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 EP: Instability: Jets & Wakes II |
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Chair: Paul Durbin, Iowa State University Room: 200D |
Sunday, November 22, 2009 4:15PM - 4:28PM |
EP.00001: Near Wake Flow Topology of a Blunt Trailing Edge Profiled Flat Plate Lakshmana Sampat Doddipatla, Arash Naghib Lahouti, Horia Hangan, Kamran Siddiqui Wake flows behind two dimensional bodies are unstable due to formation of spanwise von Karman vortices accompanied by three dimensional streamwise instabilities, also referred to as rolls and ribs, respectively. These three dimensional instabilities lead to two distinct instability modes (Mode A and Mode B), or a combination of the two, depending on the flow Reynolds number and the profile geometry. It has been observed that the ribs wrap around the rolls, progressively distorting them. Therefore, enhancing the action of streamwise vortices can lead to early suppression of the spanwise von Karman vortices accompanied by the reduction of fluctuating lift and base drag. The present investigation seeks to better understand these near wake instabilities for blunt trailing edge profiled bodies of various aspect ratios, for flow Reynolds numbers ranging from Re(d)=500 to Re(d)=2200, and various inlet conditions. Planar Laser Induced Fluorescence (PLIF) visualizations and measurements are performed in the near wake to study and characterize the topology of streamwise and spanwise vortices. [Preview Abstract] |
Sunday, November 22, 2009 4:28PM - 4:41PM |
EP.00002: ABSTRACT WITHDRAWN |
Sunday, November 22, 2009 4:41PM - 4:54PM |
EP.00003: Bifurcations in axisymmetric wakes and their stabilization by base bleed Enrique Sanmiguel-Rojas, Patricio Boh\'{o}quez, Alejandro Sevilla, Carlos Mart\'Inez-Baz\'an We investigate the instability properties of the laminar incompressible flow around a cylindrical body with a rounded nose and length-to-diameter ratio 2, at zero angle of attack, combining experiments, three-dimensional direct numerical simulations and a global linear stability analysis. The direct numerical simulations and the global stability results are in excellent agreement in their prediction of a first stationary and three-dimensional bifurcation in the wake, which takes place at $Re_{c1}\approx$ 325. Moreover, both the experiments and the numerical simulations show the existence of a second oscillatory bifurcation at $Re_{c2}\approx$ 400. However, although the global stability analysis does also predict the existence of an oscillatory bifurcation, it is at a considerably larger value of $Re\approx$ 518. The disagreement between the global linear analysis and the experimental and numerical results in the prediction of the oscillatory bifurcation is investigated and justified in terms of the flow field used to performed the stability analysis. In addition, we report the existence of two critical values for the bleed coefficient, defined as the bleed-to-free-stream velocity ratio, to re-stabilize the wake to its axisymmetric steady state. [Preview Abstract] |
Sunday, November 22, 2009 4:54PM - 5:07PM |
EP.00004: Surface signatures of momentum/zero-momentum wakes in stratified fluids S.I. Voropayev, H.J.S. Fernando, C. Nath The momentum (over- and under-thrusted) wakes of self-propelled bodies in stable density stratified fluids were studied using scaled laboratory experiments and theoretical analysis. Particle image velocimetry (PIV) and a highly sensitive cooled Infra-Red (IR) camera were used for flow diagnostics. Two problems of broad interest that have not been received adequate study were the focus of this research: (i) thermal (IR) surface signatures of wakes of submerged/surface self-propelled bodies; and (ii) thermal (IR) and hydrodynamic (PIV) surface signatures of submerged/surface turbulent jets. The measurements and modeling delineated the mechanisms of vertical turbulent transport, the types of flow structures formed at the surface and their dependence on bulk wake properties on governing parameters. The physics-based models and parameterizations so developed help extrapolate laboratory results to oceanic environments. [Preview Abstract] |
Sunday, November 22, 2009 5:07PM - 5:20PM |
EP.00005: Vortex statistics in a bluff body wake with varying inlet conditions Bengt E.G. Fallenius, Jens H.M. Fransson For many years the flow in the wake behind bluff bodies have been the subject of extensive studies. One reason is the interesting flow phenomena that occur therein, such as the von K\'{a}rm\'{a}n vortex shedding. Another is the reduced pressure that contributes highly to the drag force. Learning how to control the vortices and increasing the pressure in the wake can lead to reduction of noise and vibrations in structures as well as improved energy efficiency for vehicles. This study investigates experimentally how changes of the boundary layer on the bluff body, i.e. the inlet condition for the wake flow, affects the vortex structures in the wake. The bluff body boundary layer is modulated by applying varying strengths of continuous suction or blowing through the surface of the bluff body, which is made of a permeable material. A high number of instantaneous velocity fields of the wake flow have been acquired by means of particle image velocimetry. Each velocity field has then been processed by a computer program, developed in order to detect, analyze and store information about the characteristics of small-scale vortices. Statistics for the vortex structures in the bluff body wake have then been compared for different inlet conditions. [Preview Abstract] |
Sunday, November 22, 2009 5:20PM - 5:33PM |
EP.00006: Physical reduced model for the flow past a circular cylinder for $47<$\bf{\emph{Re}}$<100$ Iago C. Barbeiro, Ivan Korkischko, Julio R. Meneghini, J.A.P. Aranha The \emph{Reynolds} ($Re=\frac{U.D}{\nu}$) range considered for this study lies within the time-periodic bidimensional r\'{e}gime where any experiment started by its stationary solution should evolve to a stable limit-cycle. This transient oscilatiory ramp starts with the exponential growth of the linear unstable eigenmode and finishes bounded by nonlinear effects with multi-harmonics extra dissipation. The steady solution and the leading eigenmode are numerically obtained using FEM discretization (\emph{Taylor-Hood P2/P1 elements}) and \emph{Arnoldi} iterations, then the nonlinear evolution operator is employed to generate new modes complementing the linear eigenmode up to a given order. The full NSE is then projected onto this physical base (\emph{nonlinear Galerkin projection}) leading to a physical reduced system. This reduced model has a simple framework to track many nonlinear features like meanflow evolution and energy changes between the harmonics, clarifying the nonlinear mechanisms that takes this system to a periodic orbit. Numerical and experimental (\emph{Particle Image Velocimetry}) evidences will be presented at the time of the meeting. [Preview Abstract] |
