal motion of the control cylinder, in order to maximize the amplitude response of the FIV of the larger diameter cylinder. In testing this system, various sources of noise have been identified that disrupt the genetic algorithm's ability to find the optimal control cylinder parameters. To minimize the impact of noise on the system, a secondary genetic algorithm will be used to characterize the noise properties of the system as a function of the control cylinder parameters. Insights from this analysis will allow for modifications to both the mechanical system and the software to improve the overall performance of the system.   

Controlling chaos by the domain size

As part of the recent effort to understand dynamics and evolution on time-dependent spatial domains, we present an experimental investigation on how domain deformation may serve as a mechanism regularizing chaotic motion. Faraday waves -- standing waves formed on the free surface of a liquid layer due to its vertical vibration -- are chosen here as a paradigm owing to their historical use in testing new theories and ideas. In our experimental setup of a vibrating water container with controlled positions of lateral walls, the Faraday patterns are visualized using the Fourier transform profilometry and the wave amplitude is measured using a high accuracy laser displacement sensor: these techniques allow us to reconstruct a time history of the pattern three-dimensional landscape. Data analysis reveals that domain deformation is not only able to transform the chaotic state of two competing modes into a regular (periodic) one, but also to isolate one of the competing modes in the regime, which on a time-fixed domain of the same size would otherwise correspond to a regular or chaotic pattern competition. These experimental findings are interpreted with appropriate theoretical arguments and insights.   

Measurements of Reacting Fuel Sprays Using High-Speed Imaging

Engineers are currently looking for solutions to reduce waste and the effects transportation needs have on the environment. One method is to transition to biofuels in diesel engines. Computational Fluid Dynamics (CFD) simulations can be useful provided enough experimental data is available for validation. Hence, we need real experiments to determine the relationship between simple properties of fuels that we can measure and the results of actual combustion experiments. An optically accessible constant pressure flow rig (CPFR) is the primary experimental apparatus and it can be used to set control parameters to study the fuel-air mixing in conditions similar to diesel engines. A schlieren camera measures initial fuel-air mixing, a chemiluminescence camera measures initial combustion, and a two color pyrometry camera measures soot production. Extensive test campaigns at various injection conditions, ambient conditions, and fuel type will enable a new level of understanding of the diesel combustion process. Before significant testing can be completed, the primary focus of the work presented here is on efforts to optimize the experimental set-up by addressing some key experimental challenges that had previously limited the quality of data obtained. Significant effort was also dedicated to developing code to aid in processing a limited initial data set. This code serves as a proof of concept that can be leveraged for larger data sets to be acquired in the future. As a result, the lab is an efficient and effective work space that allows for ease with acquiring data.   

Predicting Plasma Plumes Induced by High-Powered Lasers on a Copper Plate under Different Conditions.

Industries that require material removal on a micro level, e.g., microelectronic industry, utilize high powered lasers that essentially vaporize the material away. When such high-power lasers strike materials, however, they often create plasma. The plasma plume absorbs the laser radiation before it reaches the target, reducing the rate of material removal. In the present work, plasma plumes induced by irradiation of a copper plate with a short-pulse laser under various background gas pressures are studied experimentally. The plasma is recorded using Schlierden imaging and a highspeed camera. This project is aimed to recreate the plasma environment in order to better predict its behavior. With better understanding of the plasma, we can more efficiently use these lasers to remove material by minimizing plasma absorption effects.   

Preliminary Characterization of an Iodine Plasma Source for use in Material Analysis

Electric propulsion devices using xenon as a propellant are a high efficiency solution for large conventional satellites. The high storage density of iodine would enable these devices to require less mass for use in space technologies, if used as a propellant as an alternative to xenon. The ability to reduce the mass required for electric propulsion devices would not only reduce costs of space travel but also open up new opportunities for these devices to be used in smaller, more volume constrained missions. Iodine is a strong oxidizing agent. To determine if it is a viable alternative, the erosive properties must be quantified. The object of this research project was to characterize an iodine plasma source before using it for material exposure and analysis. A double Langmuir probe was used as the method of data acquisition for the plasma conditions. The plasma characterization identified the conditions in the plasma source that will be used to properly quantify the erosive properties in iodine plasma. Preliminary results indicate a maximum electron temperature of four electron volts, and a maximum plasma density of eight inverse cubic centimeters.   

Measuring Pressure and Strain with Luminescent Coatings

Often, researchers employ probes such as pressure taps and strain gauges to measure the pressure and strain on aerodynamic objects. However, these tools lack high-resolution and full-field capabilities that may be necessary for high-speed aerodynamic testing. A combination of photoelastic coatings (PEC) and pressure sensitive paint (PSP) has the potential to provide researchers with correlated, full-field surface measurements of maximum shear strain and pressure, respectively. Photoelastic coatings use circular polarized light along with birefringent material properties to provide information about the surface strain of objects, while pressure sensitive paint utilizes oxygen-quenched luminophores to measure pressure. Benchtop test results will be presented of a dual-layer PEC/PSP coating applied to cantilever specimens subjected to static and dynamic loading and imaged with a micro-polarizer digital camera.   

Can we measure the three-dimensional orientation of \textit{Vorticella} from two-dimensional videos?

\textit{Vorticella} are aquatic suspension feeding microorganisms that live attached to surfaces and generate a feeding flow to draw in their food. They are crucial players in aquatic ecosystems, eating bacteria and debris as well as supporting larger aquatic organisms. To evaluate the impact of \textit{Vorticella} in their environments, as well as in practical applications like waste water treatment, it is important to understand \textit{Vorticella} feeding rates. Previous work has shown that the orientation of \textit{Vorticella} relative to the surface of attachment affects feeding flow and feeding rates. \textit{Vorticella} cell body orientation is defined by the polar angle, which is measured from the vertical axis, and the azimuthal angle. Previous experiments have observed \textit{Vorticella} using a horizontal microscope from which the polar angle was directly measured. The azimuthal angle was inferred as a function of the projected cell body length compared to a maximum measured cell body length. However, it is unknown how accurately this technique determines the azimuthal angle. We recorded time-lapse videos of \textit{Vorticella} simultaneously from the side and the top. We then compared the calculated azimuthal angle from the side view to a direct measurement from the top view. We report the error in the calculated azimuthal angle as a function of the organism orientation.   

Influence of characteristic components of blood on blood splashing onto a solid wall

Understanding of blood splashing on a solid wall is of great importance in forensic science since blood splashing determines the blood pattern remained on the wall. Blood is known as a complex liquid, containing platelets and blood cells such as red blood cells and white blood cells, as well as coagulant factors and liquid components such as water and proteins. In this study we investigate influence of characteristic components of blood on its splashing onto a solid wall by separating main components of the bloods. In our experiment, whole blood, containing all of the components, can be separated into three types of liquids: (i) Serum containing only liquid components of blood, (ii) PRP containing serum with coagulant factors, (iii) Plasma containing PRP with platelets. The droplets of these liquids and blood simulant are dropped from a needle at various heights and recorded by a high-speed camera. It is found that the droplet of whole blood shows a quite different behavior from droplets of the other types of liquids. We also compare the experimental results with the theories proposed recently and discuss the effect of the blood components on splashing.   

Wind tunnel testing of a NACA 0012 airfoil with passive biomimetic flow control devices

Increased demand for eco-friendly and energy-efficient transportation have led researchers to explore methods of drag reduction for decades. Previous studies have shown some success by both passive and active flow control techniques. Recent studies have shown that shark skin has the ability to passively alter the flow by inhibiting flow reversal and controlling flow separation. This study examines various 3D printed biomimetic flaps and scales inspired by sharks and birds to begin to determine optimum design parameters such as the presence of surface riblets and characteristic length. The biomimetic surfaces are placed on the upper surface of a NACA 0012 wing. Force data is acquired for baseline and biomimetic surface models (Re$_{\mathrm{\thinspace }}=$ 200,000) at fixed angles of attack (steady case), gradual pitching rate (quasi-steady case), and high frequency pitching rate (dynamic case). Results will be presented as to the correlation between the biomimetic flaps and the alteration of airfoil lift and drag.   

Particle alignment under unsteady shear in non-Newtonian fluids causing modulations of effective viscoelasticity

The effective viscoelasticity modulated by particle alignment in unsteady shear in non-Newtonian fluids was revealed by means of ultrasonic spinning rheometry (USR) [Yoshida et al., J. Rheol.(2019)]. The dispersed particles make alignments in the sheared direction in unsteady shear flows when the fluid media have sufficiently long relaxation time. USR evaluated the effective rheological properties modulated by the particle alignments; (1) the effective viscosity does not reach the value estimated by Einstein's law; (2) the effective elasticity increases significantly with the increasing volume fraction of particles in the bulk of the measurement volume. To clarify factors causing the particle alignment in unsteady shear flows/deformations, numerical tests using a simple toy model with considering spring forces connecting between the particles with specific yield stresses were examined. The numerical tests explained the importance of the relaxation process on the orientation of the particles. To conclude the experimental findings supplemented with the results of the numerical tests, we suggest that local and macro rheological characteristics are strongly modulated by the particle alignment when the test fluid media have long relaxation times.   

Direct and indirect influence of crystallization on double-diffusive convection in ammonium-chloride solutions

Crystallization inside fluid flow is a complicated, but highly fascinating phenomenon in the field of geo- or astrophysics. For instance, a magnetic field of Jupiter's moon Ganymede is thought to be sustained by crystallizing flow termed “iron-snow”, but little is known up to date. Thus, experimental tests to seek likely conditions for a presence of a dynamo are required. We have approached these phenomena via a model experiment using a test fluid of aqueous ammonium-chloride solution, which changes phase from liquid to solid under room temperature. To quantify velocity and temperature fields simultaneously, thermochromic liquid crystals (TLC) were suspended into the test fluid. We have established a masking technique to obtain velocity fields of the continuous and the solid phase. In addition, a neural network based thermometry utilizing TLC coloration has been established. With these methods, we found a direct and indirect influence of the crystals on the flow. At the beginning of the cooling process, intense precipitation of equiaxed crystals can directly modify the flow structure. Meanwhile, columnar crystals start to grow, and impede the cooling from the wall. Indirectly, crystal growth creates a stable density stratification, but an unstable temperature stratification.   

Experimental study on flow-induced vibration of tandem flexible cylinders at varying angles of inclination

Flow-induced vibration (FIV) response of a highly flexible inclined circular cylinder placed in the wake of a stationary cylinder is studied experimentally. The flexible cylinder is tension-dominated with an aspect ratio of 47 and a high mass ratio of 120. The cylinder was held fixed at both ends and placed in the test-section of a subsonic wind tunnel. The angles of inclination were varied from 0\textdegree to 45\textdegree with increments of 15\textdegree . The inclined flexible cylinder lied in the wake of an upstream stationary cylinder of equal diameter and inclination. The dynamic response of the downstream flexible cylinder is studied for center-to-center spacing range from 3 to 7 times the cylinder diameter, in the reduced velocity range of$ U$*$=$3.6-48.5 and the Reynolds number range of \textit{Re }=260-3750. Influence of inclination and cylinder spacing is investigated through studying the dynamic response of the cylinder in terms of the excited structural modes, amplitudes and frequencies of oscillations and transition between modes. Different values observed for the onset of oscillations and modal weight contributions explains that the FIV response of the system is different from that of a completely vertical cylinder for all angles of inclination larger than 15?. Dynamic response of the flexible cylinder was found to be always under the influence of the upstream one, even for large cylinder spacing of 7d.   

