Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session PT: Vortex Dynamics and Vortex Flows VI |
Hide Abstracts |
Chair: Serhiy Yarusevych, University of Waterloo Room: 200H |
Tuesday, November 24, 2009 11:40AM - 11:53AM |
PT.00001: ABSTRACT WITHDRAWN |
Tuesday, November 24, 2009 11:53AM - 12:06PM |
PT.00002: Trailing vortices from low speed flyers Rye Waldman, Jun Kudo, Kenneth Breuer The structure and strength of the vortex wake behind a airplane or animal flying with a fixed or flapping wing contains valuable information about the aerodynamic load history. However, the amount of vorticity measured in the trailing vortex is not always in agreement with the known lift generated, and the behavior of these vortices at relatively low Reynolds numbers is also not well-understood. We present the results from a series of wind tunnel PIV experiments conducted behind a low-aspect ratio rectangular wing at a chord-Reynolds numbers of 30,000. In addition to wake PIV measurements measured in the cross-stream (Trefftz) plane, we measure the lift and drag directly using a six-axis force-torque transducer. We discuss how vortex size, shape, strength and position vary in time and downstream location, as well as the challenges associated with the use of PIV wake measurements to accurate determine aerodynamic forces. [Preview Abstract] |
Tuesday, November 24, 2009 12:06PM - 12:19PM |
PT.00003: Optimal passive pitch reversal via elastic coupling Michele Milano, Robert Spade, David Jurjevich We consider a prototypical experimental setup, comprising a pitching and heaving rectangular plate. The plate is heaving sinusoidally at constant frequency and variable amplitudes, and a rotational spring generates the pitching motion passively. The rotational spring is simulated by a servo motor driven by a model following controller, and a genetic algorithm optimizes the spring parameters so as to maximize the average lift produced. We present results showing the relationship between optimal parameters for linear and nonlinear springs, and we also investigate the effect of the tip region flow on optimality. PIV measurements for the optimal cases highlight critical features of the flow, showing that the passive pitch reversal is due mainly to leading edge vortices. [Preview Abstract] |
Tuesday, November 24, 2009 12:19PM - 12:32PM |
PT.00004: Evolving Structure of Tip-Vortex Generated by Helicopter Rotor Blade in a Hover Yong Oun Han, Byung Ho Park, Yong Joon Son In a hover the vortex trail forms a helical trace with respect to the vertical rotor hub axis, and the slip stream becomes contracted rather drastically within one revolution of rotor blade, making the angular momentum of the tip vortex grow quickly. And, as wake age grows, tip vortex experiences diffusion, distortion and stretching during its evolving process. Vortex diffusion proceeds continuously just after matured until disappeared, but distortion and stretching eventuates far downstream with both being correlated. At this stage the helical trace cannot be maintained. Two bladed rotor provides information about vortex distortion as well as diffusion within one revolution of both blades with simplicity. This system makes it possible to observe the change of vortex structure before and after 180 degrees of wake age within one revolution of blade. It is naturally expected that tip vortex affected by the second blade may experience the distortion including the vortex diffusion. This paper aimed primarily to investigate change of vortex structures without and with the second blade effect by the use of experimental devices. It was resulted that tip vortices generated by the first blade satisfy Landgrebe's model of their locations even after they were accelerated by the second blade in downstream. Swirl velocity components follow Vatistas' n=2 model on both regions without loss of vortex circulation. [Preview Abstract] |
Tuesday, November 24, 2009 12:32PM - 12:45PM |
PT.00005: Effect of Liquid Viscosity on the Instability of Polygonal Pattern within a Hollow Vortex Core Amr Mandour, Hamid Ait Abderrahmane, Georgios H. Vatistas, Lyes Kadem, Hoi Dick Ng The present study deals with the effect of the viscosity on the speed and limits of endurance of the polygonal pattern, observed within the hollow-core vortex generated by rotating a flat disc near the bottom of a cylindrical tank. Previous investigations have used water, where the Froude number (Fr) appears as the main control parameter. Varying the viscosity of the fluid introduce another independent control parameter, Reynolds number (Re). Using image and signal processing techniques, the influence of these two control parameters on the dynamics of the polygonal pattern is thoroughly investigated. The viscosity of the rotated fluid is varied by mixing glycerol with water and the mixture viscosity is measured using Zahn cup viscometer. The disc rotational speed is measured and controlled accurately by a PID controller loop using LabView environment. A 3-D bifurcation diagram (Fr,Re,$N)$, where the stability region of \textit{N-gon} is delimited, is given. Moreover, the influence of the viscosity on the evolution of the \textit{N-gon} speed is also obtained. [Preview Abstract] |
