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 KK: Aerodynamics |
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Chair: C. Randall Truman, University of New Mexico Room: Salt Palace Convention Center 250 E |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KK.00001: Flow-induced flutter of thin elastic films C. Randall Truman, Michelle Gallegos, Hugh Smyth An experimental investigation of the aeroelastic performance of thin elastic films in flutter mode was carried out. The flow velocity of a uniform air stream was varied, as were film length, width and tension. Several commercially-available polymer films were tested. A laser vibrometer was used to acquire vibration characteristics of the film, including flutter frequency and film acceleration. Correlations between these parameters will be used to design a device to disperse particles from an adhesive film within an air stream. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KK.00002: The Pressure Disturbance Produced by and Oscillating Fence on a Pitch Oscillating Wing Manjinder Saini, William Lindberg, Jonathan Naughton The ability of an oscillating fence to produce changes in the pressure distribution on a pitch-oscillating NACA 23012 wing is studied using time-resolved pressure measurements and Particle Image Velocimetry (PIV). The fence is located at 35\% of chord, and the effect of fence Strouhal numbers is studied by varying the fence frequency between 20 and 80 Hz. The fence is tested under three different oscillation conditions with mean angles of attack of -5, 0 and +3 degrees and a pitch oscillation range of $\pm 5$ degrees about the mean angle. The oscillating fence actuator produces a large vortical structure that induces large changes in the unsteady pressure distribution. The implication of these vortical disturbances for flow control is important because the local pressure changes produced are largely independent of the angle of attack during oscillation. However, all the cases studied show that the vortical structures and the resulting pressure disturbances are a function of the fence frequency. The experiments also indicate that the overall effect of fence for one complete oscillation cycle is a strong function of mean angle of oscillation. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KK.00003: An Experimental Study of Flexible Membrane Airfoils at Low Reynolds Numbers Masatoshi Tamai, J.T. Murphy, Hui Hu Thin and flexible membrane wings are unique to flying and gliding mammals, such as bats, flying squirrels and sugar gliders. These animals exhibit extraordinary flight capabilities with respect to maneuvering and agility that are not observed in other species of comparable size. We conducted the present study to try to leverage the flexible membrane wing feature of bats and other flying and gliding mammals to Micro-Air-Vehicle (MAV) designs for improved aerodynamic performance. A comprehensive wind-tunnel experimental investigation was conducted to study the flow behavior around flexible membrane airfoils compared with their rigid counterparts. In addition to aerodynamic force measurements using force transducers, a high-resolution Particle Image Velocimetry (PIV) system was used to conduct quantitative flow field measurements around the flexible membrane airfoils. The objective of the present study is to elucidate fundamental physics and to explore the potential applications of such non-traditional, bio-inspired flexible membrane airfoils to MAV designs for improved aerodynamic performance. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KK.00004: Effect of Pitch Rate on Time Evolution of Surface Topology on a Delta Wing Tunc Goruney, Donald Rockwell A basic delta wing of moderate sweep angle, representative of Unmanned Combat Air Vehicles (UCAVs) and Micro Air Vehicles (MAVs), undergoes a pitching maneuver. Near-surface flow patterns are visualized by a technique of high-image-density particle image velocimetry for a wide range of pitch rates. Five different universal states are defined during the relaxation process following cessation of the pitching motion. These states involve distinct patterns that can be defined in terms of topological features such as negative (separation) and positive (reattachment) bifurcation lines, saddle points, foci, and nodes. Such universal states can be identified for all pitch rates, extending over an eightfold range. Irrespective of the severity of the flow distortion at the end of the pitching maneuver, the relaxation of the flow involves the same sequence of universal states. The time delay to occurrence of the first universal state is very sensitive to the pitch rate. The delay between subsequent states is, however, nearly independent of pitch rate. Due to the highly three-dimensional nature of the flow, the flow patterns and topological states will also be visualized by stereoscopic particle image velocimetry. [Preview Abstract] |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KK.00005: Effects of Sweep Angle on Flow Features and Leading Edge Vortices of Thin, Cambered Wings at Re=5,000 John McArthur, Geoffrey Spedding The aerodynamic performance of wings at chord-based Re $<$ 10$^{5}$ is highly sensitive to the laminar boundary layer separation and possible reattachment, and even simple fixed geometries can have complex, three-dimensional, unsteady flows. It has been suggested that fixed wings with strong sweep (such as those of the swift in certain configurations) can induce favorable three-dimensional effects in the form of a stable leading-edge vortex (LEV), but no systematic parametric study has been performed to test the idea. New results from an array of qualitative dye-visualization experiments on fixed wings with constant aspect ratio and varying sweep ($\Lambda $=0, 20, 40, 60\r{ }) will be presented. There is always significant three-dimensionality in the flow over the suction surface, even for $\Lambda $=0\r{ }. Structures that might be termed LEVs appear at $\Lambda $=60\r{ }, but they are small and not significant in the dynamics. Selected quantitative force and flow field measurements confirm the qualitative findings, and sweep, by itself, is not sufficient to generate a significant stable LEV. Implications for swifts and micro-air vehicles are considered. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KK.00006: Wing Warping and Its Impact on Aerodynamic Efficiency Ben Loh, Jamey Jacob Inflatable wings have been demonstrated in many applications such as UAVs, airships, and missile stabilization surfaces. A major concern presented by the use of an inflatable wing has been the lack of traditional roll control surfaces. This leaves the designer with several options in order to have control about the roll axis. Since inflatable wings have a semi-flexible structure, wing warping is the obvious solution to this problem. The current method is to attach servos and control linkages to external surface of the wing that results in variation of profile chamber and angle of attack from leading edge or trailing edge deflection. Designs using internal muscles will also be discussed. This creates a lift differential between the half-spans, resulting in a roll moment. The trailing edge on the other half-span can also be deflected in the opposite direction to increase the roll moment as well as to reduce roll-yaw coupling. Comparisons show that higher L/D ratios are possible than using traditional control surfaces. An additional benefit is the ability to perform symmetric warping to achieve optimum aerodynamic performance. Via warping alone, an arbitrary span can be warped such that it has the same aerodynamic characteristics as an elliptical planform. Comparisons between lifting line theory and test results will be presented. [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KK.00007: Active concentration of vorticity along the leading edge of a semi-circular wing David Williams, Jesse Collins, Tim Colonius Leading-edge vorticity concentration plays a key role in lift enhancement for insect flight, swept wings on aircraft, and in unsteady flows through the formation of the dynamic stall vortex. Using 16 spatially localized pulsed-blowing actuators, we are able to concentrate the vorticity at the leading edge of a wing with a semi-circular planform. The experiments are done in a wind tunnel with a model chord Reynolds number of 68,000. Peak vorticity values double those of the unforced case result in an 80 percent increase in lift on the wing relative to the unforced post-stall lift. The semi-circular wing obtains lift coefficients approximately 35 percent larger than a rectangular planform wing with a comparable aspect ratio. The sweep of the wing's leading edge is believed to establish a spanwise transport of vorticity, contributing to the stabilization of the leading edge vortex. Closed-loop control of the wing plunging motion in an unsteady flow stream is demonstrated by modulating the strength of the leading-edge vorticity via a proportional-derivative controller. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KK.00008: Flow Control of the Stingray UAV at Low Angles of Attack John Farnsworth, John Vaccaro, Michael Amitay The effectiveness of active flow control, via synthetic jets and steady blowing jets, on the aerodynamic performance of the Stingray UAV was investigated experimentally in a wind tunnel. Global flow measurements were conducted using a six component sting balance, static pressure, and Particle Image Velocimetry (PIV) measurements. Using active control for trimming the Stingray UAV in pitch and roll at low angles of attack has similar effects to those with conventional control effectors. The synthetic jets were able to alter the local streamlines through the formation of a quasi-steady interaction region on the suction surface of the vehicle's wing. Phase locked data were acquired to provide insight into the growth, propagation, and decay of the synthetic jet impulse and its interaction with the cross-flow. The changes induced on the moments and forces can be proportionally controlled by either changing the momentum coefficient or by driving the synthetic jets with a pulse modulation waveform. This can lead the way for future development of closed-loop control models. [Preview Abstract] |
Tuesday, November 20, 2007 9:44AM - 9:57AM |
KK.00009: Flow Structure along the 1303 UCAV Mehmet A. Kosoglu, Donald Rockwell The 1303 Unmanned Combat Air Vehicle is representative of a variety of UCAVs with blended wing-body configurations. Flow structure along a scale model of this configuration was investigated using dye visualization and particle image velocimetry for variations of Reynolds number and angle-of-attack. Both of these parameters substantially influence onset and structure of the leading-edge vortex (LEV) and a separation bubble/stall region along the tip. The onset of formation of the LEV initially occurs at a location well downstream of the apex and moves upstream for increasing values of either Reynolds number or angle-of-attack. In cases where a separation bubble or stall region exists, quantitative information on its structure was obtained via PIV imaging on a plane nearly parallel to the surface of the wing. By acquiring images on planes at successively larger elevations from the surface, it was possible to gain insight into the space-time features of the three-dimensional and highly time-dependent structure of the bubble or stall region. Time-averaged images indicate that maximum velocity defect decreases in magnitude and moves downstream with increasing elevation from the surface. [Preview Abstract] |
Tuesday, November 20, 2007 9:57AM - 10:10AM |
KK.00010: Feature detection and Proper Orthogonal Decomposition of time resolved velocity data for flow separation over an elliptical leading edge. Daniel Morse, James Liburdy In this study the flow characteristics over a fixed surface, flat, low aspect ratio thin wing are investigated. Of interest is the dynamic separation process for a range of angle of attacks, and chord Reynolds numbers, particularly the time dependent nature of the vortex development, convection and interactions. Angle of attack is varied from 14\r{ } to 20\r{ }. The Reynolds number based on chord length ranges from 14,700 to 66,700; this corresponds to a velocity range between 1.75 and 5.0 m/s. Time Resolved Particle Image Velocimetry (TRPIV) is used to obtained time resolved velocity information near the leading edge. Using discrete vortex detection schemes coupled with a high pass filtering and Proper Orthogonal Decompostion (POD) analysis, the time dependent characteristics of this flow is elucidated. Methods of vortex detection include the $\lambda _{2}$ method proposed by Jeong and Hussain [1995] and Large Eddy Simulation (LES) filtering. The POD reveals a low number of high energy, dominant modes of velocity variation for most cases. [Preview Abstract] |
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