Effect of Seed Density on Dispersal of Seeds from Wet Splash Cup Plants

Splash cup plants use raindrops to disperse their seeds. Plants are approximately 4-30cm tall with \textasciitilde 5mm-diameter fruit bodies. When raindrops fall into their conical fruit bodies, the splash ejects the seeds up to 1 m away from the parent plant. Understanding how the seeds are projected may enable a deeper understanding of dispersal after drop impact in other contexts and of splash cup plant evolution. Previous work using 3D printed cones as fruit body mimics found that maximum dispersal occurs with a 40$^{\circ}$ cone angle, defined as the angle between the side of the cone and the horizontal. Later work found that seeds, which were not accounted for in the original study, decrease dispersal distance. Seed density has also been found to correlate inversely with the average dispersal distance of seeds projected from a dry cup. In this study, we investigate the effect of seed density on dispersal from a cup that contains both seeds and water since this situation is commonly found in nature after one splash has occurred. We use 3D printed cones, a low-density polyethylene seed mimic, a high-density glass seed mimic, and high-speed video to analyze the splash. Our results show that lighter seeds travel further than heavier seeds, and the optimal cone angle remains 40$^{\circ}$ $_{\mathrm{\thinspace }}$for both densities.   

Inertial, Aerodynamic and Elastic scaling of a passively pitching insect wing

Evidence suggests that prominent features of insect wing pitching behavior are affected by inertial and aerodynamic forces with largely passive contributions from the wing hinge joint, which acts like a torsional spring. Motivated by the robotic applications of insect-inspired passive-pitching flapping wings, we study the scaling relationship between aerodynamics, inertia, and elasticity in the regulation of wing pitch and in the generation of lift forces in hovering flight. We measure forces and wing kinematics using an under-actuated robotic model with a prescribed wing stroke and an elastic wing hinge. Our data show that suitably defined dimensionless parameters accurately predict aerodynamic performance for wings of varying geometrical, physical, and operational parameters. We also observe a dependency of pitching kinematics on these dimensionless parameters, providing a connection between lift coefficient and pitch angle characteristics. Our results illustrate the trade-off between contributions to lift by quasi-steady and rotational dynamics associated with wing translation and wing rotation at stroke reversal. These results will be of value both in understanding the mechanics of insect flight as well as in the future design of under-actuated flapping aerial robots.   

Waterbowls: Reducing Impacting Droplet Interactions by Momentum Redirection

Droplets impacting a solid surface can transfer mass and energy to that substrate. While superhydrophobic surfaces can restrict this transport by making drops bounce off, the liquid-solid contact is still extensive. Recent studies aiming to limit this transport further have focused on reducing the contact time. Here, we remark that flux-based transport phenomena scale with the contact area as well as time leading us to define an Interaction Parameter as the integral of the contact area as a function of time to describe the drop-substrate interaction. We design superhydrophobic surfaces with a macroscopic structure that redirects the momentum of the spreading lamella upwards, thereby restricting the liquid-solid contact. We show that, when optimally designed, such surfaces can reduce the interaction parameter by an order of magnitude compared to a regular superhydrophobic surface and provide design guidelines for the macroscopic structure. Finally, we demonstrate that a well-designed surface can reduce heat transfer between a simulated rain and a solid surface by a factor of two.   

Particle Focusing in the Poiseuille Flow of Colloidal Dispersion

We recently reported the lateral migration and focusing of non-colloidal particles, suspended in the nanoparticle colloidal dispersion, by its normal stress differences in microchannel flow at Brownian-motion-dominant low Péclet number conditions [1]. In this presentation, we will demonstrate the key experimental results of the normal stress difference-driven particle focusing in the colloidal dispersion [1]. We will also show that the experimental results are in consistency with the existing theories on the colloidal rheology [2] and the lateral particle migration [3] in viscoelastic fluid [1]. In addition, it will be demonstrated that the second normal stress difference in the colloidal dispersion generates the secondary flow in non-circular channel and the recently observed viscoelastic properties of blood plasma [4] can be elucidated by the colloidal dynamics of the blood plasma-constituting protein molecules [1]. Finally, we will present the characterization of the viscoelasticity of micelle solution based on the microfluidic methods applied to the colloidal dispersion. \textbf{References} [1] Kim \textit{et al}., \textit{Sci. Adv. }5, eaav4819 (2019). [2] J. F. Brady and M. Vicic, \textit{J. Rheol.} 39, 545--566 (1995). [3] P. Brunn, \textit{J. Non-Newt. Fluid Mech.} 7, 271--288 (1980). [4] M. Brust \textit{et al., Phys. Rev. Lett.} 110, 078305 (2013).   

Analysis of Shear Layer Structures from Time Resolved Schlieren Images of Supersonic Multi-Stream Rectangular Nozzle Flow

Operational guidelines for new jet engines result in extreme flow physics. Using a Single-Expansion-Ramp-Nozzle [SERN], Syracuse University and The Ohio State University study the effects of variable operating conditions, nozzle configurations, and deck plate geometries on jet development and far-field acoustics. Identifying noise sources will allow the implementation of control systems to reduce the overall sound pressure levels. Building off previous APS presentations, Ruscher 2015 and Tenney 2018, this project uses time resolved (100 and 400 kHz) Schlieren imaging. Through filtering, these Schlieren images show dark ``blobs'' convecting along the top shear layer, and seemingly synchronized ``bands'' propagating off the exit shock wave. Upon analysis, these flow structures occur at a frequency of 34kHz. The frequency of the phenomena are due to von-Karman streets from the third-stream splitter plate, triggering Kelvin-Helmholtz Instabilities [KHI] in the shear layers. Analysis is occurring on the relationship between the KHI occurrences and resulting aeroacoustics. Ongoing work includes processing alternate image orientations and new data acquisition for variable splitter and deck plates.   

Using Traveling Water Beads for Particle or Vapor Capturing

Here, we report a new dehumidifier design with 200{\%} higher water condensation rate per volume of the device compared to the traditional dehumidifiers while decreasing air-side pressure drop due to providing straight gas flow channels for the gas stream. This structure consists of cold fresh water beads traveling down along an array of vertically aligned strings, on which the counterflowing hot humidified air stream is condensed. The water beads are generated using the intrinsic instability of liquid films flowing down a vertical thread. Offering direct contact between the water and air stream, very high interface-to-volume ratios and long resistance times for heat/mass transfer, our unique dehumidifier is cable of achieving superior heat and mass transfer rates. Additionally, using our bead generating method, we developed an efficient particle capturing unit, in which an array of traveling beads is used as the collection section to capture the charged ultra-fine particles (UFP) in the counterflowing air stream. Studies show that UFPs can enter the blood and penetrate the cell membranes. Our experimental results show that this design can remove more UFPs with significantly lower water consumption rate compared to the traditional particle capturing systems.   

Plasma Assisted LCF Perovskite Ion Transport Membrane Processing of Methane.

Ion Transport Membranes (ITM) are ceramic based materials that allow the permeation of ions, commonly oxygen, through the structure at high temperatures. ITMs can be utilized in oxy- combustion, molecular separation of hydrogen reactors, and is effective in Carbon Capture and Sequestration (CCS) methods. Nevertheless, such high operating temperatures are not completely ideal, mainly due to its environmental impact and high energy input. In this project, it is aimed to reduce the temperature required to enhance oxy combustion reactions by applying a low-temperature plasma-catalysis to a LaCaFeO3 (LCF) perovskite ITM. The experimental reactor is placed inside a ceramic casted furnace where the reaction takes place on each side of the membrane, monitoring the products on the sweep side using a Quad-Pole Mass Spectrometer (QPMS). Future works aim to use solar energy to power the reactor.   

Plasma Assisted LCF Perovskite Ion Transport~Membrane Processing of Methane.

Ion transport membranes (ITMs) have played a key role in the study of processing and converting natural gases.~ The membranes are composed of ceramic-based materials and allow the permeation of ions at temperatures ranging from 700${^\circ}$ to 950${^\circ}$. Due to low energy conversion efficiency, the high temperatures related to the membrane separation system are inefficient for industry use. The experiment at hand focuses on treating LaCaFeO (LCF) perovskite ITM with low-temperature plasma in order to lower the activation energy and heighten the performance of the ITM. The reactor was controlled in a two-inch diameter quartz tube placed inside an alumina ceramic furnace. Metal inlets sealed the opening ends of the quartz tube and allowed the insertion of the LCF perovskite ITM, feedstock gas(air), sweep gas or reactive gas(CH4), and plasma electrode (kanthal wire). To generate the plasma, the electrodes were connected to a 110.V PVM/DDR plasma drive. The study was inconclusive due to the damaging of the LaCaFeO3 perovskite ITM. The bulk of the ITM, as well as the adhesive used to attach the membrane, assisted in the ITM rupturing. Moving forward we hope to find a solution to these problems in order to run the reactor and receive data.~   

Particle enrichment and instability on a fluid interface

We investigate the displacement of a suspension of non-colloidal particles by an immiscible fluid inside a vertical Hele-Shaw cell with a gap less than twice the nominal particle size. We find that the particles move slower than the invading fluid and accumulate on the interface. The particle enrichment can cause an interfacial instability reminiscent of the classic Saffman-Taylor instability. However, unlike the classic viscous fingering patterns, the invading fluid penetrates into regions surrounding clusters of high particle concentration. In this way, the high-concentration clusters deform the otherwise flat interface. Although this effect is enhanced by the presence of many particles in a cluster, we show that the instability can also occur in the case of a single particle for a narrower range of parameters. In this poster, we present experimental results and discuss the competition between viscous drag and interfacial energy giving rise to this instability.   

Flexible Airfoils and Their Effect on Flow Separation

In the US alone there are 5,000 planes in flight at any given moment and 52,000 wind turbines in operation. Any object that is subject to high wind speeds or varying attack angles has the potential to have flow separation. Flow separation increases drag which results in a less efficient aerodynamic system. Previous research has shown that active shape changing airfoils can reduce flow separation. However, since the shape change is active it introduces parasitic cost to the system, detracting overall energy capture. In this project, a passive method of reducing flow separation with flexible airfoils is investigated. The flexible airfoils have shown to increase airfoil performance as compared to a rigid design. While the flexible airfoils can increase airfoil performance, it cannot withstand the same wind speeds as its rigid counterpart. The performance improvement is speculated to be a result of boundary layer reattachment post the point of stall, reducing the flow separation and increasing lift when compared to the rigid design.   

Numerical Study of the Hydroacoustic Characteristics of a Marine Propeller

Propulsion of surface and under water ships with low noise characteristics depends on a number of factors in terms of their safety and operational performances, and it is crucial to predict and control their underwater noise characteristics. In this respect, the main scope of this study is to calculate numerically the propeller noise, which is one of the main sources of underwater noise. Therefore, propeller noise is studied numerically for non-cavitating conditions. Flow around the propeller is solved with a commercial CFD software, while hydro-acoustic analysis is performed using a model based on Ffowcs Williams-Hawking equation. Flow around a propeller is solved using a RANS solver with the SST k-? turbulence model. Then, transient solution is performed with second order implicit pressure-based solver. Velocity and pressure coupled via SIMPLE algorithm Numerical Methods and Flow Solver. Time dependent pressure data is used as the input for the FWH equation to predict far-field acoustics.   

