Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session F2: Student Poster Competition (6:15 - 7:00) PM |
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Room: Level 3 Lobby |
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F2.00001: GENERAL FLUID DYNAMICS |
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F2.00002: Reynolds Number Effects on Mixing Due to Topological Chaos Sangeeta Warrier, Spencer Smith Topological chaos has emerged as a powerful modeling tool to investigate fluid mixing. While this theory can guarantee a lower bound on the stretching rate of certain material lines, it does not indicate what fraction of the fluid actually participates in this minimally mandated mixing. Indeed, the area in which effective mixing takes place depends on physical parameters such as the Reynolds number. To help clarify this dependency, we numerically simulate the effects of a batch stirring device on a 2D incompressible Newtonian fluid in the laminar regime. In particular, we calculate the finite time Lyapunov exponent (FTLE) field for two different stirring protocols, one topologically complex (pseudo Anosov) and one simple (finite order), over a range of viscosities. After extracting appropriate measures indicative of mixing from the FTLE field, we see a clearly defined range of Reynolds numbers for which the relative efficacy of the pseudo Anosov protocol over the finite order protocol justifies the application of topological chaos. The Reynolds number dependance of these mixing measures also reveals several other intriguing phenomena. [Preview Abstract] |
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F2.00003: Integral Method and Eigenspace Decomposition for RANS Turbulent Mixing Flows Wade Spurlock, Eric Parish, Daniel Israel Integral methods can be used to obtain low-order dynamical systems which approximate the solution of RANS models. Such techniques are beneficial for calibrating coefficients and verifying a model for flows away from self-similarity. We apply an integral method approach to turbulent mixing flows and compare analytic results to full-field RANS simulations in xRage, a code developed at Los Alamos National Laboratory. Flows that exhibit late time self-similarity are considered. Eigenspace decomposition is then used to identify dominant solution characteristics and visualize higher dimensional closure models. Results are shown for the temporal shear and Rayleigh-Taylor layers. [Preview Abstract] |
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F2.00004: On the stability of homogeneous three-dimensional turbulent flows Ananda Mishra, Sharath Girimaji Flows experiencing spatially uniform deformation rate, appellated as homogeneous flows, are the most elementary cases to exhibit hydrodynamic instabilities. While the stability characteristics of homogeneous flows subject to planar strain and rotation are well-established, those of three-dimensional flows are not. We address the stability characteristics of general incompressible flows undergoing three-dimensional streamline convergence, divergence and swirl. Two of the salient findings are:(i) flow stability is completely contingent upon the third invariant of the background velocity gradient tensor - flows with a positive third invariant are stable while those with a negative value are unstable; and, (ii) with the sole exception of two-dimensional elliptic flows, inertial effects are destabilizing and pressure effects are stabilizing. [Preview Abstract] |
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F2.00005: 2D CFD Analysis of an Airfoil with Active Continuous Trailing Edge Flap Dylan Jaksich, Jinwei Shen Efficient and quieter helicopter rotors can be achieved through on-blade control devices, such as active Continuous Trailing-Edge Flaps driven by embedded piezoelectric material. This project aims to develop a CFD simulation tool to predict the aerodynamic characteristics of an airfoil with CTEF using open source code: OpenFOAM. Airfoil meshes used by OpenFOAM are obtained with MATLAB scripts. Once created it is possible to rotate the airfoil to various angles of attack. When the airfoil is properly set up various OpenFOAM properties, such as kinematic viscosity and flow velocity, are altered to achieve the desired testing conditions. Upon completion of a simulation, the program gives the lift, drag, and moment coefficients as well as the pressure and velocity around the airfoil. The simulation is then repeated across multiple angles of attack to give full lift and drag curves. The results are then compared to previous test data and other CFD predictions. This research will lead to further work involving quasi-steady 2D simulations incorporating NASTRAN to model aeroelastic deformation and eventually to 3D aeroelastic simulations. [Preview Abstract] |
