Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session GE: Flow Control Actuation and Unsteady Flows |
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Chair: Noel T. Clemens, University of Texas at Austin Room: 003A |
Monday, November 24, 2008 8:00AM - 8:13AM |
GE.00001: Investigation of pulsed plasma jet for supersonic flow control Venkat Narayanaswamy, Noel Clemens, Laxminarayan Raja A high-frequency ($\approx $5 kHz) and high velocity-amplitude (250 -- 300 m/s) pulsed plasma jet actuator is currently undergoing development in our laboratory for application in supersonic flow control. The actuator is composed of an array of three jets that are pitched and skewed to form three pulsed streamwise vortices at high-bandwidth. The actuator has been used to control the separation shock motion in a Mach 3 compression-ramp shock / boundary layer interaction. Planar laser scattering visualization of a seeded CO$_{2}$ fog was performed in planes parallel to the wall 0.012954 to study the disturbance caused by the actuator. Imaging was performed at distances of 0.25$\delta $, 0.5$\delta \quad {\rm g}$ and 0.9$\delta \quad {\rm g}$ above the wall, where $\delta $ is the boundary layer thickness. It was found that individual pulsed plasmajet causes considerable heating of the surrounding fluid, which is seen till a distance of 15 jet diameters downstream of the actuator and 0.5$\delta $ from the wall. The disturbance is convected at a velocity which is very close to the local velocity of the unforced boundary layer. Particle image velocimetry will be performed in order to obtain quantitative information of the flowfield around the disturbance with attention to the magnitude of the longitudinal vorticity that is generated by the pulsed plasma jet. [Preview Abstract] |
Monday, November 24, 2008 8:13AM - 8:26AM |
GE.00002: Vortex Generation in Jet Vectoring Plasma Actuators Jamey Jacob, Ceren Ozturk, Michael Bolitho Jet vectoring flow control is demonstrated using plasma synthetic jet actuators (PSJA). The PSJA is a geometric variant of a plasma actuator, consisting of a symmetric electrode array that results in a counterflowing region of dielectric barrier discharge plasma. Quiescent flow PIV measurements of the PSJA reveal that the flowfield on actuation resembles that of a zero-mass flux or synthetic jet that is useful for flow control, particularly separation reduction, except with jet formation under both steady and pulsed operation. Like synthetic jets, unsteady pulsed actuator operation results in formation of multiple vortex rings and the jet momentum is found to be controlled by the pulsing frequency. While increasing the input power increases the maximum jet velocity, an optimum range of pulsing frequencies and actuator dimensions are observed to exist in order to maximize jet momentum. By asymmetrically varying the plasma input parameters, it is possible to control the jet angle and it is shown that vectoring using high frequency pulsing similar to synthetic jets is more effective than vectoring by modifying steady control inputs. Differences in control effectiveness are due primarily to the time scales associated with the vortex formation. Cross-stream vortex generation for separation control is demonstrated and the changing vortex characteristics from the different PSJA parameters are observed and discussed. [Preview Abstract] |
Monday, November 24, 2008 8:26AM - 8:39AM |
GE.00003: Precursor Flow Separation Detection Using Plasma Actuators Patrick Bowles, Thomas Corke A method for detecting incipient flow separation using plasma actuators is presented. The detection scheme relies upon the receptivity of the flow to unsteady disturbances that are introduced by the flow actuator. The receptivity to the unsteady disturbances is heightened as the flow approaches the separation limit, and subsequently can be detected downstream. This is demonstrated on a dynamically pitching airfoil that progresses through a dynamic stall cycle. A plasma actuator is located at the leading edge and pulsed at a frequency that is optimal to re-attach the flow. A pressure sensor monitors the unsteady pressure disturbances on the suction-side of the airfoil. Short-time Fourier analysis and Wavelet analysis capture the time-frequency behavior of the pressure sensor time series. Simultaneous flow visualization using a high-speed camera aid in elucidating the fluid response to the actuator input. The method not only provides a precursor for flow separation, but also an indicator when conditions exist where active re-attachment control is no longer needed. A closed-loop, feedback control scheme based on this is demonstrated. [Preview Abstract] |