Analysis of Unsteady Wall Jet Created by a Coaxial-Rotor in Ground Effect

The study of unsteady characteristics of the outwash of a coaxial multi-rotor configuration drone in-ground effect conditions is presented. A modular, thrust scaled experimental setup consisting of three bladed rotor is used for the current study. The outwash of the coaxial rotor is studied using planar, time-resolved particle image velocimetry (PIV). The experiments are carried out at torque matched trim conditions with varying rotor axial separation distances operating at a tip Reynolds number around 150,000. The effect of axial separation distance on the instantaneous interactions between the tip vortex structures in the outwash region and its influence on the gusts generated in the outwash are investigated. The variation of the outwash characteristics with varying ground heights for the drone is also discussed.   

Circular towing: a restricted case

Dynamic equilibria of strings in fluids are relevant to a variety of situations including fiber sedimentation and cable towing. Here I consider a highly restricted special case of steadily rotating planar configurations experiencing purely normal linear drag forces, for which analytical results may be obtained. The resulting shapes approach Fermat spirals at sufficiently large radius.   

Analysis of the self-starting capability of a new hybrid vertical axis wind turbine with a fluid-structure interaction approach

Vertical axis wind turbines (VAWTs) provide promising solutions for distributed wind energy harvesting in both urban and rural areas. However, it is challenging to guarantee satisfactory self-starting capability and high power efficiency simultaneously in a VAWT design. We have recently designed a new hybrid Darrieus-Modifed-Savonius (HDMS) VAWT to address this challenge. The aerodynamics of the new hybrid design is analyzed using a fluid-structure interaction approach based on high fidelity computational fluid dynamics. We find that compared to the Darrieus VAWT, the HDMS design has better self-starting capability due to the torque provided by the inner MS rotor at small tip speed ratios (TSRs); the HDMS design can maintain high power efficiency at large TSRs with an appropriately sized MS rotor. The key flow physics is that the HDMS design can keep accelerating at small TSRs due to the inner MS rotor, and can suppress dynamic stall on the Darrieus blades at large TSRs. The effects of the turbine configuration, inertia and loading on the self-starting capability and power efficiency are further studied.   

Dynamics of overshooting convection in a rotating spherical shell

Overshooting convection is a physical process by which turbulent convective motions generated in a convectively unstable region can propagate into a stably stratified zone that lies either on top or on bottom of the convective one. This process can lead to mixing of chemical species, thermal mixing, as well as contribute to the transport of magnetic fields and angular momentum. Thus, convective overshooting has direct and significant implications in stellar dynamics. Motivated by the Sun, we investigate these dynamics via numerical simulations that solve the anelastic Navier-Stokes equations in a spherical shell containing a convection zone with an underlying stable region. We present results of our runs which span a range of parameters and illustrate the dependence of convective overshooting on the intensity of the turbulence and degree of stratification of the convective region, the relative stability of the stable zone, the transition width between the two regions, as well as the rotation rate. These results can be particularly useful for gaining a better understanding of convective overshooting processes in stars and for improving existing models prescribed in 1D stellar evolution calculations.   

Modelling nutrient delivery to cells grown in a multiscale perfusable system

Growing cells \emph{in vitro} expedites the process of testing viable drugs and reduces the need for animal testing. However, current methods to grow 3D structures eventually result in the formation of a necrotic core due to lack of nutrient access. One way to circumvent this is to use 3D bioprinting and photopatterning techniques to engineer multiscale perfusable systems that enhance nutrient delivery to cells. While these techniques offer a high degree of control over the configuration of the perfusable channels, it is not clear how the channels or cells should be distributed in order to maximize nutrient transport and avoid necrotic zones. To tackle these problems, it is imperative to have knowledge of the fluid flow within the perfusable hydrogel system; advection is the dominant nutrient transport mechanism. Thus, understanding and being able to control the flow within the bioreactor is paramount. In this talk, we use mathematical modelling to investigate how the nutrient transport to the growing cells is affected by experimentally controlled parameters, such as channel distribution, cell density, and the flux rate of nutrient fluid through the perfusable hydrogel structure.   

Transition to collective motion in two-dimensional microswimmer suspensions

A collection of microswimmers immersed in an incompressible fluid is characterised by strong orientational interactions due to the long-range nature of the hydrodynimic fields generated by individual organisms. As a result, suspensions of 'pusher' swimmers exhibit a state often referred to as collective motion or 'bacterial turbulence', which is dominated by jets and vortices compromising many microswimmers. The onset of collective motion can be understood within a mean-field kinetic theory for dipolar swimmers. In 3D, the theory predicts that the instability sets in at the largest scale available to the suspension. Here, we present a mean-field kinetic theory for a suspension of dipolar swimmers confined to a 2D plane embedded in a 3D fluid. We analyse the stability of the homogeneous and isotropic state, and find two types of instability: one is the analogue of the orientational instability in 3D systems, while the other is associated with strong density variations absent in 3D. In contrast to 3D suspensions, both instabilities occur at the smallest possible scale, and we discuss their implications for the ensuing collective motion.   

Coupled fluid-structure interaction and mass transport in aortic valves

Near-leaflet biotransport processes play an important in calcific aortic valve disease initiation and bioprosthetic aortic valve thrombosis. The solution to these transport processes involves coupled blood flow, nonlinear structural mechanics, and convective mass transport problems. Herein, 2D simulations are carried out where a two-way coupled fluid-structure interaction (FSI) model of an aortic valve is coupled to continuum advection-diffusion transport equations. Two classes of problems representing aortic valve complications are studied. First, constant biochemical concentration is imposed at the aortic root. Next, constant biochemical flux is imposed at the moving leaflet. Subsequently, biochemical transport near the leaflet is studied. The results show a close connection between vortex structures and biochemical concentration patterns. Distinctions in concentration patterns on the aortic and ventricular side of the leaflet are shown and implications for calcification and thrombosis discussed.   

Force exerted by a Stokeslet on confining boundaries

Solutions to the Stokes equation can be constructed by combining suitably placed Stokelets and other singular solutions, that simultaneously satisfy the equation of motion and the boundary conditions. This approach has proven especially fruitful in describing the motion of small solid bodies and self-propelled particles. Recent debate on the pressure exerted by microswimmers on the walls of the enclosing container, together with the observations of the apparent viscosity of microswimmer suspensions being strongly affected by their presence, stresses the need to evaluate the forces exerted by microswimmers on solid boundaries. \\ Here, we study two archetypal problems: a Stokeslet next to a single flat boundary, and a Stokeslet confined in-between to parallel walls. This allows us to calculate forces exerted on the walls by microswimmers, and we find that while in the case of a single wall microswimmers exerts no total force on the wall, the force becomes nonzero in the latter. We estimate the pressure exerted on the wall by the typical dilute bacterial suspension used in experiments.   

An implicit adaptive high-order flux reconstruction framework for scale-resolving simulation of unsteady flows over moving complex geometries

We present a recent development of an implicit adaptive high-order flux reconstruction framework for moving domain simulation at intermediate and high Reynolds numbers. In this framework, the high-order flux reconstruction method is used for spatial discretization; the explicit first stage, singly diagonal implicit Runge-Kutta (ESDIRK) method is used to perform time integration; a dual-time stepping approach is used to assist convergence while maintaining temporal accuracy; a matrix-free implementation of the restarted generalized minimal residual (GMRES) method is employed to solve the large linear system; and a flow-resolution-based p-adaptation algorithm is adopted to apportion the computational resources to critical flow regions. The arbitrary Lagragian and Eulerian (ALE) approach is used to enable moving domain simulation with body-fitted unstructured meshes, and the radial basis function (RBF) interpolation is employed to handle mesh movement and deformation. Several challenging 2D and 3D unsteady flow cases in the moderate to high Reynolds number range are used to demonstrate the capability of the new numerical framework.   

Bio-inspired flows in unsteady environments. Part II: crosswind gusts

Autonomous underwater vehicles (AUVs) and unmanned aerial vehicles (UAVs) usually need to carry out tasks in unstructured and dynamic flow environments. This poses a number of challenges that cannot easily be addressed by approaches developed for highly controlled environments, such as uniform flows frequently used in experiments and numerical simulation This work studies the impact of crosswind gusts on the performance of flapping wings/fins at moderate to high Reynolds numbers (i.e., in the range from 10e$+$4 to 10e$+$6). A high-order accurate flux reconstruction flow solver with moving/deforming body-fitted unstructured meshes is used to perform the numerical simulation. We find that dynamic stall in a crosswind gust is very different from the stalled flow at a large geometric angle of attack (AoA) due to the different transient dynamics of the leading and trailing edge vortices. Reynolds numbers can significantly affect the vortex structures over the suction surface of the foil The effects of relative position between the gust and foil and gust strength are also discussed in this study.   

Bio-inspired flows in unsteady environments. Part III: mean flow shear

Autonomous underwater vehicles (AUVs) and unmanned aerial vehicles (UAVs) usually need to carry out tasks in unstructured and dynamic flow environments. This poses a number of challenges that cannot easily be addressed by approaches developed for highly controlled environments, such as uniform flows frequently used in experiments and numerical simulation This work studies the impact of mean flow shear on the performance of flapping wings/fins. A hyperbolic tangent mean flow shear profile is superposed onto the uniform freestream to generate a shear flow A high-order accurate spectral difference flow solver with moving/deforming body-fitted unstructured meshes is used to perform the numerical simulation, and dynamic mode decomposition is applied to analyze coherent flow structures. We find that flapping motion can significantly promote unsteady lift generation in mean flow shear; the stronger the shear is, the larger the lift is. At the same time, the lift coefficient is much larger than that predicted by the Kutta-Joukowski theory under the same flow conditions. Thrust generation is almost not affected by the mean flow shear.   

Non-Newtonian effects on the slip and mobility of a self-propelling active particle

Self-propelling Janus particles generate concentration gradients along their surface by exploiting the asymmetry in surface activity. This gives rise to a `slip' at the particle surface, which propels the particle without the requirement of external concentration gradients. In this work, we study the influence of viscoelasticity (second-order-fluid model) and shear-thinning/thickening (Carreau model) on the slip and mobility of an axisymmetric active particle. Using matched asymptotic expansions, we provide an analytical expression for the modification of slip. Using reciprocal theorem, we demonstrate the influence of fluid rheology on particle mobility for low Peclet numbers. The current study also provides insights into the transport of complex fluids through phoretic pumps.   

Computational simulation of guidewire motion in a blood vessel

The guidewire is made of a thin stainless-steel wire, inserted into the human body and moved through the blood vessel, and is an essential tool for the treatment and diagnosis of vascular diseases. In this study, Kirchhoff rod theory is applied to develop a guidewire model as an elastic rod, and to simulate moving within a given blood vessel. Particularly, the inherent characteristics (shape, strength, torque, and elasticity) of the guidewire are applied to the model, and the reaction of the guidewire to the axial movement and rotation of the operation portion is simulated. The blood vessel is presented with single branch, and the movement of guidewire along the shape of the vessel is examined. It will be also discussed the tip shape of the guidewire that must be selected to navigate to the desired path. The development of guidewire simulations can provide a safe environment for practitioners to practice as often as necessary while avoiding bioethics issues. In addition, it is possible to find an optimal pathways and controls for moving the guidewire to the clinical target with minimal stress on the environments within the vessel.   

