Session N01: Poster Session (3:20-4:05pm)

A rate-and-state framework for surging glaciers

Glacier surges are quasiperiodic accelerations of ice flow, principally triggered by increased slip at the ice-bed interface. Mechanisms for the initiation, propagation and termination of glacier surges remain open questions, pointing to gaps in fundamental understanding of ice-bed interactions. In this theoretical study, we present results from our model for a rate-and-state description of the granular subglacial till that is two-way coupled to the overlying ice. This rate-and-state framework captures the coupled evolution of porosity and water pressure, thereby unlocking a rich variety of behaviours and dynamical feedbacks as the system is perturbed away from steady-state. Our findings have implications for glacier surge dynamics, mechanics of ice-bed interactions and granular medium descriptions of subglacial till.   

A Feature of Vortical Axis in Coalescence of Vortical Regions in a Homogeneous Isotropic Turbulence

The present study analyses the dynamics of coalescence of vortical regions with the behavior of vorticity lines in a homogeneous isotropic turbulence. A process of coalescence of several vortical regions shows that two vortical regions approach and coalesce, and that another vortical region coalesces with them. While a single and large vortical region is formed by the coalescences, all vortical regions do not coalesce and the core regions of respective vortices with intense swirling remain separately. Then the bundle of vorticity lines exist in the organized vortical region that pass respective regions of the coalesced vorticies. On the other hand, the bundles of vorticity lines in the core regions of respective vortices tend to pass through non intense core region of coalesced vortex. It is noted that vorticity vectors of two vortical regions to coalesce have the same direction. It shows that the characteristics of two vortical regions in the coalescence of vortical regions are different from these of anti-parallel vortex tubes in the reconnection of vortex.   

Two-Dimensional Aerodynamic Analysis of Flight in the Smallest Insects

Two-dimensional immersed boundary simulations were performed to determine how stroke plane angle and wing flexibility affect aerodynamic efficiency and energetic pseudo-efficiency for the smallest flying insects. Experimental data pertaining to small insect flight is limited and therefore, their flight mechanisms are still largely unknown. The immersed boundary method was used to solve the fully-coupled fluid-structure interaction problem of a flexible wing immersed in a two-dimensional viscous fluid. We considered five different strokes: a horizontal stroke, three hybrid strokes, and a vertical stroke. We also considered five different wing flexibilities ranging from rigid to highly flexible. Aerodynamic efficiency was defined as the ratio of the average vertical force coefficient to the average total force coefficient and energy pseudo-efficiency was defined as the ratio of the average vertical force generated by a wing to the average power delivered by the wing to the surrounding fluid. Our results indicate that at Reynolds numbers relevant to small insect flight, aerodynamic efficiency and energy efficiency decrease with increasing stroke plane angle regardless of wing flexibility and a rigid wing is aerodynamically and energetically more efficient than flexible wings.   

Interstitial fluid flows of bones

Cancer cells metastasize to bones at the late stages of the disease, leading to high mobility and mortality rate in patients. In this work, we investigate flow structures within the pores of an in-vitro bone model to understand the mechanical micro-environment surrounding cancer cells. The bone scaffold is immersed in a fluid flow inside a bioreactor. Cancer cells are seeded to grow on the surface of the scaffold for 23 days before being harvested for analysis.  Based on the micro-Computed Tomography scans from the in-vitro experiments, we created a full-scale 3D surface mesh of the scaffold using open-source software Slicer3D and Meshmixer. The computational grid was generated using the commercial software Gridgen Pointwise. We performed Computational Fluid Dynamics (CFD) simulations with the immersed boundary method to investigate the flow patterns inside the pores of the scaffolds. Post-processing of the results was carried out using the open-source software Paraview and Blender to provide a high-resolution visualization of the flow within the scaffold's complex porous geometry. The flow velocity and the shear stress distributions inside the scaffold are shown to be convoluted and very sensitive to the pore size. The computational results show a distinctive difference on the shear stress distribution on the scaffold's sides. Our results suggest that there is a link between interstitial flow patterns and cancer cell growth.   

A Fourier-based technique for efficient discovery of linear PDEs

Machine learning techniques have been used to identify the form and coefficients of partial differential equations (PDEs). However, these methods require large amounts of data in the presence of noise. We present a new method based in Fourier analysis for determining the coefficients of a PDE when its form is already known. In contrast to a traditional PDE discovery method, our approach presents two major advantages. Firstly, it is robust against noise as it relies on the large Fourier modes that are insensitive to noise. Secondly, it is data-efficient since those lower modes can be reconstructed using a few data points. We demonstrate these advantages by testing our method on the heat and the wave equation. Our goal is to fully integrate this method into a machine learning framework to be able to tackle the sparse regression problem for more general PDEs while requiring only small amounts of data.   

Regime classification for stratified wakes from planar velocity field snapshots using convolutional neural networks

Previous studies have shown that stratified wakes can be classified into various regimes from topological features present in the velocity field, which in turn depend on the Reynolds number and Froude number.  In this study we use a machine learning-based technique to develop a wake regime classifier from very limited data: single 2D-2C velocity snapshots in the vertical plane.  Specifically, we use convolutional neural networks (CNNs) which are often used for image classification due to their ability to "learn" distinguishing features in the images and generate translation-equivariant feature maps.  This also makes CNNs ideal for the present application. We train the CNN on a labelled dataset of velocity field snapshots available from direct numerical simulations.  Classification accuracy is then evaluated using an experimental dataset that does not always have the same field of view as the numerical dataset that the CNN has been trained on. The resulting accuracy is therefore a measure of the robustness of the classifier to real-world measurements. This study complements previous work on the development of an expert-defined decision-tree-based classification system and holds promise for the development of automated fluid pattern classifiers.   

Multi-layer network analysis of fluid-structure interactions

The coupling interactions between deformable structures and unsteady fluid flows occur across a wide range of spatial and temporal scales in many engineering applications. These interactions make it challenging to predict the flow physics and structural loads accurately. We propose two multi-layer network approaches to characterize the interactions between the fluid and structural layers in a 2D laminar flow over a flat plate at 50 degrees angle of attack. In one approach, the grid vortices and bound vortexlets form the nodes of the network with the edges defined by induced velocity. In the other approach, coherent structures (fluid modes) contributing to kinetic energy and structural modes contributing to strain energy constitute the network nodes. The energy transfers between the modes are extracted using a perturbation approach. The interactions are further simplified using network community detection. The present work aims to create a network-theoretic framework for reduced-order modeling of multiphysics systems.   

CFD Analysis of Fan Wall Downstream Flow for UAS Testing

The push towards greater autonomy of aerial vehicles requires an extensive knowledge of the environment and forces that will act on these vehicles during their flights. However, creating a somewhat analytical or perfect control system is an extremely difficult physics problem in the face of the nearly unpredictable nature of turbulence. Thus, experimentation is critical for incremental improvement. To achieve a repeatable experimentation process for UAS testing, a "fan wall" consisting of numerous individual fans, called the Windshaper, will be used to build a faux wind tunnel testing system that allows for various different profiles and disturbances to be generated. To effectively use this system to test UASs, a CFD model for the inside of the testing facility must be generated. This model can help prescribe the bounds of a valid test space, by portraying the distance before the downstream breakdown of the flow occurs for a given profile. Furthermore, the model will be critical in assessing the safety of containing a wind-generating device in an open-air test section. The sensitivity of the simulation to its turbulence model and meshing fidelity must also be verified. Ultimately, the accuracy of the model will then be validated with actual pressure measurements.   

Sliding uphill: Using Surface Evolver to study the curvature-driven propulsion of floating films.

Whereas a solid object will roll down an incline, capillary forces can create situations in which solids appear to move uphill. We study an initially-planar ultrathin elastic film, which propels itself up a curved meniscus into a flat interfacial region. This behavior is driven by the ability of the sheet to readily wrinkle, thereby approximating different surface topographies. In particular, previous work showed that a thin film and droplet will deform together into a three-dimensional shape that minimizes the exposed liquid surface area [1]. In our problem, gravity and surface tension must both be taken into account in an analogous geometric optimization. To understand the energies driving this propulsion, we use Surface Evolver simulations to measure the equilibrium energy when the sheet this pinned at different radial positions along the liquid bath. We investigate how these energy gradients depend on the liquid volume and sheet radius. These measurements can be linked back to the experimentally-measured velocities by estimating the drag on the sheet due to the fluid.

[1] Paulsen et al., Nat. Mater. 14 (2015).   

Data-Analysis of the Coarse-Grained Velocity Gradient Tensor

We present the co-evolved statistics of the Lagrangian velocity gradient tensor and material volume, coarse-grained on a range of scales in high fidelity direct numerical simulation data. Defining the velocity gradient tensor over a representation of fluid volume, we obtain a joint dynamical system whose statistical evolution is closely related to a variety of turbulence characteristics, such as: flow topology, deformation of material volume, energy cascade and intermittency. Therefore this analysis searches for commonalities and differences of the joint system that depend on scale. This poster will provide additional technical details on the data-analysis used to enable our machine learning of the coarse-grained velocity gradient tensor.   

The analysis of the topological characteristics of bundle of axes lines

This study focuses on the bundle of axes lines in an isotropic homogeneous turbulence, and analyses the topological characteristics of vorticity lines and eiven-vortical-axis lines passing the vortex region. The characteristics of bundles of axes lines are analysed using the theory of local axis geometry. This theory analyses quantitatively the characteristics of the bundle of axes lines with respect to the passage of vortical region by focusing on the gradient tensor of the subjected vector field with respect to swirl plane and evaluating the physical quantities derived from the tensor such as swirlity and sourcity in the swirl plane. The results of analysis using this theory show that vorticity lines have a swirling feature, whereas the eigen-vortical axis lines pass the core region without swirling. It is also shown that the difference of these axes lines derived from vortex stretching and topological feature of the vortex itself.   

An Analysis of Three-Dimensional Vortical Flow Structure in a Homogeneous Isotropic Turbulence

The present study investigates velocity structure of vortices in a homogeneous isotropic turbulence with a low Reynolds number. It applies physical quantities that have been proposed to specify the topology of local flow, where swirlity indicates the unidirectionality and intensity of azimuthal flow in arbitrary plane, and symmetry of vortical flow does the skewness of vortical flow. In the analysis of the velocity structure, the vortex center is identified as a maximum points of the swirlity in a vortical region, using its Hessians. If the symmetry of vortical flow does not exhibit the axisymmetric feature in the vortex center, then the elliptic direction of the vortical flow is set to have uniform direction in the statistical flow structure. The results show that the statistical vortical flow is asymmetric, and that the azimuthal flow is dominant in the vortical flow. The radial flow is also asymmetric, and has both inflow and outflow, while the axial flow is weak. These characteristics specify the particular features of a vortex associated with topological stability or pressure minimum.   

Numerical modelling of Cavitation in Blood Vessel using Immersed Boundary Method

Controlled oscillation of microbubbles in a blood vessel can be used to modify the permeability of the vessel wall. This technique,called sonoporation, can  potentially be used for gene therapy and targeted drug delivery for  the treatment of    cancer  and other diseases.   This   project   focuses   on   developing   an   Octree-based Fluid-Structure   Interaction   (FSI)   model   that   simulates   the interaction   of   an   oscillating   axisymmetric   bubble   subjected   to focused ultrasound waves  with the  walls of  a blood vessel. The model uses the Immersed Boundary Method (IBM) to simulate the interaction of viscoelastic tissue and blood plasma separated by an elastic blood vessel.  It also  uses an "all-Mach"  formulation to simulate   compressible   multi-phase   simulation   of   bubbles   and blood. The multiphase flow solver,  developed  inside the platform Basilisk,  is based on the Volume-of-Fluid method  and  includes surface tension and viscous forces in the formulation. The study also focuses on the understanding of the bubble-vessel interaction for varying vessel radius and vessel  compliance  and also on the frequency of the ultrasound waves.    

Atomisation of a pulsed liquid jet by Volume of Fluid method

Simulation of a pulsating dense cylindrical liquid oil jet injected into a stagnant air phase is performed at Reynolds number Re=5800, Weber number We=5555 and density ratio r=28 using the Volume of Fluid method and octree adaptive mesh refinement, using the Basilisk code. The grid refinement above 900 cells per initial jet diameter is achieved. This is to our knowledge the most detailed simulation of this type of flow. Simulations provide direct evidence of many underlying mechanisms including characteristic sheet perforation due to droplet impact and sheet rupture due to stretching and thinning. The numerical curvature provides a diagnostic of the presence of weak spots. After holes form from weak spots, characteristic ligament networks are seen. The probability distribution function (PDF) of droplet sizes is obtained. It converges slowly with the minimum grid size ∆ and has a bimodal character. The position of the two peaks is a shifting showing towards small scales as ∆ is decreased, indicating the dependence of the mechanisms involved on the grid size, with sheet perforation the likely culprit. At large scales, however, the PDF is seen to be converging.   

Computational modeling of blood clots

Abnormalities in blood clot contraction result in life-threatening consequences: insufficient clotting can cause excessive bleeding, while excessive clotting can cause thrombosis and lead to heart attack or stroke. Better understanding of the physics governing the clot contraction process may provide novel insights for treatments and diagnoses for diseases related to abnormal clotting. Because blood clotting is a complex biological process involving platelets, fibrin network, red blood cells, and flow, it is difficult to study experimentally without disrupting the clotting process. Using dissipative particle dynamics, we developed an integrative, mesoscale, computational model of red blood cells within platelet-fibrin clot to investigate the interactions between those components during the clot contraction process under blood flow. We study how red blood cells that are initially moving with the flow get captured and deform inside contracting clots. We examine how blood flow affects clot retraction and structure as well as the flow disruption by contracting clots.   

Use of Skeletonization Techniques for the Analysis of Thin Structures in Multiphase Flows

Measurements of velocity, vorticity, and energy profiles have long been used for the analysis and validation of fluid flow simulations, but proper analysis of multiphase flow simulations requires the consideration of the geometric properties of the flow. Previous Volume-of-Fluid (VOF) studies reported on the statistical analysis of droplet sizes in primary atomization. We expand the analysis to the formation of ligaments and thin films. First, we apply standard methods of skeletonization used in image processing such as thinning and levelset methods to identify and localize thin films and ligaments from the VOF color function. We then measure the length, shape, and thickness of identified structures and perform deterministic and statistical analysis of these measurements. Last, we demonstrate the use of skeletonization techniques to improve the efficiency and physical accuracy of primary atomization simulations, using skeleton characteristics as criteria for adaptive mesh refinement and the identification of locations for film perforation. In the future, skeletons could also be used to replace thin structures with thin film asymptotic approximations which would greatly speed up simulations where the bottleneck is the large range of length scales in the fluid flow structures.   

Study of granular flow in a wedge-shaped hopper using DEM simulations

Granular materials are made up of particles or discrete solids that flow like liquids. The flow of material through a hopper is a fundamental industrial unit operation and a granular flow problem in which material flows under gravity and leaves the storage bin through the outlet at the bottom of the bin. Using a discrete element method (DEM), the interaction between particles is evaluated using Newton’s laws of motion. It is important to understand and model such granular flows in terms of parameters such as grain size, solid fraction, wall roughness, particle-particle interactions, and others that affect them. The continuity equations and radial momentum balance are solved in a wedge-shaped hopper for a smooth wall and radial gravity flow. Our computational results fit the theory developed by Savage (1965) and Prakash and Rao (1988). The present study also includes parametric analyses to investigate wedge-shaped hopper rheology in depth.