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F2.00006: Experimental Investigation of Passive Shock Wave Mitigation using Obstacle Arrangements Monica Nguyen, Qian Wan, Veronica Eliasson With its vast range in applications, especially in the defense industry, shock wave mitigation is an ongoing research area of interest to the shock dynamics community. Passive shock wave mitigation methods range from forcing the shock wave to abruptly change its direction to introducing barriers or obstacles of various shapes and materials in the path of the shock wave. Obstacles provide attenuation through complicated shock wave interactions and reflections. In this work, we have performed shock tube experiments to investigate shock wave mitigation due to solid obstacles placed along the curve of a logarithmic spiral. Different shapes (cylindrical and square) of obstacles with different materials (solid and foam) have been used. High-speed schlieren optics and background-oriented schlieren techniques have been used together with pressure measurements to quantify the effects of mitigation. Results have also been compared to numerical simulations and show good agreement. [Preview Abstract] |
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F2.00007: A Shock-Driven Mechanism for Standing-Wave Patterns in Vertically Oscillated Grains Alex Gilman, Jon Bougie We develop a simplified model for standing-wave pattern formation in vertically oscillated granular layers based on an instability in shocks found in these layers. When layers of particles are oscillated at accelerational amplitudes greater than that of gravity, the layers leave the plate, and shocks are created upon re-established contact with the plate. Additionally, standing-wave patterns form when the accelerational amplitude exceeds a critical value. For a given layer depth and accelerational amplitude, varying driving frequency alters the shock strength as well as pattern wavelength; increasing layer depth produces stronger shocks and longer pattern wavelengths for a given frequency. We demonstrate relationships between properties associated with shocks and properties associated with standing wave patterns, and present a simple mechanism by which a non-uniform shock front drives standing-wave configurations. We justify this mechanism using mathematical relationships derived from a continuum granular model. We then compare these mathematical relationships to full numerical solutions of continuum equations to Navier-Stokes order for uniform, frictionless, inelastic spheres. [Preview Abstract] |
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F2.00008: Computational Analysis of Wake Field Flow between Multiple Identical Spheres Wesley Brand, Morton Greenslit, Zach Klassen, Jay Hastings, William Matson It is well understood both that objects moving through a fluid perturb the motion of nearby objects in the same fluid and that some configurations of objects moving through a fluid have little inter-object perturbation, such as a flock of birds flying in a V-formation. However, there is presently no known method for predicting what configurations of objects will be stable while moving through a fluid. Previous work has failed to find such stable configurations because of the computational complexity of finding individual solutions. In this research, the motions of two spheres in water were simulated and combinations of those simulations were used to extrapolate the motions of multiple spheres and to find configurations where the lateral forces on each sphere were negligible and the vertical forces on each sphere were equivalent. Two and three sphere arrangements were simulated in COMSOL Multiphysics and Mathematica was used both to demonstrate that combinations of two sphere cases are identical to three sphere cases and to identify stable configurations of three or more spheres. This new approach is expected to simplify optimization of aerodynamic configurations and applications such as naval and aerospace architecture and racecar driving. [Preview Abstract] |
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F2.00009: Vortex breakdown of a co-flowing swirling jet with density difference under normal and inverted gravity fields Shunsuke Tsutsumi, Adzlan Ahmad, Hiroshi Gotoda We study vortex breakdown (VB) of a co-flowing swirling jet with a density difference under normal and inverted gravity fields. The density difference is created by issuing CO2 from an inner tube into ambient air. The formation region of unstable VB for a CO2 jet is larger under inverted gravity than under normal gravity. The trends of the changes in the stagnation point height are given particular attention while investigating the stable breakdown region. A physical model derived by considering the momentum balance in the flow is adopted to reasonably interpret the decrease in the stagnation point height of stable VB under inverted gravity with increasing inner swirl number of the inner jet, or the increase in the stagnation point height with increasing bulk flow velocity of the outer jet, for a swirling jet with a density difference. [Preview Abstract] |