A Mechanism by Which Nose Bluntness Suppresses Second-mode Instability

A physical mechanism by which nose bluntness suppresses second-mode instability is proposed. Considered are 7 degree half-angle straight cones of various nose bluntness radii at tunnel conditions relevant to the AFOSR-Notre Dame Large Mach 6 Quiet Tunnel (Lakebrink et al. 2018). It is shown that second-mode suppression is achieved via entropy layer modulation of the basic state density gradient. A weakening of the density gradient disrupts the acoustic resonance necessary to sustain second-mode growth. These results are consistent with the thermoacoustic interpretation (Kuehl 2018) which posits that second-mode instability can be modeled as thermoacoustic resonance of acoustic energy trapped within an acoustic impedance well. Furthermore, the generalized inflection point criteria of Lees and Lin (1946) is applied to develop a criteria for the existence of second-mode instability based on the strength of the basic state density gradient. Lakebrink, M. T., K. G. Bowcutt, T. Winfree, C. C. Huffman and T. J. Juliano 2018. Journal of Spacecraft and Rockets. 55 (2) 315-321. Kuehl, J. 2018. AIAA Journal, 1-8, 10.2514/1.J057015. Lees, L., and Lin, C.-C., 1946. NACA TR 1115.   

Observations of Thermodynamic and Kinematic Properties During the Morning Transition in a Wind Turbine Array Boundary Layer Using an Instrumented Unmanned Aerial Vehicle

Observation, simulation and modeling have shown that wind farms alter downstream atmospheric properties as turbulent wakes generated by turbines enhance vertical mixing. With a large portion of wind farms hosted within an agricultural setting, changes to the wind turbine array boundary layer (WTABL) are important as they can potentially impact crop productivity, along with inflow to downstream turbines. The authors, and others, have demonstrated changes to thermodynamic properties within the WTABL during daylight observations made by small unmanned aerial systems (sUAS). The obtainment of permission to fly at night and at higher altitudes, along with the enhancement of the sUAS instrumentation suite with fast-response three-dimensional sonic anemometers, enabled observations during predawn hours and through the morning transition. This work details observed changes to thermodynamic and kinematic properties during a series of overnight field campaigns undertaken during the summer of 2019 around a utility-scale wind turbine.   

Experimental study on resonance heat generation due to shock wave compression

The presentation addresses an experimental study of heat generation phenomenon associated with the conversion of flow kinematic energy to thermal energy through acoustic resonance. The phenomenon has been examined using a set of a sonic nozzle and a resonance tube. An underexpanded jet from the nozzle enters the tube and its shock waves compress the gas in the tube and reflect from the end. The repetition of the compression in a resonant manner results in an abrupt increase of gas temperature inside. The results indicate that various geometrical and flow parameters including a nozzle diameter, a tube inlet diameter, a distance between the nozzle and the tube, a nozzle stagnation pressure, a mass flow rate and a working fluid, affect heat generation characteristics. High-speed Schlieren imaging of supersonic flow in the vicinity of the nozzle exit and the tube inlet successfully identifies and reveals that corner space adjacent to the inlet edge of the tube should be large enough to allow the reflecting flow to expand and escape the tube for the occurrence of the resonance. The application of helium as working gas compared with nitrogen shows that the temperature increase becomes two times greater and it reaches beyond 1000 K in less than two seconds.   

Stability of Semi-Extrapolated Finite Difference Schemes

When numerically solving partial differential equations, finite difference methods are a popular choice. Several factors come into play when choosing a finite difference method, such as stability, accuracy, and computational cost. In response to the small stability regions of explicit methods and the computational cost of implicit methods, we've developed a novel discretization technique called semi-extrapolation. Semi-extrapolation generates explicit schemes from implicit schemes by applying extrapolation in an unconventional fashion. Extrapolation can severely curtail stability, however, we've found that semi-extrapolation can improve stability, as compared to analogous explicit methods. In our presentation, we'll introduce our semi-extrapolation technique and discretize the Advection-Diffusion Equation according to semi-extrapolated methods and mainstream methods. Then, we'll discuss the stability regions of the schemes and analyze how the stabilities of the semi-extrapolated schemes compare to the stabilities of analogous schemes.   

Accuracy and Computational Cost of Semi-Extrapolated Finite Difference Schemes

When numerically solving partial differential equations, finite difference methods are a popular choice. Several factors come into play when choosing a finite difference method, such as stability, accuracy, and computational cost. In response to the small stability regions of explicit methods and the computational cost of implicit methods, we've developed a novel discretization technique called semi-extrapolation. Semi-extrapolation generates explicit schemes from implicit schemes by applying extrapolation in an unconventional fashion. Semi-extrapolation can improve stability, however, we've also found that semi-extrapolation can have unexpected and interesting effects on accuracy. In our presentation, we'll introduce our semi-extrapolation technique and discretize the Advection Equation and the Advection-Diffusion Equation according to semi-extrapolated and mainstream finite difference methods. Then, we'll examine the computational costs and accuracies of semi-extrapolated methods. Included in this examination will be a comparison against the costs and accuracies of mainstream methods and a discussion regarding how stability influences the accuracy of semi-extrapolated schemes.   

State-free Front Tracking for Compressible Multi-material Problems

High energy density physics (HEDP) is a rapidly growing field studying interaction of matter and energy under conditions of extreme temperature, pressure and density. Numerical models capturing hydrodynamic instabilities and shocks are of crucial importance for understanding HEDP and designing different experimental components. Interface tracking methods, and specifically the front-tracking method with a fixed Eulerian mesh and a moving Lagrangian interface, have been applied to these types of problems with a best-in-class success {Glimm2002}.   

A Modified Cut-cell Approach for Inclined Boundary Conditions on Computational Fluid Dynamics

A simulation method of fluid dynamics based on cut-cell approach was modified to deal with distorted boundary conditions. Most conventional cut-cell methods focus on a flux control across computational grids. On the other hand, we combined interpolation of fluid velocity and re-defined wall shear stress by using normal distance and projected fluid velocity. This combination was implemented in the structured computational grid. In addition, Detached Eddy Simulation approach with non-slip wall and logarithmic wall function model were used for calculation on turbulent flow in this study. Two-dimensional laminar flow with an inclined channel and air flows around a cylinder in three-dimensional fields were calculated to demonstrate applicability of our approach. As a result, we were able to obtain fair flow fields without any numerical instability, and its calculation efficiency was much higher than that based on unstructured approach from the viewpoint of accessibility of computational memory.   

Hytrac: A Hydrodynamic Front-Tracking Code for the Study of High Energy Density Multi-Material Flows

Inertially Confined nuclear Fusion (ICF) is an established research field pursued in laboratories worldwide; most notably in the US National Ignition Facility. First Light Fusion (FLF) is exploring alternative ICF directions, with the prime focus being sustainable power generation. Hydrodynamics and mixing of materials are critical to the design of an ICF target, where high-energy densities bring additional complexities to CFD models. Interface-tracking is a challenging numerical problem in terms of accuracy and robustness. Hytrac, an AMR front-tracking code, was developed with the aim of overcoming these challenges, to enable reliable and rapid iteration of complex target geometries and optimisation. It has been verified in detail, with standard compressible fluid tests and methods, and validated against in-house experimental capability. Hytrac is parallelised using HPX, for efficient load balancing. It includes state-free tracking methods, and the more established Glimm approach. It also supports fluid nodes of arbitrary order, includes thermal conduction, thermal nonequilibrium, advanced physics, exact and approximate Riemann solvers, and several algorithms for space reconstruction and explicit time solution   

A parallel pore-scale multiphase flow tool using the lattice Boltzmann method

The main focus of our study is to mimic multiphase flow in realistic three-dimensional rock models that enables us to gain a better insight into the effect of pore-scale phenomena on real reservoir problems. We developed a fluid flow simulator using a D3Q19 multiphase multi-relation-time (MRT) lattice Boltzmann (LB) model. The present LB model is an extended Color-Gradient approach with improved numerical stability and can handle multiphase flow simulations with low capillary number and high viscosity ratio. To improve the computational efficiency of the LB simulations to a reasonable level for industrial applications, the model has been applied to a parallel scheme written in C$++$ using the Message Passing Interface (MPI). We herein introduce the capability of our tool for multiphase flow simulation in porous media and present its application to CO$_{\mathrm{2}}$ sequestration in geological formations. The model has been applied to the simulation of CO$_{\mathrm{2}}$ and brine in sandstone rocks, by employing three-dimensional micro-CT images of rock samples. Injection of supercritical CO$_{\mathrm{2}}$ into the brine-saturated rock sample is simulated and complex displacement patterns under various reservoir conditions are identified.   

A comparison between functional derivative-based global sensitivity analysis and mixed Karhunen-Lo\`{e}ve active subspace analysis of flows through porous media with uncertain material properties

Flows through porous media, such as aquifer and biological tissues, are usually affected by the uncertain material properties. Quantifying the flow uncertainty due to the material uncertainty, and identifying the flow sensitivity to material properties are important practices to understand the underlying flow physics. Recently we have developed a functional derivative-based global sensitivity analysis (GSA) method and a mixed Karhunen-Lo\`{e}ve active subspace analysis method for surrogate modeling of models with high-dimensional inputs and functional outputs. We have applied these two approaches to analyze the pressure field from biotransport in porous tumors. We find that the functional derivative-based GSA method is effective in finding input parameters in the parameterization of material properties that the pressure field is sensitive to; and the mixed Karhunen-Lo\`{e}ve active subspace analysis method can identify a linear combination of input parameters that contribute most to the formation of coherent structures in the flow field. The connection between the two methods is studied in the context of biotransport in porous biological tissues.   

Stable long-wavelength convection with Dirichlet thermal boundary conditions through a Batchelor-Nitsche instability

It is a well known fact that the onset of Rayleigh-B\'{e}nard convection occurs via a long-wavelength instability when the horizontal boundaries are thermally insulated. In this work, we consider three-dimensional Rayleigh-B\'{e}nard convection in a cell of infinite extent in the $x$-direction and having vertical walls located at $y=0$ and $y=\delta$ and horizontal boundaries located at $z=0$ and $z=h$. We assume stress-free boundary conditions and thermally conducting walls. We put forth a set of values of the parameter $\delta$ for which we show the existence and stability of long-wavelength convection.   

Insights into droplet impact shape dynamics

The underlying mechanics related to the initial shape transition is being studied experimentally using high speed imaging techniques for a droplet impacting on various substrates. Two traditional (Glass and PDMS) and two bio-inspired surfaces were used. A very distinct change in the shape of the deforming droplet during the early phase of the droplet spreading was observed when the impact Weber number of the droplet is varied from 1 to 50. The nature of the capillary waves (wavelength) on the droplet surface changes quite significantly. This shows the existence of a critical Weber number beyond which the shape of the droplet during the initial transient of the spreading phase changes. The wavelength of the capillary waves was found to be a function of the impacting Weber number. This work provides some basic insights related to this transitional behavior using basic dimensional analysis and scaling theories.   

Dynamics of droplet spreading on Non-Newtonian liquid films

We investigate the dynamics of a liquid drop as it spreads along the interface of a liquid film. We consider the case of a liquid layer which exhibits non-Newtonian characteristics and is described by the Ostwald–de Waele constitutive law. In the limit of both a thin droplet and a thin subphase, we use lubrication theory to derive equations for the positions of the interfaces. We use a finite-element formulation to obtain numerical solutions of the evolution equations. The effects of the physical parameters and rheological characteristics on the interface shapes are studied.   

How wettability controls nanoprinting .

