Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session KQ: Wake Stability |
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Chair: Hui Hu, Iowa State University Room: Hilton Chicago Stevens 2 |
Monday, November 21, 2005 4:10PM - 4:23PM |
KQ.00001: Wake development and control for an airfoil with blunt and divergent trailing edge M. El Gammal, H. Hangan, B.E. Thompson The wake development downstream of an airfoil with a blunt and divergent trailing edge is experimentally investigated with conventional hot-wire anemometry. Two distinct wake development regions are identified. (i) a near-wake region where the vortex shedding is robust, the wake is highly asymmetric and the wake mean flow direction is curved; (ii) a far-wake region where momentum thickness reaches an asymptotic value, distributions of mean flow and turbulence quantities are almost symmetric, curvature of the mean flow becomes negligible and self-preserving state is reached. The effect of attaching rectangular vortex generators to the pressure and suction sides of the blunt trailing edge on the vortex shedding phenomena is quantified. The results clearly indicate vortex shedding suppression when the vortex generators are placed at a distance that equals twice the integral length scale in the spanwise direction. Based on these results, it is concluded that the streamwise components of the horseshoe vorticies generated by the vortex generators are responsible for the early suppression of the von Karman rolls; hence weakening the vortex shedding and accelerating the flow transition toward the far wake state. The effectiveness of this mechanism depends on the vortex generators placement in the spanwise direction. [Preview Abstract] |
Monday, November 21, 2005 4:23PM - 4:36PM |
KQ.00002: Experimental Measurements of a Model Submarine Wake Damien Bretall, Deborah Furey, Paisan Atsavapranee, Kimberly Cipolla High resolution stereo-PIV measurements were made over ten body lengths downstream of a 1/18$^{th}$ scale submarine model in the Deep Water Tow Basin at NSWCCD. The submarine model is an unclassified generic submarine shape (ONR Body-1) composed of an axisymmetric body, four stern appendages (control surfaces) and a propeller. This body is 5.8 m long, 0.49 m in diameter. Block gages on the struts measured streamwise force on the body and provided loading details for setting propeller speed. The model was towed through a stationary laser sheet oriented perpendicular to the tow direction to obtain three-dimensional velocity fields. The objective of the study was to quantify the submarine wake and rate of decay of the coherent vortices. These data will be used in conjunction with measurements obtained on a model towed array to validate computational models for array shape and dynamics. Results with and without the propeller will be presented. Approximately 40 instantaneous vector fields were obtained for each location. Mean and fluctuating streamwise and cross-stream velocities and vorticity were computed. [Preview Abstract] |
Monday, November 21, 2005 4:36PM - 4:49PM |
KQ.00003: Critical properties of forced wakes Gilles Bouchet, Benjamin Thiria, Jos\'{e} E. Wesfreid We present direct numerical simulations of a flow behind an oscillating cylinder around its axis, at moderate Reynolds number. This flow geometry represents the very typical situations observed in flow control studies. We worked on lock-in and non lock-in regimes and, in this latter case, we analyzed the critical behavior of the global mode as a function of the forcing amplitude, as well as the forcing frequency. We confirmed the results of previous experimental works dealing with the scaling properties of theses global modes, scaling with the growth rate of perturbations, themselves depending on forcing parameters. Owing to the scaling, we have been able to renormalize the global modes. [Preview Abstract] |
Monday, November 21, 2005 4:49PM - 5:02PM |
KQ.00004: Late-wake Vortices of Maneuvering Bodies in Stratified Fluids Richard Morrison, Sergey Voropayev, Harindra Fernando Laboratory experiments were conducted in a large flow facility to investigate the formation and evolution of large vortices that form in stratified late wakes of maneuvering and self-propelled bodies. The maneuvers included acceleration, deceleration and turning, whence a significant momentum is imparted to the fluid. Previous small-scale experiments conducted at Re = 1000 [Phys. Fluids, 1999, 11(6), 1682] showed that the late-wake vortices of maneuvering bodies are much bigger and different in dynamics and morphology from that of the steadily moving bodies. The present experiments delved into such differences at much higher Reynolds numbers (Re = 50,000). New findings include: the effect of internal wave radiation on momentum balance in the wake, surface signatures of stratified wakes, and transverse-propagating dipoles that form during the motion of bodies in curved paths. State of the art flow diagnostic techniques were employed for flow measurements, and the flow phenomena and measurements were explained using simple dynamic arguments and phenomenological models. [Preview Abstract] |
Monday, November 21, 2005 5:02PM - 5:15PM |
