Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session S21: Vortex Dynamics VIII |
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Chair: Enrico Fonda, Universita di Trieste Room: 324-325 |
Tuesday, November 22, 2011 3:05PM - 3:18PM |
S21.00001: Particle-vortex interactions in quantum turbulence Enrico Fonda, Katepalli R. Sreenivasan, Daniel P. Lathrop We study quantum turbulence in a thermal counterflow of He II, using sub-micron and micron-sized frozen particles. For low heat fluxes, these particles can either trace the motion of the normal component or track quantized vortices when they get trapped on their cores. For high heat fluxes the increased number of particle-vortex interactions and scattering events result in a different state in which the particles track neither the vortices nor the normal component. Analyzing the trajectories of tracers of different size and density for a wide range of heat fluxes, we investigate the particle-vortex interaction mechanism. The results are used to test analytical and numerical models, and explain the discrepancy between two previous counterflow experiments. [Preview Abstract] |
Tuesday, November 22, 2011 3:18PM - 3:31PM |
S21.00002: Effects of a flexible margin on Robojelly vortex structures Alex Villanueva, Kelley Stewart, Pavlos Vlachos, Shashank Priya An Unmanned Underwater Vehicle (UUV) inspired by jellyfish morphology and propulsion mechanism, termed ``Robojelly,'' was used to analyze the effects of the flexible margin on jellyfish propulsion. The natural animal has a bell section which deforms at a different phase then the rest of the bell. This lagging section, referred to as flexible margin or flap, is delimited by the bell margin and an inflexion point. The flap was replicated on the robotic vehicle by a flexible passive material to conduct a systematic parametric study. In a preliminary experiment, Robojelly was tested without a flap and with a flap. This revealed a thrust increase over an order of magnitude. We hypothesize that the length of this passive flap affects the vortex ring circulation strength of the jellyfish which can lead to higher efficiency and thrust. Velocity field measurements were performed using planar Time Resolved Digital Particle Image Velocimetry (TRDPIV) to analyze the change in vortex structures as a function of flap length. The robot input parameters stayed constant over the different configurations tested thus maintaining a near constant power consumption. Results clearly demonstrate that the flap plays an important role in the propulsion mechanism of Robojelly and provides an anatomical understanding of natural jellyfish. [Preview Abstract] |
Tuesday, November 22, 2011 3:31PM - 3:44PM |
S21.00003: 3D Reconstruction of the Vortex in a Human Right Ventricle Model using High Speed PIV Arash Kheradvar, Ahmad Falahatpisheh This work aims to characterize the formation process and translation of the vortex, which forms along with the trans-tricuspid jet in a realistic model of a human right ventricle (RV). A clear model of the RV made of silicone rubber was carefully casted in real size from echocardiographic data of an adult human heart. The RV model was used in our heart pulsed-flow simulator at KLAB at UCI to perform experiments. Bioprosthetic heart valves in appropriate sizes were used at tricuspid and pulmonary positions. Multi-planar high-speed PIV was performed to capture and reconstruct the 3D flow field with a 1-millisecond time gap between each two velocity frames. $\lambda_{2}$ iso-surfaces were used to illustrate the evolution of vortex cores. The highly asymmetric shape of the RV chamber results in a complex 3D trans-tricuspid vortex that forms and translates toward right ventricular outflow tract, and finally departs RV from pulmonary valve. Through this study, -for the first time- the formation, evolution and pathway of the RV vortex have been characterized \emph{in vitro}. [Preview Abstract] |
Tuesday, November 22, 2011 3:44PM - 3:57PM |
S21.00004: Experimental study of asymmetric heart valve prototype M. Vukicevic, S. Fortini, G. Querzoli, A. Cenedese, G. Pedrizzetti The mechanical heart valves (MHVs) are extremely important medical devices, commonly used for diseased heart valves replacement. Despite the long term of use and constant design refinement, the MHVs are very far from ideal and their performance is very diverse from that of the native ones. It has been approved that small variations in geometry of valvular leaflets influence the significant change in the intraventricular vortical flow, known as one of the most important factors for the overall functionality of the heart. We have experimentally examined the home-made heart valve prototypes, exclusively modeled for the mitral valve replacement. The performance and energetic properties of the prototypes have been compared with those in the presence of standard MHVs. The analysis was based on the testing of intraventricular fluid dynamics, usually missing criteria for the quality of the valve performance. It has been shown that the asymmetric prototype, with unequal leaflets and D-shaped orifice produces flow patterns and energetic properties close to those found in the healthy subjects. Thus, the break of symmetry in the standard bi-leaflet MHV prosthesis, at least from the fluid dynamics point of view, is worthwhile to be considered for the design of MHVs for the mitral valve replacement. [Preview Abstract] |
Tuesday, November 22, 2011 3:57PM - 4:10PM |
S21.00005: The whale footprint Adrien Benusiglio, Christophe Clanet In their wake whales leave patches of very calm water, with few waves, that can last for several minutes known as whale footprints. The same phenomenon of damping of waves appears near pears of bridges, in the turbulent wake of ships or in rivers re-emergences, thus it appears to be the result of the interaction of a turbulent flow with surface waves. To understand this phenomenon we study the interaction of a single vortex ring with surface waves. We investigate the influence of the size and circulation of the vortex ring. We measure the time of interaction and the size of the resulting patch of calm water. We then verify if our model can explain the whale footprint. [Preview Abstract] |