Using large scale molecular dynamics, we study in detail the impact of nanometer droplets of low viscosity on substrates and the effect of the wettability between the liquid and the plate. We show the maximal contact diameter during the nanodroplet impact (Dmax) as well as the time required to reach it (tmax) agree with experimental data at the macroscale showing similarities between droplet impacts at the nano and the macro scales. The comparison between the MD simulations and different models reveals that most of these models do not consider all the effects we observe at the nanoscale. Moreover, most of their predictions for the impact at the nanoscale do not correspond to the simulation results. We have developed a new model for Dmax which agrees not only with the simulation data but also the experimental observations and it also considers the effects of the liquid-solid wettability. We also propose a new scaling for tmax with respect to the impact velocity which is also in agreement with the experimental observations. We then present a new way to collapse in a master curve the evolution of the micro to nanometer drop contact diameter during impact for different wettabilities and different impact velocities.   

Controlling the pinning of a receding contact line in a flow coating process.

Capillary flow coating is a simple and effective technique to print and assemble ordered nanoparticle-based structures over patterned surfaces. The technique makes use of a nanoparticle suspension confined between two plates. Solvent evaporation and sliding movement of the top plate induce an internal flow that leads to the accumulation of nanoparticles at the bottom receding contact line and to their deposition on the bottom plate. Nevertheless, a comprehensive understanding of the process remains elusive, and in this respect the dynamics of wetting at the receding contact line is known to play a critical role. With the help of large-scale molecular dynamics simulations, we investigate the dynamic contact angle at the receding contact line as well as contact-line pinning on substrate heterogeneities. We develop a model to predict the pinning time of a receding contact line as a function of the displacement speed of the top plate on both chemical and topographical heterogeneities. Confirmation of the dynamic nature of contact-line pinning and justification of the contact line settling time allow us to better describe the time evolution of the receding angle in presence of heterogeneities.   

Hydrodynamics-Dominated Wetting Phenomena on Hybrid Superhydrophobic Surfaces

On chemically heterogeneous rough surfaces, determining the true apparent contact angle is a challenging task, because the global minimum surface energy is hardly attainable. The equilibrium contact angle is associated with the global minimum surface energy of the droplet-air-substrate system. However, in practice, the most stable contact angle is the measurable contact angle of the system. We present a series of 3D simulations using various initial conditions to reach possible meta-stable states for a water droplet on four micro-patterned hybrid substrates. The surface energy and energy barriers are computed. The apparent contact angles compared with experiments, global contour of the droplets, and liquid-solid area fractions are presented. Our results reveal a hydrodynamics-dominated wetting behavior on these hybrid surfaces, and capture several meta-stable Cassie-Baxter states, which cause a large contact angle hysteresis. For a given micro-patterned hybrid substrate, a critical impact speed can be found, above which the impact method cannot overcome more energy barriers to reach a lower energy state. Furthermore, a smaller variation of the measured contact angles is observed on the substrate with a lower heterogeneity of topology and chemistry.   

Spreading on viscoelastic solids: Are contact angles selected by Neumann's law?

The spreading of liquid drops on soft substrates is extremely slow, owing to strong viscoelastic dissipation inside the solid. A detailed understanding of the spreading dynamics has remained elusive, partly owing to the difficulty in quantifying the strong viscoelastic deformations below the contact line that determine the shape of moving wetting ridges. Here we present direct experimental visualizations of the dynamic wetting ridge, complemented with measurements of the liquid contact angle. It is observed that the wetting ridge exhibits a rotation that follows exactly the dynamic liquid contact angle, as was previously hypothesized [Karpitschka et al., Nature Commun. (2015)]. This experimentally proves that, despite the contact line motion, the wetting ridge is still governed by Neumann's law. Furthermore, our experiments suggest that moving contact lines lead to a variable surface tension of the substrate. We therefore set up a new theory that incorporates the influence of surface strain, for the first time including the so-called Shuttleworth effect into the dynamical theory for soft wetting. It includes a detailed analysis of the boundary conditions at the contact line, complemented by a dissipation analysis, which shows, again, the validity of Neumann's balance.   

Droplet dynamics during condensation on tailoring nanostructured surfaces.

Liquid condensation from the atmosphere on a solid surface is commonly found in nature. To enhance energy conversion, nanostructured surfaces is with high potential to utilize the water generating applications. In this work, the droplet condensation on the tailored surface has been investigated by a 2D phase field model which is favorable to study the interfacial dynamics under microscopic scale. To model the liquid extracted from the air, both liquid and vapor are initially coexisted in random distribution. Spinodal decomposition process is then used here in which the fluid starts from an unstable thermodynamic state, and the homogeneous phase separates into coexisting phases spontaneously. Initially the small liquid droplet generated along the nanostructured surface and gradually merged into a coalescence droplet. By analyzing the droplet formation, the phase change dynamics can be studied and its relation with the spatially distributed structures on surface. The overall performance enhancement created by surface nanostructured was examined in comparison to a flat surface. Our understanding of this work provides more insights into the nanostructured surface topography on mass and heat transfer to improve the energy efficiency.   

Three-dimensional front-tracking model for evaporation of drops

We present the development of a full three-dimensional model to simulate the evaporation of falling drops. The evaporation model is based on the simultaneous solution of the mass, momentum, energy and vapor mass fraction conservation equations for incompressible fluids, properly adapted to incorporate the possibility of mass transfer at the boundary between the phases. The mass reduction of the drop is influenced by local thermodynamic conditions which in turn are modified by the dynamics of the drop motion. The vapor mass fraction at the interface is computed through the Clausius-Claperyon relation. The set of equations is defined in the whole domain, including the interface, and are solved using the finite volume/front-tracking method. The solution of the resulting linear equations systems are solved using the CUSP library implemented in a GPU in order to reduce the computational time. The validation for the evaporative flux calculation was done by comparison with a one-dimensional analytical solution for the evaporation of a planar surface. The simulation results for a static drop showed very good agreement with the $d^2$ law. Simulations for a single and multiple falling drops are also presented.   

Proper Orthogonal Decomposition Analysis of Turbulent Cryogenic Liquid Jet Injection Under Transcritical Conditions

Liquid-rocket and high-pressure diesel engines operate at pressures and temperatures that exceed the critical pressure and temperature of the liquid fuels during injection. The turbulent flow features and turbulent fluctuations are impacted by the transition from subcritical to supercritical conditions that in turn affect the turbulent mixing between the fuel and oxidizer in the combustor. LES and the proper orthogonal decomposition (POD) algorithm are employed to study the turbulent flow and dominant unstable flow modes at transcritical and supercritical conditions for cryogenic nitrogen jet injected into a warm nitrogen environment. The effects of the transition from transcritical to supercritical conditions on mixing layer behavior are demonstrated. Real-gas thermodynamic properties at supercritical conditions are considered via implementing the cubic Peng--Robinson equation of state and Chung's method for obtaining fluid transport properties. The results show that the presence of large density gradients in the mixing layer at transcritical conditions smear out the turbulent coherent structures in the radial direction and turbulence shows anisotropic behavior near the mixing layer that retards the overall mixing process.   

Slip flow-enhanced streaming current in graphene oxide nanochannels

In recent year, fast transport of water in carbon-based nanochannels due to the slip effect has attracted much attention. In this work, the pressure-driven streaming currents through sub-1nm nanochannels reconstructed by the layered material of graphene oxide (GO) was investigated experimentally and theoretically. The results show that the measured values of streaming current are 2\textasciitilde 3 orders of higher than the predicted values calculated from electrokinetic model under no-slip assumption. It is inferred that the streaming current is greatly enhanced due to the presence of water slippage in sub-1nm partial wetting GO nanochannels. In addition, it is found that the slip length is strongly dependent on the KCl concentration, i.e., surface charge density. The estimated slip length is from 1 to 22nm. The lower surface charge density (KCl concentration) reveals the larger slip length. It is believed that our finding is beneficial to develop a higher efficiency of electro-kinetic power generator and electro-osmotic pump.   

Investigation of Corner Flows in Complex Supersonic Rectangular Jet Nozzles

The understanding of complex turbulent flows is of upmost importance when designing propulsive systems for next-generation-plus aircraft. Due to the highly unsteady nature of these flows, innovative techniques are required to extract the key physics insights that effect the overall performance of jet nozzles. The experiments in this study investigate the flow from a rectangular jet nozzle, specifically a Single Expansion Ramp Nozzle (SERN) over a proximal surface. The primary area of investigation is the flow in the corner regions and their influence on the downstream shear layers. The experiments conducted utilized a variety of measurement techniques including: Particle Image Velocimetry (PIV) to obtain high-resolution velocity measurements, high frequency response pressure transducers to measure the unsteady deck surface pressure, and simultaneous far field acoustic measurements. These measurement techniques will help determine the effects of the jet nozzle corner regions on both the wall (deck plate side) and free (SERN side) shear layers. Additionally, this study aims to determine the influence of the corner flows on the downstream separation region as well as their role in near wall turbulence dynamics and unsteady deck loading.   

Development of multiple-color fluorescence image velocimetry

Scalar image velocimetry (SIV) is the technique to extract velocity vectors from scalar field measurements. The usual SIV involves minimizing a cost function, that penalizes the deviation from the one scalar transport equation. This method can lead to multiple solutions and additional condition must be applied to select the best one if the full scalar gradient is zero over the volume. In addition, this technique is not applied for images with large displacement between two instances. Here we propose to minimize these problem with the reconstruction of the velocity field by using two different dyes and image deformation. Conceptually, we argue that having a double set of convergence criteria will result in a much more accurate velocity field. This improved SIV scheme is applied to the coaxial round free jet in liquid phase. The spatial velocity fields $\mathbf{u}\left( \mathbf{x},t \right)$ thus obtained demonstrate the good agreement of the velocity field solution with the continuity condition$\nabla \cdot \mathbf{u}=0$. The correlations between ${\partial {{u}_{i}}}/{\partial {{x}_{i}}}\;$ and $\sum\nolimits_{j\ne i}{{-\partial {{u}_{j}}}/{\partial {{u}_{j}}}\;}$ lie in the range 0.94-0.96 for the proposed method. The PDF of velocity fields also represent Gaussian distribution.   

Visualization of flow structure and its changes on flap with vortex generators

For the aircraft design, the high-lift systems have an important role because of satisfying the demand for improving the lift for the take-off and landing configurations and increasing the payload. Vane-type vortex generators (VGs) are sometimes installed on the flap as a passive flow control device to improve the high-lift performances. However, the effect of VGs on the flow physics of the flap is still in discussion. In this study, the performance of VGs installed on the flap of a half-span model with a three-element high-lift wing was investigated by low-speed wind tunnel tests. In the tests, VGs were installed where chordwise velocity became nearly maximum on the flap, and flow visualization was conducted with VGs in different size. The test results revealed that the interaction between longitudinal vortices generated by the VGs and cross flow on the flap influenced flow separation patterns behind the VGs, which depended on the size and installation spacing of the VGs. We also evaluated the lift coefficient of the wind tunnel model to investigate the relationship between the flow structures and aerodynamic performance of the flap with VGs.   

Control of flow and sound around leading-edge slat of 30P30N airfoil using plasma actuator

We studied active control of flow and noise around a multi-element airfoil (30P30N) using time-resolved particle image velocimetry (TR-PIV), microphone and dielectric barrier discharge plasma actuator (DBD-PA). The angle of attack of the wing model fixed in the hard wall test section was $6^{\circ}$ to $10^{\circ}$, and the Reynolds number based on the stowed chord length was $1.1\times 10^{5}$ to $1.5\times 10^{5}$. As a result of acoustic measurement, characteristic peaks were confirmed in the range of $St=1.0$ to $4.0$, which is based on the slat chord length. Periodic disturbance ($St=0.3\sim 3.9)$ was applied to a shear layer formed from the slat cusp using a DBD-PA placed at the lower of the slat. The noise peak frequencies were modified in synchronization with the disturbance frequencies of $St=1.0\sim 2.8$, these Strouhal number is close to the vortex shedding frequency in the case where the DBD-PA was deactivated. It was confirmed by TR-PIV and dynamic mode decomposition that the flow structures at the characteristic peak frequencies modified by the disturbance frequencies. The results show that the noise and the flow field around the leading edge slat could be controlled by the periodic disturbance.   