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F2.00010: FILM AND INTERFACES |
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F2.00011: Surfactant Spreading on Thin Viscous Fluid Films Caitlyn Bonilla, Nathaniel Leslie, Jeanette Liu, Dina Sinclair, Rachel Levy We examine the spreading of insoluble lipids on a viscous Newtonian thin fluid film. This spreading can be modeled as two coupled nonlinear fourth-order partial differential equations, though inconsistencies between the timescale of experiments and simulations have been reported in recent research. In simulations, we replace traditional models for the equation of state relating surfactant concentration to surface tension with an empirical equation of state. Isotherms collected via a Langmuir-Pockels scale provide data for the equation of state. We compare the timescale of simulation results to measurements of the fluorescently tagged lipid (NBD-PC) spreading as well as the height profile, captured with laser profilometry. [Preview Abstract] |
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F2.00012: Using falling soap film to visualize flow in a wavy channel Vontravis Monts, Osazuwa Edokpolo, Zachary Mills, Alexander Alexeev The disturbances created by the walls of a sinusoidal shaped channel lead to the development of unsteady, time periodic flow. This periodic flow is the result of vortex shedding occurring along the crests of the channel walls. We used a falling soap film to investigate the influence of the channel geometry on the flow. In falling soap films, variations in the thickness of the film correspond to streamlines in the flow. These thickness variations are made visible by reflecting monochromatic light off the film. This allows for soap films to be an accurate, but inexpensive method of visualizing two dimensional flows. In our experiments we used a gravity driven soap film flowing through a wavy channel of several periods and used a high speed camera to record the resulting flow. The collected footage was then analyzed to collect data on the flow. From this data we were able to characterize the dependence of the size of the vortices and their shedding frequency on the amplitude and period of the sinusoidal channel walls as well as the Reynolds number of the flow. [Preview Abstract] |
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F2.00013: Wakes and flow-induced oscillations of tandem cylinders in a flowing soap film Wenchao Yang, Josam Waterman, Mark Stremler We investigate the wake dynamics and flow-induced oscillations of a tandem two-cylinder system aligned vertically in a flowing soap film. The cylinders interact with the soap film as circular disks. The upstream cylinder is fixed in place, while the downstream cylinder is free to oscillate as a pendulum that is driven by interactions with the wake of the upstream cylinder. The soap film is a convenient system for investigating quasi-2D dynamics and considering how they compare with the typical 3D system. Wake structures are visualized by the film's interference fringes; both these and the cylinder locations are recorded with a high-speed camera system. The force response of the downstream cylinder is measured with a microcantilever laser-mirror sensor system. Varying the distance between the cylinders reveals multiple modes of behavior, including variations in the force response and the regularity of the oscillations. [Preview Abstract] |
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F2.00014: Electrohydrodynamics of a surfactant-covered drop Andrew Oberlander, Malika Ouriemi, Petia Vlahovska We present an experimental study of the behavior of a drop covered with insoluble surfactant in a uniform DC electric field. Steady drop shapes, drop evolution upon application of the field, and drop relaxation after the field is turned off are observed for a polybutadiene (PB) drop suspended in silicon oil (PDMS). The surfactant is generated at the drop interface by reaction between end-functionalized PB and PDMS. The experimental data is compared with the theory of Nganguia et al (2013) for the steady shapes, and a new model developed by us which accounts for polarization relaxation. The latter effect turns to be significant for our system of very low conductivity fluids, for which the Maxwell-Wagner time is of the order of tens of seconds. We will discuss the complex interplay of shape deformation, surfactant redistribution, and interfacial charging in droplet electrohydrodynamics. Our results are important for understanding electrorheology of emulsions commonly found in the petroleum industry. [Preview Abstract] |
