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 BR: Vortex Dynamics and 3D Vortex Flows II |
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Chair: Israel Wygnanski, University of Arizona Room: Salt Palace Convention Center 251 F |
Sunday, November 18, 2007 10:34AM - 10:47AM |
BR.00001: Drag Reduction on a Circular Cylinder using Spatially Distributed Forcing James W. Gregory, Christopher O. Porter, Daniel M. Sherman, Thomas E. McLaughlin This work investigates the use of spatially-distributed open-loop forcing for control of vortex shedding from a circular cylinder. Force-shaped plasma actuators were used to control the flow, with the aim of reducing drag on the circular cylinder at a Reynolds number of 6500. Traditional approaches to cylinder drag reduction have typically involved two-dimensional forcing of the flow field using blowing and suction. Spatially-distributed forcing, however, involves a spanwise modulation of the forcing on the flow. Kim and Choi (Phys. Fluids 17, 033103, 2005) showed in their computations that a spanwise distribution of blowing and suction significantly altered the spanwise development of vortex shedding, reduced the strength of the vortices, and reduced the drag by 40{\%}. The current experiments implement the method of Kim and Choi with a new type of plasma actuator where the momentum addition can be directed either normal or tangential to the surface, as well as spatially tailored in a spanwise fashion to optimize control efficacy. Force-shaped plasma actuators were applied to a 2-inch diameter circular cylinder and wake profile measurements were made at several spanwise locations to evaluate the resulting drag reduction and modification of the wake structure. [Preview Abstract] |
Sunday, November 18, 2007 10:47AM - 11:00AM |
BR.00002: Airfoil Pitch Control Using Trapped Vorticity Concentrations Daniel Brzozowski, John Culp, Ari Glezer Closed-loop feedback control of the attitude of a free pitching airfoil is effected without moving control surfaces by alternate actuation of nominally-symmetric trapped vorticity concentrations on the suction and pressure surfaces near the trailing edge. The pitching moment is varied with minimal lift and drag penalties over a broad range of angles of attack when the baseline flow is fully attached. Accumulation (trapping) and regulation of vorticity is managed by integrated hybrid actuators (each comprised of a miniature [O(0.01c)] obstruction and a synthetic jet actuator). In the present work, the model is trimmed using a position feedback loop and a servo motor actuator. Once the model is trimmed, the position feedback loop is opened and the servo motor acts like an inner loop control to alter the model's dynamic characteristics. Position control of the model is achieved using a reference model-based outer loop controller. [Preview Abstract] |
Sunday, November 18, 2007 11:00AM - 11:13AM |
BR.00003: On the Near Wake of an Inclined Circular Cylinder with Active Flow Control Lutz Taubert, Israel Wygnanski Coherent structures in the near wake of a circular cylinder at different angles of inclination were investigated by 3D-PIV. Active flow control in the form of oscillatory, zero mass flux excitation generated by internal actuators was applied through two slots located symmetrically on the circumference of the cylinder or a single slot at varying positions. The diameter was 3" with L/D larger than 16, the Reynolds number for the flow normal to the cylinder was 30000. Pressure measurements were taken in the wake and on the surface to determine the forces on the model. Drag and lift were controlled by periodic perturbations emanating either symmetrically from a pair of slots or a from single slot. In addition to the changes in the radial flow patterns significant modifications of the axial flow were observed. Single pulse and short wavetrain perturbations were applied to investigate the stability of the flow. Single-point hotwire measurements were taken for time-resolved analysis. Phase-locked 3D-PIV data was acquired to access the spatial development of the introduced disturbances in the near wake. [Preview Abstract] |
Sunday, November 18, 2007 11:13AM - 11:26AM |
BR.00004: Control of cellular vortex shedding over a cone O. Ramesh, Rahul Chopde Vortex shedding over 3D geometries is known to be complex involving the so-called cellular vortex shedding phenomenon, wherein cells of constant frequency are formed. In the present work we consider the passive control methodology of Strykowski and Sreenivasan (of placing a small control cylinder outside the undisturbed wake of the main cylinder) to the cellular shedding problem. It is interesting that this control methodology which was originally meant for 2D flows works well in the present 3D flows also. Similarly, the vortex shedding was quenched by this methodology even for other cellular shedding configurations such as a uniform flow over a stepped cylinder or a shear flow over a uniform cylinder also. The smearing of vorticity due to the control rod (envisaged in the 2D scenario of Strykowski and Sreenivasan hypothesis) may be expected to be non-uniform all over the span in a 3D geometry as the gap between the main cylinder and the control cylinder varies with the span. It is hypothesised that the control rod works on the global instability modes and alters their growth rates all across the span even when the gap between the cone and the control cylinder varies along the span. [Preview Abstract] |
Sunday, November 18, 2007 11:26AM - 11:39AM |
BR.00005: Regimes of Flow Structures Generated by an Oscillating Fence in a Flat Plate Boundary Layer Michael Hind, Jonathan Naughton, William Lindberg Oscillating fences are commonly used to control wall bounded flow by transferring high momentum fluid from the free-stream into the boundary layer. Recent flow visualization studies on a laminar flat plate have shown that the coherent structures (vortices) generated by an oscillating fence can be classified into four regimes based on the ratio of the fence oscillation frequency to the natural shedding frequency of vortices from a static fence at the maximum oscillating fence height. Two- dimensional PIV is currently being used to quantify the vortex characteristics and refine the range of each flow regime. The PIV images are taken in a tow tank, with water as the working fluid, and are used to measure the following features: vortex shedding frequency, vortex strength, vortex advection speed, vortex dissipation rate, and effect of shed vortices on the boundary layer downstream of the oscillating fence. The static fence Strouhal number, based on vortex natural shedding frequency, fence height, and free-stream velocity, is $\sim$0.2 and shows little variation over the range of experimental Reynolds numbers (200 - 700, based on free-stream velocity and fence height). [Preview Abstract] |