Reduction of drag acting on the Ahmed body using plasma actuators

Ahmed body is a scientific model of automobile, and it is known that the rear end of this model has an important role in aerodynamic characteristics such as drag and the wake structure. In this study, we used a dielectric barrier discharge plasma actuator array (DBD-PA) at the rear end of the Ahmed body with a slant angle of $\phi =25^{\circ}$ and $35^{\circ}$ to control the drag acting on the model. The experiment was conducted in the wind tunnel at $Re=3.95\times 10^{4}$. The DBD-PA were uniformly installed in the spanwise direction at the edge between the slant surface at the rear end and the roof, and burst control was applied by changing the excitation frequency (burst frequency $f^{+})$ and the ratio (burst ratio, BR). As a result, at $\phi =25^{\circ}$under $f^{+}\le 400 $Hz, the drag coefficient tends to decrease as BR increases. However, when $f^{+}=1000 $Hz, where the burst frequency was large, drag was most reduced at $BR=30\% $. The drag coefficient hardly decreased at \quad $\varphi =35^{\circ}$. We hope to present a results of the one-dimensional array of the DBD-PA, which introduce disturbance distributed in the spanwise direction with the temporal phase difference allowing to create lambda type vorticies into the shear layer.   

Transient Evolution of Flow Profiles in Shear Banding Wormlike Micellar Fluids

We report experiments on spatiotemporal evolution of flow field in model shear-banding viscoelastic micellar solutions in a Taylor-Couette cell. Our goal is to systematically study the effects of fluid elasticity on transient evolution of flow fields. By varying surfactant concentration, salt concentration, and temperature, we vary the fluid elasticity in the range 4.21 \texttimes 10$^{\mathrm{4}}$ to 8.57 \texttimes 10$^{\mathrm{6}}$ while keeping entanglement density fixed at 5.1 \textpm 0.7, viscosity ratio fixed at 40.3 \textpm 2.9 Pa\textbullet s, and curvature fixed at 0.085. Our experiments show as shear strain increases, shear stress shows an overshoot followed by a decay towards steady state. Simultaneously with shear stress decay, fluid moves in the opposite direction to that of the imposed motion in a subset of the gap ($i.e.$, back flow). Consistently with theory, the back flow strengthens as elasticity number increases. However, at very high elasticity numbers, the transient backflow disappears, contrary to the same theoretical predictions. In addition to the back flow, a multiple shear band structure forms in the transient flow at high elasticity numbers. These transient multiple bands persist to steady state. We surmise that the lack of back flows at high elasticity numbers is linked the formation of transient multiple bands.   

Ultra-fine roughness effect on a flat plate boundary layer transition

To reduce the viscous drag around an airfoil, delaying turbulence transition is one of the effective ways. In this study, we analyzed the influence of very small wavy roughness on the two-dimensional boundary layer transition with Direct Numerical Simulation, by resolving each small roughness. First, we found that the transition considerably depended on the streamwise grid resolution. Insufficient resolution in the streamwise direction may mislead to delay the transition. In the study, as many as 400 grid points were used in the boundary layer thickness. Second, it was noted that the transition delayed when the roughness wavelength was longer, even if the roughness height was increased to 1.5 times of the inflow boundary layer thickness. We examined the vortices generated based on the stability analysis. The vortex scales converted as frequencies were compared with the neutral stability curve of the Blasius flow for the Tollmien-Schlichting wave. Almost vortices located in the unstable region and some were shifted in the stable one. The small wavy roughness modified the near-wall pressure distribution. In the long-wavelength case, transition delayed with the background boundary layer modification.   

Instability analysis of Poiseuille flow between two parallel walls partially obstructed by porous surfaces

Plane Poiseuille flow is widely encountered in studies of diverse fields such as filtration, biomechanics, and geological problems. This work explores the effect of porous geometrical parameters, namely depth ratio and porous resistivity on the stability of Poiseuille flow over various porous surfaces. The most unstable mode is determined by numerically solving the eigenvalue problem derived from coupling between Navier-Stokes and Brinkman equations. Comparison of critical Reynolds number versus porous resistivity graphs for different depth ratios shows that there is an instability mode shift from the porous to the fluid with increasing the depth ratio. Also, the most stable mode occurs at smaller porous resistivity as the depth ratio becomes larger. To validate the theoretical analysis, we performed a set of experiments in which a Water-Glycerin solution flows through a channel partially obstructed by porous medium. The porous medium is modeled as square arrays of cylinders and is installed on the bottom wall of the channel. We found a good agreement between the steady-state and perturbed velocity profiles obtained analytically and experimentally.   

Radial distribution function of Lennard-Jones fluids in shear flows from intermediate asymptotics

The microstructure of a suspension of particles is ruled by the probability of finding a reference particle in a position with respect to a target one, the pair correlation function. Its description under shear flow has been a challenge for theoretical methods due to the singularly-perturbed boundary-layer nature of the problem. Previous approaches have been limited to hard-spheres (HS) and suffer from various limitations in their applicability. Here, we present an analytic scheme based on intermediate asymptotics which solves the Smoluchowski equation with shear in spherical coordinates including both intermolecular and hydrodynamic interactions with the intent of describing of the pair correlation function for realistically interacting particles in shear flows. First, the method has been validated through a comparison with the rdf of a HS fluid under strongly sheared conditions. Finally, we have been capable of studying the microstructure of a complex interacting fluid such as the Lennard-Jones at varying values of the attraction strength: a new depletion effect is predicted in the microstructure of the LJ fluid under shear, a feature to our knowledge never discovered before.   

Development of lubricant impregnated organogel surface for sustainable high drag reduction

Lubricant-infused surfaces (LIS) where micro/nanostructured surfaces are infused with lubricating liquids have attracted much attention due to their slippery properties for drag reduction. However, most state-of-the-art LIS technologies require complex fabrication processes and suffer from depletion problem of the infused lubricant, which limit their scalability and sustainability of high performance. Thus, a new strategy is proposed to overcome these problems by utilizing lubricant-impregnated organogel surfaces (LIOS) for efficient and sustainable drag reduction. In this LIOS, a lubricating liquid is dispersed in a solid 3D cross-linked network assimilated through physical or chemical interactions. Owing to its distinctive liquid absorption and retention capacity, the proposed LIOS might work out the lubricant depletion problem. The LIOS exhibited an extremely low sliding contact angle of 1.2° ± 0.2°, indicating a highly slippery surface. Due to the high slippery feature, the organogel had a large slip length of 209.4 $\mu $m, which gives rise to high drag reduction. The present results demonstrate a new strategy for LIS system of low cost and sustainable high drag reduction.   

Enhancement for long –term integration results of one-way nested regional ocean model by employing boundary small eddy additions technique

Generating small-scales structures during dynamical downscaling using the nested regional oceanographic models have been studied by the numerous studies. Those studies generated fine-scales feature in the nested regional circulation model with the large scale information at the lateral boundary incorporating with the local forcing. Recent studies however, found that the small-scales motions are under-developed seriously by downscaling work and such errors in the small scales can be transferred to the larger scales by inverse cascading and this could lead degrading the whole scale motions. This mainly limits the jump ratio of resolutions to small requesting the higher cost for downscaling. From that point of view, this work proposed an efficient technique, named “boundary small eddy additions (BSEA)” to enhance the quality of simulation and allow the higher jump ratio of the down scaling. The BSEA adds artificially the small-scale motions to the boundaries of a nested model based the spectral information of large-scale and can greatly improve the quality of the reproduction, even at much higher spatial resolution difference between the driving and the nested models.   

1.5-layer flow along a slope and western boundary

Analytic and semi-analytic solutions are derived for two important classes of layered geophysical flows: a topographically controlled flow along a slope and a western boundary current. Specifically, a similarity solution approach is used to solve the 1.5-layer shallow water equations. Case A: An analytic solution taking the form of an inverse tangent function is found to describe the flow of a bottom intensified (lower-layer), weak current moving along a broad shelf/slope. Case B: An ordinary differential equation is derived to describe the flow of a 1.5-layer (upper-layer) viscous western boundary current. This equation is solved numerically to study the effect of a deformable layer interface on the structure of the western boundary current and the results are compared with rotating table laboratory experiments. Both cases are formulated as idealized, two-layer, rotating fluid basins with sloping bottom topography. Kuehl, J. J. 2014. Geophysical Research Letters, 41. Ibanez, R., J. Kuehl, K. Shrestha and W. Anderson 2018. Nonlinear Processes in Geophysics, 25, 201-205. Kuehl, J. J. and V. A. Sheremet 2014. Journal of Fluid Mechanics, 740 97-113. Zavala Sanson, L. and G. J. van Heijst 2002. Journal of Fluid Mechanics, 471, 239--255.   

Measuring Energy Flux using PIV Data

Determining the energy flux of an internal wave from the experimentally measured velocity field was made possible by the work of Lee et al. (Lee et al., Phys. Fluids, 26, 2014). This method is used in our work to measure the amount of energy dissipated when internal waves reflect from sloping boundaries by comparing the incoming energy flux to the outgoing energy flux through a surface near to the reflection region. We also use numerical simulations of the Navier-Stokes equations in the Boussinesq limit where the energy flux is known from the pressure and velocity fields. There is good agreement between our experimental and numerical simulation data, and we find that there are high rates of energy dissipation during reflection process at the critical angle when the boundary flows are strongest. The results are consistent with Dettner et al. (Phys., Fluids, 25, 2013) who showed that strong boundary flows are excited by tidal motion over model topography, but the conversion of tidal energy into internal waves is weak.   

Cross-stream migration of a particle in a non-isothermal flow in a microchannel using inertial focusing

Cross-stream migration of particles is observed in an axial flow in the presence of finite inertia in inertial focusing. It is a passive separation method where particles migrate to equilibrium positions. These positions mainly depend on the two counteracting forces namely the wall lift force and the shear gradient force. The equilibrium positions can be altered by changing the velocity profile of the fluid. In this work, the migration characteristics of a particle in a parallel plate microchannel is studied numerically in a pressure driven flow under the influence of constant temperature gradient in the transverse direction. The viscosity variation in the lateral direction is considered since the fluid viscosity is dependent on the temperature. An immersed boundary method is used to study the particle migration. The particle equilibrium positions depend on the applied temperature gradient. The equilibrium positions are not symmetric since the velocity profile is not symmetric. The effect of temperature gradient, Reynolds number and particle size on the particle migration are analyzed.   

