Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session H12: Vortex V |
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Chair: Mark Stremler, Virginia Polytechnic Institute and State University Room: 26B |
Monday, November 19, 2012 10:30AM - 10:43AM |
H12.00001: Vortex evolution behind tandem cylinders under forced vibration Yingchen Yang, Tayfun Aydin, Alis Ekmekci Flow past two circular cylinders in tandem arrangement has been studied experimentally employing the hydrogen bubble visualization technique. The two cylinders had the same diameter ($D$ = 6.35 mm), and were subjected to forced in-phase vibration in the cross-flow direction. The Reynolds number based on the cylinder diameter was \textit{Re} = 250. Both the vibration frequency ($f_{e})$ and center-to-center pitch ratio ($P/D)$ were varied in certain ranges, whereas the vibration amplitude ($A)$ was fixed at $A/D$ = 0.25. The flow visualization resulted diverse and highly-repetitive vortex patterns. They were classified into two typical modes: a low-frequency mode and a high-frequency mode. The difference between the two modes is on the number of vortices formed per vibration cycle. For the low-frequency mode, the number is four; for the high-frequency model, it is two. In both modes, the vortex formation is phase-locked to the cylinder motion. For a specified mode with a fixed vortex number per cycle, the way the vortices evolve in the wake can be somewhat different by changing the vibration frequency and pitch ratio. These affecting factors have been examined in this work, and the associated vortex patterns have been characterized and compared. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H12.00002: Characterization of vortex-induced vibration of a flexible cylinder Jessica Shang, Howard Stone, Alexander Smits In this study, the phenomena of 3D vortex-induced vibration (VIV) of a flexible cylinder (diameter D) is shown to be distinct from 2D VIV. We seek to identify correlations between wake regimes and vibration responses for a low mass-ratio ($m^*$ = 1.2), flexible ($E =1.2$ MPa, natural frequency in water $f_N = 0.37$ Hz) cantilevered cylinder undergoing cross-flow for reduced velocity $U^*$ = 20-120 ($U^* = U / f_N D$). A P+S wake mode appears for a range of $U^*$; the onset of this range may be correlated with a hysteretic jump to an upper branch in the transverse amplitude response ($A^*_Y = A_Y / D$) at several locations along the midspan. This asymmetric wake mode does not present a unique transverse frequency response ($f^*_Y = f_Y / f_N$) in the cylinder. The upper branch in the amplitude response gives way to an abrupt decrease in $A^*_Y$ to a lower branch, accompanied by a bifurcation in $f^*_Y$. The bifurcation takes place over a narrow range of $U^*$ where the lower $f^*_Y$ gradually transfers power to a higher $f^*_Y$, and may demarcate a wake transition regime between laminar and turbulence states. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H12.00003: In-line and cross-flow multi-frequency vortex-induced vibrations of a long flexible cylinder are phase-locked under wake-body synchronization Remi Bourguet, George Karniadakis, Michael Triantafyllou A slender flexible body with bluff cross-section immersed in cross-flow exhibits vortex-induced vibrations. The vibrations are excited by the flow under a condition of lock-in defined as the synchronization between vortex formation and body displacement. Within a sheared current, the possible occurrence of the lock-in condition at a number of different locations can lead to broadband vibrations involving a wide range of excited frequencies and structural wavenumbers. In a previous study focusing on the vortex-induced vibrations of a flexible cylinder at a single frequency in each direction, we have found that the lock-in condition is established through counter-clockwise figure-eight trajectories where the body moves upstream at the extremes of the cross-flow oscillation. In the present work, on the basis of direct numerical simulation results, we show that this mechanism can be generalized to multi-frequency responses: even if the trajectory shape substantially departs from a figure eight, the phase difference between the components of the in-line and cross-flow vibrations locally involved in the lock-in phenomenon remains within a particular range, associated with counter-clockwise figure-eight orbits in the mono-frequency case. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H12.00004: Enhancing Vortex Induced Vibration of a Circular Cylinder by Using Roughness Strips Ashwin Vinod, Arindam Banerjee The current experimental work focuses on studying the effects of surface roughness on vortex induced vibration (VIV) of an elastically mounted circular cylinder which is free to vibrate in a direction transverse to the flow. Our objective is to identify configurations which lead to high amplitudes of vibrations and a greater range of synchronization that can be successfully used for energy harvesting. Different configurations such as smooth cylinders, cylinder with zero roughness strips, and prescribed roughness (using sand paper) were used. Experiments were also conducted with the zero roughness strips at different angles around the cylinder to verify the effect of the position of the strip. All results were also found to be dependent on the spring stiffness. Variations were observed in the amplitude and frequency response profiles for the different cases investigated. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H12.00005: Use of targeted energy transfer to delay K\'{a}rm\'{a}n vortex shedding and suppress vortex-induced vibration in flow past a cylinder Ravi K.R. Tumkur, Ramon E. Calderer, Lawrence A. Bergman, Alexander F. Vakakis, Arif Masud, Arne J. Pearlstein For two-dimensional flow past a circular cylinder whose motion is constrained by a linear spring to be perpendicular to the mean flow, we report computations showing that ``targeted energy transfer'' using a nonlinear energy sink (NES; consisting of a mass, a linear damper, and an essentially nonlinear spring) not only can reduce the amplitude of the cylinder motion, but can also increase the critical Reynolds number (\textit{Re}) at which the K\'{a}rm\'{a}n vortex street first appears. Absent the NES, the critical \textit{Re} at which vortex shedding (and hence cylinder motion) sets in is determined as a function of the stiffness of the linear spring. Over a wide range of stiffness, the NES is shown to delay the onset to higher \textit{Re}. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H12.00006: Using LCS to identify vortex shedding on a cylinder in cross-flow Melissa Green The transition from steady separation to unsteady vortex shedding downstream of a circular cylinder in cross-flow is examined using a Lagrangian coherent structure analysis. Velocity data is gathered from both 2D and 3D simulations at a Reynolds number shortly after transition ($Re=100$). At transition, when flow begins to entrain into and detrain from the cylinder wake, the wake as described using LCS undergoes a distinct qualitative change. This event in the evolution of the LCS will offer new information about possible timing and location at which to implement effective flow control to mitigate the shedding and unsteady forces on the cylinder body. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H12.00007: Experiments on the flow around yawed and fixed cylinder: Forces and Flow measurements Guilherme R. Franzini, Rafael S. Gioria, Ivan Korkischko, Julio R. Meneghini, Andre L.C. Fujarra Flow around yawed and fixed circular cylinders were experimentally investigated at a Recirculating Water Channel facility. The total hydrodynamic loads were measured by using a 6DOF load cell and flow measurements were carried out using the 2D PIV technique. The cylinders were yawed in angles up to 45 degrees for both upstream and downstream orientations. For all the experiments, the aspect ratio is close to 13 and the lower end of the model is kept close to the channel floor. For the force measurements, the Reynolds number considering only the component normal to the cylinder axis lies in the interval 4000 $<$ Re$_{n} <$ 14000. The PIV measurements were carried out at Re$_{n}$ to 9000. Differences were observed in the force coefficients plots depending on the orientation. Low frequencies component fD/U$_{n}<$ 0.08 were observed in lift force spectra in the case of upstream orientation. The Koopman decomposition applied to the PIV snapshots helped to a better understanding of the results. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H12.00008: Flow around an inclined cylinder with different end plates boundary conditions Rafael Gioria, Guilherme Franzini, Julio Meneghini, Andre Fujarra Numerical simulations of the flow around an inclined circular cylinder are carried out using the Lattice Boltzmann Method with Reynolds number Re=9,000 based on the normal component of the free-stream. The wall at the tips of the cylinder are modeled by two different boundary conditions: no-slip condition on the bottom wall and slip boundary conditions on the top wall. The resulting flows are different for an inclined cylinder with bottom tip upstream and another one with bottom tip downstream. This is related to the different boundary conditions on top and bottom. These differences are assessed via force coefficients comparison, near wake probes analysis and near wake visualization of the flow simulations. The main different feature observed is a streamwise vortical structure near the lower tip of the cylinder inclined upstream. This flow structure is not observed at the downstream orientation. The dominance of the upstream end of the cylinder in the wake and the asymmetry of the boundary conditions chosen for the walls seem to be responsible for the differences between upstream and downstream results. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H12.00009: Reorientation of Vorticity on a Rapidly Accelerating Finite Aspect Ratio Plate Jochen Kriegseis, Matthias Kinzel, David Rival In recent studies the competition between the developing leading-edge vortex (LEV) and tip vortex (TV) has been considered from an Eulerian perspective. Such analyses are limited in that little is understood regarding the reorientation of vorticity layers from the attached boundary layers (BL). The vortex formation is fed by both the shear layers as well as the original BL vorticity. The purpose of the present work, therefore, is to uncover the influence of the BL on the formation of these vortices. 3D Particle Tracking Velocimetry (PTV) experiments have been performed so as to measure the flow around a low aspect ratio plunging flat plate. From the PTV results, Lagrangian structures have been identified that originate from the plate surface. The mass contribution of the boundary layer to the formation of the LEV and TV is discussed. Moreover, the reorientation of the BL vorticity during the process is tracked. By studying the reorientation of mass-containing vorticity, the close connection between the BL at the first instant of motion, and the salient vortices at later stages of the formation process is illustrated. Finally, the Lagrangian structures are compared with direct-force measurements to elucidate the influence of the BL vorticity distribution on the unsteady loadings. [Preview Abstract] |
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