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 G10: Instability: Jets, Wakes and Shear Layers IV |
Hide Abstracts |
Chair: Matthew Juniper, University of Cambridge Room: 25C |
Monday, November 19, 2012 8:00AM - 8:13AM |
G10.00001: Resonant Oscillations of Shallow Flow Past a Cavity: Exchange Coefficients and Depthwise Variations Burak Ahmet Tuna, Egemen Tinar, Donald Rockwell Fully turbulent shallow flow past a cavity can give rise to highly coherent oscillations, which arise from coupling between: the instability of the separated layer along the cavity; and a standing gravity wave mode within the cavity. These oscillations yield large increases in the time-averaged entrainment and mass exchange coefficients between the cavity and the main flow. Such increases are due to substantial enhancement of turbulent stresses in the separated shear layer during the coupled oscillation, relative to the stresses associated with no coupling. Patterns of the flow structure have been characterized as a function of elevation above the bed (bottom surface) of the shallow flow. At elevations close to the bed, the time-averaged streamlines are deflected inwards towards their center of curvature. This streamline deflection is due to radial migration of flow along the bed, which arises from a radial pressure gradient. In addition, patterns of normal and shear Reynolds stresses are substantially altered as the bed is approached. These changes of stresses with depth are, in turn, associated with degradation of coherent, phase-averaged patterns of vortex formation in the separated shear layer. All of the foregoing aspects of the depthwise variations of the flow patterns are related to corresponding changes of the entrainment and mass exchange coefficients. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G10.00002: Shallow Flow Past a Cavity: Self-Excited Oscillations due to Resonant Coupling Maxwell Wolfinger, Donald Rockwell, Cem Ozen A fully turbulent shallow flow past a cavity can give rise to highly coherent oscillations. Coupling between the instability of the separated shear layer along the cavity and a gravity standing wave mode within the cavity results in: sharp spectral peaks of fluctuating pressure along the cavity wall; and substantial modification of the flow patterns along and within the cavity. Onset of the fully coupled, highly coherent oscillation of the shear layer-cavity system occurs as follows. As the inflow velocity along the cavity increases, the instability frequency of the separated shear layer approaches the frequency of the gravity standing wave mode. When these frequencies are coincident, the instability frequency locks-on to (remains the same as) the standing wave frequency, and highly ordered, time-dependent deflections of the free-surface occur. The peak amplitude of the unsteady pressure fluctuation occurs during this locked-on state. Moreover, quantitative imaging in the form of particle image velocimetry reveals large-scale vortex formation in the separated shear layer, which is associated with substantial changes of time- and phase-averaged flow patterns within the cavity. In turn, these features of the flow are associated with large increases of Reynolds stresses in the separated layer along the cavity. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G10.00003: LES Investigation of instabilities in cavity flow with a top boundary Aarthi Sekaran, Gerald Morrison The effect of the influence of a top boundary on cavity instabilities is studied using Large Eddy Simulations (LES). The motivation for the geometry was the flow over the cavities of a hole-pattern seal, where sudden changes in instability modes could lead to large variations in the rotordynamic stability of the system. A single, two-dimensional cavity is modeled at different conditions to study the occurrence of phenomenon such as the shear layer instability and the wake mode instability. The simulations are successfully able to capture both modes and this is verified via a spectral analysis of the data. The study also details the occurrence and development of each instability mode and discusses its physical effect on the overall flow behavior in the system. Qualitative comparisons are then made with cavities without top boundaries in order to determine particular differences due to the presence of the same. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G10.00004: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 8:52AM - 9:05AM |
G10.00005: Kelvin-Helmholtz instability in a confined geometry Paul Boniface, Luc Lebon, Mathieu Receveur, Fabien Bouillet, Laurent Limat Growth of kelvin-Helmholtz instabilities received many attention in the case of wakes or shear layers with thickness increasing in the downstream direction. In contrast with these ``unstationary'' situations, few works investigate stationary situations in which a Kelvin Helmholtz raw grows and saturates inside an imposed geometry. One of the sole exception is an experiment developed by M. Rabaud \& Y. Couder where vortices develop between concentric rotating disks. We developed an experiment in a different geometry: a recirculating belt is running at the free surface of a long rectangular tank of larger width. In some conditions a regular rectilinear raw of vortices develop between the flow dragged by the belt and the flow recirculating beside the belt. When the belt is on the central axis of the tank, with free surface on each side, recirculation flows on each side. In this case two regular raws can develop. Vortices of these two raws can interact and a 180$^{\circ}$ phase shift between them is observed. Properties of these vortices have been investigated and we tried to relate it to available models. Parallels with jet instability can be made. We also have explored the occurrence condition of the recirculation: at low reynolds number it can occur via the bottom of the tank. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G10.00006: Evolution of inviscid Kelvin-Helmholtz instability from a piecewise linear shear layer Anirban Guha, Mona Rahmani, Gregory Lawrence Here we study the evolution of 2D, inviscid Kelvin-Helmholtz instability (KH) ensuing from a piecewise linear shear layer. Although KH pertaining to smooth shear layers (eg. Hyperbolic tangent profile) has been thorough investigated in the past, very little is known about KH resulting from sharp shear layers. Pozrikidis and Higdon (1985) have shown that piecewise shear layer evolves into elliptical vortex patches. This non-linear state is dramatically different from the well known spiral-billow structure of KH. In fact, there is a little acknowledgement that elliptical vortex patches can represent non-linear KH. In this work, we show how such patches evolve through the interaction of vorticity waves. Our work is based on two types of computational methods (i) Contour Dynamics: a boundary-element method which tracks the evolution of the contour of a vortex patch using Lagrangian marker points, and (ii) Direct Numerical Simulation (DNS): an Eulerian pseudo-spectral method heavily used in studying hydrodynamic instability and turbulence. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G10.00007: 3D optimal perturbations developing in homogeneous mixing layers in presence of subharmonic vortex-pairing Adriana Lopez-Zazueta, Laurent Joly, Jerome Fontane Many experimental and numerical studies have found that the pairing of primary Kelvin-Helmholtz (KH) vortices in mixing layers generally inhibits the growth of infinitesimal three-dimensional disturbances, delaying the transition to turbulence. In this work, we investigate the existence of 3D perturbations that grow fast enough to survive the subharmonic merging instability. For this purpose, we perform a numerical study of the transient linear evolution of 3D perturbations emerging in a homogeneous time-evolving mixing layer which undergoes pairing. We look for the optimal perturbation that yields to the largest gain of energy at a specific time horizon, by the use of an optimization method which solves iteratively the linearized direct and adjoint Navier-Stokes equations. In particular, we consider the influence of the time horizon relative to the saturation times of both the primary KH and the subharmonic pairing instabilities. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G10.00008: Methodologies for solving Vortex Wave Interaction problems to obtain edge states Andrea Isoni, Hugh Blackburn, Philip Hall, Spencer Sherwin The interaction of waves with streamwise vortices (Vortex Wave Interaction Theory or VWI Theory) is relevant in motivating the ``self-sustained processes'' and in delineating perturbations to shear flows which may become either laminar or turbulent. It has been recognised that a streamwise vortex velocity field (U,V,W) can be decomposed into an O(1) axial U-component, ``streak'' field, and a O($R^{-1}*$) roll field (V,W). As identified by Hall and Smith 1991, an equilibrium solution can be produced by an interaction of the non-linear wave terms with the roll field within the critical layer. VWI theory has been investigated numerically on a Couette flow using three different approaches, which we refer to as asymptotic, regularised and hybrid methods. In the first approach, the roll equations are subjected to jump conditions along the critical layer as proposed by Hall and Smith 1991. In the second approach, a body forcing, which regularises the jump conditions, is added on the roll equations as discussed in Hall and Sherwin 2010. In the third approach, a forcing term proportional to the divergence of the Wave Reynolds Stresses is imposed on the roll equations. In this presentation we will discuss the merits of each of these approaches and the connection with the lower branch solution. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G10.00009: Identifying instability mechanisms in swirling shear flows by using all components of the structural sensitivity Matthew Juniper, Ubaid Qadri Four different physical mechanisms can cause or support instability in swirling shear flows (Gallaire and Chomaz 2003, PoF 15(9) 2622-2639). These are: axial shear, inertial waves, centrifugal instabilities, and azimuthal shear. In relatively simple flows, such as a Rankine vortex with plug axial flow, analytical methods can identify the physical mechanisms active in each region of the flow. In more complex flows, such as a vortex breakdown bubble, analytical methods cannot be applied and, in any case, regions of the flow are not easily delineated. When considering the stability of perturbations on top of a base flow, the structural sensitivity quantifies the effect of altering the feedback between the perturbation velocity vector and the perturbation momentum equation. We examine the nine components of this structural sensitivity, firstly for simple flows such as solid body rotation, secondly for complex swirling flows. The first analysis identifies the signature of each physical mechanism, such as the Kelvin-Helmholtz instability and the Coriolis mechanism. The second analysis compares these signatures with those found in different regions of the complex swirling flows. In this way, we identify the physical mechanisms that are active in each region of the more complex flow. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G10.00010: Experimental Study of LES Models in Turbulent Stratified Flows Duo Xu, Jun Chen Stratification caused by density difference leads to significant changes of flow structure. To achieve accurate and realistic results of stratified turbulent flows in large-eddy simulation (LES), the the behavior of subgrid-scale (SGS) models is crucial. In this study, two-dimensional high resolution velocity and scalar (density) dataset from a turbulent stratified jet, obtained through applying a combined Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) technique, is used to study the behavior of SGS models. A strong correlation between SGS stress and scalar flux is observed. In particular, variations of turbulent Prandtl number in stable stratification and unstable stratification regions are explored from the experimental data. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700