Monday, November 24, 2008 8:39AM - 8:52AM |
GE.00004: Role of Negative Ions and Crosstalk in DBD Flow Actuation M.G. Mungal, H. Do, W. Kim, M.A. Cappelli Phase-locked particle image velocimetry (PIV) is used to study the mechanism of induced-flow in the near field of a radio frequency (RF) dielectric barrier discharge (DBD) actuator mounted in the separated flow region of a bluff body. For a single pair of electrodes, flow actuation is found to be asymmetric, with suction towards the buried downstream electrode when it is biased positively relative to the upstream exposed electrode. Lesser flow is seen on the reverse voltage swing, where the buried electrode should attract positive ions. This phenomenon is enhanced when oxygen is added to the flow, suggesting that oxygen negative ions, possibly O$_{2}^{-}$, play a dominant role in plasma actuation. When using multiple pairs of electrodes, the PIV images further confirm the importance of negative ions in the actuation process. Furthermore, the flow actuation now becomes nearly symmetric owing to crosstalk interactions between adjacent upstream/downstream electrodes of neighboring actuator pairs. [Preview Abstract] |
Monday, November 24, 2008 8:52AM - 9:05AM |
GE.00005: Numerical simulation of a viscous compressible flow around a NACA 0012 airfoil: Low Reynolds number regime Fernando Valenzuela, Ruben Avila The two dimensional Navier-Stokes equations for a compressible fluid have been numerically solved to investigate the effect of the viscosity and the compressibility on the instability of the laminar air flow around a NACA0012 airfoil at zero incidence angle of attack. The numerical simulations are carried out at two $Re$ numbers (5,000 and 10,000), and at the $Ma$ number in the range 0.2-0.9. The non-dimensional compressible Navier-Stokes equations have been solved by using a high order Spectral/$h$$p$ Element Method (SEM) that uses a Discontinuous Galerkin approach. The mesh consists of 2320. The computed values of the pressure coefficient $C_p$, the lift coefficient $C_L$ and the drag coefficient $C_D$, have been compared with numerical simulations and experimental data available in the literature. The Mach number countours allow to identify the presence of shock waves in the transonic flow regime. We may conclude that the discontinuous Galerkin method is capable to predict the transition stages that the laminar flow undergoes as the $Ma$ number increases (at fixed $Re$ number). [Preview Abstract] |
Monday, November 24, 2008 9:05AM - 9:18AM |
GE.00006: Low Reynolds Number Low Aspect Ratio Leading Edge Separation Dynamics Daniel Morse, James Liburdy The low Reynolds number flow conditions for a low aspect ratio wing are investigated using time resolved PIV with specific emphasis on the leading edge vortex generation at high angles of attack. The flow is highly three dimensional and the flow visualization shows very strong tip vortices which extend over a major portion of the wing. The separation bubble consists of a triangular shaped region extending back to about one third of the chord length. Three component velocity data are obtained at the centerline, one half span and at the wing tip. The time variation of the leading edge vortex shedding is studied in each of these regions using a swirl detection algorithm. The frequency of shedding is shown to be an order of magnitude higher than the dominant frequency within the recirculation region. The tip vortex flow dominate near the edge of the wing. [Preview Abstract] |
Monday, November 24, 2008 9:18AM - 9:31AM |