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F2.00015: Enhancing capillary rise on a rough surface Melissa Chow, Jason Wexler, Ian Jacobi, Howard Stone Liquid-infused surfaces have been proposed as a robust alternative to traditional air-cushioned superhydrophobic surfaces. However, if these surfaces are held vertically the lubricating oil can drain from the surface, and cause the surface to lose its novel properties. To examine this failure mode, we measure the drainage from a surface with model roughness that is scaled-up to allow for detailed measurements. We confirm that the bulk fluid drains from the surface until it reaches the level of the capillary rise height, although the detailed dynamics vary even in simple surface geometries. We then test different substrate architectures to explore how the roughness can be designed to retain greater amounts of oil. [Preview Abstract] |
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F2.00016: ABSTRACT WITHDRAWN |
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F2.00017: Mechanism of the lift force acting on a levitating drop over a moving surface Masafumi Saito, Yoshiyuki Tagawa, Masaharu Kameda The purpose of this study is to understand the levitation mechanism of a drop over a moving surface. In our experiment we softly deposit a silicon-oil drop onto the inner wall of a rotating hollow cylinder. With sufficiently large velocity of the wall, the drop steadily levitates. The drop reaches a stable angular position in the cylinder, where the drag and lift balance the weight of the drop. The lift force, which is vital for the levitation, is generated inside a thin air film existing between the drop and the wall. Here three-dimensional shape of the air film plays a crucial role for the magnitude of the lift force. Note that, although the shapes of some levitating drops had been reported, the lift estimated from the shape had not been validated. Using interferometric technique, we measure the three-dimensional shape of the air film under the drop. We then calculate the lift by applying the lubrication theory. This lift is compared with that estimated from the angular position. Both lifts show a fair agreement. In addition, we investigate the shapes of the air film under drops with various sizes, viscosities and wall velocities. We discuss effects of these parameters on the shape and the lift. [Preview Abstract] |
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F2.00018: BIO AND BIO-INSPIRED |
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F2.00019: The recreation of a unique shrimp's mechanically induced cavitation bubble Ryan Miller, Christopher Dougherty, Veronica Eliasson, Gauri Khanolkar The \textit{Alpheus heterochaelis}, appropriately nicknamed the ``pistol shrimp,'' possesses an oversized claw that creates a cavitation bubble upon rapid closure. The implosion of this bubble results in a shock wave that can stun or even kill the shrimp's prey (Versluis et al., 2000). Additionally, the implosion is so violent that sonoluminescence may occur. This light implies extreme temperatures, which have been recorded to reach as high as 10,000 K (Roach, 2001). By developing an analogous mechanism to the oversized claw, the goal of this experiment is to verify that cavitation can be produced similar to that of the pistol shrimp in nature as well as to analyze the resulting shock wave and sonoluminescence. High-speed schlieren imaging was used to observe the shock dynamics. Furthermore, results on cavitation collapse and light emission will be presented. [Preview Abstract] |
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F2.00020: Swimming \textit{Vorticella convallaria} in various confined geometries Luz Sotelo, Donghee Lee, Sunghwan Jung, Sangjin Ryu \textit{Vorticella convallaria} is a stalked ciliate observed in the sessile form (trophont) or swimming form (telotroch). Trophonts are mainly composed of an inverted bell-shaped cell body generating vortical feeding currents, and a slender stalk attaching the cell body to a substrate. If the surrounding environment is no longer suitable, the trophont transforms into a telotroch by elongating its cell body into a cylindrical shape, resorbing its oral cilia and producing an aboral cilia wreath. After a series of contractions, the telotroch will completely detach from the stalk and swim away to find a better location. While sessile \textit{Vorticella} has been widely studied because of its stalk contraction and usefulness in waste treatment, \textit{Vorticella}'s swimming has not yet been characterized. The purpose of this study is to describe \textit{V. convallaria}'s swimming modes, both in its trophont and telotroch forms, in different confined geometries. Using video microscopy, we observed \textit{Vorticellae} swimming in semi-infinite field, in Hele-Shaw configurations, and in capillary tubes. Based on measured swimming displacement and velocity, we investigated how \textit{V. convallaria}'s mobility was affected by the geometry constrictions. [Preview Abstract] |
