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"Some Experimental Observations on Unsteady Aerodynamics for Micro Air Vehicles"

Location: EB3-2232, 11:00 AM, Friday, September 9, 2011

Speaker: Dr. Michael Ol

Title: "Some Experimental Observations on Unsteady Aerodynamics for Micro Air Vehicles"

The currently fashionable field of Micro Air Vehicles (MAVs) has as one of its benefits a resurgent interest in classical aerodynamics. Topical applications involve basic questions of flow separation, vortex formation and vorticity transport, lags in flowfield response to imposed transients, wake interactions and fluid-structure interaction. We will focus on two problems involving flat plates of various aspect ratios: variations on linear pitch ramps, and combinations of translation/rotation in hover. These cases are illustrative of some of the core questions in unsteady aerodynamics for MAV applications, of which the main issue is the extent to which quasi-steady aerodynamic models are useful. We find that for moderate dimensionless frequencies of motion, peak lift coefficient occurs at times and incidences commensurate with peak strength of the main vortical structures - that is, of the leading edge vortex. This is a pleasingly intuitive observation that unfortunately breaks down for high rates of motion. Some trends are reassuringly simple; for example, lift and drag histories for the pitching plate collapse fairly well in accordance with scaling relations first proposed 40 years ago. The main effect of so-called “dynamic stall” is an extension of the usual 2*pi*alpha lift curve to surprisingly high angles of attack, with amplitude in linear proportion to motion rate. Such trends are good news for MAV engineers seeking tractable aerodynamic models for vehicle sizing, performance estimation and flight control design. Other trends are a bit surprising. An example is that aspect ratio appears to be entirely irrelevant for a plate in translating/rotating hovering motion. The talk will aim to propose conjectures for arriving at aerodynamic models and for their physical basis."

Bio: Dr. Michael Ol is a researcher at the Air Force Research Lab, Wright-Patterson Air Force Base in Dayton, Ohio. He and his colleagues conduct experiments in unsteady aerodynamics in the Horizontal Free-Surface Water Tunnel at AFRL. He is an Associate Fellow of AIAA and a member of the AIAA Fluid Dynamics Technical Committee.

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