Keywords: granular materials, wedge-shaped hopper, DEM simulations   

Effect of Droplet Modes on Secondary Fragmentation

A Newtonian drop undergoes secondary (non-vibrational) breakup under impulsive acceleration when the flow Weber number is greater than critical Weber number (We_cr). For a spherical droplet, We_cr depends on density ratio ρ as well as drop (Oh_d) and ambient (Oh_o) Ohnesorge numbers. However in nature, droplets are rarely spherical. Natural droplets almost always show free surface oscillations in various modes. These non-spherical shapes can have a significant impact on droplet deformation and breakup, depending on its mode as well as the phase of the oscillation. To illustrate this behavior, droplets with different oscillation modes are studied using numerical simulations and compared against analogous spherical droplets. Significant changes to both breakup morphology and We_cr are observed due to a change in initial droplet shape.   

Roles of 2-Dimensional and 3-Dimensional Vorticity Transport Mechanisms in Assisting LEV Attachment on a Revolving Wing

An aspect ratio 9.5 rectangular wing is revolved in a cylindrical domain at 45-degree angle of incidence and a Reynolds number Re_c = 300, based on the wing velocity two chord lengths from the axis of rotation (z/C = 2.0). Four cases are considered which include combinations of linearly-varying or constant inflow velocity profile, and the presence or absence of rotational accelerations, emulating pure rotation, pure translation, and two hybrid cases. Each case exhibits a strikingly different behavior of the leading-edge vortex (LEV), demonstrating that inflow shear is an important factor governing LEV behavior, in addition to the rotational accelerations. Vorticity transport analyses were conducted in chordwise planar control regions, at z/C = 2.0. There is frequently a moderate to strong correlation between the nominally two-dimensional vorticity transport mechanisms (shear-layer flux and surface diffusive flux) and between the three-dimensional transport mechanisms (spanwise convective flux and tilting flux). Although 2D fluxes typically dominate, inflow profile and rotational accelerations are shown to modify flux contributions, resulting in varied LEV strengths and trajectories.  The physical and mathematical foundations governing the observed behaviors will be discussed.   

Exploration of Autoencoder Loss Functions for Reduced-order Modeling of Fluid Flow Data

Data-driven reduced-order models (ROMs) have seen many practical applications in fluid mechanics to reduce the dimensionality of a problem. Fluid flows are notoriously difficult to reduce due to the need to capture phenomena that occur at order-of-magnitude varying scales. An autoencoder is a neural network ROM that can capture non-linear behaviours. In previous literature, the mean-squared error (MSE) has been the prominent loss function in the autoencoder algorithm. However, for non-normalized loss functions such as the MSE, errors in large-scale fluctuations can mask out smaller-scale ones, even though both are equally important to resolve for an accurate representation of the flow field. Furthermore, the MSE is a scalar-based loss while the velocity field is a vector quantity. The use of normalized and vector-based losses will be explored in autoencoder networks to produce a more accurate ROM.

A case study on the incompressible flow past a NACA0012 airfoil at angles of attack from 0^o to 10^o is conducted. The autoencoder loss function is studied in detail, and an alternative to the MSE is proposed which also incorporates conservation principles. Locations on the airfoil surface and in the wake region will be probed to show a more complete picture of the reconstruction error.   

Numerical study of proppants transport and distribution in rock fractures

The proppants' transport, displacement, and distribution in rock fractures were numerically studied. A realistic three-dimensional rock fracture geometry was generated using the Brown method. Lagrangian proppant tracking and the Eulerian approach for fluid flow were used to simulate the proppants laden flow inside the rock fracture. The pressure gradient drives the four-way coupled proppant-laden flow in the streamwise direction. Different horizontal and vertical fracture configurations were considered. Coverage of rock fracture by proppants were reported for various test cases with different proppant sizes and rock fracture apertures. It was shown that the fracture coverage changes dramatically by including a realistic surface roughness compared with a smooth channel fracture model. It was also shown that the particle diameter and the mean aperture significantly affect the fracture coverage, and the optimum proppant size for a given aperture was evaluated. As a result of proppants packing over time, the flow rate in the fracture varies with time. Propagation and placement of proppants into rock fractures from the injection until the steady flow is reached were presented.   

Numerical Investigation of Flow over Orthoconic Structure

Orthoconic cephalopods were classic fossil nautiloids of underwater communities whose locomotion in swimming and special annulations around the shell are understudied. We intend to investigate the marine ecology and hydrodynamic characteristics of straight orthoconic shells in a moving fluid, which can shed light on potential applications in morphologic design for underwater vehicles. The geometry of the orthoconic shell model is determined based on measurements of the sculpture and size from fossil specimens. Direct numerical simulations of incompressible flow over the orthoconic shell are performed using two different angles of attack (AoA): 0° and 180°, to mimic different swimming directions of orthoconic cephalopods. The Reynolds number considered in the present work ranges in the order from O(10) to O(1000). Flow features and hydrodynamic forces around the orthoconic shell and in the wake are characterized. From the preliminary results, critical transition from steady to unsteady flow falls between Re=500 and 1000 for the AoA 0° case. In the future, analysis of an annulated orthoconic shell model will be investigated, which could provide insights into the control of the canonical flow over a canonical cone structure in engineering applications.   

Header Optimization for Efficient Flow Distribution in Ceramic Heat Exchangers

Recent advancements in the field of ceramics enable fabrication of heat exchangers capable of operating in high-temperature, high-pressure environments. However, effectively integrating complex ceramic heat exchanger core geometries with cycle fixtures requires modification of the classic header design. In this work we present a header design capable of directing two independent, turbulent flows through a multiscale ceramic heat exchanger core. Optimized for uniform flow distribution and minimal pressure loss, this header design offers a scalable and manufacturable solution for ceramic heat exchanger operation in extreme cycle environments.   

Influence of the Boundary Layer State on the Wake of a Wall-Mounted Square Cylinder

The influence of the incoming boundary layer state on the near wake of an AR 4 square cylinder is experimentally investigated. Two cases are considered at Reynolds number ~ 10⁴ and the obstacle protrudes (i) a laminar boundary layer (LBL) with a relative thickness to height of δ/h = 0.05; and (ii) a turbulent boundary layer (TBL) with δ/h = 0.18. Large scale vortex structures are educed through phase-aligning planar PIV measurements using reference pressure measurements on the sides of the obstacle. A mean pair of contra-rotating streamwise vortices, known as dipole, appears in both cases. In the LBL case, the dipole is accompanied by an additional contra-rotating pair of descending vortices, which arises as a result of the interaction between successive Kármán vortices from the opposite sides. Along with the descending vortices, Reynolds stresses and productions terms are significantly higher in the LBL case. Closer to the plate, phase-averaged results show higher interactions of the opposite side shear flow and the forming Kármán vortex in the TBL case, which is evidenced by complementary oil-film visualizations.   

Focusing patterns of spherical particles suspended in square tube flows of viscoelastic fluids

The particle focusing in square tube flows was investigated experimentally for spherical particles suspended in aqueous solutions of polyvinylpyrrolidone (PVP). The location of each particle center in the tube cross-section was detected near the outlet by an “enface” observation method, and the distribution of particles was obtained for various PVP concentrations, blockage ratios and Reynolds numbers (Re). In 4wt% PVP solutions, the particles were focused on the tube center and near the corners at low Re, whereas, at slightly elevated Re, all particles migrated toward the tube center. At elevated Re, the particles suspended in 1wt% PVP solutions were focused at the center of channel faces, in accord to those in Newtonian fluids. An increase in PVP concentrations at the same Re shifted the focusing position to the equilibrium position on the diagonal. These transitions of the particle focusing pattern were accounted for by the interplay between the inertial lift and elasticity-induced lift.   

Quantifying flow through thin and flexible electroosmotic micropumps with droplet shape analysis

Electroosmotic (EO) micropumps transport fluid without any moving parts. The ease of miniaturizing these pumps into thin flexible devices enables novel applications in implantable drug delivery and lab-on-a-chip systems. However, it also complicates conventional methods of measuring pump performance. It is not trivial to attach fluidic interconnects to these microscale devices for mass flow measurements. Also, these pumps may be optically incompatible with fluorescent particle tracking. In this study, we introduce a connected droplets technique to measure pump performance by quantifying pump parameters such as zeta potential, velocity, and flow rate. We fabricated a thin biocompatible EO pump consisting of an epoxy SU-8 passive layer sandwiched between two polymer electrodes. The pumping action takes place through a microchannel passing through the multilayer stack. We placed droplets on either side of the microchannel and observed the evolving droplet shapes as the pump drove fluid from one droplet to the other. By comparing these observations to theory and multi-physics simulations, we deduced the key parameters describing the flow. The advantages of this technique include simplicity, low cost, and measurement of zeta potential without specialized instruments.

Eddy covariance measurements for prediction of optical turbulence

Differential heating and turbulence in the atmosphere results in regions of spatially varying temperature, density, and humidity. Laser light traveling along a path with varying optical properties will experience excessive beam spread, loss of coherence, and diminished irradiance on target. This phenomenon is called optical turbulence, and is typically quantified by the index of refraction structure parameter C_n². We aim to develop both traditional and machine learning models for determining C_n² from sonic anemometer data. High frequency wind velocity, temperature, and gas flux through the atmosphere has been measured at three different heights within the surface layer. This data will be applied to similarity theory and direct methods to calculate C_n². Using statistical regression models and supervised machine learning techniques, a new model will be developed and trained to identify C_n² for a local environment and thus predict laser performance.   

The effect of ambient flow on Vorticella convallaria 3D orientation and feeding behavior

Vorticella convallaria are microscopic sessile suspension feeders which live attached to substrates and are ubiquitous in aquatic environments. They depend on a self-generated current to feed and help maintain the health of aquatic ecosystems by consuming bacteria and detritus. They can also improve efficiency of wastewater treatment and bioremediation. Their environmental impact is mediated by their feeding rate, which is reduced when recirculating eddies deplete the supply of food particles in their feeding current. Previous work found that when organism feeding currents interact with ambient flow, feeding rates are highly dependent on organism orientation relative to the surface and the flow, with some orientations leading to recirculation. We thus hypothesized that individuals actively orient themselves with positions most favorable for feeding. We cultured organisms attached to the bottom surface of a flow chamber and exposed them to unidirectional laminar flow at four speeds. We recorded the 3D orientation of individuals over a span of 20 minutes using a simultaneous top and side view microscope. We determined that the orientation of the cell body and stalk are impacted by flow and observed changes to their feeding patterns and feeding rate as flow speed increased.   

Not Participating

Swimming droplets leave a repulsive trail when they dissolve in water. This leads to self-avoidance and memory effects in the active droplet dynamics. Classical polymer physics models would predict a super diffusive scaling of 3/2 for the mean-squared displacement (MSD) in the limit of a self-avoiding random walk with no noise.  Here we find that the ensemble-averaged and time-averaged MSD reveal a discrepancy due to the history dependence of the droplet motion. To capture this behavior, we derive a theoretical Non-Markovian model (NMM), in which concentration gradient of the dissolved oil repels the particle motion. Interestingly, the ensemble-averaged MSD can be equally well fit with the Markovian active Brownian particle (ABP) model and the NMM that takes into account the trail repulsion. We therefore implement a deep-learning method known as convolutional neural network (CNN) to classify experimental trajectories by the two models. The network is trained and tested on trajectories simulated with parameters obtained from fitting both models to ensemble-averaged MSD. Then experimental trajectories are classified using the well-trained network. It turns out that more than 85% of the experimental trajectories are identified as NMM. The success of the NMM in fitting the data allows us to extract the active noise in the system, arising from hydrodynamic effects, which can be interpreted as an effective temperature of ~ 10⁶ k_BT. Our findings indicate that even though the MSD scaling laws do not change because of the memory effect, there are distinguishing features that select NMM over ABP.    

How does the presence of water in a splash cup affect seed dispersal distance?

Splash cups use falling raindrops as a method of seed dispersal. These plants launch seeds from millimeter-sized cups and use a raindrop's kinetic energy to send their seeds a horizontal distance up to ten times their height. Understanding the biomechanics of this dispersal method can be more broadly applied to better understanding similar phenomena such as soil erosion, the spread of pesticides, or 3D printing. While previous work has focused on cups that are initially dry, splash cups in nature are often found partially full of water. Here, we study how the presence of water in a splash cup changes patterns of seed dispersal. We released water drops above 3D-printed splash cup models that contained one seed and were either empty or half-full with water. We varied the raindrop's impact location relative to the center of the cup, as well as the steepness of the cup walls, to determine optimal values for dispersal distances. Our findings suggest that the presence of water affected dispersal distances in a complex and non-linear fashion. Furthermore, the presence of water shifts the optimal raindrop impact location towards the center of the cup.   

Piezoelectric energy harvesting from cantilevered-rectangular bluff bodies using fluid-structure interaction

Rigid rectangular bluff bodies of varying B/D ratios attached to two piezoelectric cantilevers are subjected to vortex-induced vibrations (VIV) and flutter in a suction-type wind tunnel to study the influence of different flow-induced vibrations. The cantilevered-rectangular bluff bodies were arranged in a parallel configuration to aid in vibration frequency control. Images of the vibrating cylinders were captured at high speeds and compared with the voltage levels generated as a function of incident wind speed. The ratio of natural frequency to vibration frequency proved to be an indicator of operating efficiency. The energy harvesting efficiency of the bluff bodies was also calculated and was shown to be dependent on the frequency ratio. The maximum power harvested was found to be 1.8μW. An operating limit for each bluff body of a given B/D ratio was established in terms of non-dimensional parameters such as Reynolds number (Re)-(1500-5500), Strouhal number (St)-(0.06-0.16), Dimensionless frequency (0.1-0.7), Scruton number (Sc) and reduced velocity (4-14).  Beyond the operating limit, no positive effect on the resultant voltage was observed. These results will further aid the design and development of vibration-based ambient energy harvesters.    

Stroke frequency and size effects on metachronal paddling

Numerous aquatic organisms use a locomotion strategy known as metachronal paddling across a wide range of body sizes, from microscopic paramecia to lobsters nearly a meter in body length. This range of body size results in vastly different fluid dynamic effects on the swimming regime. Despite broad application of metachronal paddling in nature, the effect of paddle-based Reynolds number on the paddling wake and swimming performance is unknown. Using a paddling robot with fixed stroke amplitude and metachronal phase lag, we vary stroke frequency and fluid viscosity to investigate these effects. Varying stroke frequency allows us to examine the fluid dynamic effects of an organism stroking faster or slower, while varying fluid viscosity allows us to examine the different fluid dynamic effects acting on organisms of different sizes. We found that while viscosity has a stronger effect on the direction of the wake than frequency, the frequency does affect cycle to cycle interactions in the wake. Interestingly, the Strouhal number remained relatively unchanged (approximately St=0.25) for Reynolds numbers ranging from Re=50 to Re=50,000, indicating that metachronal paddling is robust across different sizes.   

Leading-edge vortex evolution over an impulsively rotated wing

The flow over a rotating wing is 3D and unsteady in nature and is influenced by phenomena unique to the rotating frame. However, conventional PIV techniques are unable to comprehensively characterize these rotating frame physics. Hence, a novel methodology termed "Rotating Three-Dimensional Velocimetry" (R3DV) is developed to quantify the spatio-temporal evolution of the flow field in the rotating reference frame. The flow field in R3DV is imaged through a stationary plenoptic camera viewing a hub-mounted mirror that rotates with the wing. Experiments over an impulsively rotated flat plate showed a uniform shear layer separation along the span of the wing, that rolled up to form the leading-edge vortex (LEV). As the shear layer continuously feeds vorticity to the LEV, it strengthens in time. After the LEV saturates, the vortex separates from the shear layer and moves downstream. During the LEVs evolution, a region of secondary vorticity was observed due to the flow induced by the LEV. As time progressed, this region grew between the LEV and the shear layer, which led to the separation of the LEV. Following this, a continuous shedding of secondary LEVs was observed.   

