Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L42: High Reynolds Number Flows |
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Sponsoring Units: DFD Chair: Marcel Ilie, University of Central Florida Room: D138 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L42.00001: On the stable hovering of an asymmetric body in oscillatory airflows Bin Liu, Annie Weathers, Stephen Childress, Jun Zhang A free rigid body, built with up-down asymmetry can hover in a vertical oscillatory airflow if the airflow amplitude and frequency exceed certain thresholds. The key to free hovering lies in the difference in drag coefficients as the airflow passes the object in two opposite directions. The hovering motion is surprisingly stable and robust, lasting for thousands of oscillation periods. We describe a series of flow visualizations of vortex shedding by the hovering object, which show how correcting moments restore its orientation, leading to stable hovering. This study may shed light on the stability of the hovering flight of insects. [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L42.00002: Modeling flexible flapping wings oscillating at resonance Alexander Alexeev, Hassan Masoud Using a hybrid approach for fluid-structure interactions that integrates the lattice Boltzmann and lattice spring models, we study the three-dimensional aerodynamics of flexible flapping wings at hovering. The wings are a pair of flat elastic plates tilted from the horizontal and driven to oscillate according to the sinusoidal law. Our simulations reveal that resonance oscillations of flexible wings dramatically increase aerodynamic lift at low Reynolds number. Comparing to otherwise identical rigid wings, flexible wings at resonance generate up to two orders of magnitude greater lift. Within the resonance band, we identify two operation regimes leading to the maximum lift and the maximum efficiency, respectively. The maximum lift occurs when the wing tip and root move with a phase lag of 90 degrees, whereas the maximum efficiency occurs at the frequency where the wing tip and root oscillate in counterphase. Our results suggest that the resonance regimes would be optimal for the design of microscale flying machines using flexible flapping wings driven by simple kinematic strokes. [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L42.00003: Vortices within vortices: Hierarchical vortex structures in experimental, two-dimensional flow Douglas H. Kelley, Nicholas T. Ouellette The topology of a fluid flow is concisely described by its critical points (locations of zero flow) and the manifolds (streamlines) that connect them. Streamlines that carry fluid away from a critical point and then return it to the same critical point from another direction are known as homoclinic manifolds. Rare in three-dimensional flow, homoclinic manifolds are common in two-dimensional flow and form unambiguous topological boundaries useful for defining vortex edges. Approximating two-dimensional flow with an electromagnetically driven, stably stratified solution in a 90~cm x 90~cm pan, we use particle tracking to measure the velocity field and locate its critical points and their manifolds. Strikingly, homoclinic manifolds are often nested~--- the flow contains vortices within vortices. Its regions can thus be classified by an embedding number, an integer defined as the depth of vortex nesting. We will discuss the dynamics of this hierarchical vortex embedding number, particularly as a function of flow speed (Reynolds number). [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L42.00004: Evolution of Triangles in Quasi-Two-Dimensional Flow Nicholas Ouellette, Sophia Merrifield, Douglas Kelley The anomalous transport of scalar fields in complex flow has recently been explained by considering the nontrivial shape dynamics of clusters of fluid elements. Here, we study the dynamics of three-particle clusters--Lagrangian triangles--that minimally parameterize planes as they are advected in a quasi-2D electromagnetically driven experimental flow. We report results for the shape distributions as a function of the initial triangle size, and discuss the impact of the flow structure on the subsequent triangle evolution. This work is supported by the National Science Foundation. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L42.00005: Creating Turbulence with vortex rings Kelken Chang, Gregory P. Bewley, Eberhard Bodenschatz We report measurements of the small-scale statistics of turbulence created by interacting vortices at a Taylor microscale Reynolds number of 500. We study the flow using Lagrangian particle tracking technique, in which the three-dimensional motion of passive oil particles in air is followed optically using multiple high speed cameras. We compare the results with measurements obtained in a nearly homogeneous and isotropic turbulent flow at comparable Reynolds number. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L42.00006: Vortex ring refraction at large Froude numbers Kerry Kuehn, Matthew Moeller, Michael Schulz, Daniel Sanfelippo We have experimentally studied the impact of a planar axisymmetric vortex ring, incident at an oblique angle, upon a sharp gravity-induced interface separating two fluids of differing densities. After impact, the vortex ring was found to exhibit a variety of subsequent trajectories, which we have organized according to both the incidence angle, and the ratio of the Atwood and Froude numbers, $A/F$. For relatively small angles of incidence, the vortices tended to penetrate the interface. In such cases, the more slowly moving vortices, having values of $A/F \ga .004$, tended to subsequently curve back up toward the interface. Quickly moving vortices, on the other hand, tended to refract downward, similar to a light ray entering a medium having a higher refractive index. A simplistic application of Snell's law of refraction cannot, however, account for the observed trajectories. For grazing angles of incidence, fast moving vortices tended to penetrate the interface, whereas slower vortices tended to reflect from the interface. In some cases, the reflected vortices executed damped oscillations before finally disintegrating. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L42.00007: DNS of the Velocity and Temperature Fields in a Model of a Small Room John McLaughlin, Xinli Jia, Goodarz Ahmadi, Jos Derksen This talk presents the results of a numerical study of the velocity and temperature fields in a model of a small room containing a seated mannequin. Results are also presented for the trajectories and ultimate fate of small particles that are introduced through the air inlet as well as particles that are entrained by the mannequin's thermal plume. The study was motivated by an experimental study performed at Syracuse University. In the experimental study, air entered the room through a floor vent and exited through a ceiling vent on the other side of the room. A mannequin was seated facing the floor vent. The mannequin could be electrically heated so that its surface temperature was 31C. The objective of the simulations was to obtain a more detailed understanding of the flow in the room. Of specific interest were the effects of the mannequin on the ultimate fates of small particles. The importance of the thermal plume around the mannequin was of particular interest since the thermal plume plays a role in transporting particles from near the floor to the breathing zone. The simulations were performed with a single phase version of a lattice Boltzmann method (LBM) that was originally developed for two-phase flows by Inamuro et al. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L42.00008: Large-eddy simulations of particle-laden turbulent swirling flows Marcel Ilie In many combustion devices, a swirling flow is used to stabilize the flame through a recirculation zone. Swirling flows, however, are prone to instabilities which can trigger combustion oscillations and deteriorate the performance of the combustor. The presence of fine particles makes swirling flows of particular interest from a combustor efficiency point of view. Depending on the strength of swirl, a number of recirculation zones and central vortex breakdown regions are identified in many swirl-stabilized flames. In general these characteristics make swirling flows and flames to exhibit highly three-dimensional, large-scale turbulent structures with complex turbulent shear flow regions. The present research concerns the influence of swirl characteristics on the particle dispersion and total deposition. A Lagrangian particle tracking algorithm using large-eddy simulation is proposed. The influence of particle characteristics such size, density and shape on the particle dispersion and total deposition is subject of investigation as well. The present research shows that the total particle deposition increases with size and density. It was also observed that particles of ellipsoidal shape are more prone to deposition. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L42.00009: Lattice Boltzmann and Pseudo-Spectral Methods for Decaying Turbulence Li-Shi Luo, Yan Peng, Wei Liao, Lian-Ping Wang We conduct a comparison of the lattice Boltzmann (LB) and the pseudo-spectral (PS) methods for direct numerical simulations (DNS) of the decaying turbulence in a three dimensional periodic cube. We use a mesh size of $128^3$ and the Taylor micro-scale Reynolds number $24.35 \leq \mbox{Re}_\lambda \leq 72.37$. All simulations are carried out to $t \approx 30 \tau_0$, where $\tau_0$ is the turbulence turnover time. We compare instantaneous velocity $\mathbf{u}$ and vorticity $\mathbf{\omega}$ fields, the total kinetic energy $K(t)$, the dissipation rate $\varepsilon(t)$, the energy spectrum $E(k,\, t)$, the rms pressure fluctuation $\delta p(t)$, the pressure spectrum $P(k,\, t)$, and the skewness $S_u(t)$ and the flatness $F_u(t)$ of velocity derivatives. Our results show that the LB method compares well with the PS method in terms of accuracy: the flow fields and all the statistical quantities --- except for $\delta p(t)$ and $P(k,\, t)$ --- obtained from the two methods agree well with each other when the initial flow field is adequately resolved by both methods. Our results indicate that the resolution requirement for the LB method is $\eta_0 / \delta x \geq 1.0$, where $\eta_0$ and $\delta x$ are the initial Kolmogorov length and the grid spacing, respectively. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L42.00010: ABSTRACT WITHDRAWN |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L42.00011: Fluid-Structure Interaction based on Lattice Boltzmann and p-FEM Benjamin Ahrenholz, Sebastian Geller, Manfred Krafczyk Over the last decade the Lattice Boltzmann Method (LBM) has matured as an efficient method for solving the Navier-Stokes equations. The p-version of the Finite Element Method (p-FEM) has proved to be highly efficient for a variety of problems in the field of structural mechanics. The focus of this contribution is to investigate the validity and efficiency of the coupling of two completely different numerical methods to simulate transient bidirectional Fluid-Structure Interaction (FSI) problems with very large structural deflections. In this contribution the treatment of moving boundaries in the fluid solver is presented, the computation of tractions and displacements on the boundary as well as the explicit coupling algorithm itself. In addition, efficiency aspects of the two approaches for two- and three-dimensional laminar flow examples at intermediate Reynolds numbers are discussed. Finally we give an outlook on modeling turbulent FSI problems. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L42.00012: Lattice Boltzmann Methods for thermal flows: applications to compressible Rayleigh-Taylor systems Luca Biferale, Mauro Sbragaglia, Andrea Scagliarini, Kazuyasu Sugiyama, Federico Toschi We compute the continuum thermo-hydrodynamical limit of a new formulation of Lattice Kinetic equations for thermal compressible flows, recently proposed in [{\it Sbragaglia et al. ``Lattice Boltzmann method with self-consistent thermo-hydrodynamic equilibria'', J. Fluid Mech. {\bf 628} 299 (2009)}]. We show that the hydrodynamical manifold is given by the correct compressible Fourier-Navier-Stokes equations for a perfect fluid.We also apply the method to study Rayleigh-Taylor instability for compressible stratified flows and we determine the growth of the mixing layer at changing Atwood numbers up to $At \sim 0.4$. Both results show that this new Lattice Boltzmann Methods can be used to study highly stratified/compressible systems with strong temperature gradients, opening the way to applications to Non-Oberbeck-Boussinesq Convection and compressible Rayleigh-Taylor turbulence. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L42.00013: The shape of fair weather clouds Yong Wang, Giovanni Zocchi It is well known that cumulus clouds are formed under the influence of thermals - convection currents which channel moist air upwards. Here we introduce a simple physical model which accounts for the shape of cumulus clouds exclusively in terms of thermal plumes or thermals. The plumes are explicitly represented by a simple potential flow generated by singularities (sources and sinks) and with their motion create a flow field supporting the cloud. We discuss the parametrization of this model, which attempts a description of the cloud starting from the coherent structures in the flow. We use the model to explore transitions which occur in the dynamical state of the cloud. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L42.00014: On the dynamics of cartoon dunes Christopher Groh, Ingo Rehberg, Christof A. Kruelle The spatio-temporal evolution of a downsized model for a barchan dune is investigated experimentally in a narrow water flow channel. We observe a rapid transition from the initial configuration to a steady-state dune with constant mass, shape, velocity, and packing fraction. The development towards the dune attractor is shown on the basis of four different starting configurations. The shape of the attractor exhibits all characteristic features of barchan dunes found in nature, namely a gently inclined windward (upstream) side, crest, brink, and steep lee (downstream) side. The migration velocity is reciprocal to the length of the dune and reciprocal to the square root of the value of its mass. The velocity scaling and the shape of the barchan dune is independent of the particle diameter. For small dunes we find significant deviations from a fixed height-length aspect ratio. Moreover, a particle tracking method reveals that the migration speed of the model dune is one order of magnitude slower than that of the individual particles. In particular, the erosion rate consists of comparable contributions from low energy (creeping) and high energy (saltating) particles. Finally, it is shown that the velocity field of the saltating particles is comparable to the velocity field of the driving fluid. [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L42.00015: Supernova Shear and Magnetic Field Amplification Cyril Allen A core collapse supernova marks the death of a star over 8 times the size of the sun. Sometimes in the aftermath of these explosions a spinning, magnetized, neutron star can be left behind, also known as a pulsar. It has recently been discovered that pulsar spins can arise through a spiral spherical accretion shock instability (SASI) of a supernova. This instability produces a strong shear flow inside the supernova shock wave, which might lead to amplification of the star's magnetic field.~ To study this possibility, hydrodynamic simulations have been modified to include a tracer of the magnetic field by adding the magnetic induction equation to the code. Diagnostics were added to the code to measure the overall field strength and shear flow generated by the SASI.~ I found the magnetic field could be amplified by a factor of 100 in only 20 milliseconds. This raises the possibility that shear-induced field amplification might be able to contribute to the energy of the supernova explosion and explain the high magnetic fields of the pulsar left behind. [Preview Abstract] |
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