Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session M15: Vortex Dynamics: Applications |
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Chair: Robert Kerr, University of Warwick Room: 203 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M15.00001: Helicity Annihilation in Trefoil Reconnection: Simulations Robert M. Kerr The simulated evolution and self-reconnection of a perturbed trefoil vortex knot is compared to the Scheeler et al, PNAS 111 (2014) experiment. To have a single initial reconnection, as in the experiments, the trefoil is perturbed by 4 weak vortex rings. Visualizations show that the simulations and experiments undergo similar topological changes. Quantitative comparisons using the helicity and global topological number show that both are preserved for a long period before reconnection begins, as in the experiments. Unlike the experiments, once reconnection begins, a significant fraction of the helicity is dissipated and the global topological number changes by a discrete amount in a fixed time. Helicity spectra and physical space correlations show that the change in helicity is associated with the appearance of negative helicity at lower wavenumbers and in the outer regions of the trefoil. Furthermore, using a range of Reynolds numbers, with the highest comparable to the experiments, it is demonstrated that a Reynolds number independent fraction of the initial helicity is dissipated in a finite time. This observation does not violate any current mathematics restricting the strong growth of Navier-Stokes norms as the viscosity goes to zero due to the structure of the trefoil. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M15.00002: 3D characterization of leading-edge vortex formation and growth Kyohei Onoue, Kenneth Breuer We examine the vorticity transport mechanisms responsible for regulating the stability and strength of the leading-edge vortex (LEV) on rapidly pitching plates with different planforms (swept vs. rectangular) in a uniform airflow. All experiments are carried out using a cyber-physical experimental setup (Onoue et al. 2015, JFS vol. 55) and synchronized 3D PIV measurements. In the case of a swept wing, two distinct regions of intense spanwise flow are observed around the LEV centroid---a feature conspicuously absent on a rectangular pitching plate. The interaction between these spanwise flows and the LEV core seems to play a role in prolonging the LEV residence time at the cost of the vortex circulation growth rate and magnitude. Detailed control volume analysis is performed to elucidate the flow physics at work. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M15.00003: The Flow Dynamics of the Garden-Hose Instability Fangfang Xie, Xiaoning Zheng, Michael Triantafyllou, Yiannis Constantinides, George Karniadakis We present for first time full simulations of flow-structure interactions in a flexible pipe conveying incompressible fluid. We show that the Reynolds number plays a significant role in the onset of flutter in a fluid-conveying pipe under similar boundary conditions as for the classic garden-hose problem. We investigate the complex interaction between structural and fluid dynamics and obtain a phase diagram of dynamic transition between states as a function of two non-dimensional parameters, the fluid-tension parameter, and the Reynolds number. We observe that the precise flow patterns inside the pipe determine the type of induced motion. For unsteady flow, symmetry along one direction leads to in-plane motion whereas breaking of the flow symmetry results in out-of-plane motion. Above a critical Reynolds number, as the pipe vibrates, complex flow patterns result as there is continuous generation of new vorticity due to pipe wall acceleration, which is subsequently shed in the confined space of the pipe interior. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M15.00004: Flow development over low aspect ratio cantilevered circular cylinders in the laminar shedding regime Chris Morton, Mohammad Saeedi, Robert Martinuzzi The flow development over a cantilevered circular cylinder of aspect ratio 4 at Re $=$ 300 has been investigated numerically by employing a laminar flow solution to the Navier-Stokes equations. The results show that two distinct wake modulation frequencies are detectable downstream of the cylinder, differing from higher Reynolds number turbulent flow cases where only one dominant frequency is present. In particular, there is a low frequency modulation with a well-defined narrow-band peak (f$_{\mathrm{m}})$, and a high frequency contribution from the shedding of vortices (f$_{\mathrm{v}})$. The fluctuating loading on the cylinder in the streamwise direction is tightly coupled with the low frequency modulation, while the transverse direction forces show only a weak correlation with the vortex shedding frequency. Coherent flow structures have been analyzed using proper orthogonal decomposition (POD) to provide insight into the nature of vortex formation and associated coupling with the detected low frequency modulation. The temporal coefficients obtained from the POD analysis have been used to construct a low order model for the investigation of the overall flow development. While the high frequency component is known to be related to the formation and shedding of vortices, the low frequency component is shown to be associated with a modulation in upwash and downwash intensity. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M15.00005: Vortex-induced vibration of a harbor-vibrissa-shaped cylinder: Experimental measurements. Yingzheng Liu, Shaofei Wang Influence of the unique surface variation of the harbor seal vibrissa on its vortex-induced vibration was extensively investigated in a low-speed wind tunnel. Toward this end, a scaled-up model of the harbor-vibrissa-shaped cylinder was employed for measurement, while a circular cylinder sharing the same hydraulic diameter, mass ratio and nature frequency was used as baseline case. Two configurations with and without the approaching Karman-vortex street were respectively tested for both cylinders at various free stream flow speeds. Here, the Karman-vortex street was generated by placing a circular cylinder far up stream. A laser displacement sensor having a high time-spatial resolution was used to capture the cross-stream displacement of the cylinders. The fluctuating pressure distribution on the surface and the wake flow pattern were captured by microphone array and the planar Particle Image Velocimetry, respectively. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M15.00006: Abstract Withdrawn
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Tuesday, November 24, 2015 9:18AM - 9:31AM |
M15.00007: A thin-walled Taylor column surrounding a bathtub vortex in rotating tank. Chin-Chou Chu, Kuan-Ruei Lai, Yin-Chung Chen, Chien-Cheng Chang Numerical simulations and laboratory experiments were jointly conducted to investigate a bathtub vortex under the influence of a protruding cylinder in a rotating tank. The flow pattern depends on Rossby number (\textit{Ro}$=U$/\textit{fR}), Ekman number (\textit{Ek }$=\nu $/\textit{fR}$^{2})$, and height ratio, $h$/$H$, where $R$ is the radius of the cylinder, $f$ the Coriolis parameter, $\nu $ the kinematic viscosity of the fluid, $h$ the vertical length of the cylinder and $H$ the height of the tank. Steady-state solutions obtained by numerically solving the Navier-Stokes equations in the rotating frame are shown to have good agreements with flow visualizations measurements. The bathtub vortex exhibits an interesting two-celled structure with an inner Ekman pumping and an outer up-drafting motion. The two regions of up-drafting motion are separated by a notable finite-thickness structure, identified as thin-walled Taylor column. The Taylor column sets a barrier to the fluid flow that flows into the inner region only through the narrow gaps, one above the Taylor column and one beneath it. Moreover, the dependence of thickness and height of the thin-walled Taylor column on angular velocity ratio of cylinder to background rotation ($\omega $/$\Omega )$, ranging from -8/3 to 8/3 are also discussed. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M15.00008: Effects of Canard on the Flowfield over a Wing Arash Nayebzadeh Surface and flowfield pressure measurements have been done over delta wing/canard configuration in a variety of canard vertical and horizontal locations and angles of attack. The experimental model consisted of wing, canard and a body to accommodate pressure tubing and canard rotation mechanism. All the tests have been performed at subsonic velocities and the effect of canard were analyzed through comparison between surface and flowfield pressure distributions. It was found that vortex flow pattern over the wing is dominated mainly by canard vertical position and in some cases, by merging of canard and wing vortices. In addition, the pressure loss induced by canard vortex on the wing surface moves the wing vortex toward the leading edge. In the mid canard configuration, canard and wing vortices merge at x/c greater than 0.5 and as a result of this phenomenon, abrupt pressure loss induces more stable vortex flow over the wing. It is also shown that canard plays a vital role in vortex break down over the wing. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M15.00009: Characterization of a vortical gust generator using PIV Esteban Hufstedler, Beverley McKeon A heaving plate has been used to generate aperiodic vortical gusts in a free-surface water tunnel as part of an effort to experimentally investigate the interaction between a wing and an incoming parallel vortex. Particle image velocimetry measurements provided information about the growth and evolution of the vortices over a range of heaving and freestream speeds. Vortex tracking methods were used to examine the circulation and movement paths of the vortices. Preliminary results of vortex-airfoil interactions will also be presented, with a view to identifying the gust response and tolerance. [Preview Abstract] |
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