Characterizing the thrust production of partially-submerged flapping propulsors

Archer fish jumping out of the water to capture prey utilize varying tail stroke kinematics throughout their jump. In this study, we experimentally investigate the propulsive performance of flexible plates breaching the air-water interface inspired by these jumps. Analyzing the performance across a range of stroke kinematics and plate flexibilities at different fixed submergence depths reveals how propulsive dynamics change throughout the course of the jump. We measure net thrust using a load cell and wake patterns using 2D PIV. Additionally, we compare both jumping and forward in-water swimming configurations using the same stroke kinematics to investigate the effects of the free surface on the generated thrusts and wakes. Understanding how propulsive dynamics change when partially-submerged allows us to develop flapping profiles for water exit to maximize quantities such as net thrust generated or efficiency.   

Energetics in the Wake of Self-Propelling Pitching Airfoil

The present study aims to understand the role of flexibility in self-propelling bodies, mainly how it affects the speed and efficiency of propulsion. We experimentally investigate the energetics of the wake of a rigid airfoil (NACA0015 section) and a composite airfoil (NACA0015 section with a flexible appendage) pitching about 1/3rd chord-length of rigid section. We employ Particle Image Velocimetry (PIV) to obtain the instantaneous velocity field in the wake of the self-propelling foil for different pitching amplitude and frequencies. Complex wake patterns are observed in both cases. Since the net force on a self-propelling body is zero, energetic efficiency is defined by a Power Loss Coefficient (CPL) which is essentially the non-dimensional kinetic energy of the wake. While there are specific interesting comparisons between the rigid and the composite airfoil for the same set of parameters, the rigid airfoil has a wider jet-like wake. In contrast, the wake of the composite airfoil is narrower in comparison making it more efficient. The presence of a flexible appendage results in a pattern of vortex shedding that is nearly aligned to the center-line. With these insights, we try to obtain valuable pointers to design efficient underwater or micro-aerial vehicles.     

Roughness Effects of a Model Seabed on Flow Around an Underwater Vehicle

In this work, we report an experimental investigation of the roughness effects of a model seabed on the flow around an underwater vehicle. The seabed is modeled by the bottom wall of a test flume with or without an array of roughness elements. These roughness elements consist of an array of hemispheres with an approximate pitch-to-height ratio of 3. The underwater vehicle is also modeled by the hull of a scaled DARPA submarine. The vehicle is mounted on the model seabed and subjected to an open channel turbulent flow of Reynolds number 2.7 x 10⁴ (based on the free stream velocity and the body length). Particle image velocimetry is used to obtain detailed velocity measurements of the flow upstream of the vehicle, over the vehicle, and downstream of the vehicle with or without roughness elements on the model seabed.  Mean velocities and higher order moment turbulent statistics are assessed to determine the effect of the roughness elements. The results indicate significant increments in mass flux, momentum flux and streamwise turbulence intensities due to the presence of the wall roughness.  The flow is also marked by anisotropic Reynolds stresses with shear components that are muted behind the vehicle.  Overall, these results have implications on the kind of turbulence models suitable for simulating the flow, as well as the quantified drag associated with flows with such wall roughness effects.   

Characterization of the Wind Flow Generated by the Windshaper

A fan array wind generator known as the Windshaper is operated and analyzed using quantitative data-collection experiments conducted at Syracuse University. The wind-flow generated from the Windshaper is characterized through the collection of air pressure measurements using a Scanivalve pressure scanner. The Windshaper used in testing is a device made up of 18 fan modules placed within a 1.5m x 0.75m wall. Each module consists of nine pixels with each pixel having two fans attached face to face rotating in opposite directions. This enables the experimenter to control the generated wind-flow by creating a downstream, upstream or mixed flow. Each fan in the Windshaper is individually controlled using a graphical user interface. This allows for the creation of various fan-array profiles to be analyzed. The data collection performed brings an understanding of how the turbulence created from the wind generator behaves. This data the potential to be compared to the flow behavior created by more traditionally used wind tunnels which are immobile and more expensive when compared to the Windshaper. This data also has the potential to assist in understanding the turbulence disturbances that can be generated for experimentation with Unmanned Arial Vehicles.   

The Effect of Seed Quantity and Material Density on Dispersal from Wet Splash Cup Plants

Splash cup plants harness energy from falling rain to disperse their seeds distances up to 10x their plant height; the resulting splash accelerates the raindrop's initial velocity up to 5.5x. Understanding the biomechanics involved in this unique process could shed light on the evolution of these plants, how foliar diseases spread, and the mechanics of soil erosion. Splash cup plants are <10cm tall with 5mm diameter conical flowers–the cup–which house its seeds and are ubiquitous where potential splashing can occur. In nature, the cup can be wet before any dispersal occurs. Seed material density may also change dispersal distance. Previous work examined the effect of seed density in wet cups using 5 seed mimics and found that lighter seeds went longer distances. These plants may start with varied numbers of seeds. So, we examined the effect of seed density in wet cups for varying numbers of initial seeds. We used 3D-printed cup models half full of water to compare the dispersal distance of glass seed mimics to that of much lighter plastic mimics. We also varied the cup's internal cone angle and initial raindrop impact location to examine their interplay with seed density. Our results demonstrate that plastic seed mimics traveled about twice as far as the denser glass mimics.   

Using Background Oriented Schlieren to Diagnose Changes in the Density Profile Produced by Self-Interacting Internal Wave Beams

The meridional overturning circulation is regulated by diapycnal mixing in the abyssal ocean. This mixing is produced by the nonlinear interactions of internal waves (IWs) which are created when the barotropic tide flows over topography. Mathematically, an oscillating fluid over a stationary ridge is equivalent to a stationary fluid beneath an oscillating ridge. This allows for a laboratory experiment where we studied the evolution of the density field by oscillating an artificial ridge in a linearly stratified tank. Using a novel Background Oriented Schlieren (BOS) software package, as well as a unique BOS setup, we obtained the evolution of the density and velocity fields, showing evidence of mixing in the tank. We measured the evolution of the tank locally using a conductivity probe to validate the results of BOS. Our results showed that an oscillating topography caused not only a local change in the stratification but  a global change to the density field in the tank. This experiment showed that IWs can efficiently mix the deep ocean far from their origin, which is required in order to sustain the Meridional Overturning Circulation.   

Analyzing and benchmarking an active grid turbulence generator in a laboratory water tunnel

Active grid turbulence generators used to produce turbulent flow in laboratories have become commonplace in wind tunnels. However, usage of active grids in water tunnels is rare, and our facility at Lehigh University is one of the few water tunnels that allow for active grid turbulence generation. We will discuss results from an extensive experimental campaign where we explore the influence of parameters including grid operating protocol, free-stream inlet velocity (mesh Reynolds number, Re_M), winglet rotational velocity (Rossby number, Ro), and winglet blockage ratios on produced turbulent flow. Turbulence is characterized by turbulence intensities, integral length scales, Taylor Reynolds numbers, and anisotropic ratios. Results suggest the three most influential parameters include the winglet blockage ratio, Re_M, and Ro. Similar to trends in wind tunnel studies, a larger blockage ratio contributes to larger turbulence intensities. However, unlike in wind tunnel experiments, there is little evidence that Ro impacts turbulence intensities despite directly influencing integral length scales. Results of this study encourage the plausibility of tailoring desired turbulent flows by carefully manipulating active grid operating conditions.   

Turbulent flow over a backward-facing step with a porous insert

This paper presents an experimental investigation of the effects of a porous insert downstream of a turbulent flow over a backward-facing step. Three cases are studied, namely two porous insert configurations of a backward-facing step flow, and a reference unobstructed backward-facing step flow. In each of these test cases, the expansion ratio of the test channel is maintained at 1.25, and the Reynolds number based on the step height and the maximum velocity is fixed at 6230. Particle image velocimetry is used to obtain extensive velocity measurements of several streamwise-wall-normal planes covering the region upstream of the step, as well as the recirculation and redevelopment regions of each case study. The results indicate a substantial reduction of the reattachment length depending on the location of the porous insert. An evaluation of first and higher order moment turbulent statistics also shows that while the inserts lead to a retardation of streamwise mean flow, they result in significant increments in turbulence intensities and Reynolds stresses, modifications in the directions of the transport of turbulent kinetic energy, and variations in Reynolds stress anisotropy in the recirculation region. Furthermore, the recovery of the mean flow is observed to be impeded by the presence and location of the porous inserts. The results indicate that the placement of porous inserts in a backward-facing step flow may result in heat transfer improvements.   

Selection of features for an image-based machine learning model to predict atmospheric optical turbulence

Direct measurements of atmospheric effects on light propagation often require equipment and access that are unavailable in remote or complex environments. In these situations imaging data may be able to provide estimates of atmospheric turbulence levels suitable for performance predictions of laser-based systems. To select the most significant image features, a supervised machine learning model is developed using partial-reference image data and scintillometer-based measurement of atmospheric optical turbulence via the index of refraction structure parameter, C_n². Both the images and the scintillometer data come from a 1-km over-water path adjacent to the Chesapeake Bay. The specific image features identified with the partial-reference model will then be used to develop machine learning models for atmospheric optical turbulence using no-reference image data.   

Investigation of acoustic spectrum in a simplified model of aortic valve stenosis

In classifying heart murmurs, a qualitative approach is usually employed where the “pitch” and “volume” of the murmur is identified by a medical professional. The production of this sound is a result of the narrowing, or stenosis, of the aortic valve. This study aims to provide a quantitative assessment of a narrowing in a pulsing flow, modeled after the heart and aortic valve. It is desirable to find a non-invasive technique to assess the severity of aortic valve stenosis. Utilizing dynamic similarity to match our model with the healthy human heart, a set of 3D-printed restrictions will be tested at varying Reynolds numbers to assess the intensity of the acoustic spectrum. These model restrictions span the range of a healthy aortic valve to a severely restricted valve, and the Reynolds numbers are varied to match the corresponding flow associated with these restrictions. Through a spectral analysis of a sound signal obtained by a contact microphone, a relationship between the dominant frequencies and the narrowness/flow conditions will be formed and investigated. This will form a basis for future understanding of the severity of actual heart murmurs in relation to their acoustic signatures.   

Experimental Investigation of Settling-Initiated Instabilities in a Two-Layer Stably-Stratified Hele-Shaw Cell

Fingering instabilities can be initiated by settling of sediment in an initially stable step-stratification. Rayleigh-Taylor (RT) and Double-diffusive (DD) instabilities were investigated with initially stably stratified particle-laden fresh water over top of either salt water or dextrose water in a Hele-Shaw cell. Particle sizes were chosen such that settling rates were slower than, comparable to, or faster than diffusion rates of salt in order to separate out regimes likely to be dominated by either RT or DD or containing both simultaneously. Silicon dioxide nanoparticles with sizes 500 nm, 700 nm, 1000 nm or soda-lime glass microparticles of nominal sizes 3- 6 microns, 8 – 12 microns, or 35 – 45 microns were used. Considerable difficulties with aggregation of particles were encountered, especially with nanoparticles and with salt water.  These issues were largely overcome by a glycerol ligand exchange technique with the nanoparticles and substitution of dextrose solution for the saline, thus eliminating the interaction with the dissolved sodium and chlorine ions. Results are presented in light of linear stability analysis in the literature as well as in light of Darcy calculations.   

Inertial focusing of red blood cells suspended in square tube flows of blood plasma

Particles and biological cells suspended in laminar tube flows are known to migrate laterally due to the lift force generated by the effects of inertia, particle deformability, medium viscoelasticity, and so on. As a result, they are often observed to pass through specific locations in the downstream cross-section of the tube. In the present study, we investigated experimentally the lateral migration of red blood cells (RBCs) suspended in blood plasma flowing through square capillaries. A cross-section of the tube near the outlet was observed from the downstream side, and the position of each RBC center in the tube cross-section was detected to obtain the RBC distribution. RBCs were found to be focused around the tube centerline at low flow rates, due the effect of RBC deformability, whereas an increase in flow rates induced the RBC focusing off-center near four points located on the diagonal of the cross-section. Additional experiments using glutaraldehyde-hardened RBCs and various suspending media (Newtonian and viscoelastic fluids) indicated that the RBC focusing on the diagonal could be caused by a combined effect of deformability of RBCs and viscoelasticity of plasma as well as inertia.   

The behavior of turbulent flow adjacent to irrotational flow in different geometries

Turbulent flow adjacent to irrotational (or non-turbulent) flow leads to the mixing process along with entrainment, momentum transfer, and viscous diffusion. In this study, we conduct direct numerical simulations (DNS) of freely decaying compressible turbulence with a "hole" consisting of (irrotational) fluid at rest. We investigate the behavior of mixing and its statistics depending on different geometries of the irrotational flow, which are initially planar, cylindrical, and spherical. As turbulent flow penetrates to the non-turbulent flow, the mixing zone grows, and turbulence fills up the non-turbulent hole. We suggest how to measure turbulence characteristics and examine dissimilarities caused by different hole dimensions. Our results are compared with scaling derived by Barenblatt et al. (1987).   

Physics Informed Machine Learning of Smooth Particle Hydrodynamics: Solving Inverse Problems using a mixed mode approach

While modern machine learning tools have been successfully applied to many fluid dynamics applications, including Smooth Particle Hydrodynamics (SPH), it still remains a great challenge to encode underlying physical structure into machine learning algorithms. In this work we show how a mixed mode approach (using both forward and reverse mode automatic differentiation) along with classical analytic techniques, such as the adjoint method, can be used to solve inverse problems for SPH (in both physical parameter space and function space). In addition, our mixed mode approach allows us to introduce the physical (and numerical) structure of SPH into the machine learning algorithm which can be used to learn a fully parameterized SPH model from Lagrangian flow data.    

Vortex Dynamics in a Rapidly Rotating Container with Axial Forcing

We have studied vortex dynamics that formed in rotating tanks due to axial forcing. A cylindrical, water-filled tank of 15cm radius is spun on a turntable; forcing is provided by a pump drawing water through 2 through-holes at tank bottom, which is fed back homogeneously at the radial boundary. Control variables include the axial forcing volumetric pump flow rate (Q), rotation frequency (ω), and fluid volume within the tank i.e. watermark height (h) on the cylinder. Cylinder rotation forms a parabolic deformation on the surface, while often distinct tendril-like vortices are observed that extend all the way to the tank bottom. Different flow regimes are observed: vortex dipoles; tripoles; and a "critical" regime whose vortex surface connects to one through-hole which resembles a bathtub whirlpool. This study focuses on determining the existence and finding the boundary between these flow types in control variable space, and finding correlation between the variables and other internal vortex dynamics. Approximately 100 experiments were conducted at differing values of Q, ω, and h, with consequent regimes observed and analyzed.   

Role of Continuous Particle Size Distributions on Gravity Driven Flow

The defining feature of granular gases is the gradual energy dissipation via particle-particle collisions. Fundamental understanding of granular hydrodynamics is vital in both industry (powder processing) and nature (sandstorms). Many numerical models of granular flows have investigated either single species or binary mixtures. However, true granular materials are often composed of a distribution of different particle sizes and masses. Due to species segregation in granular mixtures under external excitation such as gravity or vibrating walls. It remains unclear if homogeneous or binary granular mixtures can correctly capture the hydrodynamics of continuously distributed particle sizes and/or masses. To directly investigate more realistic granular mixtures, a modified direct simulation Monte Carlo (DSMC) method for continuous particle size distributions is employed. A Poiseuille granular flow is studied where particles are driven by isothermal walls and gravity. Here, we consider the case of normally distributed particle sizes with equal mass density. The hydrodynamic description for continuous particle size distributions is compared to the homogeneous and binary cases.   

Understanding cavitation mass transfer at different scales

Cavitation is the formation of vapor-filled cavities in a liquid due to local depressurization. While cavitation inception is a small-scale phenomenon which typically occurs at the micron scale, subsequent conversion from liquid to vapor phase causes the formation of large bubbles in the liquid.  Despite the abundance of mass transfer models that deal with cavitation, characterizing this mass transfer at different scales with no ad-hoc assumptions for inception has proven difficult. Here, we present and analyze a new model based on the Navier-Stokes-Korteweg equations which can predict cavitation inception at small scales and also quantify the mass transfer at larger scales accurately.    

