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 E40: Flow Instability: Vortex Flows |
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Chair: Paul Fontana, Seattle University Room: Sheraton Back Bay D |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E40.00001: Towards an understanding of vortex shedding frequency in conventional and quasi-two-dimensional flows. Paul W. Fontana I investigate mean flows and the role played by surface friction and surface tension in generating them in a quasi-two-dimensional vortex shedding experiment, thereby elucidating the connection between quasi-two-dimensional effects and shedding frequency. We have previously shown that quasi-two-dimensional effects in a vertical soap film channel produce anomalously low frequencies compared with conventional observations, and that the Strouhal number ($\mbox{St} = fD/U_\infty$, where $f$ is the shedding frequency, $D$ the cylinder diameter, $U_\infty$ the upstream flow speed) is not uniquely determined by the Reynolds number ($\mbox{Re} = DU/\nu$, where $\nu$ is the kinematic viscosity) [\em Bull.~Amer.~Phys.~Soc.\em\,~\textbf{57}(17), R10.7 (2012)]. Vortex shedding by circular cylinders is an archetypal flow instability, yet its physical mechanism remains poorly understood. There exists no rigorous theory predicting the shedding frequency, but evidence points to nonlinear mutual interaction between the mean flow and the shedding mode. I explore how quasi-two-dimensional effects influencing the shape the mean flow may therefore be responsible for the shedding behavior seen in the experiment. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E40.00002: Viscoelastic Fluid-Structure Interactions Anita Dey, Jonathan Rothstein, Yahya Modarres-Sadeghi When a flexible object such as an elastic sheet is placed perpendicular to the flow of a Newtonian fluid, the structure can oscillate due to the shedding of separated vortices at high Reynolds numbers. If the same flexible object is placed in non-Newtonian flows, however, the structure's response is still unknown. Unlike Newtonian fluids, the flow of viscoelastic fluids can become unstable at infinitesimal Reynolds numbers due to a purely elastic flow instability. In this talk, we will discuss the fluid-structure interaction between a wormlike micelle solution at high Weissenberg number and a flexible elastic sheet in cross flow. Elastic flow instabilities have been observed for wormlike micelle solutions in a number of flows. Here we will study what happens when elastic flow instabilities occur in the vicinity of a thin flexible polymer sheet. We will show that the time varying fluid forces exerted on the structure can grow large enough to cause a structural motion which can in turn feed back into the flow to modify the flow instability. The static and dynamic responses of the flexible sheet will be presented for a series of flexible sheets oriented at different angles to the flow direction, for varying fluid flow rates, and for varying fluid compositions and properties. In addition, measurements of velocity profiles and flow-induced birefringence will be presented in order to quantify the time variation of the flow field and the state of stress in the fluid. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E40.00003: Azimuthal instability of vortex rings generated by an oscillating disk Jian Deng, C. P. Caulfield We report the instabilities of vortex rings generated by an oscillating disk. Assuming sinusoidal variation in the azimuthal direction with mode number, $m$, a Floquet linear stability analysis is performed. We study the dynamics for a range of the two control parameters, the Keulegan-Carpenter number $KC=2\pi A/c$ and the Stokes number $\beta=fc^2/\nu$, where $A$ is the amplitude of oscillation, $f$ is the frequency of oscillation, $c$ is the diameter of the disk, and $\nu$ is the kinematic viscosity of the fluid. We observe two distinctive flow regions in the ($KC,\beta$) parameter space. First, in the low $\beta$ region, the flow breaks its symmetry with a single wavenumber mode getting a positive growth rate. Second, in the high $\beta$ region, high-order unstable modes emerge, with the highest mode number $m=9$ recorded. Furthermore, we carry out Direct Numerical Simulations (DNS) on the fully three-dimensional Navier-stokes equations. The results reproduce the main features of the high-order unstable modes predicted by the Floquet analysis, exhibiting the highest mode number $m=6$. We conjecture that the inconsistence in the highest mode number between the Floquet linear stability analysis and the DNS implies the non-linear characteristic of the current problem. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E40.00004: Liner stability analysis of the two-dimensional Taylor-Green vortices in a stratified flow Shota Suzuki, Makoto Hirota, Yuji Hattori The linear stability of the two-dimensional Taylor-Green vortices in a stratified fluid is studied by modal stability analysis and short-wavelength stability analysis. By modal stability analysis it is found that the growth rate of the most unstable mode depends on the horizontal Froude number $F_{h} $ and the stratification effects on the growth rate change as $F_{h} $ becomes small or stratification becomes strong. There are three regions of $F_{h} $ where the stratification effects are different: the stabilizing region where the elliptic instability is dominant at large $F_{h}$, the region where the growth rate has maximum, the slightly destabilizing region where the zigzag instability is dominant at small $F_{h}$. In order to reveal the mechanism of the behavior of the growth rate in the second region, we investigate the local stability of the flow near the vortex center and the flow near the boundaries between vortices by short-wavelength analysis. As a result, it is found that the competition between stabilizing elliptic instability near the vortex center and destabilizing hyperbolic instability near the boundaries occurs in the weakly stratified region. The relation between modal stability and the competition of short-wavelength stabilities will be discussed. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E40.00005: Vorticity amplification and its effects on flow separation from simplified landing gear wheels Philip McCarthy, Graham Feltham, Alis Ekmekci In the presence of weak streams of inbound vorticity, the stagnation region of bluff bodies have been shown to support mechanisms for the collection and amplification of said vorticity into large-scale, discrete vortex structures. For extremely low aspect ratio cylinders, such as those which represent simplified aircraft landing gear wheels, these discrete vortex structures tilt around the sides of the geometry, orientating their axes in the streamwise direction. Once the oncoming vorticity is collected and amplified into discrete vortices, they are shed from the stagnation region and this cycle repeats itself periodically. The present work investigates the effect of the vortex tilting and subsequent shedding on the behaviour of the outboard side flow separation region present on simplified landing gear wheels. Experiments were conducted in a recirculating-type water tunnel on a two-wheel landing gear model, with the upstream vorticity source being a 100 µm platinum wire. Hydrogen bubble visualisations were first used for qualitative understanding of the flow, accompanied by 2D-PIV for vortex identification and tracking of the growth and movement of the observed structures. Finally, the side separation bubble has been characterised using 3D velocity measurements (using V3V). [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E40.00006: ABSTRACT WITHDRAWN |
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