Van Hove Singularities due to hydrodynamic interactions among the spheres in two dimensional flow system

Dispersion relation for the phonon-like collective vibration modes due to the hydrodynamic interactions among the spherical particles with periodic separation a in quasi two dimensional flow shows peaks at the wavelength of 4.759a. In analogy to Van Hove singularities in solids, the density of states for hydrodynamically interacting systems becomes infinite at this wavelength with vanishing group velocities. Existence of these singularities is verified by computer simulation of hydrodynamic phonons in two cases: for periodic boundary conditions and for the system with broken symmetries. However, in unbounded systems the collective vibration decays before reaching the singularity   

The rising velocity of a slowly pulsating bubble in a shear-thinning fluid

We study the rising motion of small bubbles that undergo contraction, expansion or oscillation in a shear-thinning fluid. We model the non-Newtonian response of the fluid using the Carreau-Yasuda constitutive equation, under the assumptions that the inertia of the fluid and of the bubble are negligible, and that the bubble remains spherical. These assumptions imply that the rising velocity of the bubble is instantaneously proportional to the buoyancy force, with the proportionality constant given by the inverse of the friction coefficient. We evaluate the friction coefficient as a function of the rheological parameters and of the instantaneous expansion/contration rate of the bubble. Our results show that the radial motion of the bubble reduces the viscosity of the surrounding fluid, and markedly decreases the friction coefficient of the bubble. We find that the average rise velocity of a bubble undergoind radial pulsations is increased. We compare our predictions with the experiments performed by [S. Iwata, Y. Yamada, T. Takashima, and H. Mori. J. nonNewton. Fluid Mech., 151(1-3):30--37, 2008], who found that the rise velocity of bubbles that undergo radial pulsations is increased by orders of magnitude compared to the case of bubbles that do not pulsate.   

Strategic placement of an obstacle eliminates droplet break-up in the flow of a microfluidic concentrated emulsion

Droplet microfluidics has enabled a wide range of high throughput applications, such as digital polymerase chain reaction (dPCR) and antibiotic screening. However, few studies have attempted to increase the throughput of the drop interrogation process. Previously, strategic placement of a circular post near a narrow exit reduced conflict between interactions among living organisms or particles. Inspired by such work, we placed a circular post close to the constriction entrance of a tapered microchannel. The results of our experiment demonstrate that the effects of this placement on droplet break up are noteworthy. If the obstacle position and size is properly selected, the probability that the droplet will break decreases by up to 99{\%}, thereby enabling a 3-fold increase in drop interrogation rate. Droplet break-up depends on drop-drop interaction and drop deformation. Optimal obstacle placement immediately before? the constriction reduces drop deformation, which in turn reduces drop break-up. Strategic obstacle placement is therefore an attractive strategy for increasing droplet throughput.   

Parametric Study of the Wetting Transition of a Moving Meniscus.

In this study, we investigated the wetting transition of a moving meniscus in a grooved microchannel through a detailed parametric study based on measurement by an optical tool and micro-particle image velocimetry to avoid the transition in designing the microchannel. The parameters investigated were pitch, flow rate, and height of a microchannel. The contact angle, contact speed, and interfacial pressure difference were analyzed according to the parameters. We found that the pitch is most effective, the flow rate is moderately effective, and the height is least effective on that. The height even does not affect the contact angle because the solid-fluid interaction at the groove edge is stronger than the fluid-air interaction. As the critical correlation, the contact angle, which is dependent on the pitch and the flow rate, and the height affect the air pressure between the grooves, which governs the air penetration flux and mainly determines the wetting transition. Therefore, a powerful way to delay the wetting transition is to reduce the degree of air pressure variation, specifically with a low pitch and a tall height with a low flow rate. Eventually, understanding dominant input parameters in relation to the wetting transition will be very useful in the design stage of microfluidic applications.   

Natural gas supercritical properties: a multiscale molecular simulations study

A typical deficiency of continuum approach relies on the models describing averaged fluid properties. In particular, those models usually do not consider important molecular features occurring at the atomistic level, which influences the macroscopic regime. A suitable strategy to describe these phenomena occurring over temporal and spatial scales is to combine the continuum mechanics with higher resolution methodologies within a multiscale scheme. In this context, molecular dynamics (MD) simulations provide reliable values to describe fluid basic properties. The obtained quantities could serve as input parameters to address multiscale issues observed in numerous scientific and industrial applications, including the Oil & Gas challenges. In present study, we take advantage of the predictive role handled by MD simulations to determine natural gas properties in the supercritical region, since there exist a lack of experimental data describing the mixture properties. In light of the multiscale approach, we discuss how the gas density, viscosity, and diffusion could feed lower resolution methodologies to address Oil & Gas interest systems, including nanoflows in porous media and membrane’s technology for gas separation.   

Single Pixel Resolution Optical Flow for Low-rank Flow Fields

Single pixel resolution optical flow for quasi-steady fluid motion is proposed, while the conventional methods have been smoothed temporally or spatially. The proposed method does not use any spatial or temporal smoothing but utilizes the prior information that the fluid fields can be expressed by low-rank dataset. The new objective function with the restriction of low-rank approximation is formulated and solved by fast iterative shrinkage thresholding algorithm (FISTA) and randomized singular value decomposition (rSVD). The use of both FISTA and rSVD help us to speed up to solve the optimization problems. This algorithm gives us the single-pixel resolution flow fields form the pair images of flows, such as particle images. In the presentation, the details of objective functions and the results of numerical experiments will be reported.   

Characterization of drag coefficients of a sphere in non-Newtonian fluids by energy dissipation rate

Accurate prediction of the drag coefficient of a single particle in non-Newtonian fluid is often required for analysis of fluid particle systems: for examples, prediction of behavior of rock fragments in drilling system, design and operation of slurry pipelines and solid-liquid separation processes. In this study, we introduce an effective viscosity in terms of energy dissipation rate to define the viscosity of the non-Newtonian fluid, and estimate the settling velocity and drag coefficient using effective viscosity. This method is established on the balance of the energy dissipation rate such that the external power is dissipated within the system as viscous dissipation in a laminar regime. The effective viscosity is a function of the effective shear rate, and the effective shear rate is determined by the apparent shear rate, which is the ratio of flow velocity to characteristic length, and the two flow numbers which depend geometrical characteristics of flow field only, almost independent of rheological property of a fluid. $^{\mathrm{1}}$This work is supported by Korea Agency for Infrastructure Technology Advancement grant funded by Ministry of Land, Infrastructure and Transport (17IFIP-B133614-01, The Industrial Strategic Technology Development Program) and by the National Research Foundation of Korea (NRF-2019R1A2C1003974).   

Transition to turbulence in randomly packed porous media: energy and mixing characteristics

Transition to turbulence in randomly arranged porous media is observed in nature in applications such as cardiovascular, respiratory and biological systems. The mechanisms driving the transition to turbulence through these flows are not very well identified. This work describes the parameters influencing on overall mixing during the transition process from the perspective of energy and dispersion characteristics by addressing the following questions: (a) what are the dominant mechanisms for energy growth emanating from swirl as opposed to turbulent kinetic energy budget, and (b) how does the inertial effects of vortical structures enhance the flow transport properties such as tortuosity and dispersion. Using time-resolved PIV, flow structures are investigated in the range of macro-scale Reynolds numbers from 100 to 1000 to show the pore- versus macro-scale effects on the energy of flow and swirl structures, turbulence production and dissipation, as well as dispersion, and their contribution in interpreting the overall flow mixing. We also show Lagrangian mixing characteristics based on Eulerian local pore velocity variances and relate these to macro-scale bed characteristics for uncovering the transitional processes in randomly distributed porous media flows.   

Gravity-Induced Ripening Undermines Capillary Trapping Stability

Capillary (residual) trapping has long been considered as one of the safest CO2 sequestration mechanisms due of its hydrodynamic stability. Here we show, for the first time, that the long-term thermodynamic stability of capillary trapping could be compromised because of gravity, which has thus far been neglected in studies of bubble ripening. The interaction of gravity with molecular diffusion causes the vertical redistribution of trapped bubbles in a geologic porous medium, where bubbles at the top grow at the expense of bubbles at the bottom. The result is the formation of a gas cap at the top of the reservoir posing subsequent leakage risks. Accurate predictions of CO2 storage stability must therefore account for gravity-induced ripening. Here, we analyze the evolution of a population of trapped bubbles over time and develop simplified pore-scale and continuum models capturing its salient physics. The models reveal that the upward migration of CO2 may be hindered through the judicious selection of CO2 storage sites.   

Prediction of scattering properties for gas molecules on solid surfaces

In high Knudsen number flows, thermal and flow properties of gas are strongly influenced by the characteristics of reflected gas molecules at solid surfaces. The investigation of scattering properties of gas molecules on solid surfaces therefore is important to analyze the flow fields for micro/nano scale flows or rarefied gas flows. However, the scattering properties of gas molecules depend on many factors such as atomic species, pressure and temperature in a flow field, and solid surface states. It is difficult to completely consider these factors for the investigation of the gas-surface interaction. The purpose of this study is to construct an effective model for predicting the scattering behavior of gas molecules on solid surfaces with adsorbate using machine learning approaches. The molecular velocity distribution functions of the reflected gas molecules were obtained by molecular dynamics simulations for the gas-surface interaction. The parameters of a Gaussian function model which expresses these velocity distribution functions for various adsorbed surfaces were predicted using machine learning. The velocity distribution functions based on the constructed model properly reproduce the results of molecular dynamics analysis.   

Fundamental Differences between Large-Eddy Simulation of Incompressible Turbulence vs. Premixed Turbulent Combustion

In contrast with RANS where the modeled terms are of leading order, the LES framework requires that the modeled subfilter-scale (SFS) contributions be of lower order than the leading-order terms. This will be the case if the resolved-scale (RS) contributions to the triadic sum of advective nonlinearities in spectral space dominate the SFS triads, requiring an effective grid that resolves well Reynolds stress motions. Turbulent combustion deviates from the LES framework is several key ways, primarily in the existence of the chemical source terms that lead to the release of thermal energy at scales generally unresolved. In this study we quantify the dominant SFS contributions to the key nonlinearities that underlie RS evolution in LES of premixed turbulent combustion to isolate fundamental deviations from the LES framework. With a new method to remove spurious spectral content in inhomogeneous directions, we apply a concurrent physical-Fourier space methodology to compressible DNS of flame-turbulence interactions to isolate the triadic structure of advective nonlinearities and the quadratic structure of chemical nonlinearities in the Fourier representation from which the dynamically dominant contributions are determined. We find that when the RS fluctuations in momentum and energy are well resolved, the relative SFS contributions are very different depending on the type of nonlinearity and the evolving RS vs. SFS content of individual species. \textit{Supported by AFOSR.}   

Numerical Study of Detonations in Multiphase Flows

The detonation phenomenon is of great interest in the engineering and scientific community. Much work has been done for gaseous detonations and their processes are relatively well understood. However, multidimensional, multiphase detonations develop characteristics that are more complicated to predict and understand. Many practical engineering applications aim to utilize liquid fuels due to their convenient nature, thus, a need arises for a greater understanding of liquid spray detonations. This poster focuses on the effects of varied initial conditions and physical models in two-dimensional liquid droplet JP-10 detonations. The effects of equivalence ratios, droplet size, droplet distribution, and particle breakup will be discussed. Different methods for particle tracking and generation will be discussed as well. Data was generated using the FLASH code developed by the Flash Center for Computational Science and modified for this work to include reactions and active particles using the particle-in-cell method. The multiphase results will be compared to data from gas phase simulations and other multiphase simulation codes.   

Space charge in pneumatically assisted electrospray

The importance of coulombic repulsion due to space charge on the trajectories of ions and droplets in pneumatically assisted electrospray depends on numerous physical properties of the spray. The influence of space charge has practical implications for systems that use electrospray, including ion sources for mass spectrometers. We develop a simplified theoretical model system representing an electrospray plume with high-pressure nebulizing gas. We solve Poisson’s equation and the continuity equation for this model system to find the radius of the plume, and use this result to quantify the effect of space charge on the trajectories of charged liquid droplets in the spray. These results are applied to ion sources for liquid chromatography-mass spectrometry by considering the effects of space charge on the trajectories of ions of various mobilities. We find that the effects of space charge can be ignored in the case of modest electrospray currents, which correspond to low liquid flow rates. At higher currents and flow rates, the magnitude of the electric field from the space charge of the spray itself is comparable to that of electric fields from externally applied sources, suggesting that the role of space charge influences ion source behavior at high flow rates.   