Actuation of a flap array inspired by bristling shark scales for turbulent boundary layer separation control

A method for reducing flow separation with actively actuated flaps inspired by bristling shark scales was studied experimentally using digital particle image velocimetry (DPIV). The flaps in this study had a rectangular shape and were designed with a protrusion height in the bottom 10% of the boundary layer height, which is similar to that of the flexible scales found on the mako shark. The flaps were embedded into a flat plate, actuated in unison with frequencies of 1, 4, and 8 Hz within a water tunnel in a separating turbulent boundary layer flow. These tests were then compared to a corresponding smooth plate case. In the first set of tests, a zero pressure gradient case, it was found that the actuation of the flaps increased the overall momentum in the boundary layer near the wall, when compared to the flat plate case.  This momentum increase helps support the hypothesis that the flexible scales of the mako shark help to energize the boundary layer when self-actuated by the flow. In the second set of tests the model was placed in a separated region induced by a rotating cylinder to impose the presence of an adverse pressure gradient. The cases with the actuating flaps showed a reduction in the size of the separated region compared to the smooth wall case.    

Collective behavior of boundary-aligning mutually-repelling self-propelling particles

Inspired by the group behavior of larval zebrafish in dishes, we introduce a new model for self-propelling particles in circular confinements. The swimmers mutually repel if in each-other's proximity, but they align with the confining boundaries. We present simulations of the collective motion of such particles for different drop sizes and particle densities  when the initial distribution is uniform and isotropic. Last, we show phase diagrams based on the magnitudes of wall aligning and particle repulsion for varying swimmer densities and domain sizes.    

Computational and experimental investigation of the shear stress on a coastal foredune

Flow over coastal dunes can be an important source of particulate matter to the atmosphere from dust emissions. For onshore winds, a foredune can be the first major obstacle coastal winds encounter and can modify the developing boundary layer. We conduct numerical and experimental investigations of the flow over a mature foredune located in Oceano Dunes State Vehicular Recreation Park, Oceano CA. High resolution topography of approximately 10 cm resolution was acquired through photogrammetry and inlet boundary condition for a CFD was developed from flow measurements from multiple sonic anemometers mounted on a mobile tower. An additional mobile tower collected validation data at four locations throughout the foredune. Time averaged flow simulations using a K-omega SST turbulence model allow for the quantification of shear, which can be used to predict sediment mobilization. Flow visualization reveals that natural plant stabilization of the foredune results in highly aerodynamic forms with little flow separation.   

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Dispersion occurs when waves of different wavelengths propagate with various phase speeds such that an envelope of mixed wavelengths spreads out in space. The conduit equation which possesses a bounded, nonlocal dispersion relation is an accurate model of viscous fluid conduit interfacial waves. Laboratory measurements of glycerin viscous fluid conduits are compared with theoretical predictions from the linear theory for the conduit equation. Periodic interfacial waves are generated by periodically varying the flow rate of dyed, diluted glycerin injected into glycerin exterior fluid. To perform harmonic analysis on the experimental waves, Fourier transforms and the cosine fitting method are implemented to investigate wave properties such as amplitude, wavenumber and frequency. Periodic traveling wave solutions in the subcritical regime and spatial-decaying waves in the supercritical regime are obtained. Measurements of wave profiles and the wavenumber-frequency dispersion relation quantitatively agree with the conduit equation. A downshift of the critical frequency is observed which is explained by the full two Stokes fluid system. This study presents important linear wave features of the conduit system and provides a foundation for complex nonlinear wave dynamics.   

On the directionality of thermal and salinity gradients in double diffusive convection

Double diffusive convection is a phenomenon that describes a new form of convection driven by two different density gradients (having different molecular diffusivities). The formation of horizontal double diffusive layers (DDLs) has been explained as a result of interaction between thermal and salinity gradients. DDLs can be formed both by creating a vertical or horizontal thermal gradient on a pre-existing vertically stable salinity gradient. The proposed work focusses mainly on the major differences between the characteristics of DDLs formed by imposing vertical and horizontal thermal gradient. The development of a fluent model for the investigation of the aforesaid phenomena has been proposed, explaining the flow behavior inside DDLs. The presence of a specially oriented convective rolls has been found in the case where the fluid particles move due to the buoyancy created by lateral thermal gradient. Compared to this, the characteristics of DDLs found in the presence of vertical thermal gradient are different, where, elements from same horizontal plane move vertically and mix with surrounding, creating a horizontal layer. A combination of Rayleigh numbers (solutal and thermal) and different analytical scales have been quantified to support the understanding of different DDLs.   

Ground noise prediction of drone delivery networks

Over the past few years, unmanned aircraft systems (UAS) have been experiencing significant growth in the United States and around the world. In a parallel fashion, online retail sales have also experienced drastic growth, which in turn has focused significant attention on drone delivery networks to increase the volume and speed of commercial packages to retail customers. One of the environmental concerns that these networks pose is noise, which will become a prominent problem that will scale with the number of drones flying simultaneously. Although field testing can provide direct measurements, it does not provide prognostic noise assessments or guide noise control strategies. In this work, we develop an effective and efficient noise evaluation tool that can be utilized to estimate various noise metrics on the ground. The outputs of this tool are represented by noise metrics contours (such as Leq or SEL) at the ground level that can be superposed on a street map.   

Potential of Active Particles for Heat Transfer Enhancement

We present results of numerical calculations of the effective thermal conductivity of nanofluids containing self-propelled nanoparticles. The translational and rotational dynamics observed in the simulations follow the behavior expected from the standard theoretical analysis of Brownian and self-propelled nanoparticles. The superposition of self-propulsion and rotational Brownian motion causes the behavior of the self-propelled nanoparticles to resemble Brownian diffusion with an effective diffusivity that is larger than the standard Brownian value by a factor of several thousand. As a result of the enhanced diffusion (and the convective mixing resulting from the motion), we observe a discriminable increase of the effective thermal conductivity of the solution containing self-propelled nanoparticles. While the increases we observe are in the range of several percent, they are significant considering that, without propulsion, the nanofluid thermal conductivity is essentially not affected by the Brownian motion and can be understood within the effective medium theory of thermal conduction. Our results constitute a proof of concept that self-propelled particles have the potential to enhance thermal conductivity of the liquid in which they are immersed, an idea that could ultimately be implemented in a broad variety of cooling applications.   

A multiphase tracking of perfusion through in silico dense tumor domain

Dense fibrous constitution of solid tumors exerts high resistance to diffusive transport; additionally, the scarcity of blood and lymphatic flows hinders convection. Such formations are associated with over 85% of cancers including pancreatic cancer, which is this project's clinical condition of interest. The complexity of fluidic transport mechanisms in such tumor environments is still not well-explored. To that end, computational fluid dynamics (CFD) modeling presents a cost-effective strategy for a systematic investigation on how different physicochemical factors tend to affect plasma uptake and outflow at the tumor vasculature. In this talk, we will present our findings for a simple biomimetic tumor geometry with three different fenestra opening sizes, viz. 0.3, 0.8, and 1.3 µm, thereby mimicking varying degrees of leakiness. The plasma percolation into the tumor extracellular space is tracked and characterized, through simulating a reduced order 3-phase system that comprises plasma, RBCs (red blood cells or erythrocytes), and air voids. The exercise assumes transient flow, viscous-laminar model. We are also applying the same in silico framework to track transport in realistic geometries built from imaging data of solid pancreatic tumors engrafted in mouse xenograft models.   

Optimization of Soft Robot Swimmer Using Lighthill's Large-amplitude Elongated-body Theory

Long slender organisms demonstrate remarkable proficiency in varied terrain and especially water. The success of these organisms inspires novel designs in various fields especially robotics. Recent advances in robotic fabrication techniques have led to a new class of "soft robots". Soft robots are made using highly compliant materials and actuated using pneumatics, electromagnetism and tendons. Because of the low elastic modulus for the materials used to construct soft robots, they are inherently back drivable and compatible for interacting with humans and animals. These same properties, many degrees of freedom and under-actuation, create challenges in the area of modeling and control. We address these challenges by modeling a long, slender, soft robotic swimmer as a visco-elastic rod wherein Lighthill's large-amplitude elongated-body theory is used to represent the nonlinear hydrodynamic forces at a low computational cost. We model the actuation from the swimmer's body using a parameterized internal bending moment. Doing so, we find the optimal locations, size and number of finite fluidic actuators. Finally, we explore the control of the swimmer for best thrust and propulsion efficiency and discuss how to co-optimize the design and control of the soft swimming robot.    

A time-independent solver for diffusive transport in 2D eroded geometries

To describe diffusive transport of quantities such as heat or chemical concentrations in porous media, the diffusion equation must be solved in complex unbounded geometries. I will describe a numerical method that uses the Laplace transform to recast the time-dependent diffusion equation into a series of time-independent PDEs. These elliptic PDEs are solved using a boundary integral equation method, and the Laplace transform is inverted by carefully choosing a well-behaved Bromwich integral. By combining these techniques, high-order accuracy in both space and time are achieved. I will use this method to study the effects of erosion on diffusive processes.   

Compact apparatus for experimental characterization of UAV propeller generated internal flow

UAV motor-propeller propulsion suites combine the key merits of component tunability, high flow rate generation, and portable power sources. These advantages have the potential to benefit a broad range of devices that require active air flow under power constraints, especially those operating at static pressure conditions exceeding the capabilities of small axial case fans. However, the use of UAV motor-propellers for closed systems and internal air delivery remains understudied. In this work, we present a compact apparatus optimized for characterizing the performance of small UAV motor-propeller in generating internal flow under various pressure conditions. Compact sensors are integrated to probe both the static and dynamic characteristics of the internal air flow. We also discuss a framework of theoretical calculations and CFD simulations used to both guide the design and validate the measurements of our proposed setup. Finally, our experimental results demonstrate that a small quadrotor motor and propeller can deliver close to 25 L/s while overcoming static pressure drop of over 70 Pa. These metrics match the challenging conditions imposed by complex interior airways. Overall, this work encompasses a holistic effort in understanding and harnessing the superior aerodynamic performance of UAV propellers for internal flow generation.    

Optimization of Microparticle Suspensions for Shock Accelerated Particle Studies

At Los Alamos National Laboratory’s Horizontal Shock Tube (HST) facility we are studying the acceleration of shocked isolated micron-scale liquid and solid droplets in a gas. Unsteady forces on microparticles driven by a shock wave are not well understood and difficult to model and therefore standard drag coefficients may not predict the motion of particles. Since drag coefficient is dependent on particle diameter, controlled seeding of flow field is required. This work details how we optimized our microparticle seeding systems and monitored resulting distributions for both liquid and solid particles. Controlling the polydispersity of the size distribution proved critical for minimizing random and systematic uncertainty of resultant drag measurements.  This was done for liquid droplets by carefully optimizing the spray injector and particle solution concentration, and for the solid particles by sifting. Additionally, for solid particles, moisture and electrostatic forces caused clumping which we are addressing by redesigning the particle circulation system using grounded stainless steel tubing. As a result, we saw substantial reductions in estimated drag, which can be attributed to reduction in uncertainty and spread of particle diameters used in drag coefficient analysis.    

Study on the effect of the turbulent bursting phenomenon on sediment entrainment using 3D particle image velocimetry and particle tracking velocimetry

Analyzing the cause of sediment entrainment is the most fundamental knowledge to understand the various morphodynamic phenomenon in a natural river. Earlier researchers proposed that the sediment starts to move when the magnitude of the momentum of flow acting on the sediment particle exceeds a certain threshold value. However, recent studies revealed that the sediment is transported by the instantaneous turbulent motions within the boundary layer, making the threshold disappeared. Such motions are generally created by the turbulent bursting phenomenon where a sequence of cyclic events repeatedly occurs and are in charge of creating large momentum near the bed. In order to examine the effect of the turbulent bursting phenomenon on sediment entrainment, this study performed laboratory experiments based on optical measurement techniques. 3D particle image velocimetry and particle tracking velocimetry were applied to measure the velocity field and to identify the sediment movement, respectively. Octant analysis and proper orthogonal decomposition were applied for the measured velocity field to find the dominant turbulent motion inducing sediment entrainment. Finally, the variation of the pressure field was analyzed when the sediment starts to move.   

Fluid Dynamics of the Human Left Atrium under Simulated Normal and Atrial Fibrillation Conditions: A in vitro Tomographic PIV Study

Atrial fibrillation (AFib) is the most common cardiac arrhythmia in the US. AFib-induced flow changes would cause thromboembolism events inside the left atrium (LA), leading to a significantly increased risk of stroke. In this study, we performed an in vitro experimental flow study to quantitatively characterize the pulsatile flow in a left atrium model under normal and AFib conditions using tomographic Particle Image Velocimetry. The flow boundary conditions were numerically simulated via a system-level Windkessel model and validated using the clinical literature data. The experimental flow was then generated using a programmable closed-loop pulsatile flow simulator. The results show that the transient flow structures are highly three-dimensional and time-dependent in the left atrium chamber due to the complex inlets and the flow waveforms. The four pulmonary-vein-inlet jets give rise to a large vortical flow structure, which gains energy during systole and then dissipates during diastole. Compared with the “double-peak” flow waveform of the normal healthy LA, the “single-peak” AFib waveform causes a loss of flow momentum towards the end of the cycle. The results imply the important role of the lost pulsatility in the AFib-related flow stasis and pathology.   

Separation characteristics of dispersed oil-water pipe flows—CFD modelling and experiments.

The separation of liquid-liquid dispersions in pipes is common in many industrial sectors. However, it is challenging to predict the characteristics of the flow evolution due to the complex interfacial nature of separation mechanisms. Therefore, in this study, experiments along with Design of Experiments (DoE) techniques and Computational Fluid Dynamics (CFD) simulations were performed to investigate the flow of silicone oil and water in a horizontal pipe. Several cases with different mixture velocities (0.52- 1.04 m/s) and oil fractions (15%-60%) have been explored. OpenFOAM (version 8.0) was used to perform Eulerian-Eulerian simulations coupling with the population balance models. Good consistency is observed between the simulated and experimental results. The blending factor in the OpenFOAM solver is found to impact the simulation. Also, it may provide a feasible compensation mechanism for the mesoscale uncertainties of drop-drop coalescence models. Overall, this study aims to improve the physical understanding of drop-drop and drop-interface coalescence and the evolving characteristic layers during the separation of a dispersed flow.   

Dynamic ground effects on undulatory motion in a stingray-like robotic fin.

Rays and skates have been observed with highly distinctive swimming strategies ranging from pure flapping to pure undulation. Traditional experimental studies of undulatory motions relied on an intricate series of servo motors, making it difficult to systematically investigate the wavelength and amplitude of the fin. To build on those previous studies, we designed a robotic fin that generates easily-customizable undulation with a single motor and tunable cam-shaft system. Our apparatus is capable of testing different permutations of amplitude, wavelength, and frequency of the fin undulation, and it's compatible with various planforms via interchangeable skins. To quantify hydrodynamic performance, we measured the forces produced as the fin swam in a recirculating water tunnel. Specifically, streamwise thrust and transverse lift were measured by bi-axial load cells, and energy consumption was measured by a torque sensor on the actuator. Building on our previous works, we plan to use our fin to study how planform, aspect ratio, and undulation strategy affect near-ground swimming performance. With a better understanding of the hydrodynamics that governs near-ground swimming, we hope to better explain ray fin morphology and improve bio-inspired underwater vehicle design.    

