Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H18: Vortex Dynamics: Dipoles, Pairs and Instabilities |
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Chair: Ellen Longmire, University of Minnesota Room: 2004 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H18.00001: Interaction of a vortex dipole with a deformable cantilevered plate Eugene Zivkov, Serhiy Yarusevych, Sean Peterson The coupled interaction of a vortex dipole impacting the tip of a deformable cantilevered plate is investigated both numerically and experimentally. Numerically, a strongly coupled fluid-structure interaction code is used to simulate the impact at three dipole Reynolds numbers, \textit{Re} $=$ 500, 1500, and 3000. These Reynolds numbers are representative of flows over small-scale energy harvesting devices, and the plate properties model an ionic polymer-metal composite. Of particular interest is the vortex dynamics and the attendant plate response, with the underlying implications to energy harvesting. As the dipole approaches the plate, secondary vortical structures are generated at the plate, with finer structures present at higher Reynolds number. The dipole breaks up after the initial impact, which is followed by complex vortex interactions of secondary structures. The initial impact produces the largest plate deflection, followed by a more complex response attributed to plate interaction with multiple secondary vortices. The plate response to the initial impact is not strongly dependent upon the Reynolds number. However, the secondary vortex dynamics, and the associated plate loadings, exhibit strong Reynolds number dependence. To validate the numerical results, a similar dipole-plate interaction is modelled experimentally and characterized using flow visualization and time resolved, planar particle image velocimetry. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H18.00002: Numerical Study of Transport in Viscous Vortex-Dipole Flows Ling Xu, Robert Krasny Material transport in viscous vortex-dipole flows is studied numerically using a high order finite difference method and the Lamb-dipole as the basic unit in the initial condition. The vorticity field and streamline pattern are displayed, and material curves are tracked in order to visualize particle trapping and detrapping in the dipole. Results are presented for evolution of one dipole, and the interactions of two and three dipoles. [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H18.00003: Interaction of monopolar and dipolar vortices with a shear flow: a numerical study Leon Kamp, Vitor Marques Rosas Fernandes, Gert-Jan van Heijst, Herman Clercx Interaction of large-scale flows with vortices is of fundamental and widespread importance in geophysical fluid dynamics and also, more recently for the dynamics of fusion plasma. More specifically the interplay between two-dimensional turbulence constituted by a collection of unsteady eddies and so-called zonal flows has gained considerable interest because of the relevance for transport and associated barriers. We present numerical results on the interaction of individual monopolar and dipolar vortices with typical sheared channel flows (Couette and Poiseuille). Contrary to monopolar vortices, dipolar ones tend to retain their compactness while propagating through the shear flow along curved pathways without much distortion. Simulations on the interaction of a driven turbulent field with mentioned channel flows are used to explore the suppression of turbulence and turbulent transport and the pronounced role played by the boundaries on these. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H18.00004: Interaction of dipolar vortices with a shear flow: experimental results Vitor Marques Rosas Fernandes, Leon Kamp, Herman Clercx, Gert-Jan van Heijst Interaction of large-scale flows with vortices is of fundamental and widespread importance in geophysical fluid dynamics and also, more recently for the dynamics of fusion plasma. More specifically the interplay between two-dimensional turbulence constituted by a collection of unsteady eddies and so-called zonal flows has gained considerable interest because of the relevance for transport and associated barriers. We present an experimental study with a two-fluid-layer setup of the interaction of Lamb-like dipolar vortices with a quasi-two-dimensional channel flow that is driven electromagnetically. Dipoles are injected into the sheared flow perpendicularly and obliquely. Using particle image velocimetry we evaluate the evolution of the dipolar vorticity. Results are confronted with two-dimensional numerical simulations. Dipoles turn out to be quite robust structures despite the shearing action imposed by the background flow. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H18.00005: Embbeded dipolar vortices driven by Lorentz forces in a shallow liquid metal layer Cinthya G. Lara, Sergio Cuevas We present an experimental and numerical study of the vortex pattern that results from the action of a localized Lorentz force in a thin liquid metal layer (GaInSn) contained in a square box. The fluid motion is generated by the interaction of a uniform D.C. current and a non-uniform magnetic field produced by square-shaped permanent magnet much smaller that the container. Unlike the simple vortex dipole created by a localized Lorentz force in a layer of electrolyte, a more complex vortex pattern is formed in a liquid metal layer. Experiments show the appearance of two ``embedded'' vortex dipoles with a quasi-stagnat zone in the region of highest magnetic field intensity. The observed pattern can be explained by noticing that the localized magnetic field acts as a magnetic obstacle for the imposed flow. Using the Ultrasonic Doppler Velocimetry technique, we obtained the velocity profiles along the symmetry axis. We developed a quasi-two-dimensional numerical model that takes into account the effect of the boundary layers adhered to the bottom wall, the Hartmann friction and the induced effects. Numerical simulations show a satisfactory qualitative and quantitative agreement with the experimental results. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H18.00006: Deflection of a vortex pair by a flat plate Monika Nitsche, Jason Archer We investigate the inviscid evolution of a counterrotating vortex pair in the presence of a flat plate. The plate is positioned downstream of the initial vortex pair position, centered on its trajectory. If the plate lies normal to the incoming vortex trajectory, the vortices travel around the plate and leave on the opposite side without changing direction. If the plate is inclined relative to the incoming vortex pair, the vortices are deflected and leave the plate at an angle. Changes in the outgoing angle are highly sensitive to changes in the plate inclination, which under certain conditions lead to singular behaviour. The observations are applied to separate an incoming stream of vortex pairs. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H18.00007: Interaction regimes of unequal viscous vortex pairs in the presence of background shear Patrick Folz, Keiko Nomura The interaction of two co-rotating viscous vortices in linear background shear is investigated through two-dimensional numerical simulations. In general, equal co-rotating viscous vortices will merge if brought within a critical separation distance. The mutually induced strain causes core detrainment which eventually leads to mutual entrainment and the flow transforming into a single vortex with combined strength. Unequal vortices, depending on the degree of asymmetry, may or may not merge depending on the relative timing of core detrainment and core destruction. When background shear is present, advective motion of the vortices is altered. With sufficiently strong adverse shear, the vortices will separate. Merger may be enhanced or inhibited by favorable or adverse shear respectively. Prior studies of interacting invsicid pairs identified several interaction regimes based on a merging efficiency, i.e., the circulation of the final vortex (or vortices) relative to the initial circulation. Here, a similar method is developed for viscous flows, and is used to objectively identify the observed interaction outcomes. A categorization of possible interactions is presented. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H18.00008: Evolution of Vortex Pairs Subject to the Crow Instability in Wall Effect Daniel Asselin, C.H.K. Williamson In this research, we examine the effect of a solid boundary on the dynamics and instabilities of a pair of counter-rotating vortices. An isolated vortex pair is subject to both a short-wave elliptic instability and a long-wave Crow (1970) instability. Near a wall, the boundary layer that forms between the primary vortices and the wall can separate, leading to the generation of secondary vorticity. In the present study, we are examining the long-wave Crow instability as it is modified by interaction with a wall. Several key features of the flow are observed. Strong axial flows cause fluid containing vorticity to move from the ``troughs'' of the initially wavy vortex tube to the ``peaks.'' This process is associated with distinct differences in vortex concentration at the peak and the trough, which lead to the establishment of an axial pressure gradient. Furthermore, the primary and secondary vortices interact to form additional small-scale vortex rings. The exact number and orientation of these small-scale rings is highly dependent on the extent to which the Crow instability has developed prior to interaction with the ground. Finally, significant changes to the vortex dynamics, including circulation, core size, and topology, are also observed during and after interaction with the boundary. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H18.00009: Kelvin-Helmhotz instability and B\'enard-Von Karman vortex street in a confined geometry Luc Lebon, Paul Boniface, Mathieu Receveur, Laurent Limat We have experimentally investigated the appearance of Kelvin-Helmhotz vortices in a confined geometry: in a closed rectangular tank a tape is pulled at high speed on the water surface. This induces a flow in the same direction as the tape, and by conservation a backward flow in the opposite direction. With an appropriate choose of the experiment parameters (water height, tape speed) the backward flow takes place on the sides of the tank: this creates a strong shear that can induces a Kelvin-Helmhotz instability on each side of the tank. As long as the tape width stays small enough compared to the tank width, we can observe the appearance of well organized vortex rows on each sides of the tank. In this case, the vortex rows are coupled like a B\'enard-Von Karman vortex street, but without the classical forcing of a wake behind an obstacle. All our experiments are in agreement with a theoretical prediction by Rosenhead which extended the B\'enard-Von Karman vortex street stability calculation to a confined geometry. Our work seems to be one of the first experimental verification of this 80 years old model. [Preview Abstract] |
Monday, November 24, 2014 12:27PM - 12:40PM |
H18.00010: First instabilities of the wake behind a rotating sphere Jose Eduardo Wesfreid, Maciej Skarysz, Sophie Goujon-Durand, Jacek Rokicki The wake behind a sphere, rotating about an axis aligned with the streamwise direction, has been experimentally investigated in a water tunnel using LIF visualizations and PIV measurements. The measurements focused on the evolution of the flow regimes that appears depending of two control parameters, namely the Reynolds number Re and the dimensionless rotation or swirl rate $\Omega$ which is the ratio of the maximum azimuthal velocity of the body to the free stream velocity. In the present investigation, we covers the range of Re smaller than 400 and $\Omega$ from 0 and 1.5 . Different wakes regimes such as an axisymmetric base flow, a low frequency frozen state, and an single and double helicoidal mode are represented in the (Re, $\Omega$) parameter plane. [Preview Abstract] |
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