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‘Unsteady Vortex Formation of Low-Aspects-Ratio, Bio-Inspired Propulsors’
Department of Mechanical and Aerospace Engineering Seminar
Friday, Nov. 2, 2:15-3:10 p.m., Watson Theatre, Watson Hall
Dr. Matthew Ringuette
Department of Mechanical & Aerospace Engineering
The State University of New York at Buffalo
In recent years substantial effort has been focused on understanding animal propulsion, e.g., that of fish, insects and birds. The application is small, highly maneuverable bio-inspired autonomous vehicles used for information gathering. The Reynolds number (Re) range is 102–104, and this coupled with low aspect ratio (AR) propulsors at high angles of attack creates flows dominated by separation and unsteady, 3-D vortex formation. More research is needed to understand the time-varying flow structure and its relationship to the forces. The focus of this talk is the characterization of these vortex flows using experiments with very simplified models and motions, which nonetheless produce highly complex flow structures and provide substantial insight. Two cases are considered, a rigid trapezoidal fin executing a rotational
starting motion, and rigid flat-plate wings rotating from rest; the latter is a simplified hovering half-stroke. The diagnostics are dye visualization, particle image velocimetry (PIV) and force measurements.
The fin generates a symmetrical ring-like vortex dominated by the tip vortex (TV), and substantial root-to-tip velocity. For large rotational amplitudes, the vortex sheds and a secondary one is generated while the plate is still moving, indicating saturation of the first. For different velocity programs, the TV circulation exhibits saturation as plateaus. Its behavior is complex due to an interaction with the root-to-tip flow, so a range of saturation times is defined. The lower bound indicates the initial TV pinch-off, and is reasonably predicted by a simple scaling.
The rotating-wing experiments focus on AR effects. Rectangular wings of AR = 2 and 4 with a fixed angle of attack of 45° are tested in a glycerin-water mixture, with a matched Retip = 5,000. The time-varying, 3-component volumetric velocity field is reconstructed using phase-locked, phase-averaged stereoscopic PIV in multiple chordwise planes. For both ARs the flow is initially a vortex loop consisting of a leading-edge vortex (LEV), the TV and the trailing-edge vortex. The AR = 2 case has greater spanwise velocity and a more helical LEV. After about 20° of rotation, the outboard LEV for each AR lifts up and is arch-like. For AR = 4 this is progressive and followed by breakdown. The AR = 2 flow is more coherent, and the greater influence of the TV contributes to a stronger flux of LEV vorticity to the tip, which mitigates lift-off.
Inboard the LEV is “stable,” and for both ARs has similar spanwise velocity and vorticity flux distributions. The AR = 2 lift coefficient shows a higher growth after startup.
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