Dynamics of a multiple flexi-splitters in the wake of an elliptic cylinder

Flow induced vibrations in bluff body configurations are widely encountered in various industrial and scientific applications in the fields of aerospace, coastal, offshore, and petroleum engineering. In this study, the influence of flexible splitter plates attached in the frontal and rear surfaces of an elliptic cylinder is investigated and presented in detail. For this purpose, an in-house algorithm based on the Immersed Boundary Method (IBM) is employed such that elastodynamic equations for Lagrangian Structures and Navier-Stokes equations for the fluid flow are solved. The results of flow pattern, pressure distribution, aerodynamic forces and tip-displacement amplitudes are investigated for the case of flexible splitters attached to both frontal and rear surfaces. Finally, the influence of dual-splitters and tri-splitters attached to both frontal and rear surfaces for wide ranges of Reynolds number (Re = 50 - 150), aspect ratio (AR = 0.5 - 1.0) of the elliptic cylinder, splitter plate length (L = 1D - 6D) and flexural rigidity (EI) are discussed and presented in-detail.   

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Engineers and fluid dynamicists have studied vortex induced vibrations (VIV) to develop passive and active methods to reduce oscillations and decrease stress in cross flow dynamic structures. Changes in cross-flow characteristics causes increased vibrations and stress on the structure, leading to higher maintenance costs or degraded mission performance. To mitigate effects of VIV, the design of cylindrical structures in cross-flow can be altered by introducing perturbations on the surface. The aim of the current experimental study is to understand the phenomena of VIV on a perturbed cylinder and compare with a smooth cylinder using a cyber-physical system (CPS). A CPS uses a combination of the physical fluid forces from the flow and cyber implemented forces from a dynamics model to better quantify fluid-structure interaction. A perturbed cylinder will be compared to a smooth cylinder by measuring the force on the cylinder, the amplitude, and frequency of oscillation in an open surface water channel. Additionally, the vortex street in the wake will be analyzed and compared between the cylinders using a dye flow visualization technique. This will enable an improved understanding and correlation between the wake structure and VIV and paves a path forward for control of VIV.   

Comparing active and passive flexibility in oscillating propulsors

Passively flexible swimmers show increased efficiency over rigid hydrofoils due to resonance. Biological swimmers have been observed controlling the flexibility of their propulsors, presumably to achieve higher thrust and efficiency while swimming. Our goal is to prove there is a theoretical propulsive benefit to active (time dependent) flexibility in basic oscillating propulsion. We introduce active flexibility into previous passive flexibility models through a pseudo spectral method. The fluid is assumed 2 dimensional, inviscid, and the dynamics linearized in small amplitude deflections. Passively and actively flexible flat plates are compared over a wide parameter space of temporal and spatial flexibility distributions. We optimize for thrust and efficiency across the parameter space. By theoretically proving there is a benefit to active flexibility we tie foundational unsteady hydrodynamics to biological observation.   

Advection-based temporal reconstruction technique for undersampled PIV data

The present work investigates the use of an advection-based supersampling technique to increase the temporal resolution of PIV datasets. The approach utilizes the available spatial information to determine the velocity of a fluid particle at some intermediate time. A Taylor series expansion is used to obtain the instantaneous fluid trajectory from past and future time instants. The velocity field is then estimated using a temporally based weighted average of the forwards and backwards predictions. The performance of the technique is directly compared with a bilinear interpolation approach for two synthetic PIV datasets: the 1-DOF vortex induced vibrations (VIV) of a circular cylinder within the laminar vortex shedding regime (Re=150), and a turbulent round jet (Re=10,000). A proper orthogonal decomposition (POD) is used to identify coherent wake structures and compare their energy distribution before and after application of the super-sampling method. The technique demonstrates the ability to reconstruct key wake dynamics for both datasets, indicating that the required acquisition rate requirements of PIV measurements can be reduced through consideration of the available spatio-temporal information.   

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Hypothesis: Deposits of biofluid droplets on surfaces (such as respiratory droplets formed during an expiratory) are composed of water-based salt-protein solution that may also contain an infection (bacterial/viral). The final patterns of the deposit formed and bacterial aggregation on the deposits are dictated by the composition of the fluid and flow dynamics within the droplet.

Experiments: This work reports the Spatio-temporal, topological regulation of deposits of respiratory fluid droplets and control of bacterial aggregation by tweaking flow inside droplets using non-contact vapor-mediated interactions. Desiccated respiratory droplets form deposits with haphazard multiscale dendritic, cruciform-shaped precipitates when evaporated on a glass substrate. However, we showcase that short and long-range vapor-mediated interaction between the droplets can be used as a tool to control these deposits at nano-micro-millimeter scales. We morphologically control hierarchial dendrite size, orientation and subsequently suppress cruciform-shaped crystals by placing a droplet of ethanol in the vicinity of the biofluid droplet. Active living matter in respiratory fluids like bacteria is preferentially segregated and agglomerated without attenuation of its viability and pathogenesis.

Findings: The nucleation sites can be controlled via preferential transfer of solutes in the droplets; thus, achieving control over crystal occurrence, growth dynamics, and the final topology of the deposit. For the first time, we have experimentally presented a proof-of-concept to control the aggregation of live active matter like bacteria without any direct contact. The methodology can have ramifications in biomedical applications like disease detection and bacterial segregation.   

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We consider the flow of a nematic liquid crystal (NLC) film placed on a flat substrate containing embedded interlaced electrodes, which leads to a spatially varying electric potential. Due to their polar nature, NLC molecules in the film interact with the (nonuniform) electric field generated, undergoing dielectrophoresis, which in turn leads to instability of a flat film. Implementation of the long wave scaling, appropriate in the limit where the film height is small compared to the inter-electrode spacing, leads to a partial differential equation that predicts the subsequent time evolution of the thin film's surface. The film evolution equation is coupled to a boundary value problem that describes the interaction between the local molecular orientation of the NLC and the electric potential. We investigate numerically the behavior of an initially flat film for a range of film heights, and discuss the possible relevance of our work for industrial applications such as dynamic optical shutters and controllable liquid lenses.   

Assessment of the role of numerical errors in setting contact lines to motion in two-phase flow simulations

The physics of the moving contact line is an active topic of research in fluid dynamics and chemistry, and understanding the problem well has important implications for applications such as coating flows, thin films, microfluidics, and droplet impacts. In this work, we explore the behavior of contact lines from two-phase flow simulations of a Navier-Stokes solver fully coupled with a diffuse-interface model for capturing an interface under finite surface tension. Specifically, we examine conditions in which only static contact angles are set as the boundary condition on a no-slip wall. The role of discretization error and mesh size on creating dynamic contact lines with finite velocity are quantitatively assessed. Our results have implications in consideration of inner-outer models under finite slip length for dynamic contact lines.    

Non-Linear System Controlled UAV Experiment

To test the flight control performance of a micro Unmanned Aerial Vehicle(UAV) under a turbulent atmosphere, a wind wall composed of small fans is utilized to create a turbulent environment. The wind wall used is known as WindShape, which is an open wind tunnel, consisting of 3x3 fans in cells. These cells can be manipulated to emulate turbulent air flow formed by eddies in wind. This will allow many variations in the turbulent environment generated by changing the configurations of each cell. In order to take measurements on fluid flow data and investigate the UAV’s response to the turbulence generated, hot wire, pressure transducers, and motion capture system are utilized. By using those tools, a flow field can be replicated in order to investigate the physics of the flow and examine how the UAV reacts. The pressure transducers yield low frequency velocity profiles. The hot wire probes yield high frequency fluctuation velocity profiles and the motion capture system will display movements of the UAV from the fluid flow.   

Phase field simulations of dynamic wetting by the Volume of Fluids method.

Practical simulation of real dynamic wetting flows are still challenging. In the Volume-of-Fluid method a contact line is moved using empirical information, and a slip Navier Slip condition is used for velocity. In the phase-field (PF) method the Cahn-Hilliard equations are formulated from the thermodynamics of an immiscible two-component mixture and a phase function is used to represent the moving interface. While the PF method has the advantage of needing less empirical fitting, it is considerably more expensive.

We show how VoF simulations can produce results essentially equivalent to those from the more expensive PF method. We introduce a computational boundary at a specified short distance from the wall in the VoF simulation. There we apply a Navier condition for tangential velocity, and an apparent contact angle that accounts for the interface curvature at the wall. The basis for the model is a theoretical relation between total diffusive mass transport across the interface and curvature at the wall for the PF model.

We compare full PF simulations with VoF simulations using these boundary conditions. The input parameters for these VoF simulations are the same as for the full PF solution and the VoF solutions reproduce the PF solutions, at a much lower cost.   

Aerodynamic Characteristics of Owl-inspired Leading-Edge Serrations"

Owls are stealth raptors. It has been suggested that their stealth capabilities stem from their wings’ unique structure. One of these prominent elements is the so-called leading edge serrations: rigid miniature hook-like patterns placed at the primaries of leading edge. It has been hypothesized years ago that leading edge serrations alter the adjacent flow field partially to suppress aerodynamic noise, impact its aerodynamic performance and function as a passive flow control mechanism. Herein, we investigate the flow characteristics around an owl wing with serrated leading-edge geometry at intermediate chord-based Reynolds number (Re_c ~ 50,000). The flow around a Barn (Tyto alba) owl wing with and without serrations at different angles of attack are studied. The flow field is resolved by employing a DNS (Direct Numerical Simulation) approach, where unsteady incompressible Navier-Stokes equations are solved in a cartesian grid with sufficient resolution to resolve all the relevant flow scales. Disparities in the boundary layer structure as well as wake flow dynamics between the serrated and the unmodified wing are quantified to assess the influence of the serration geometry. Finally, the aerodynamic performance due to the presence of serrated leading edge is evaluated.   

Constriction-length dependent instabilities in the microfluidic entry flow of polymer solutions

Real-world porous media are often mimicked in laboratories by microfluidic models with an abrupt contraction-expansion or a channel-centered cylinder. Various parameters have been investigated for the flow of polymer solutions through contraction and/or expansion microchannels. However, the majority of the previous studies has been focused upon the contraction flow. Little is understood about how the upstream contraction flow may be influenced by the high shearing flow inside the finite constriction as well as the transverse stretching and subsequent relaxation in the downstream expansion flow. We experimentally study the rheological responses of such flow instabilities in planar contraction-expansion microchannels differing only in the constriction length with three different polymer solutions.   

In vivo characterization of Lagrangian Coherent Structures Using Lagrangian Descriptors: Application to Left Ventricular flows

Despite the availability of in vivo, instantaneous, and three-dimensional intracardiac flow data, their clinical analysis from a Lagrangian perspective remains limited due to their high computational cost. As an example, identifying Lagrangian coherent structures (LCS) in cardiac flows is not routinely performed in clinical settings despite their ability to identify mixing and stagnation regions along with locations of elevated shear stresses. Here, we explore a recently developed approach, “Lagrangian descriptors”, which quantifies the finite time Euclidean arc-length of Lagrangian trajectories released from a grid of initial positions. Through the evaluated arc-lengths of a set of trajectories, signatures of the LCS (computed from the same initial condition) are captured. Notably, the Lagrangian descriptor approach extracts the LCS within the flow at least five times faster than the common geometrical approach (i.e., using finite-time Lyapunov exponents). In this work, we apply, for the first time, the Lagrangian descriptors approach to in vivo 4D-MRI velocity fields inside left ventricles. The results show the ability of this approach to rapidly reveal the LCS within the left ventricle and how their organization can be altered under healthy and pathological conditions.   

Modeling the El Niño Southern Oscillation with Neural Differential Equations

El Niño Southern Oscillation (ENSO) is the largest inter-annual variability phenomenon in the tropical Pacific and its influence goes beyond tropics to higher latitudes via atmospheric and oceanic teleconnections; therefore, it has a significant impact on global climate predictions.

We used Taken’s delay embedding theorem to reconstruct, from a univariate time series, the chaotic attractor towards which the ENSO system tends to evolve.  

We considered the Nino3.4 sea surface temperature (SST) index data starting from the pre-industrial era and spanning the entire period 1870-2016 as univariate time series and we used a Neural Ordinary Differential Equation (NODE) to model the embedding function. 

After learning the optimal embedding dimension and the associated time delay, our NODE model successfully captures the essential features of ENSO, the two most important features of which are ”phase locking” to the seasonal cycle, which is the concentration of abnormal ENSO events during November and December, and the predictability barrier, which is related to the increase of model uncertainties during spring. Finally, our model exhibits robust short-term prediction skills, outperforming more complex and computationally expensive models on the same tasks.    

Learning the correction factor in closed macroscale equations using conventional neural networks

We use conventional neural networks (CNNs) to estimate the correction factor in closed macroscale equations. Presence of nonlinear operator poses a great challenge in direct computation of the closure factor using conventional analytical techniques. To tackle this problem, we develop a deep architecture that can identify the key points in streamlines and map them to the correction factor. We feed the snapshots of the microscale field to the deep neural network in order to predict the correction factor. To improve the fidelity of the prediction, we identify some source terms arising from the upscale PDEs and boundary conditions and add them to the output of the CNN and input of the feed forward network deep network. We show that this architecture can map the snapshot of the field to the correction factor with high fidelity. In fact, the effect of the source terms contribution on fidelity is more prominent than the field grid resolution. The advantageous of this framework is that one can estimate the high-fidelity correction factor without knowing the physical parameters of system.   

Application of the multi-fluid model to study a sea spray effect on the vertical transport of momentum in a hurricane

Recently a significant evidence has been emerged that sea spray strongly affects the dynamics and structure of a hurricane by influencing the balance between heat influx and momentum outflux through the ocean surface. In this work we present a study of the spray effect on the air-sea exchange of momentum with recently developed Eulerian multi-fluid model. Such an approach enables us to quantify the effect of the spray on the vertical momentum transport in a marine atmospheric boundary layer (MABL) of a hurricane much more precisely compared with a more traditional mixture-type approach. Particularly it allows us to accurately describe the suppression of turbulence intensity by the spray in the air due to two different mechanisms: the turbulence attenuation, which results from the inability of spray droplets to fully follow turbulent fluctuations and the vertical transport of spray against gravity by the turbulent flow, also known as “gravity lubrication”.

The numerical and analytic results obtained for a spray laden MABL using the multi-fluid model show that the accelerating effect of the turbulence suppression dominate over decelerating effect of spray inertia increasing the wind velocity and reducing the drug coefficient. The falloff of the drag coefficient from its reference values is stronger pronounced at higher altitudes. Significant reduction of the drag coefficient occurs for spray volume fraction in the range 10^-5 - 10^-4, which  is significantly lower than that previously predicted by mixture-type models.   

Vortex ring interaction with buoyant droplet diffusion flame

In general droplet diffusion flames, the detachment of vortices from flame downstream tip decides the shedding/Pinch-off height. It is curious to see the effect of vortex ring additionally introduced to the droplet diffusion flame. The incident vortex ring is characterized by Vortex Reynolds number (Re_v). During vortex interaction, the flame tends to move above the equatorial plane of droplet with low Re_v and with increase in strain rate partial extinction happens at very high Re_v. The flame oscillations are observed while interaction of vortex which tends to increase the heat release intensity area. When the vortex moves away from the downstream point of flame, the oscillations tend to dampen. Buoyant diffusion flame behaviour is observed for before and after interaction with vortex. Flickering frequency of the flame follows (g/h)^1/2 for buoyant diffusion flame. But while Vortex interaction, an additional force is added to the system by vortex which leads to a low Richardson numberand scaling used for buoyant flame will not obey. Hence a new scaling is proposed which incorporate the vortex momentum effects instead of gravity, agrees with the experimental observations.   