Sunday, November 22, 2009 5:33PM - 5:46PM |
EP.00007: Floquet stability analysis of the wake of a circular cylinder undergoing VIV Rafael S. Gioria, Julio R. Meneghini In some flow situations, bluff bodies undergo vortex-induced vibration (VIV). The amplitude of oscillation of a circular cylinder undergoing vortex-induced vibration has a three branch response in respect to the reduced velocity ($V_R = U_\infty / d f$, where $U_\infty$ is the free stream velocity, $d$ the cylinder diameter and $f$ the natural frequency of the oscillation of the cylinder). One remarkable feature of the behavior is the jump in the amplitude between the upper and the lower branch: hysteresis is observed on this jump. This hysteretic phenomena is still subject of investigation. In this work, we investigate the circular cylinder three-dimensional wake behavior on the three different branches of response with the intention of acquiring better understanding of the hysteretic jump on the amplitude response of VIV. In order to realize this investigation, Floquet stability analysis of the two-dimensional wake of a circular cylinder in VIV is conducted. We choose two Reynolds numbers ($\mathrm{Re}$) close to the secondary transition observed on the wake of a circular cylinder undergoing imposed oscillations: $280$ and $300$. The stability analysis is realized for each of the three branches of VIV response. We expect to observed subcritical unstable modes in the hysteretic region as these modes also present hysteresis on the transition. [Preview Abstract] |
Sunday, November 22, 2009 5:46PM - 5:59PM |
EP.00008: Characterizing cylinder and hydrofoil wake dynamics Morten Kjeldsen, Bjarte G. Seim, Roger E.A. Arndt A number of high speed PIV measurements of wakes trailing a NACA 0015, c=0.081m, and a cylinder D= 0.0127m, in the speed range 2 through 9 m/s have been made in the high speed water tunnel at SAFL- UMN. The cylinder vortex shedding follows closely St=0.2, while that off the hydrofoil is more irregular. Although the hydrofoil shows a more irregular nature a measure for both shedding frequencies and vortex strength of is of great interest also for drag analysis. The direct approach mapping individual structures, e.g. vorticity based analysis, can be obscured by the quality of the measurements made, hence other methods to reveal frequency and strength are in demand. A study were the mapping of time variation of the main flow direction impulse flux integrated over the wake at, minimum two, downstream positions has been made. A cross-correlation analysis of the impulse flux can reveal structure transport speeds, the frequency spectrum will reflect the shedding frequency, while the temporal variation represents the strength. For the hydrofoil it's shown that a significant cross- correlation is present. In terms of spectra even the more structured shedding from cylinders are hard to capture, and finally the vortex strength found using the cited algorithm seems somewhat unreliable. A thorough comparison between the suggested measure and traditional measures is given. [Preview Abstract] |
Sunday, November 22, 2009 5:59PM - 6:12PM |
EP.00009: A New Mechanism for Mixing Enhancement in Turbulent Mixing Layer Wei Zhao, Guiren Wang It is well known that in shear layer, even under the most effective modes---sub-harmonic modes, the mixing under active forcing is only nearly two times enhanced compared to that without forcing, and saturation will be reached for the further increased forcing amplitude. Hence, the potential of the mixing enhancement is limited for active forcing based on receptivity and instability in mixing layer. Recently we observed that, in a confined mixing layer in a pipe, such saturation could be overcome, so that extremely fast mixing enhancement could be achieved by further increase of the forcing amplitude. However the mechanism behind the high receptivity and fast mixing is still not clear. Preliminary data shows that the optimized forcing frequency does not change with Reynolds number based on bulk flow velocity and pipe diameter in the range of moderate Reynolds number. Therefore we postulate that the phenomenon might be related to the resonance frequency of the pipe flow and the frequency corresponding to the fast mixing augmentation should be scaled with some Strouhal number. The dependence of the optimized frequency on various parameters is investigated. [Preview Abstract] |
Sunday, November 22, 2009 6:12PM - 6:25PM |
EP.00010: Reduced Order Modeling for Beam Propagation through a Shear Layer Jurgen Seidel, Casey Fagley, Stefan Siegel, Thomas McLaughlin The performance of airborne platforms emitting or receiving light beams is severely hampered by the flow field around the turret mounted on the air vehicle. From a fluid dynamics point of view, the flow separating from the turret develops large, coherent structures. The goal of this research is to improve system performance by mitigating these structures using feedback flow control. While developing a feedback flow control system is a multi-step process, the most important step is the design of a Reduced Order Model of the flow field under consideration. A blowing and suction slot is used to actuate the flow field. Three dimensional simulations have been resolved and show a large amount of controllability of the optical path difference (OPD). Proper orthogonal decomposition techniques are then applied to open loop simulation data. A low dimensional model is realized by non linear system identification techniques. The accurate model of the flow field is then utilized to develop control strategies to mitigate the optically detrimental coherent structures and simulate closed loop behavior of the flow field. The chosen control algorithm will be simulated in a CFD environment for verification. [Preview Abstract] |
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