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F2.00021: Bio-inspired propulsor using internally powered flexible fins Peter Yeh, Alper Erturk, Alexander Alexeev Using experiments and three dimensional numerical simulations, we study the underwater locomotion of internally powered flexible plates. The flexible plate is composed of Macro-Fiber Composite (MFC) piezoelectric laminates. A sinusoidally varying voltage is applied to the MFCs, causing bending and generating thrust similar to a flapping fin in carangiform motion. In our fully coupled FSI simulations, we model the swimmer as a rectangular elastic plate actuated by a sinusoidal internal moment. The steady state swimming velocity and thrust are measured experimentally and compared to our numerical simulations. Our results can be used to design underwater self-propelling vehicles driven by internally powered flexible fins. [Preview Abstract] |
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F2.00022: The Effects of Including Piezoelectric Film as Part of a Wing Surface Charlotte Sappo Micro air vehicles (MAVs) are size- and weight-restricted, unmanned, flying vehicles that often exploit biology for inspiration. Membrane wings, one commonly employed biological adaptation, improves aerodynamic efficiency. These efficiency gains are due to the passive deformations and vibrations of the membrane. Piezoelectric films have the potential to further utilize these vibrations through the conversion of this motion into measureable electrical energy. In this investigation, an amplifier circuit was designed to measure the charge generated by a flexible polyvinylidene fluoride (PVDF) film adhered to a rectangular wing frame (aspect ratio of 2). The trailing edge was unattached and free to vibrate. The circuit consisted of two charge amplifiers, to convert the high impedance charge of the piezoelectric film into an output voltage, and an instrumentation amplifier, to reject common-mode noise. Amplifying and filtering the output signal appropriately, through the use of the feedback capacitance and resistance, was discovered to be of the utmost importance for this endeavor. Results from shaker and wind tunnels tests are presented. [Preview Abstract] |
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F2.00023: Effects of Fluid Shear Stress on Cancer Stem Cell Viability Brittney Sunday, Ursula Triantafillu, Ria Domier, Yonghyun Kim Cancer stem cells (CSCs), which are believed to be the source of tumor formation, are exposed to fluid shear stress as a result of blood flow within the blood vessels. It was theorized that CSCs would be less susceptible to cell death than non-CSCs after both types of cell were exposed to a fluid shear stress, and that higher levels of fluid shear stress would result in lower levels of cell viability for both cell types. To test this hypothesis, U87 glioblastoma cells were cultured adherently (containing smaller populations of CSCs) and spherically (containing larger populations of CSCs). They were exposed to fluid shear stress in a simulated blood flow through a 125-micrometer diameter polyetheretherketone (PEEK) tubing using a syringe pump. After exposure, cell viability data was collected using a BioRad TC20 Automated Cell Counter. Each cell type was tested at three physiological shear stress values: 5, 20, and 60 dynes per centimeter squared. In general, it was found that the CSC-enriched U87 sphere cells had higher cell viability than the CSC-depleted U87 adherent cancer cells. Interestingly, it was also observed that the cell viability was not negatively affected by the higher fluid shear stress values in the tested range. In future follow-up studies, higher shear stresses will be tested. Furthermore, CSCs from different tumor origins (e.g. breast tumor, prostate tumor) will be tested to determine cell-specific shear sensitivity. [Preview Abstract] |
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F2.00024: Design of a millifluidic device for thermophoretic analysis of biomolecules Ryan Phelps, Tyler Sharby, Jennifer Kreft Pearce Thermophoresis is the migration of particles due to a temperature gradient, which is enhanced in small channels due to the high temperature gradients that can be achieved. Thermophoresis can be used to analyze biomolecules such as proteins and DNA. It can also be used to study the absorption of small molecules to lipid membranes. For this experiment a millifluidic device is used. The channel in which the sample is injected is 500 microns wide. The temperature gradient is produced by hot and cold water baths. ~This device is a low cost alternative to the commercially available systems for thermophoresis-based analysis of biological molecules. [Preview Abstract] |