Turbulent wall flows large-scale roughness heterogeneity: flow response to oblique alignment

The physics of wall turbulence -- ducts, boundary layers, and pipes -- affects the aero-/hydro-dynamic signature of an array of flows. Such flows often occur under the ``fully rough'', inertial-dominated limits for which viscous effects can be readily neglected and skin friction is driven by form drag (i.e., turbulent mixing). Rough surfaces composed of a complex height distribution are common in engineering and geophysical flows. Prognostic flow description is confounded by the presence of large-scale heterogeneity in surface geometry: that is, ``patches'' of differing roughness type, with spatial extent at least equal to the depth of the flow (i.e., duct half height, pipe radius, or boundary layer depth). When the prevailing transport direction is aligned orthogonal and parallel to such a heterogeneity, the flow responds with formation of an internal boundary layer or with counter-rotating rolls, respectively. These surface-driven secondary flows can completely disrupt outer layer dynamics, and thus have direct implications for wall modeled large-eddy simulation predicated upon outer-layer content. Moreover, realizations of precise orthogonal/parallel alignment are expected to be rare, and oblique alignment is likely the norm (for example, during vehicle maneuver). Results of wall turbulence response to such oblique arrangements are shown.   

Autonomous RANS/LES hybrid models with data-driven subclosures

We investigate the use of artificial neural networks (ANNs) to adapt classical Reynolds averaged Navier-Stokes (RANS) turbulence models for use in subgrid large-eddy simulation (LES) closures with the framework suggested by Perot \& Gadebusch Phys. Fluids 19, 115105 (2007) . In this study, we consider the application of a slightly-modified $k-\epsilon$ model to simulate stationary and decaying homogeneous isotropic turbulence (HIT) at a range of grid resolutions. These modifications dynamically account for (I) grid resolution relative to resolved motions and (II) backscatter from unresolved to resolved scales. In this framework, a modified turbulent viscosity accounts for the former, and the latter is determined empirically as a multiplicative factor in the modeled turbulent stress tensor. We leverage artificial neural networks to establish a universal form of this backscattering factor as a function of filter size and resolved flow statistics via a-priori analysis of direct numerical simulations. We perform simulations of stationary and decaying HIT at multiple grid resolution with the model and show via \textit{a-posteriori} analysis that the use of an ANN to model complex physical phenomena, such as local upscale transfer, is both attractive and practical.   

Magnus effect near flat ground

This research is aimed to conduct experimental and numerical analysis of the Magnus effect when a circular cylinder is approaching a flat ground. The Reynolds number is fixed at 2000. Normalized parameters include the translation-rotation speed ratio $\alpha $, declining velocity ratio $\beta $ (translation-downward), and the gap ratio, denoted by SG ($=$gap/D), where D$=$2cm is the diameter of the cylinder. The range of interest for $\alpha $ is from 0 to \textpm 0.2, and SG from 5 to 0.5, Three types of flow behaviors are identified according to the rotation of the cylinder: (i) non-rotating ($\alpha \quad =$ 0), (ii) rotating counterclockwise ($\alpha $ \textgreater 0) and (iii) rotating clockwise ($\alpha $ \textless 0). In the first case ($\alpha \quad =$ 0), the ground effect mitigates eddies behind the cylinder and leads to a higher lift and drag. In the second case ($\alpha $ \textgreater 0), as SG is decreasing, the lift and drag drops while the vortex shedding frequency increases. The vortex around the cylinder is alleviated by the ground effect, and the separation occurs at a lower portion behind the cylinder. In the last case ($\alpha $ \textless 0), as SG is decreasing, the drag increases while the vortex shedding frequency decreases. The vortex is strengthened by the ground effect, and the separation occurs at a higher location with the same reasoning. Further, stability analysis is applied to the three distinguished types of motion to examine their stability. In comparison, the phenomena of the flow patterns are consistent in both static and dynamic cases, yet the forces exerted on the cylinder are smaller in the dynamic cases.   

Project micro-meddy: doubly-diffusive experiments with heated vortices

We report on experiments that feature anticyclonic vortices embedded in linearly stratified, rotating tanks. When heated with respect to their surroundings there are two conspicuous instability features. First, prominent early, is that the circumferential edge appears serrated with cusp-like features from lateral intrusions, which are surmised to be due to thermal convection. Second, prominent later, a stepped layer develops above the vortex due to thermohaline diffusive convection. Observations are described from both the Coriolis platform (13m diameter tank, \textasciitilde 1.5m diameter vortices) and a prototype smaller vessel (0.3m diameter tank, \textasciitilde 0.1m diameter vortices). Our observations are considered with respect to previous laboratory work as well as to geophysical vortices that are thermally distinct from their environs, such as Atlantic Meddies.   

The trajectory of a leading-edge vortex following separation from an oscillating hydrofoil

Oscillating hydrofoils operating at high angles of attack shed leading edge vortices (LEVs) into the wake during their flapping cycle. Predicting the path that these vortices follow is of critical importance when attempting to optimize the interactions between multiple foils operating in close proximity. Here, we report on Particle image velocimetry (PIV) measurements of the flow field~generated by an oscillating hydrofoil at various pitch amplitude and reduced frequency. Using the Q-criterion (Haller 2005), the LEV location was identified and tracked both on the hydrofoil and in the wake. We find that a larger pitch amplitude generally resulted in an earlier LEV shedding from the foil together with the generation of a wider wake behind the foil; higher reduced frequency usually delayed the LEV shedding from the foil leading to a narrower wake. The effects of endplates were also explored. Here we find that that with endplates on the foil the LEV appeared~more coherent~and stayed closer to the foil. Once shed into the wake, the LEV shed with endplates generally travelled at the same~cross-stream~(Y)~position, while the LEV shed without endplates continued to travel away from the centerline, resulting in a wider wake.   

Controlling the forces on tandem stationary cylinders using oscillatory rotational motion

In a typical tandem circular cylinder set-up where the cylinders are held fixed, the forces acting on the downstream cylinder are primarily due to its own vortex shedding as well as the vortex interaction from the upstream cylinder. The upstream vortex upon interacting with the downstream cylinder displaces the boundary layer on it, which then leads to larger forces on the downstream cylinder compared to the case of an isolated cylinder. The movement of the stagnation point can however be controlled in a smart way by rotating the downstream cylinder in an appropriate manner. An oscillatory rotational motion with a frequency corresponding to the Strouhal frequency of the upstream cylinder is specified. However, the amplitude and phase of this rotational motion is actively monitored so that the forces on the downstream cylinder are controlled. A detailed analysis using the flow contours is conducted to explain the effect of oscillatory rotation motion on the forces of the downstream cylinder. Numerical simulations are performed at Reynolds number of 100 as well as at 0,000. This study will help in developing active feedback control for determining the forces acting on the downstream cylinder.   

Experimental Observation of Modulational Instability in Crossing Surface Gravity Wavetrains

The coupled nonlinear Schr\"odinger equation (CNLSE) is a wave envelope evolution equation applicable to two crossing, narrow-banded wave systems. Modulational instability (MI), a~feature of the nonlinear Schr\"odinger wave equation, is characterized (to first order) by an exponential growth of sideband components and the formation of distinct wave pulses, often containing extreme waves. Linear stability analysis of the CNLSE shows the effect of crossing angle, $\theta$, on MI, and~reveals instabilities between $0^\circ < \theta < 35^\circ$, $46^\circ < \theta < 143^\circ$, and~$145^\circ < \theta < 180^\circ$. Herein, the~modulational stability of crossing wavetrains seeded with symmetrical sidebands is determined experimentally from tests in a circular wave basin. Experiments were carried out at $12$ crossing angles between $0^\circ \leq \theta \leq 88^\circ$, and strong unidirectional sideband growth was observed. This growth reduced significantly at angles beyond $\theta\approx 20^\circ$, reaching complete stability at $\theta$ = 30--40$^\circ$. We find satisfactory agreement between numerical predictions (using a time-marching CNLSE solver) and experimental measurements for all crossing angles.   

Science for the People: The dual nature of Science

Science for the People is an organization dedicated to building a social movement around progressive and radical perspectives on science and society. We are STEM workers, educators, and activists who believe that science can be a positive force for humanity and the planet. Science is not an abstraction removed from society. Science is produced by our labor. But the conditions of this production and the use of science are not controlled by the general public. Science has been used historically against marginalized communities for example to justify racism. Today, we see anti-scientific movements such as anti-vaccines arising out of the mistrust between the scientific community and the people. We must ensure that science serves all people, including the most marginalized. Until this is addressed, the STEM field will lack diversity and any solutions to this are patches. While knowledge is won with our labor and can be used to advance common goals, we recognize that technical knowledge alone never delivers justice. Because of this, our members are committed to educating scientists and university students about the role that science and technology play in shaping the world, and using our knowledge to work with specific grassroots campaigns.   

Creation and research at the arts \& sciences chair in Paris

The "Arts \& Sciences" Chair at the École polytechnique, the École nationale supérieure des Arts Décoratifs-PSL and the Daniel and Nina Carasso Foundation was created in September 2017 with the ambition of bringing together the arts \& sciences to imagine tomorrow, by promoting collaborative practices and a citizen approach. In a context of climate emergency and constant technological and digital developments, artists and scientists have in common that they can play a "scouting" role. The Arts \& Sciences Chair offers a research and creation program to question our interdependence with the environment, human and non-human, whether it is to explore our relationship to plants and climate or human-machine interactions. Several examples of art installations involving rain, fog, oxygen, waves, instability and time will be presented.   

Data- driven design and predict heat exchanger performance: configuration complexity and difficulty of prediction

Data-driven methods have shown potential capabilities in accelerating future designs, optimizations, and evaluations. However, due to the complexity of fluid flow and geometry configurations of industrial devices, it is challenging to train effective data-based models. In the present research, we explore the relationship between device configuration complexities and reliable prediction algorithms. Taking a 2D plate heat exchanger as an example, we first use computational fluid dynamics solver to generate fluid flow/heat transfer data and then we train machine learning models to predict device performance. By increasing the complexity of the geometry configurations, we repeat the process and study the relationship between geometry configurations and machine learning models. Through the study, we can conclude the suitable algorithm for the specific plate heat exchanger application.   

Effects of foil shape on fish-like swimming.

Aquatic animals have evolved a diversity of propulsive mechanisms to locomote effectively through water. Fishes produce hydrodynamic thrust by acceleration of water through movement of their body and tails, while simultaneously reducing the resistance to their motion through morphological design, phased kinematics, and behaviors. In this study, the effect of the shape of a fish-like caudal fin as well as changes in the frequency of the linear motion of the fin is investigated. Many numerical tests have been done due to optical limitations, and it's time for experimental test to confirm the numerical data. Experiments were conducted on a square fin, triangle fin, and forked triangle fin in order to compare and observe the changes in efficiency and thrust produced. Furthermore, the amplitude and frequency of linear oscillations were varied to see the data at different speeds within the optimal Strouhal number range at which fish are more efficient. The results of this research can be used to confirm the data collected from numerical experiments and further be applied to future biomimicry.