GE.00007: Laminar separation and turbulent reattachment for the Eppler 387 airfoil in its transitional Reynolds number range John McArthur, Geoff Spedding Previous studies have shown that the Eppler 387 airfoil at chord-based Re = 60k experiences a large drag increase at moderate lift coefficients and angles of incidence, and a subsequent drag decrease at higher lift coefficients and angles of incidence. This drag increase has been blamed on the formation of a laminar separation bubble, where laminar separation occurs early on the wing surface, and a turbulent shear layer reattaches to the surface. However, evidence for this was only found at higher Re where this drag increase is negligible. Also, the flow field for this case has never been measured. The present study makes these flow field measurements using a custom PIV setup, and also makes force measurements using a custom force balance. The flow field measurements measure the recirculating flow in the bubble, and average profiles are constructed downstream of reattachment. Relating the flow fields to the forces explains how laminar separation and turbulent reattachment affect the forces produced by this airfoil. [Preview Abstract] |
Monday, November 24, 2008 9:31AM - 9:44AM |
GE.00008: Effect of spanwise flexibility on aerodynamics of a plunging wing Hikaru Aono, Satish Kumar Chimakurthi, Hao Liu, Carlos E.S. Cesnik, Wei Shyy The effect of spanwise flexibility on the aerodynamic performance of a plunging wing is investigated numerically. To solve the fluid-structure interaction problem, a computational aeroelasticity framework has been developed based on an implicit coupling procedure between a pressure-based Navier-Stokes finite volume solver and a quasi-3D finite element solver capable of handling geometrically nonlinear composite beam-like and plate-like dynamic deformations. Three different variations in the spanwise flexibility of a NACA0012 cross-sectional rectangular wing of aspect ratio 3 are considered at chord Reynolds number of 3x10$^{4}$, reduced frequency (based on semi-chord) of 1.82, for prescribed pure plunge actuation at the leading edge. The computed results in terms of time histories of thrust coefficient, wing shape deformation, and flow structures are compared to experimental data. Spanwise flexibility of the plunging wing affects the amplitude and phase lag of the wing tip displacement, and hence the instantaneous angle-of-attack and associated flow structures. Together, they can substantially modify the aerodynamic force. [Preview Abstract] |
Monday, November 24, 2008 9:44AM - 9:57AM |
GE.00009: Shape-Changing Bodies in Fluid: Ratcheting, Plummeting, and Bursting Michael Shelley, Saverio Spagnolie We explore the dynamics of a shape-changing body in two-dimensions, and we examine a fluid-ratchet mechanism for locomotion in an oscillating fluid. Our study relates to the experimental work of Childress et al. (Phys. Fluids 2006) on a passive flexible body in an oscillating flow. We find that a shape-changing body can transport with a non-zero mean velocity, even in a direction opposing gravity. The body's transport can be understood in terms of vortex-body rearrangements whereby initially drag-type vortex dipoles are swept past the body to become thrust-type. We also consider the velocity burst experienced by a shape-changing body during an expansion in an initially seeded direction of motion. This phenomenon may contribute to the burst velocities of various aquatic organisms during evasive maneuvers, or predatory lunges. [Preview Abstract] |
Monday, November 24, 2008 9:57AM - 10:10AM |
GE.00010: A Vortex Array Model of the Unsteady Wake of a Two-dimensional Pitching Airfoil Ahmed Naguib, Manoochehr Koochesfahani Motivated by recent interest in MAV aerodynamics, the present study is focused on obtaining a simplified, vortex-array model of the unsteady flow in the wake of an airfoil undergoing small-amplitude but high-reduced-frequency pitch oscillations. The model is used to predict the mean and unsteady velocity field in the wake of a NACA 0012 airfoil executing a sinusoidal as well as non-sinusoidal pitch oscillation. The model predictive accuracy is assessed by comparison to the LDV measurements of the streamwise velocity by Koochesfahani (\textit{AIAA J.} \textbf{37}, 1999) at a chord Reynolds number of 12,000 and a reduced frequency as high as 10. The results demonstrate the ability of the vortex-array model to successfully reproduce the experimentally measured mean and phase-averaged streamwise velocity profiles in the wake of the airfoil. Moreover, by using the model to reconstruct the complete velocity field in the wake, the mean streamwise force acting on the airfoil is computed for different frequencies, amplitudes and waveforms of the oscillation. [Preview Abstract] |
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