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F2.00025: Lattice-Boltzmann-based simulations of membrane protein dynamics Tyler Sharby, Ryan Phelps, Michael Antonelli, Jennifer Kreft Pearce The cell membrane is a complex structure composed of a phospholipid bilayer and embedded proteins. Recent work has shown that regions of different mobility exist in the membrane due to a variety of factors and that protein motion can be significantly subdiffusive due to the presence of stationary obstacles. We present work that shows that the combination of stationary obstacles and regions of different mobility can lead to aggregation of proteins in certain regions of the cell membrane. The concentration of stationary proteins is below the percolation threshold. The mechanism of this process is hydrodynamically-mediated interactions of diffusing proteins with themselves, as in hydrodynamic memory, and with obstacles. [Preview Abstract] |
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F2.00026: Microfluidic mixing using a heterogeneous array of actuated synthetic cilia Matthew Ballard, Puja De, Alexander Alexeev We use three-dimensional numerical simulations to examine mixing of an initially segregated viscous fluid solution in a microchannel containing a heterogeneous array of actuated synthetic cilia. We model the cilia as elastic filaments attached to the channel walls and actuated by an external periodic force. Fluid flow is modelled using a lattice Boltzmann model treating concentration as a scalar, coupled with a lattice spring model simulating the elastic cilia. To investigate the effects of the oscillating heterogeneous cilia on microfluidic mixing of fluid solutions of different diffusivity, we consider the effects of cilia relative size, elasticity, spacing, and oscillation pattern. We demonstrate that arrays of heterogeneous cilia can provide enhanced mixing over that achievable with a homogeneous array of identical cilia. Thus, our findings further the understanding of how heterogeneous arrays of active bio-mimetic structures can be used to enhance mixing in microfluidic devices. [Preview Abstract] |
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F2.00027: THERMAL AND COMBUSTION |
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F2.00028: Dynamic behavior of thermoacoustic combustion oscillations in a lean premixed gas-turbine model combustor with and without active control Ryosuke Tsujimoto, Shohei Domen, Yuta Okuno, Yoshitake Nakagaki, Hiroshi Gotoda We experimentally study the dynamic behavior of thermoacoustic combustion oscillations in a laboratory-scale lean premixed gas-turbine model combustor with and without active control. We adopt the delayed feedback control method based on the concept of chaos control to suppress thermoacoustic combustion oscillations. The unstable periodic orbits in the attractor of uncontrolled thermoacoustic combustion oscillations are led to the desired orbits with a small diameter of the attractor when the perturbation is switched on, resulting in the notable suppression of thermoacoustic combustion oscillations. Color-recurrence plots (Gotoda et al., Physical Review E, \textbf{89}, 022910 (2014)) are used for characterizing the complexity of the combustion state with and without delayed feedback control. [Preview Abstract] |
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F2.00029: Experimental Configuration Effects on ICE Tumble Flow Evaluation Bryan Santana, Paulius Puzinauskas The generation of ICE (Internal Combustion Engine) in-cylinder charge motions, such as swirl and tumble, have shown positive effects on reducing fuel consumption and exhaust emission levels at partial engine loads. Tumble flow is commonly measured utilizing a steady-flow rig and two-dimensional PIV (Particle Image Velocimetry) systems, among others. In order to optimize the tumble flow, it is important to retrieve accurate measurements. The tumble flow values could be affected by variations in the geometry and/or design of the steady-flow rig utilized during flow tests. In this research, a four-valve per cylinder head was tested on a steady flow bench, varying several aspects of the configuration to evaluate how they influence bulk momentum as well as PIV measurements. The configuration variations included symmetrical, asymmetrical and runner-fed configurations throughout testing. Volumetric flow rate and tumble strength flow measurements were retrieved at the selected L/D ratios. Additionally, several PIV seeding particles were characterized for size and shape. Corresponding PIV flow measurements using each type of seeding were made to evaluate how the particles influence the results. [Preview Abstract] |
