Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FO: Turbulence: Control III |
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Chair: Douglas Smith, University of Wyoming Room: Salt Palace Convention Center 251 C |
Monday, November 19, 2007 8:00AM - 8:13AM |
FO.00001: Alternation of a Pair of Streamwise Vortex by Bubbles Takamasa Suzuki, Yoshihiko Oishi, Yuichi Murai, Yuji Tasaka, Yasushi Takeda The streamwise vortices sensitively govern the turbulent momentum transport in boundary layers. In order to find out the frictional drag reduction mechanism by bubbles, we experimentally investigate the interaction between the artificial streamwise vortices and the bubbles. The spanwise and the heightwise velocity field of the vortex was obtained by Particle Image Velocimetry (PIV). When the bubbles exist inside the vortex, the spanwise interval between the vortex cores is 1.6 times longer and the enstrophy is a half times smaller than the single-phase condition. These two phenomena deduce the momentum exchange reduced by the interaction. Numerical bubble trajectory on the velocity field also has shown perpendicular oscillation that provides local negative stress around the vortices. This occurs only for small bubbles, being consistent to high persistency of the drag reduction. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FO.00002: Suppression of Vortex Induced Vibration of a Circular Cylinder Suchuan Dong Prevention of undesirable vortex induced vibrations (VIV) of cylindrical structures is important to many engineering applications. In this project we investigate the effects of three flow control schemes - suction-only, blowing-only, and a scheme combining suction/blowing - on the free oscillation of a cylinder subject to vortex induced vibrations. Results of three-dimensional direct numerical simulations show that the combined suction/blowing scheme is the most effective among the three in terms of VIV reduction, and the cylinder oscillation can be completely suppressed. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FO.00003: Lessons learned from polymer drag reduction on near-wall turbulence Yves Dubief We use the unique drag-reduction ability of polymer additives to study the response of the self-sustaining process (SSP) of near-wall turbulence. Recent numerical studies of turbulent polymer flows have demonstrated that polymers are responsible for the weakening of near-wall vortices. The intermittent interaction between vortices and polymers occurs predominantly in bi-axial extensional flows that characterize the upward and downward fluid motions induced by vortices. The present study focuses on the correlation of the elastic properties of polymer solutions and the corresponding SSP of the drag-reduced near-wall turbulence. Polymers are used to outline self-induced energy transfers that occur between vortices and bi-axial extensional flows. The study of energy fluxes through vortex cores is correlated with equilibrium states that vortices reach within the SSP in response to given elastic properties of polymer solutions. The talk will end with a comparison between polymer drag reduction and other control techniques. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FO.00004: DNS of turbulent flow past a bluff body with a compliant tensegrity surface Anish Karandikar, Thomas Bewley Direct numerical simulation (DNS) is used to study turbulent incompressible flow past a bluff body with a compliant surface. We use a 3D time-dependent coordinate transformation to account for the motion of the bluff body surface. Spatially, the flow domain is discretized using a dealiased pseudospectral method in the axial and azimuthal directions, while the radial (wall-normal) direction is discretized using a finite difference scheme. The grid is stretched in the azimuthal direction, which is handled spectrally. This leads to a unique challenge when solving the Poisson equation in the fractional step method for the time march, which we address with both multigrid and preconditioned BiCGStab algorithms. We are presently extending this flow code with a model for the compliant bluff body surface based on the ``tensegrity fabric'' paradigm which combines compressive members (bars) and tensile members (tendons) in a stable, flexible network. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FO.00005: On the Mechanisms of Shear Layer High-Frequency Control. Part I: Experimental Studies Bojan Vukasinovic, Ari Glezer The high-frequency fluidic actuation and the ensuing evolution of small- and large-scale motions in a turbulent shear layer are investigated experimentally. The high-frequency (substantially higher than the natural flow frequencies) actuation is effected through the boundary layer just upstream from the shear layer origin. The ensuing flow is characterized in the spatial and spectral domain by high-resolution diagnostic tools. It is shown that open-loop, high-frequency control modifies the shear layer energy distribution within the finite domain. Immediately downstream from the control origin, a significant increase in both production and dissipation of turbulent kinetic energy is detected, which indicates enhanced energy transfer from the mean flow. This initial domain is spatially followed by a larger domain in which the production of turbulent kinetic energy is suppressed, i.e., the shear layer becomes stabilized with respect to the turbulent kinetic energy. Beyond this spatial range of influence, significant energy content is re-established at frequencies lower that in the uncontrolled flow, as governed by the local shear layer momentum thickness. Therefore, evolution of shear layer dominant scales becomes both spatially delayed and reduced in frequency, as a consequence of the direct high-frequency control. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FO.00006: On the Mechanisms of Shear Layer High-Frequency Control. Part II: Stability Studies Z. Rusak, J.J. Choi Linear temporal and spatial stability analyses and nonlinear parabolic stability equations are used to describe the effect of upstream low- and high-frequency (HF) fluidic actuation on the dynamics of small- and large-scale motions in a base shear layer. The stability analyses show that at low forcing frequencies, the periodic vortical perturbations first grow over a distance related to position where the imposed perturbations' frequency matches twice the local natural frequency. Beyond this distance, perturbations decay exponentially. At high forcing frequencies, above twice the layer's maximum natural frequency, the perturbations always decay with distance. The nonlinear parabolic stability computations demonstrate that the nonlinear interaction between the various modes generates a zero mode change which in turn modifies the perturbations' kinetic energy (PKE) distribution in each mode. HF actuation causes in an initial domain along the layer a significant increase in layer thickness, a reduction in local natural frequency and a production of PKE. This is followed by a region, which depends on the actuation amplitude, where shear layer thickness is moderately increased and PKE is more spread and lower in magnitude. Beyond this region, the effect of HF forcing decays and large-scale, low-frequency modes reappear and dominate the flow. Results support the experimental finding of Vukasinovic {\&} Glezer (2007). The research was supported by The Boeing Company and AFRL, the FLOWCAD program, and by the NSF, Award No. ECS-0523957. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FO.00007: A generalized mean-field model for the natural and high-frequency actuated flow around a high-lift configuration Dirk M. Luchtenburg, Bert G\"unther, Bernd R. Noack, Rudibert King, Gilead Tadmor A low-dimensional model is proposed for the flow around a high-lift configuration. The resolved dynamics include natural vortex shedding, the lift increasing effect of high-frequency forcing and transients. This model utilizes steady state and transient uRANS simulation data for the extraction of an empirical basis. The form of the dynamical system has been derived from a generalized mean-field consideration. The system parameters are determined with a calibration technique. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FO.00008: Adaptive Extremum-Seeking Control of Subsonic Cavity Flows Kihwan Kim, Cosku Kasnakoglu, Andrea Serrani, Mo Samimy Flow over a shallow cavity produces strong pressure fluctuations that result from the coupling between flow and flow-induced acoustics in the cavity. These fluctuations could lead to strong resonant tones that could cause structural damages to the surfaces exposed to the flow. This work is focused on the development and implementation of extremum-seeking control to attenuate pressure fluctuations in subsonic cavity flows. First, a simple but effective feedback control law based on a reduced-order Galerkin model of the flow dynamics is implemented as an inner-loop controller to suppress the cavity flow fluctuations. The magnitude of the limit cycle in the closed-loop, obtained from a pair of surface pressure sensors, is employed as a performance output, and its static characteristics with respect to control parameters are identified analytically and experimentally. Then, a single-input single-output adaptive extremum-seeking scheme is applied to the closed-loop control in such a way that the control parameters can be tuned in real time towards the optimal value where the limit cycle magnitude is minimized. The extremum-seeking control is tested to verify its effectiveness under various flow conditions. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 9:57AM |
FO.00009: Active Control of Supersonic Jets in Ideally and Non-ideally Expanded Flow Regimes Using Plasma Actuators Mo Samimy, R. Michael Snyder, Jin-Hwa Kim, Igor Adamovich We have developed and used localized arc filament plasma actuators (LAFPAs) for control of high-speed and high Reynolds number jets. We utilize high amplitude and high bandwidth capabilities of LAFPAs to excite various instabilities of the jet. Our past effort has included successful control of Mach 0.9 jet with a nozzle exit diameter of 2.54 cm and a Reynolds number of 0.8x10$^{6}$. The current effort is focused on control of a Mach 1.3 jet operating at design and off-design flow regimes with Reynolds number over a million. The control authority and effects over various forcing Strouhal numbers and azimuthal modes in the jets with fully expanded Mach number of 1.3 (ideally expanded) and 1.4 (underexpanded) are similar to those in the Mach 0.9 jet. However, the control authority and effectiveness are significantly reduced in the jet with fully expanded Mach number of 1.2 (overexpanded). It is speculated that potential separation of the turbulent boundary layer just upstream of the nozzle exit under adverse pressure gradient and/or the oblique shock wave attached to the nozzle exit change(s) the instability characteristics of the jet rendering the control ineffective. [Preview Abstract] |
Monday, November 19, 2007 9:57AM - 10:10AM |
FO.00010: Vertical fence wake control using periodic blowing and suction of upstream flow Young-Ho Choi, Hyoung-Bum Kim In this study, we experimentally investigated that the effect of periodic blowing and suction of upstream flow on the separated flow behind the fence. The vertical fence was submerged in the turbulent shear flow and we used PIV method to measure the instantaneous velocity fields around the fence in a circulating water channel. Piston device was precisely controlled to generate the pulsating jet which is passing through the spanwise slit in front of the fence and net mass flux was zero. We changed the frequency of pulsating jet from 0.4 Hz to 1.2 Hz and maintained the area of piston was constant. The frequency and the exit velocity were determined by the velocity and the length of stroke. We divided one period of pulsating jet as 20 phases and performed phase-averaged velocity measurement. The results showed that the reattachment length continuously varied with the phase of upstream jet flow. From this study, the shortest mean reattachment length was about 60{\%} of uncontrolled reattachment length. We also found that the lower frequency was more efficient to decrease the reattachment length than the higher one with the same magnitude and the faster exit velocity was useful to control the reattachment length at the same frequency. [Preview Abstract] |
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