Tuesday, November 22, 2011 4:10PM - 4:23PM |
S21.00006: Effect of slip on vortex dynamics and forcing of a superhydrophobic cylinder Pranesh Muralidhar, Robert Daniello, Nangelie Ferrer, Jonathan Rothstein Superhydrophobic surfaces have been shown to produce significant drag reduction for both laminar and turbulent flows of water through large and small-scale channels. In this presentation, a series of experiments will be presented which investigate the effect of superhydrophobic-induced slip on the vortex dynamics in the wake of a cylinder and the change in the drag and lift forces thereof. In these experiments, circular cylinders are coated with a series of superhydrophobic surfaces fabricated from PDMS with well-defined micron-sized patterns of surface roughness. Using force measurements and PIV (Particle Image Velocimetry), we will show that these surfaces have a noticeable effect on the drag/lift and vortex dynamics of cylinders. When compared to a smooth, no-slip cylinder, we will show that the lift/drag and the amount of raw vorticity that is shed in the wake of the superhydrophobic cylinder decreases. In addition, we will show that the forcing is sensitive to changes of feature spacing, size and orientation. [Preview Abstract] |
Tuesday, November 22, 2011 4:23PM - 4:36PM |
S21.00007: VIV of cylinders with superhydrophobic coating Robert Daniello, Jonathan Rothstein For several years, superhydrophobic surfaces which are chemically hydrophobic with micron or nanometer scale surface features have been considered for their ability to reduce drag in microfluidic devices. More recently it has been demonstrated that superhydrophobic surfaces reduce friction coefficient in turbulent flows as well. In this talk, we will consider the effect of superhydrophobic surfaces on vortex induced vibrations of an elastically mounted circular cylinder. Effects on frequency and amplitude will be considered and particle image velocimetry will be used to examine vortices shed from the superhydrophobically coated moving cylinders. Several unique superhydrophobic microstructures will be used in the Cassie and Wenzel states to demonstrate the effect of microstructure and preferential slip direction on the oscillations of the cylinder. [Preview Abstract] |
Tuesday, November 22, 2011 4:36PM - 4:49PM |
S21.00008: Evolution and scaling of a start-up vortex from a two-dimensional shear layer Pooria Sattari, David Rival, Robert Martinuzzi, Cameron Tropea The formation process of a vortex due to the impulsive start of a shear layer plays an important role in a large number of industrial and biological systems. From the point of view of aerodynamic efficiency and momentum transfer, reaching higher levels of circulation prior to the release of the vortex can often be advantageous. For this reason the determination of a possible limit to vortex size would be an important step forward in our understanding of the vortex pinch-off process. In the present work, the evolution of an isolated line vortex generated by a starting two-dimensional jet is studied experimentally with the aid of time-resolved PIV. Unlike previous studies, this current experiment is not restricted by any interactions with other vortical structures or walls that potentially limit vortex size. By tracking the circulation, size and position of the vortex relative to the shear layer, we were able to identify the limit to vortex growth and therefore better understand the physical pinch-off process. Furthermore, in the absence of inherent velocity and length scales, alternative parameters for the scaling of circulation based on shear-layer characteristics are proposed instead. [Preview Abstract] |
Tuesday, November 22, 2011 4:49PM - 5:02PM |
S21.00009: Inverse cascades in an experimental turbulent von K\'arm\'an swirling flow Miguel Lopez, Javier Burguete An experimental study of the energy spectrum in a von K\'arm\'an swirling flow is presented. For the experiments we use water stirred in a closed cylindrical cavity placed in a fully developed turbulent regime ($Re \sim 10^5$). The three components of the velocity field ($v_R, v_{\theta}, v_z$) can be measured with a 1D laser doppler velocimetry, one component at a time. Different time scales can be identified in the fourier space spliting the spectrum in different cascades with different slopes. These timescales are a consequence of instabilities in the mean flow. Two different inverse cascades (slopes $-2$ and $-1/3$) appear in some regions of the flow whose origin is related to the dynamics of coherent structures. The existence of these times scales can affect the statistical characterization of the turbulent flow. [Preview Abstract] |
Tuesday, November 22, 2011 5:02PM - 5:15PM |
S21.00010: Buoyancy-Induced Columnar Vortices Mark W. Simpson, Ari Glezer Large-scale inherent instability of a thermally stratified air layer that is heated from below by a thermal ground plane in a meter-scale laboratory facility is exploited for the deliberate formation of intense columnar vortices. In nature, such buoyancy-driven vortices (``dust devils'') occur spontaneously, with core diameters of 1-50 m at the surface, heights up to one kilometer, and with induced air flow of considerable angular and linear momentum. The laboratory experiments have demonstrated the nucleation and sustainment of strong, buoyancy-driven vortices that are anchored and stabilized using an azimuthal array of flow vanes while the temperature of the thermal ground plane is maintained using a resistive heater and a controllable power source. The present investigation focuses on the fundamental mechanisms of the formation, evolution, and dynamics of the columnar vortex using stereo-PIV. It is also shown that the strength and scaling of theses vortices can be significantly altered by adjustment of the flow vanes and the amount of global sensible heat absorbed by the air flow, or the ``buoyancy flux''. The characteristics of the anchored vortices can also be altered by small modifications of the thermal ground plane that affect the flux and distribution of the ground plane vorticity that forms the core of the vortex. [Preview Abstract] |
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