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F2.00030: Laser Diagnostic Methods to Characterize Soot Evolution in Diesel-relevant Fuels Steven Overheim, Brian Fisher Soot particles are a harmful byproduct of diesel combustion and can be detrimental to the environment and our health. The purpose of this research is to gain a better understanding of how the soot formation, growth, and oxidation are directly related to the chemical structure of the fuel in a diffusion flame. Such understanding is expected to help with soot reduction methods in the future. A new method to analyze soot concentrations was developed combining previous successful methods of experimentation. The new method employs combined elastic scattering and extinction to characterize soot formation, growth, and oxidation throughout the flame. These concentrations are quantifiable through the use of a 532-nm Nd:YAG laser and carefully calibrated photodetectors as optical measuring tools. This study focused on the doping of the diffusion flame with toluene, which has an aromatic molecular structure. The diffusion flame is doped with a low concentration of toluene, 1000 ppm, in its fuel stream and compared to a methane-fueled base flame. By comparing the doped flame to the methane/oxygen base flame, the higher level of active soot formation in the doped flame was clearly observed. Future work on the project will entail further data analysis to convert measured signals into quantitative soot size and concentration information. [Preview Abstract] |
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F2.00031: On the Possibility of Condensation during Supercritical Fuel Injection Lu Qiu, Rolf Reitz Supercritical fuel injection into a nitrogen environment was simulated using Peng-Robinson equation of state. The real gas simulation was found to match the experimental injectant density much better than the ideal gas simulation, emphasizing the importance of applying realistic equation of state model. Possible fuel condensation processes were also investigated by considering the stability of the single phase by utilizing fundamental thermodynamics principles. Several conclusions from the experiments are also seen from the simulations. First, though both the injection and chamber pressures are above the critical pressure of the injectant, condensation can become possible as long as their temperature difference is large enough, and when this occurs, the fluid is able to enter the two-phase region. Condensation is found to be enhanced when the chamber temperature is further reduced, indicating that the fluid is in a state further away from the phase border. In addition, the newly formed condensed phase is found to exist only in the jet boundary where there are strong interactions between the ``hot'' injectant and the ``cold'' nitrogen. Finally, it was concluded that the local strong heat and mass exchange sent the mixture into the two-phase region by crossing the dew point line with the commencement of condensation. [Preview Abstract] |
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F2.00032: Flow Diagnostics of Swirl Stabilized Combustion with and without Porous Inert Media Carolina Vega Recalde Due to regulations, the industry and the scientific community have become interested in combustion noise and thermo-acoustic instabilities, especially those produced under lean-premixed (LPM) conditions. Instabilities are self-excited and arise when energy from combustion is added to the system faster than energy is dissipated by heat transfer. Given that porous inert media (PIM) has been shown to mitigate combustion noise and thermo-acoustic instabilities in lean direct injection (LDI) using kerosene fuel, the present study examined the flow fields produced with and without PIM. By using time-resolved particle image velocimetry (TR-PIV) and proper orthogonal decomposition (POD) techniques, the non-reacting and reacting flow fields will be studied to determine the underlying mechanisms. The purpose of this experiment is to gain more understanding of how this PIM material works. Since PIM has been shown to reduce these instabilities, modifications to combustors can be made to make them more efficient and safe for the environment. [Preview Abstract] |