Numerical investigation of centrifugal effects in high-speed turbulent boundary layers

Supersonic and hypersonic turbulent boundary layer transition has recently seen a resurgence of interest, motivated by the need to improve the design and optimization of supersonic and hypersonic vehicles, increase the efficiency of scramjet engines, or to quieten high-speed wind tunnels. The surface of realistic vehicles features regions of convex or concave curvature, which not only changes the streamwise pressure gradient but also alter the development of turbulent flow structures. Centrifugal effects come into play when the boundary layer flow evolves over a concave wall, accommodating Gortler-like vortices that feature mushroom shapes in crossflow contours of streamwise velocity. In this work, we study these centrifugal effects in a Mach 7 turbulent boundary layer flow by large eddy simulations, by varying the curvature of the wall and the Reynolds number.   

The Role of Preen Oil on Aerodynamic Efficiency as a Drag Reduction Agent.

Energy consumption is a critical aspect in the aviation industry. Some organisms form a list of morphological formations adapting to efficient locomotion, for example, using passive flow control elements to reduce drag. Some of these draw the attention to reach energy-saving goals in human-made machines. A number of birds, to well preserve their feathers and plumage, evolved into developing the uropygial gland exclusively to self-produce preen wax (oil). This oil is assumed to also serve for waterproofing, UV protection and chemo-signaling. In order to examine whether the preen oil have also an aerodynamic function (i.e.: drag reduction), controlled experiments were performed in a wind tunnel using 3D particle tracking velocimetry to measure the near wake flow of a preen oil coated 3D-printed NACA wing. The experiments included a comparison to a non-coated wing over a range of angles of attack at a Re=10⁵. Subsequent wake characteristics have been compared to examine the coated wing aerodynamics. Furthermore, to characterize the flow and shed light on the impact of oil on the wake flow mechanisms, turbulent parameters including Reynolds stresses, energy terms and spectral analysis have been determined.    

Dynamical Clustering Interrupts Motility Induced Phase Separation in Chiral Active Brownian Particles

Motility-induced phase separation (MIPS) is one of the intriguing findings in active matter systems, which has been widely studied in linear self-propulsion models such as active Brownian particles (ABPs). However, such linear models fail to describe chiral swimming patterns observed both in biological and active colloidal systems. Therefore, the self-propulsion torque is introduced into the circle-ABPs (cABPs) model.

In this work, using computer simulations and dynamic mean-field theory, we demonstrate that sufficiently fast rotation of cABPs in two dimensions generates a dynamical clustering state resulted from the short wavelength instability, which interrupts the conventional MIPS. Multiple clusters arise from the combination of the standard MIPS cohesion, and the circulating current that causes cluster disintegration. The steady state current is allowed by the detailed balance broken at continuum level. Microscopically, the current with non-zero curl originates from the motility “relieved” by automatic rotation.

This mechanism sheds light on the understanding of dynamic clusters formation observed in a variety of active matter systems, and may help examine the generalization of effective thermodynamic concepts developed in the context of MIPS.   

The geometric shape of flowing soap film channels

A flowing soap film channel is usually constructed using a pair of nylon wires, which are initially intact and pulled away from each other to create a soap film between them. When a soap film is present, these flexible wires are bent toward each other due to the action of the surface tension. This feature is used to measure the surface tension of flowing soap films [Sane et al., J. Fluid Mech. 841, R2 (2018)] by assuming that the variation of the channel width is not large enough to alter the flow. The current study investigates a more generalized case where the bending curvature is large enough to alter the flow. In soap films, the flow speed and the thickness are coupled, and therefore so do the surface tension and the bending curvature. We numerically solve for the geometric shape of the channel by using theoretical and empirical relations and find that the solution is biquadratic to the longitudinal coordinate.   

Durable Gas-entrapping Microstructured Surfaces (GEMS)

Realizing liquid-repellent surfaces without relying on coatings has remained an elusive goal due to the time-dependent degradation and perfluorocarbon unsustainability. In this context, we are investigating gas-entrapping microtextured surfaces (GEMS) comprising arrays of mushroom-shaped doubly reentrant cavities (DRCs). GEMS that have garnered much interest due to their ability to “repel” liquids regardless of the surface chemical make-up. When submerged, they entrap air and their subsequent performance, for instance for frictional drag reduction, depends on the durability of the air entrapment. This requires in depth assessment of the durability of air entrapment in GEMS and the various factors that influence it such as liquid surface tension, cavity dimensions, hydrostatic pressure, breakthrough pressure, and capillary condensation. In response, here, we combine experiments and computational fluid dynamics to investigate the stabilization of advancing liquids on DRCs of circular, square, hexagonal geometries carved on wetting substrates. For comparison, we investigated simple cylindrical cavities under similar conditions. We also explain why DRCs with sharp corners undergo faster Cassie-to-Wenzel transitions than circular DRCs. These findings will lead to superior GEMS.   

Detecting coherent structures with automatic differentiation

Transport in complex flows is difficult to quantify using traditional tools from dynamical systems theory. Lagrangian coherent structures (LCS) seek to partition the flow into long-lived manifolds, such as persistent vortices, that dominate global transport and serve as an organizing skeleton for the flow. Many popular methods for detecting LCS implicitly require the spatial gradient of a flow to be computed along trajectories, in order to measure the degree to which virtual tracer particles spread apart along different directions over time. However, in a data-limited regime, the ability to precisely compute local spatial gradients rapidly degrades---a problem worsened by the tendency of chaotic flows to rapidly disperse nearby tracers. Here, we demonstrate a new approach to estimating LCS that takes advantage of recent computational advances in automatic differentiation. These approaches allow spatial gradient information to be accumulated during forward propagation of a complex flow, improving gradient estimates without substantially increasing computational cost. We demonstrate several example applications of our approach to diverse flows, including sparse experimental datasets and three-dimensional turbulence.   

Unstable miscible displacements in Hele-Shaw cell with chemical reactions producing viscoelastic solution

Viscous fingering accompanied by chemical reactions is encountered in many processes, such as petroleum recovery, chromatographic separation, and polymerization. Therefore, the understanding of the coupling between hydrodynamics and chemical reaction in viscous fingering is quite appealing. Herein, we study the flow displacement involving a chemical reaction at the interface between displacing and displaced fluids. The two reacting fluids are considered to be miscible and Newtonian, which produces a gel-like product exhibiting viscoelastic behavior. The effect of chemical reactions on instability is examined by performing full flow simulations. The viscoelastic product is rheologically described using the White-Metzner model. It was observed that the shear-thinning behavior of the product always strengthens the instability whereas the elastic behavior of the product attenuates the finger growth as compared to the Newtonian product. Both the cases, where the product is more or less viscous than the reactants are analyzed.    

Understanding Particle-Particle Interaction during Settling under Gravity

Hydraulic fracturing is a leading technology in natural gas extraction from low permeability reservoirs. Proppants entrained in fracturing fluid stabilize the fracture and prevent premature closures. Therefore, proppant placement and settling characteristics greatly impact fracture conductivity. One key measure of reservoir permeability lies in proppant concentration at fractures. Despite its importance, minimal attention had been given to understanding the complex interactions of two proppant particles settling in static conditions. Generally speaking, an inverse correlation is known between fluid viscosity and a single particle settling velocity, but the correlation between horizontal velocity developed by particle-particle proximity during settling and settling velocity is yet of ambiguity. The present study utilizes spherical glass silica beads to investigate particle-particle settling behavior in a Hele-Shaw cell setup and questions if the quantified horizontal velocity component influences the vertical velocity component of interest. Moreover, the investigated attraction/repulsion phenomenon can unfold new characteristics impacting proppant volumetric concentration during transport. The hydrodynamic interaction of two settling particles was modeled by releasing at varying initial proximities in water. A mechanical release system was utilized to achieve high precision below surface particle release. High-speed particle image velocimetry was conducted, allowing for sophisticated particle tracking in space and time. Preliminary results of particles settling in water indicate a strong dependency of repulsion on initial proximity. Future work includes investigating repulsion characteristics as a function of rheological properties, surface properties and wall effects.   

Two-way traffic in the vascular tissue of a conifer needle

Conifer needles transport water and sugar through systems of essentially parallel tubes of tracheids (water) and sieve elements (sugar). This comparatively simple architecture is contrasted by the complex tissue surrounding them, and extending out to the bundle sheath, a cylindrical wall of cells that protects the vascular bundle against intrusion of air. This tissue consists largely of two cell types: transfusion tracheids carrying the outgoing water driven by transpiration and transfusion parenchyma carrying sugars produced in the mesophyll outside of the vascular bundle into the sieve elements. We have studied this tissue – unique to gymnosperms – by X-ray tomography on intact conifer needles and by TEM, revealing a surprising structure, reminiscent of an anisotropic “swiss cheese”, where the water moves out through the continuum, and the sugar moves in through the holes forming percolating clusters. We shall discuss main structural features of this tissue and how it can perform the complex task of conducting water and sugar in opposite directions.   

Of Direct Numerical Simulation of the Aerodynamically Rough Atmospheric Boundary Layer - Implementation of an Immersed Boundary Method for Turbulent Ekman Flow.

Direct numerical simulation (DNS) of the atmospheric boundary layer (ABL) is becoming more and more popular - along with the increasing power of high-performance computing. Nowadays, geophysically relevant domain sizes and simulation durations can be attained. To some extent, geophysical flows in nature are predominantly flows over rough surfaces, which significantly affects drag, mixing and transport properties of the flow. For such flows, a method is needed to impose solid walls while maintaining the efficient and tuned numerical methods for Cartesian meshes. This is achieved by an immersed boundary method (IBM), where three-dimensional roughness elements are fully resolved at the bottom wall of the simulation domain. The current IBM combined with compact schemes avoids the Gibbs phenomenon by preserving the homogeneity of spacial operators. However, the implementation of this IBM into the DNS algorithm poses challenges when applying the thermal boundary conditions on the roughness elements in the case of stably stratified ABL, but also with respect to potential appearance of spurious oscillations in the case of collocated grids for pressure and velocity.   

Combining high-speed planar PIV and motion tracking of a flexible cylinder in cross-flow

Most modeling studies investigating the flow dynamics in vegetation canopies are limited to rigid models.  However, most canopies embody some degree of structural flexibility, resulting in flow-structure coupling.  Studies addressing flexible canopies typically quantify either the flow or the plant motion independently.  To this end, high-speed PIV data of the flow surrounding an idealized canopy element, consisting of a flexible cylinder, together with the solid displacement field were simultaneously obtained by combining fluorescent imaging and refractive index matching (RIM).  The RIM approach involved matching the RI of an aqueous solution of sodium iodide (NaI), used as the working fluid, with that of the solid model fabricated from a clear polyurethane rubber.  The operating principle of the technique employed involves seeding the two phases with different tracers (the flow with silver-coated glass spheres and the cylinder models with fluorescent particles), facilitating independent interrogation of the dynamics of each.  This time-resolved data allowed for observation of the dynamic link between a deformable object and the surrounding flow.  The experimental method may be extended to other geometries and aid in the study of aeroelastic flow–structure interactions.   

Revealing Flow Transport Barriers in Cardiovascular Flows Using Complex Networks: Application to Edge-to-Edge Mitral Valve Repair

In cardiovascular flows, blood transport has been revealed using the Lagrangian coherent structures (LCS). Through a geometric approach, the extracted LCS highlight mixing, stagnation, and elevated shear stress regions. In geophysical flows, graph-theoretic approaches are used to represent fluid transport as a complex flow network. By using classical graph measures, one can extract the LCS along with direct measures of local dispersion and mixing. Also, from the adjacency matrix of the flow network, we can identify the coherent sets in the flow where the fluid within each set is minimally mixed with that in other sets. Notably, the geometric approach can only detect the borders of such regions. This study shows the first application of complex networks analysis to instantaneous planar velocity fields (acquired via PIV) downstream different arrangements of healthy and repaired mitral valves. Using the in- and out-degrees of the transport matrix, the instantaneous local mixing and dispersion are highlighted. Moreover, the LCS are revealed by computing the discrete finite time entropy of the network. Finally, the flow transport matrix is partitioned using a fuzzy c-means clustering algorithm to reveal the flow coherent sets which can better reveal fluid transport mechanisms.       

Study of 1st Order Numerical Scheme Physics Informed Neural Network (N-PINN) for 1D Riemann Problem

Recent research works for solving partial differential equations (PDEs) with deep neural networks (DNNs) have demonstrated that spatiotemporal function approximators defined by auto-differentiation are effective for approximating nonlinear problems, e.g. the Burger’s equation, heat conduction equations, Allen-Cahn and other reaction-diffusion equations, and Navier-Stokes equation. Meanwhile, researchers apply automatic differentiation in physics-informed neural network (PINN) to solve nonlinear hyperbolic systems based on conservation laws with highly discontinuous transition, such as Riemann problem, by inverse problem formulation in data-driven approach. However, it remains a challenge for forward methods using DNNs without knowing part of the solution to resolve discontinuities in nonlinear conservation laws. In this study, we incorporate 1^st order numerical schemes into DNNs to set up functional approximator instead of auto-differentiation from traditional deep learning framework e.g. TensorFlow package, which improves the effectiveness of capturing discontinuities in Riemann problems with constraint conditions e.g. boundary conditions and initial conditions being applied. If partial data in shockwave region of the solution is adopted for numerical physics-informed neural network (N-PINN), the results of predictions are more effective than traditional PINN set up by automatic differentiation.   

Supervised Surrogate Modeling for Hagen-Poiseuille and Womersley flows

Computational fluid dynamics plays an important role to solve real-world flow systems, but a heavy computation burden often causes a trade-off with physical accuracy. Surrogate flow models have the potential to achieve both computational efficiency and physical accuracy. We develop a numerical procedure, using neural network (NN) and Gaussian Process (GP) methods, to demonstrate the potential of machine learning approaches in building efficient and accurate surrogate models with limited runs of the original flow models and apply it to Hagen-Poiseuille and Womersley flows that involve spatial and spatial-tempo responses, respectively. Training points are generated by calling the analytical solutions multiple times with evenly discretized spatial or spatial-temporal variables. Then NN and GP surrogate models are built using supervised machine learning regression. We compare the NN and GP methods and examine the unique feature of the GP model, which also provides confidence in the prediction. The results indicate that the surrogate models can accurately represent both Hagen-Poiseuille and Womersley flow models. Our further work will be developing surrogate models for more realistic flows.   

Reynolds Averaged Modelling with Convolutional Neural Networks

The jet is a common flow, which can be observed from the nozzle for an aeration installed in a dam. Such a jet flow is one of the complicated problems since it has various sizes of eddies having different momentums. So, it is necessary to reconstruct the jet flow by reflecting the jet flow characteristics such as Reynolds stress, turbulent kinetic energy to understand and also expand its applications.

The previous studies conducting the Reynolds-averaged Navier–Stokes (RANS) model proved that the realizable k-epsilon is more efficient to do research the turbulent mechanism than other models. However, the Reynolds averaged model is hard to reflect all flow properties such as the fluctuations of velocity or pressure and the model need to be improved by including more detailed turbulent processes. As a recommendation, we use a new and suitable technique, a Convolutional Neural Network (CNN), to combine RANS and LES, which algorithm has been used to find the linear and nonlinear relationship.

This study is aimed to pursue two objectives; we try to find out the relationship between LES and RANS models and then, some turbulent processes from the LES are combined with RANS model using CNN. Finally, we compare and evaluate both results of this model and results used just RANS or LES models.   

Wake flame dynamics in a freely falling burning droplet

The moving burning droplet is an interesting area of fundamental research that gives insight into spray combustion. A freely falling burning droplet was experimentally investigated in a drop tower facility. After the droplet is ignited and is allowed to fall freely, the droplet flame transitions to a wake configuration due to forward extinction. The different wake flame behavior regimes have been identified, and the various flame transitions exhibited have been investigated. The wake flame showed similar characteristics as a laminar lifted triple-flame, and the flame stand-off increases continuously as the droplet falls. Wake flame topology changed in two regimes corresponding to different droplet diameters. The flame transitioned between premixed and diffusion due to the alteration of the fuel availability. A theoretical formulation has been proposed based on the momentum diffusion into fuel stream, which has satisfactorily estimated the flame behavior and transitions in different regimes. Furthermore, at very high Re >150, the flame stretching/shedding is observed, which is caused due to the asymmetric vortex shedding from the droplet induced by the BVK instability.   