KQ.00005: Vorticity generation and K\'arm\'an street in the flow past a magnetic obstacle Sergio Cuevas, Sergey Smolentsev, Mohamed Abdou A numerical investigation of a 2D flow of an incompressible electrically conducting viscous fluid past a localized zone of applied magnetic field, denominated a magnetic obstacle, is carried out. The applied field is produced by the superposition of two small parallel magnetized square surfaces uniformly polarized in the normal direction. Using the low magnetic Reynolds number approximation, it is shown that the flow past a magnetic obstacle may develop vortical structures and eventually instabilities similar to those observed in flows interacting with bluff bodies. In the small zone where the oncoming uniform flow encounters the non- negligible magnetic field, the induced electric currents interact with the field, creating a non-uniform Lorentz force that opposes the flow and creates vorticity. Numerical computations have been conducted for Reynolds numbers $Re=100$ and 200, and Hartmann numbers in the range $1 \le Ha \le 100$. Under these conditions, a wake is formed behind the obstacle. It may display two elongated streamwise vortices that remain steady as long as the Hartmann number does not exceed a critical value. Once this value is reached, the wake becomes unstable and a vortex shedding process similar to the one observed in the flow past bluff bodies is established. [Preview Abstract] |
Monday, November 21, 2005 5:15PM - 5:28PM |
KQ.00006: The effect of buoyancy on vortex shedding in the wake of a heated circular cylinder Zheyan Jin, Hui Hu Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) techniques were used to conduct velocity and temperature measurements in the wake of a heated cylinder to investigate the effect of buoyancy on wake behavior behind the heated cylinder. The experiment was conducted with the heated cylinder installed horizontally in the middle of a vertical water channel and approaching forced flow being downward, which results in the direction of buoyancy force being opposite to that of the approaching forced flow. The temperature and Reynolds number of the approaching forced flow were held constant during the experiment. The temperature of the heated cylinder was adjusted to let the Richardson number, which represents a ratio of buoyancy to inertial forces, varying from 0 (unheated cylinder) to about 3.8. The PIV and PLIF measurement results show that the wake behavior behind the heated cylinder changes dramatically as the Richardson number increasing. The effect of buoyancy on the wake behavior is quantified in terms of mean and fluctuation velocity and temperature distributions, vortex shedding pattern and frequency, wake closure length, drag coefficient and averaged Nusselt number of the heated cylinder. [Preview Abstract] |
Monday, November 21, 2005 5:28PM - 5:41PM |
KQ.00007: Direct numerical simulation of lock-on phenomenon in the wake of a circular cylinder Jung Yul Yoo, Ji Yong Park, Noma Park Lock-on phenomenon in the wake of a circular cylinder is investigated at the Reynolds number of 360 using direct numerical simulation. To induce lock-on, a streamwise velocity perturbation with a frequency of twice the natural shedding frequency, is superimposed on the mean velocity. The Reynolds stresses are investigated to analyze the streamwise force balance acting on the recirculation region. In the perturbed flow, the base pressure is shown to decrease mainly due to the reversal of the Reynolds shear stress. It is also shown that, with the perturbation, the strength of the primary vortices increases whereas that of the secondary vortices decreases significantly. Further, the wavelength of the secondary vortices increases by 2.5 times under the lock-on condition, which causes the three-dimensional vortical structure in the non-perturbed cylinder wake to become a two-dimensional one. [Preview Abstract] |
Monday, November 21, 2005 5:41PM - 5:54PM |
KQ.00008: Stability of inviscid vortices behind a circular cylinder Alan Elcrat, Bengt Fornberg, Ken Miller In a previous work (JFM 409(2000), 13-27 famillies of vortex patches in equilibrium with flow past a circular cylinder which is uniform at infinity were found using iterations for a nonlinear Poisson equation. These included desingularizations of the Foppl pairs. In this work we study the stability of these vortices with respect to two dimensional perturbations. In order to do this we have formulated a curve perturbation algorithm, based on the ideas of contour dynamics, which sets the normal component of velocity at a point on the boundary of the vortex patch equal to zero. The discretization is solved by a version of Newton's method; the Jacobean is factored using the singular value decomposition and a generalized inverse with the smallest singular value removed is used in the Newton iteration. This is necessary because there is always a small singular value due to the fact that there is always a nearby solution vortex in the familly. The Foppl familly is always neutrally stable with respect to symmetric perturbations in nthe sense that all of the eigenvalues are on the imaginary axis. When non symmetric perturbations are allowed there is exactly one unstable mode. A perturbation in the direction of this eigenvector implies a roll suggestive of Karman vortex shedding. [Preview Abstract] |
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