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F2.00033: Modeling the Optimal Heat Transfer Fluidization Velocity in Gas-Fluidized Beds Thomas Predey, Jon Bougie, Aleksandr Goltsiker Fluidized beds are vital to a wide range of industrial applications and are useful for studying two-phase flow. However, modeling the optimal heat transfer fluidization velocity (OHTFV) in such beds has remained difficult. Previous investigations have commonly taken one of two approaches. One such approach attempts to find a general scaling formula for homogeneous fluidized beds by taking a harmonic average between the terminal and minimum fluidization velocities. Modern approaches using computer simulations and a wide range of parameters are more commonly used in industry today, but are generally concerned with specific applications. We propose a third approach, taking into account the inhomogeneity of the fluidized bed system while limiting the input parameters to gas velocity and particle size. We use this approach to find a general formula for OHTFV that accounts for the collective behavior of the particles rather than focusing on each individual particle in the bed. We then compare this model to previous experimental results. [Preview Abstract] |
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F2.00034: Flux Variability from Turbulence and Bulk Velocity Variations in Relativistic Hydrodynamic Jets Maxwell Pollack, David Pauls, Paul Wiita We simulated relativistic hydrodynamic jets using the Athena MHD code incorporating special relativity (Beckwith \& Stone 2011). We compared the long-timescale variations produced by changes in the bulk velocity within the jet, amplified by Doppler boosting, to the short-timescale variations caused by turbulence in the flow. The flux variability due to changes in bulk velocity was calculated along a band spanning the width of the jet at a fixed distance down its stream, positioned just behind a reconfinement shock. We computed the relativistic turbulence variability by summing the results from our relativistic turbulence code over multiple zones; this required incorporating time delays. Power Spectral Densities were then computed for both turbulent and bulk velocity flux variations, and compared. For reasonable jet widths of $\sim$40 light-years, we found turbulent fluctuations on timescales of days to years and bulk-velocity variations contributing on longer timescales. We found that the slopes of the turbulent and bulk PSDs were usually between $-$1.5 and $-$2.2, in accord with observations of Active Galactic Nuclei. [Preview Abstract] |
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F2.00035: Modeling Relativistic Jets Using the Athena Hydrodynamics Code David Pauls, Maxwell Pollack, Paul Wiita We used the Athena hydrodynamics code (Beckwith \& Stone 2011) to model early-stage two-dimensional relativistic jets as approximations to the growth of radio-loud active galactic nuclei. We analyzed variability of the radio emission by calculating fluxes from a vertical strip of zones behind a standing shock, as discussed in the accompanying poster. We found the advance speed of the jet bow shock for various input jet velocities and jet-to-ambient density ratios. Faster jets and higher jet densities produce faster shock advances. We investigated the effects of parameters such as the Courant-Friedrichs-Lewy number, the input jet velocity, and the density ratio on the stability of the simulated jet, finding that numerical instabilities grow rapidly when the CFL number is above 0.1. We found that greater jet input velocities and higher density ratios lengthen the time the jet remains stable. We also examined the effects of the boundary conditions, the CFL number, the input jet velocity, the grid resolution, and the density ratio on the premature termination of Athena code. We found that a grid of 1200 by 1000 zones allows the code to run with minimal errors, while still maintaining an adequate resolution. [Preview Abstract] |
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F2.00036: Discrete Element Simulation of Density Induced Segregation in Binary Granular Mixtures Annette Volk, Urmila Ghia We study density induced segregation of binary granular mixtures under vertical vibration using the open source discrete element method (DEM) code LIGGGHTS. Published experiments of vertically vibrated binary mixtures, varying in density ratio and observed under differing intensities of vertical vibration, are simulated and the final segregation state is quantitatively compared. Simulation results compare well with experimental data when the density ratio between the binary particles is relatively small but the comparison slowly deteriorates as the density ratio increases. A sensitivity study is performed for the coefficient of restitution since this quantity is absent from the published experimental data but has been shown to affect the amount of segregation. In industrial applications, mixing/segregation time is vital to processing, and hence, the relationship between the time to reach final segregation state and the density ratio of the binary particles is also investigated. Finally, in an effort to increase computational efficiency while maintaining accuracy, the effect of domain size on both time to reach final segregation state and amount of segregation in the final state is assessed. [Preview Abstract] |
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