Developing Three-Dimensional Simulations of Bacteria with Flexible Flagella in Viscous Fluids

Escherichia coli (E. coli) and other similar bacteria move through viscous fluid environments by rotating their flagella. This behavior may be mathematically modeled as a type of fluid-structure interaction wherein the bacterial cell and flagellum are discretized into collections of spherical particles coupled by forces and torques. The flagellar flexibility can be varied using Kirchhoff rod theory and quaternions to resolve both bending and twisting in three dimensions. Our simulations are built upon the MATLAB implementation of the “methods for suspensions of passive and active filaments” in which fluid-structure interactions are accounted for through a direct pairwise evaluation of the Rotne-Prager-Yamakawa (RPY) mobility tensor. By adjusting the flagellar flexibilities, we aim to reproduce experimental observations of E. coli trajectories.   

Experimental validation of an Ab initio model for the flow and temperature distribution in the steam chamber in SAGD oil recovery.

Recently, we proposed an Ab initio model, founded on fundamental fluid mechanics, to compute the instantaneous flow and temperature distribution of steam injected in the steam chamber in a homogeneous reservoir of extra heavy oil. Our model also takes into account the steam condensation at the edge of the steam chamber and the resulting flow of water in the recovery lower pipe. Experiments were made by using a two-inches thickness iron slab having a large elliptic perforation at the middle. The perforation was filled with 3 and 6 mm diameter glass spheres and after covered with armored glass plates to avoid their explosive rupture due to the thermal shock. As usual, through the upper injection pipe, a flux of steam was injected and, due to the lower temperature of the porous matrix, water was recovered at the bottom. Through careful measurements of the spatial temperature distribution and of the recovered water we determine that there exists a unique steam flux that allows the maximum water recovering. All these results back the main predictions of the theoretical model.   

Miscible Experiments on the Rayleigh-Taylor Instability in the Linear Induction Motor Drop Tower

Incompressible, miscible experiments on the Rayleigh-Taylor Instability (RTI) using Planar Laser Induced Fluorescence (PLIF) imaging are presented. A vertical tower guides a test sled that is accelerated downward using linear induction motors. Experimental liquid pairs are prepared and placed into a test chamber attached to the sled. The sled is accelerated at a rate of approximately 15g. Upon acceleration, the stratified initially stable fluid pair within the chamber becomes unstable causing the development of RTI. The resulting RTI is imaged using PLIF by seeding the heavier fluid with fluorescein dye that is illuminated by a scanning 445nm wavelength laser beam. The indices of refraction (IOR) for the two liquids are initially matched prior to tank filling. However, the mixing of the fluids produced by RTI development alters the IOR. Variation of IOR within the fluids produces image blurring, negatively impacting PLIF imaging. Variation of IOR is minimized by modelling as a nonlinear fluid property, allowing preparation of optimized fluid pairs that reduce image blurriness. Measurements of the resulting mixing layer growth will be presented.   

On the Development of a Harmonic Pitching and Heaving System with Real-Time Control and Optimization Capabilities

Previous computational and experimental efforts have shown that bioinspired oscillating foils can be highly efficient propulsors. However, preceding laboratory implementations of oscillating foil systems have primarily focused on characterizing propulsive performance via parametric studies involving simple harmonic motions. Here, we describe the development of an oscillating pitching and heaving system for hydrofoils that is capable of generating arbitrary periodic motions as well as real-time control and optimization. The heaving and pitching motions are driven by two precision stepper motors, with built-in encoders for determining the linear and angular position of the system. Real-time measurements of hydrodynamic forces are made concurrently with a six-axis force transducer. For validation of this system, we measure the forces and torques generated by a NACA 0012 hydrofoil undergoing sinusoidal pitching and heaving motion in a free surface water channel facility, and we compare the measured thrust coefficients and propulsive efficiencies to previous studies. Ongoing work makes use of this platform for parameter space exploration and optimization of foil kinematics.   

Particle-Induced Viscous Fingering: Effects of Global Geometry

The displacement of air with a suspension inside a Hele-Shaw cell leads to the accumulation and fingering of particles near the oil-air interface. Termed "particle-induced viscous fingering", this surprising instability has been extensively investigated in radial geometry. In this study, we extend this fingering phenomenon to a rectangular geometry to remove the inherent time-dependent nature of the radial flow. To that end, we experimentally inject a mixture of silicone oil and non-colloidal particles into a confined rectangular cell. Our results demonstrate unexpected folding and merging of fingers over time, which is not observed in the radial geometry. We characterize this new nonlinear fingering regime via image processing and discuss the physical mechanism of this geometry-dependent behavior.   

Separation Control of NACA0015 Airfoil using Plasma Actuators

Separation control of NACA0015 airfoil using plasma actuators was investigated. Plasma actuators in span-wise array, which consists of 21 electrodes and has 1mm spacing between adjacent electrodes each having 9mm width, were located at the leading edge of the airfoil to distribute temporal periodic disturbance with phase variations φ (= 0 or π) into separation shear layer. The cord length of the airfoil was c = 100mm and corresponding Reynolds number was fixed at Re = 63,000. Non-dimensional frequency of the disturbance was chosen at F⁺ = 0.5 or 6.

Velocity fields was measured by conventional two-component PIV. Lift and drag were measured using a two-component force balance. Surface pressure was monitored by pressure transducers through pressure taps on the upper surface.

Improvements of lift coefficients were confirmed at PA actuation cases around stall angle. In addition, F⁺ = 6 cases showed greater improvements than F⁺ = 0.5. Among F⁺ = 6 cases, φ = π provided the higher lift compared to φ = 0 by approximately 1.4% at the angle of attack = 11degrees. Moreover, while quasi-spanwise-oriented roller vortices were formed at φ = 0, vortices arranged in a staggered manner were formed at φ = π. Hereafter, the relation between improvements of lift and 3D flow behavior will be clarified.   

Weakly nonlinear stability analysis of extensional flows and ice shelves

Ice shelves that spread into the ocean can develop rifts, which can trigger ice-berg calving and enhance ocean-induced melting. Fluid mechanically, this system is analogous to the propagation of a non-Newtonian, strain-rate-softening fluid representing ice that displaces a relatively inviscid and denser fluid that represents an ocean. Experimental observations show that rift patterns can emerge in such systems  and that the number of rifts declines in time. A recent linear stability analysis predicts some of those observations. However, such a method is limited in predicting the strongly nonlinear evolution of the observed rift patterns. Our study focuses on the weakly nonlinear stability of such a system. We consider first a Newtonian fluid, and develop an amplitude equation that describes the time evolution of the perturbed fluid interface. We use this equation to explore the evolution of rift patterns and to develop more consistent predictions of the experimental system.   

MAKING FUNCTIONAL SURFACES WITH ACRYLIC PLATES

By using sandpaper of different grit, we have polished smooth plates of acrylic to cover their surfaces with disordered but near parallel micro-grooves. This procedure allowed us to transform the acrylic surface in a functional surface; by using the capillary rise of silicone oil up to an average height h, we found that h evolves as a power law of the form h~tⁿ, where t is the elapsed time from the start of the capillary flow and n is close to 0.5, for different inclinations of the plate and different grit. Such a behavior is in agreement with the theoretical predictions for the capillary rise in very tight capillary wedges.   

Numerical study of vortex-induced vibrations of a cylinder in shear-thinning and shear-thickening power-law fluids

Vortex-induced Vibration (VIV) of a cylinder in Newtonian fluid is a model problem in Fluid-Structure Interactions (FSI) and has been studied extensively. In this work, we study the influence of shear-thinning and shear-thickening fluids on the VIV response of a 1DOF flexibly-mounted cylinder with a mass ratio of m^* = 2 at Re₀ = 15 and Re₀ = 200, respectively, defined based on the zero-shear-rate viscosity. We investigate how the VIV amplitude and frequency, flow forces, and vorticity contours change as the reduced velocity, U*, and fluid’s time constant, λ, change. When the results are compared based on Re₀, shear-thinning fluids enhance the oscillations while shear-thickening fluids suppress them. If, however, we define a characteristic Reynolds number, Re_char, based on a viscosity evaluated at the characteristic shear rate, U/D, then at a constant Re_char, the amplitude of response stays very similar for the shear-thinning, shear-thickening, and Newtonian fluids. Despite this similarity, the observed far wake is different: shear thinning amplifies the vorticity generation and reduces the extent of the wake, whereas shear thickening limits the vorticity generation and extends the wake.   

Analyzing a Turbulent Jet Flow using Proper Orthogonal Decomposition

Proper orthogonal decomposition (POD) is used in fluid dynamics in order to breakdown a variable into spatial and time coefficients. In the context of this poster, snapshot POD has been used to decompose both the streamwise and crosstream velocity planes as gathered from particle image velocimetry (PIV) preformed on a multi-aperture single expansion ramp nozzle (MARS). From this POD analysis, the most energetic modes can be identified, and coherent structures in the various mode shapes can be used to help with sensor placement. Furthermore, the instantaneous velocity field can be reconstructed using a variable number of POD modes. The more modes that are used, the more accurate the reconstruction will be, however, it was observed that using less than one third of the POD modes to reconstruct the instantaneous velocity field can accurately capture coherent structures while reducing the order of complexity of the flow field. This method of reconstruction shows promise in reducing the computational intensity of large data sets from non-time-resolved particle image velocimetry to a simpler datasets.   

Heat and salt transfer in subglacial plumes rising along a vertical ice face

Our current understanding of the small-scale fluid processes occurring in the glacier-ocean boundary layer is incomplete, and so parameterizations used in ice-ocean modeling are largely borrowed from the classic literature of heat transfer in engineered systems that do not represent glaciological conditions. The result is large departures between model predictions and observations of ocean melting of glaciers, which consequently leads to large uncertainties in projections of future sea level rise, particularly from Antarctica.

In this poster, we will present our laboratory results of heat and salt transfer across the ice-ocean interface formed between a subglacial plume and an initially smooth vertical ice face, a commonly found configuration in Greenlandic fjords. The laboratory experiments were done in a purpose-built water tank, inside a climate-controlled cold room and we simultaneously measured the temperature, salinity, and fluid velocity of subglacial plumes at high (~mm scale) spatiotemporal resolutions under realistic field conditions, using state-of-the-art particle image velocimetry (PIV) and laser-induced fluorescence (LIF) techniques for measurements. These experiments allow us to systematically re-examine the assumptions and simplifications made in existing melt-rate models, with the goal of deriving a revised and complete theory of melt at the ice-ocean interface.   

Single-Point Statistics (Second Moments and Dilatation) of a Turbulent Jet Flow

The jet flow from a multi-aperture single expansion ramp nozzle produces a time-evolving flow with a Reynold's number R = 2.76e6. The flow is formed by a core stream of Mach 1.6 and a secondary stream of Mach 1.0 which combine  behind a 'wavy' splitter plate. By collecting time-independent snapshots via particle imaging velocimetry data (PIV), a statistically steady flow field representation is obtained. To better understand the behavior of this jet flow, there is a need to use single-point statistics, second moments, and dilatation to characterize the flow and compare it to the behavior of a nominal splitter plate. In this study, instantaneous velocity data is used to produce root mean squared (RMS) and turbulent kinetic energy (TKE) data from fluctuating velocity data via Reynold's decomposition. Further, dilatation and Reynold's stress tensor components are obtained to better quantify the behavior or the jet flow. It will be shown from this study that there is noteworthy activity in our bottom shear layer and subtle differences in the shock train compared to the baseline case.   

3D-PTV wake flow analysis behind a barred owl wing

Owls are slow flying non-migratory raptors. Most owls feature stealth capabilities for hunting purposes. Nocturnal owls' species feature unique combination of their wing morphology. Two of the main features are leading-edge serrations and trailing edge fringes. The leading-edge comb is theorized to be responsible for decreasing the velocity above the boundary layer while the trailing edge fringes are thought to cause the mixing of the incoming air from the top and bottom of the wing that may cause a degradation or enhancement in the turbulent field. 3D printed wing models based on a Barred owl were used: one without features and one with the leading-edge serrations and trailing edge fringes. The wings' models were tested in an open-loop wind tunnel over three angles of attack where the near wake flow was measured using 3D-PTV ('V3V'). The volumetric data enabled to fully characterize the wake region in terms of the mean and turbulence decomposed fields. The comparison to a reference wing allowed to examine the role of the unique owl wing features in alternating the wake flow field which in consequence changed the aerodynamic loads, manifest the small-scales turbulence and the pressure field which directly impact the aerodynamic noise distribution during flight.   

Not Participating

Under classical conditions, the atomization of a liquid jet moving into a gas occurs primarily via aerodynamic interactions rather than via surface tension as is true for Rayleigh jet break-up. However, ultrasound transferred into the liquid at its source may produce a different mechanism of instability that in turn changes the atomization characteristics. Pressure fluctuation at the interface is still the root cause of instability, but these fluctuations can be driven via acoustic wave patterns transfered through the liquid rather than aerodynamic interactions. We show experimental evidence, including droplet size distribution data and high speed video, that ultrasound in fact measurably changes jet break up in multiple regimes. We perform linear stability analysis on a liquid jet that has a ultrasound induced pressure field imposed on top of a standard velocity field. We then use these results in an energy balance in order to predict the parameter space required for ultrasound to dominate aerodynamic interactions in atomization.   

Deformation and trajectory analysis of a pair of elastic particles under imposed flow

We carried out deformation and trajectory analysis of a pair of elastic particles under axisymmetric compressional and simple shear flows. The deformation dynamics of pair elastic particles are carried out initially for head-on interactions. We identified four stages during the deformation of particles: initial undeformed spherical particles, particles with flattened surfaces, the dimpled particles, and particles with fully established rims. The dynamics of the deformation of pair elastic particles is found to be fundamentally different from that of droplets, capsules, and vesicles systems under identical conditions. The effect of particle deformability on the pair trajectories (corresponds to offset interactions) of elastic particles is also studied. During these interactions, a huge negative pressure builds up before the test particle departs the reference particle, which allows its faster and asymmetric release. The trajectories exhibit significant deviation from the path followed by two rigid particles under identical conditions. Particularly under shear flow, this leads to spiraling out trajectories for both in-plane and off-plane interactions apart from the open trajectories.   

The effect of viscoelasticity in lubricated contacts in the presence of cavitation

A technique used to improve the performance of lubricants is in the addition of polymers to mineral oils. Lubricants containing polymer additives exhibit viscoelastic effects and have been observed to possess superior lubrication properties when compared to lubricants without additives. In this study, we propose a model based on the thin film approximation for viscoelastic lubricants that includes also the presence of cavitation described by the mass conserving Elrod-Adams algorithm. We use the Oldroyd-B constitutive relation to model the viscoelasticity of lubricants and assume that the product between ε, i.e. the ratio of the vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small. In doing so, it becomes possible to linearize the set of thin film equations. Results in a parabolic slider profile representing a journal bearing unwrapped geometry show an increase in load by strengthening viscoelasticity. On the other hand, for pocketed profiles, mimicking a textured surface, the influence of the viscoelasticity on the load depends on the location of the pocket. If the pocket is close to the entry (exit), the load decreases (increases) with Wi.   