Sunday, November 18, 2007 11:39AM - 11:52AM |
BR.00006: Interaction of a Fin Trailing Vortex with a Downstream Control Surface Steven Beresh, Justin Smith, John Henfling, Tom Grasser, Rusty Spillers A sub-scale experiment has been constructed using fins mounted on one wall of a transonic wind tunnel to investigate the influence of fin trailing vortices upon downstream control surfaces. Data were collected using a force balance mounted on the downstream fin to measure the aerodynamic forces of the interaction, combined with stereoscopic Particle Image Velocimetry to measure vortex properties. The fin balance data show that the response of the downstream fin essentially is shifted from the baseline single-fin data dependent upon the angle of attack of the upstream fin. Freestream Mach number and the spacing between fins have secondary effects. The velocimetry shows that the vortex strength increases markedly with upstream fin angle of attack, though even an uncanted fin generates a noticeable wake. No Mach number effect can be discerned in the normalized data, but measurements taken progressively further from the fin trailing edge show the decay in vortex strength with downstream distance. Correlations between the force data and the velocimetry suggest that the interaction is fundamentally a result of an angle of attack induced upon the downstream fin by the vortex shed from the upstream fin tip. [Preview Abstract] |
Sunday, November 18, 2007 11:52AM - 12:05PM |
BR.00007: Vorticity Dynamics in Axial Compressor Flow Diagnosis and Design. Jie-Zhi Wu, Yan-Tao Yang, Hong Wu, Qiu-Shi Li, Feng Mao, Sheng Zhou It is well recognized that vorticity and vortical structures appear inevitably in viscous compressor flows and have strong influence on the compressor performance. But conventional analysis and design procedure cannot pinpoint the quantitative contribution of each individual vortical structure to the integrated performance of a compressor, such as the stagnation-pressure ratio and efficiency. We fill this gap by using the so-called derivative-moment transformation which has been successfully applied to external aerodynamics. We show that the compressor performance is mainly controlled by the radial distribution of azimuthal vorticity, of which an optimization in the through-flow design stage leads to a simple Abel equation of the second kind. Solving the equation yields desired circulation distribution that optimizes the blade geometry. The advantage of this new procedure is demonstrated by numerical examples, including the posterior performance check by 3-D Navier-Stokes simulation. [Preview Abstract] |
Sunday, November 18, 2007 12:05PM - 12:18PM |
BR.00008: Three dimensional vorticity dynamics in rotating fluids M.D. Patterson Motivated by studies of ageostrophic dynamics associated with wind-forced anticyclonic oceanic vortices, transient spin up problems, and vortex interactions, we revisit a recently studied problem using new methodology. Wells, Clercx and van Heijst. (J. Fluid Mech. 573, 339, 2007) performed an experimental and two-dimensional numerical study of the evolution of vortices in oscillating spin-up. A rich dynamics of vortices interacting in the interior is produced in their square cell if the period of a modulation of the overall steady rotation rate is longer than the half-width advection time scale for corner vortices. They observed the evolution of dye in a horizontal slice near the free surface and treated the dynamics in the context of two-dimensional simulations. Here we interrogate the two-dimensional velocity field of the fluid at multiple levels simultaneously thereby producing a three-dimensional view of the flow. Among other things, this reveals that the vortices are launched into the interior of the fluid from the free-surface downward rather than being simultaneously released along the depth of the cell wall. Hence, from their very birth the vortices have three-dimensional structure. Vertical slices of the velocity field reveal highly localized up and downwellings, consistent with the simulations of Koszalka, Bracco and Provenzale (subjudice, 2007). Finally, our quantitative probing produces continuous velocity fields amenable to direct comparison with simulations. [Preview Abstract] |
Sunday, November 18, 2007 12:18PM - 12:31PM |
BR.00009: Vorticity POD Modes for Low-Order Modeling of Synthetic-Jet Actuated Flow Past a NACA 4415 Airfoil Undergoing Time-Periodic Pitching Motion Guy Ben-Dov, Arne J. Pearlstein, Dan Brzozowski, Ari Glezer We describe construction of two-dimensional modes by proper orthogonal decomposition (POD) of velocity and vorticity data obtained by two-component PIV measurements in turbulent flow past a NACA 4415 airfoil undergoing time-periodic pitching motion due to synthetic-jet actuation near the trailing edge. We show how modes constructed using nonsimultaneous data on adjacent spatial windows can be ``stitched together'' to produce nonoptimal (i.e., not POD) modes, that can be used to reconstruct phase-averaged data taken nonsimultaneously in the two windows. The use of ``globalized'' vorticity modes stitched together from nonsimultaneous data in low-order modeling for feedback control is also discussed. [Preview Abstract] |
Sunday, November 18, 2007 12:31PM - 12:44PM |
BR.00010: Fluidic-Driven Ducted Heat Ejector Donavon Gerty, Ari Glezer Unsteady, small-scale fluid mechanics and heat transport processes within a high-aspect ratio ducted heat ejector are investigated experimentally. The flow within the ducted ejector is entirely induced by time-periodic shedding of counter-rotating vortices from a planar vibrating reed that spans the entire width of the duct. The induced flow leads to enhanced convection heat transfer from the inner surfaces which is particularly attractive for low-power, densely-packed electronic hardware where heat is removed by direct conduction through the duct walls. The flow characteristics near the tip of the vibrating reed and within the duct are investigated using high-resolution particle image velocimetry (PIV). Of particular interest are the effects of variation in the reed motion and internal channel geometry on the induced small-scale motions and mixing and consequently on global and local heat transfer across the duct boundaries. [Preview Abstract] |
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