Tuesday, November 24, 2009 12:45PM - 12:58PM |
PT.00006: Origin and stability of radial density stratification in vortical flow Harish Dixit, Rama Govindarajan A density interface near a vortex winds into a spiral. In the absence of gravity we have shown numerically and through stability analysis that this leads to spiral Kelvin-Helmholtz and centrifugal Rayleigh-Taylor (CRT) instabilities, often reducing the life-time of the vortex dramatically. In this talk we focus on CRT instability, which is driven by centrifugal forces, so flows where the vortex core is heavier than the surroundings are expected to be unstable. Indeed, Sipp et al. (JFM, 2005) and Joly et al. (JFM, 2005), prescribing Gaussian vorticity and density profiles, find no instability in light-cored vortices. Surprisingly however, for some range of parameters, with a Rankine vortex and a step circular density jump we find that making the core lighter can lead to instability. The sharpness of the profiles are thus relevant. The mechanism for this instability, studied by constructing planar analogues in the form of stratified shear flows, bearing similarities with Craik \& Adams (JFM, 1979), will be discussed. So will the nonlinear stages of these instabilities from our direct numerical simulations. [Preview Abstract] |
Tuesday, November 24, 2009 12:58PM - 1:11PM |
PT.00007: Search for Euler Singularities with Interacting Vortex Filaments Sahand Hormoz, Michael Brenner A possible mechanism for realization of the Euler Singularity is interacting vortex filaments. Numerical simulations, however, have failed to identify initial conditions leading to a singularity. We present recent work on use of similarity solutions to assess the potential for singularity formation for arbitrary initial configurations of interacting vortex filaments. [Preview Abstract] |
Tuesday, November 24, 2009 1:11PM - 1:24PM |
PT.00008: ABSTRACT WITHDRAWN |
Tuesday, November 24, 2009 1:24PM - 1:37PM |
PT.00009: A Circulation Constraint on Enstrophy Growth in 3D Euler Robert M. Kerr, Miguel Bustamante It is shown that by careful consideration of circulation conservation in simulations of anti-parallel Euler vortices how one can discriminate between clean, potentially singular calculations and contaminated, non-singular results. For the latest case with singular behavior it is shown empirically that non-viscous enstrophy growth is consistent with the rigorous upper bound for enstrophy growth for the viscous Navier-Stokes equation, if one replaces viscosity with circulation in the argument. The new scaling laws are consistent with the older numerics presented by Kerr (1993), but many of the conclusions of the old analysis require modification. Besides a new power law for enstrophy growth, if the growth of the maximum of vorticity obeys $\|\omega\|_\infty \sim (T_c-t)^{-\gamma}$, then we find that $\gamma>1$ and that $\gamma\equiv 1$ as previously reported is unlikely. [Preview Abstract] |
Tuesday, November 24, 2009 1:37PM - 1:50PM |
PT.00010: Stability of four point vortices in a periodic strip, the ``Domm system" Vasileios Vlachakis, Hassan Aref We approach the modeling of vortex wakes and their stability by considering a system of four vortices, two of circulation $+\Gamma$, two of circulation $-\Gamma$, in a periodic strip, a system first considered by Domm in 1956. The four-degree-of-freedom ``Domm system'' can be reduced to a system with two degrees of freedom by canonical transformations (Eckhardt \& Aref, 1988). The reduced representation allows us to construct perturbations that preserve linear impulse (momentum) and kinetic energy (Hamiltonian) of the system improving upon earlier work by Dolaptschiev and Schmieden from the 1930's. We show that the only translating relative equilibria of the Domm system are the vortex streets that one already finds for two opposite vortices in a periodic strip. We also find by numerical experiments that the vortex street will dissolve into vortex pairs that escape to infinity, a mode of vortex street breakdown observed numerically by Aref \& Siggia (1981) and in soap film experiments by Couder \& Basdevant (1986). The dissolution process is extremely sensitive to the initial perturbation, suggesting that a form of chaos is involved. [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