An experimental study of stratified mixing in mean shear free homogeneous isotropic turbulence

Stratified turbulent flow has an important role in oceanic and atmospheric circulations. The interaction of turbulence with a density gradient has been studied for several decades, but there remain unknowns in quantifying the primary drivers of interfacial mass transport. We have designed an experimental study to investigate mixing at a stable density interface with a sharp gradient in which the upper layer is stirred continuously with turbulent forcing. A randomly actuated synthetic jet array is located at the top of the water tank to generate homogeneous isotropic turbulence with negligible mean shear in the upper layer, above a quiescent lower layer of sugar water. We use particle image velocimetry and laser induced fluorescence to simultaneously measure the spatio-temporal velocity and density fields, respectively. From velocity measurements, we quantify turbulence statistics such as integral length scale, turbulent kinetic energy, and dissipation. From LIF data, we investigate factors affecting the entrainment rate, generation of internal waves, and mechanisms that drive interfacial mass flux. By changing the Richardson number, the turbulent Reynolds number, and the Prandtl number, we can determine under what conditions different mixing rates and interfacial dynamics occur.   

A Pulsatile Flow Loop and Experimental measurement of In Vitro Blood Pressure in 3-D Printed Stenosed Arteries

We recently built up a pulsatile flow loop, mimicking the blood flow in the human circulatory system, to measure in vitro hemodynamics in 3-D printed arterial systems anatomically extracted from patient’s CT images.  The loop is equipped with a pulsatile heart pump (variable phase, RPM, and stroke volume), elements of resistance and compliance, as well as measurement devices. The diseased artery is segmented from computed tomography angiography (CTA) data, 3-D printed, and mounted in the loop. This pulsatile flow loop provides a unique platform to measure in-vitro blood pressure in human arteries with stenosis, a condition with reduced arterial lumen size.  Quantification of proximal pressure  and distal pressure  to arterial stenosis is critically important to assess the hemodynamic severity of stenosis via either fractional flow reserve () or trans-stenotic pressure gradient (TSPG) as invasive measurement via catheterization requires patient exposure to risk and high medical costs. Our preliminary measurement of blood pressure proximal and distal to iliac stenosis agree with the invasive measurement during an interventional treatment, which inspires more sophisticated research.   

Predicting Non-Stationary Homogeneous Variable-Density Turbulence Using Taylor-Net

Deep learning has shown the potential to significantly accelerate the numerical simulation of fluids without sacrificing accuracy, but prior works are mostly limited to stationary flows with uniform density. In real-world engineering applications, turbulent flows are mostly three-dimensional, non-stationary, and have variable-density. Here we propose Taylor-Net, a hybrid model that combines deep neural networks with the numerical Taylor series method for 3D turbulent flow prediction. Across flows with different density-ratio, our method is over 3 orders of magnitude faster than high-fidelity numerical simulations. It also achieves higher accuracy than several strong physics-informed deep learning baselines. Most importantly, the predictions of our Taylor-Net pertain consistent physical characteristics including mass conservation and turbulent energy spectrum.   

An in vitro investigation of cell injury mechanisms due to mechanical impact

Cell injuries associated with rapid mechanical load have garnered a great attention in the scopes of mechanisms of traumatic brain injury, development of reliable injury criteria, and accurate prediction of traumatic damage. We present an innovative experimental protocol that enables applications of well-controlled and repeatable acceleration-induced pressure gradients to live cells by utilizing both a drop tower system and environmentally controlled live-cell microscopy. This platform is capable of in-depth observations of individual cells and cell populations, required to reveal cell injury mechanisms at the single level while monitoring population-level cell responses, e.g., cell viability and membrane damage. Our study shows acceleration-induced cavitation is the main mechanism of cell injury and death rather than the linear acceleration itself. This result may indicate that linear acceleration, an overlooked mechanism for brain injury, must be appropriately considered due to the possibility of cavitation-induced damages.     

Experimental Study of Pool Boiling Heat Transfer on Novel Pin-Finned Surfaces

The rapid advancement of microelectronics has posed considerable challenges to the thermal management of extreme heat loads, exceeding 1000 W/cm², discharged from confined areas in many electrical systems. Boiling heat transfer associated with phase change is perhaps one of the most efficient cooling methodologies due to its large latent heat. Pin fin structures are commonly used to increase boiling heat transfer from the heated surface and have shown better performance than conventional fin-type heat sinks. This work aims to experimentally investigate the heat transfer performance of two pin-finned structures, namely solid and hollow pin fins, in a pool boiling facility. The hollow pin fin structure is designed to enhance the fin’s heat transfer performance by adding an additional artificial nucleation site. With pin fin heat sinks including a flat plane surface, different bubble growth parameters, such as bubble departure diameter, bubble growth time, bubble departure frequency, and a bubble waiting time, are thoroughly visualized and examined using high-speed imaging. Pool boiling experiments to estimate heat transfer rates and heat transfer coefficients are performed in atmospheric pressure conditions using deionized water. The obtained experimental data are compared with models and data available in the literature to assess the validity of the results. The preliminary results show that, as expected, the pin-fin heat sinks show a much better heat transfer rate when compared to that on a plane surface. Also, the hollow pin-fin structure shows better heat transfer performance when compared to the other two surfaces. This is attributed to the fact that the hollow fin has more active nucleation sites, a better rewetting phenomenon, and a favorable bubble growth and release mechanism. The overall heat transfer rate and interpretation of heat transfer enhancement on the hollow pin fin structure are studied.   

Doppler Ultrasound Assessment of Effects on Renal Resistance of Blood Flow in Human Aortorenal Systems

The renal arterial resistive index (RI) is a sonographic index of intrarenal arteries defined as (peak systolic velocity - end-diastolic velocity) / peak systolic velocity.  It assesses the ratio of the upstroke of the systolic wave in the renal artery to the end-diastolic flow rate.  The normal range is 0.50-0.70. Elevated values are associated with poorer prognosis in various renal disorders and renal transplants. We recently built up a pulsatile flow loop, mimicking the blood flow in the human circulatory system, to measure the in vitro RI using a portable ultrasound system (Philips CX50) in 3-D printed aortorenal artery systems anatomically extracted from patient’s CT images. We Preliminary study is to assess the effects of aortic pulsatility, heart rate, and renal arterial stenosis on renal RI via parameterization. It is found that RI is inversely proportional to heart rate but proportional to aortic pulsatility, meaning either increased heart rate or reduced aortic pulsatility can prevent elevated RI.  Meanwhile, reduction of the renal arterial lumen, a medical condition called stenosis, will elevate RI. These results agree with numerical simulation and medical observation.   

Wake behind circular cylinder excited by spanwise non-uniform disturbances

We experimentally investigated flow control of the wake behind a circular cylinder excited by temporal periodic disturbances with spanwise phase variations using DBD plasma actuators (PAs) array, motivated by suppressing development of large-scale wake vortices. PAs installed on the sides (+/- 90 deg from the forward stagnation point) were segmented in the spanwise direction, and the temporal phase differences, Φ = 0 or π, were given to adjacent electrodes. The width of the electrodes and gaps between them were chosen as 5 mm each. Experiment was conducted at Re_D = 8400 based on the cylinder diameter D = 23 mm, and the wake was visualized by stereo PIV. PAs were driven by 6kHz AC with voltage of 5-9 kV, and the range of St based on cylinder size was 1.0-2.2. In the case of Φ = π, a bundle of vortex lines which are selected to visualize the vortex structure formed in the following shear layer were shaped in a wavy pattern along spanwise direction with 180 degrees out of phase to the adjacent bundle. This structure, so called ‘chain-link fence structure’ was already found in planar free shear layer and planar jet, but it became evident to create it in the wake of circular cylinder in this study. The relation between the formation of the structure and drag forces will be clarified.   

Deformation and instabilities of differentially rotating drops

The Ring-Sheared Drop (RSD) module is a containerless flow device aboard the International Space Station for studying the effects of shear flow on protein aggregation and solidification at fluid interfaces with minimal wall nucleation effects. In the RSD a 2.5cm drop is constrained by a thin, stationary contact ring in one hemisphere and is sheared by the steady rotation of another ring in the other hemisphere. An earth-based analogue experiment using silicone oil drops immersed in an aqueous solution allows density matching in order to study the flow and associated deformation of the drop. The imposed shear drives a meridional flow in the drop. This leads to a pear-shaped mean deformation. The mean drop deformation has been quantified with a perturbation analysis using the capillary number (Ca) as the small parameter. The results agree with time-averaged experimental measurements, particularly at smaller ring rotation rates where Ca in the experiments is smaller. The experiment reveals an unsteady nonaxisymmetric drop deformation, which is correlated to instability in the outer flow. These experiments reveal some of the limitations of simulating microgravity using earth-based analogues.   

Wave self-focusing into an incompressible inviscid rotating fluid for the cylinder geometry

Inertial waves are driven by the Coriolis force in rotating fluids, they are valuable to study since rotating fluids occur very often in geophysics and astrophysics. The linear behavior of these waves has been extensively studied in various situations (in rotating and stratified media etc. . . ) but these waves can exhibit also nonlinear properties like electromagnetic waves in traditional non-linear optics.We are interested, here, by inertial wave-wave interaction into an incompressible inviscid rotating fluid for the cylinder geometry. Some previous studies have been devoted to possible focusing of inertial waves but mainly in the spirit of focusing toward "attractors"for those waves but not for their focusing driven by non linear processes. We analyze couplings of inertial waves and in particular their self interaction that can induce self-focusing in suitable conditions.To examine the possibility of wave focusing we shall mainly deal with a Ginzburg-Landau equation which is obtained by reduction of the Navier–Stokes equation using an asymptotic analysis.

We compute the wave coupling terms at different orders in wave amplitude, relying on an approximated associated Beltrami property. At third order it is shown that wave self-coupling can lead to wave self-focusing.   

Mechanics of Saturated Colloidal Packings: A Comparison of Two Models

A successful prediction of the response of poroelastic material to external forces depends

critically on the use of appropriate constitutive relation for the material. The most commonly

used stress–strain relation that captures the behavior of poroelastic materials saturated

with a liquid is that proposed by Biot (J Appl Phys 12:155–164, 1941). It is a linear

theory akin to the generalized Hooke’s law containing material constants such as the effective

bulk and shear modulus of the porous material. However, the effective elastic coefficients

are not known a priori and need to be determined either via separate experiments or

by fitting predictions with measurements. The main cause for this drawback is that Biot’s

theory does not account for the microstructural details of the system. This limitation in

Biot’s model can be overcome by utilizing the constitutive relation proposed by Russel

et al. (Langmuir 5(24):1721–1730, 2008) for the case of colloidal packings. We show that

in the linear limit, the constitutive relation proposed by Russel and coworkers is equivalent

to that of Biot. The elastic coefficients obtained from such a linearization are related to the

micro-structural details of the packing such as the particle modulus, the packing concentration

and the nature of packing, thereby enabling a more effective utilization of Biot’s model

for problems in the linear limit. The derivation ignores surface forces between the particles,

which makes the results also applicable to particles whose sizes are beyond the colloidal

range. We compare the predictions of Biot’s model to those of the linearized model of Russel

and coworker’s for two different one-dimensional model problems: fluid outflow driven

by an applied mechanical load, also termed as the consolidation problem, and wave propagation

in a saturated colloidal packing.   

2D dispersion dynamics in a bidisperse granular medium under controlled curvy motions of an intruder

In a 250 mm circular frame, we investigate dispersion dynamics of 2 populations of brass hollow discs: large ones (D_ext = 6mm) and small ones (d_ext = 5mm). Mixing is performed by moving a metallic intruder inside the medium along different specific and controlled paths – particles motion is captured using a camera affixed at the bottom of the bench. Using an image processing algorithm, all particles are classified and their positions retrieved through time. With that comprehensive information, for each performed path, we explore the subsequent mixing mechanism on different levels: a macroscopic level where we focus on the overall motion of centers of mass of the two populations, a mesoscopic level where the areal concentration in large/small particles is considered and lastly a microscopic level where refined local fields such us displacement or velocity around the intruder are outlined. Other systemic parameters such as the global volume fraction or the intruder’s size are also investigated in order to assess a complete rheological picture.   

Optimal energy harvesting kinematics for compliant membrane hydrofoils

Oscillating hydrofoils are increasingly being studied as an alternative to rotary turbines for extracting energy from tidal and fluvial flows due to their lower tip speed, smaller interturbine spacing and better applicability to shallow flows. Compliant membrane hydrofoils have been proposed to improve the low cycle efficiency of oscillating turbines, increasing lift and power coefficients dramatically due to their dynamic cambering. While the performance of oscillating hydrofoils has been shown to be highly dependent on their kinematics, the six dimensional parameter space of the membrane hydrofoil (heaving amplitude, pitching amplitude, pitch-heave phase, frequency, Young's modulus, and triangular to trapezoidal profile parameter) has never been comprehensively studied. In this work, the Nalder Mead optimization method is used to find the kinematic parameters which yield the optimal power coefficient of a heaving and pitching membrane hydrofoil by autonomously performing exhaustive and extensive water flume experiments. The routine illuminates trends in performance across the parameter space while avoiding time intensive mapping. Extension of this approach to the kinematics and material properties for arrays of turbines for large scale deployment is also investigated.   

A bio-inspired flapping wing robot with twist and fold capability

Bat flight is characterized by complex wing motions.  Here we present results obtained using a three degrees of freedom flapping wing robotic platform. In addition to the up/down wing flapping motion, the model can perform hand/armwing folding and wing twisting along the span of the handwing. Using this platform, we explore numerous unsteady aerodynamic effects, including handwing clapping during the upstroke, as observed in fast forward flight of the frugivorous bat, Cynopterus Brachyotis, as well as the generation of rotational lift due to rapid wing pronation during the up/downstroke transition when folding is also present – a  synergy between twist and fold motions. We record aerodynamic forces and moments of the flapping wing model placed in a wind tunnel at several flight speeds, and compare the results with quasi-steady numerical models based on blade element theory.  If time permits, we will present PIV measurements of the flows generated by the flapping wing with these complex kinematic wing motions.   

Measurement of turbulence spectrum in pipe flow using Multiple-Eye PIV

We have investigated the turbulence spectra in pipe flow using Multiple-Eye PIV system with a mirror array and confirmed that the dynamic velocity range was increased compared with 2D2C PIV.

The mirror array consists of a flat mirror arranged to form an ellipsoid, and placed it between a camera and flow target to capture images of identical particles from N (=110 in maximum) different directions. This method is known to reduce velocity random errors in proportion to N^-0.40.[A. Maekawa et al, Meas. Sci. Technol. 29, 11(2018)] by computing ensemble average of 3D particle locations identified through 3D-PTV. In this study, m sets of n different mirrors, where nm≤N, were used to capture particle images of turbulent pipe flow, and instantaneous velocity field was estimated by averaging m sets of 3D-velocity field obtained from each n images evaluated by tomographic PIV. The mean velocity was found to match well with DNS results, and the RMS values approached the DNS results as m increased. Energy spectrum was obtained by converting the time series data from frequency to wavenumber space based on Taylor's frozen field hypothesis. The attenuation of energy at high wavenumber was better captured by increasing the m.   

Delayed Leidenfrost States on Hydrophobic Surfaces

The Leidenfrost phenomenon refers to the state when a liquid droplet placed on a superheated surface starts levitating due to the build-up of vapor pressure underneath. Such a scenario can play a crucial role in droplet manipulation, spray cooling, and frictional drag reduction. This state manifests after the formation of a continuous vapor film beneath the droplet. In this context, rough hydrophobic surfaces are believed to accelerate vapor film formation due to their low adhesion energy and reduced liquid-solid contact area. Contrary to this logic, we have found that hydrophobic surfaces realized by carving arrays of doubly reentrant cavities (DRCs) actually significantly delay the emergence of the Leidenfrost phenomenon. High-speed imaging experiments reveal that water droplets remain pinned onto superheated silica surfaces with arrays of DRCs much longer than common hydrophobic surfaces where levitation is achieved immediately. The contributions of microstructure and surface chemistry on this counterintuitive phenomenon were investigated thoughtfully. These findings advance our notions of the Leidenfrost phenomenon and may open a door for applying hydrophobic surfaces to reduce fluid drags without compromising heat transfer.