Unsteady incompressible flows have been of interest in recent decades for applications of small unmanned flight vehicles and biological flyers. Both raise basic questions in the management of flow separation and the validity of unsteady aerodynamic theories that were originally developed for applications in aeroelasticity, where frequencies can be high, but amplitudes are generally low. We will review some recent experimental results on nominally 2D airfoils and wings of various aspect ratios undergoing pitch, plunge, and streamwise oscillation – periodic and transient – and will compare them with attached-flow unsteady theories. High-rate motions delay stall and perhaps paradoxically extend attached-flow results for lift coefficient to situations where the flow has fully separated. They serve as a kind of linearization mechanism. Stall delay is of course impermanent, and conservation of circulation cannot be contravened. Neither is it really practical to rapidly pitch a wing as a lift or thrust production mechanism, short of insect-like flapping. But one possibility is to revisit conventional trailing-edge flaps, albeit at high rates of deflection, relative to convective time. We will discuss water tunnel experiments on how such a scheme produces high lift and pitching moment coefficients, in separated or attached flow, without the initial negative transients that plague fluidic flow control, and again how these results may be approximated by classical unsteady attached-flow theories.
Dr. Michael Ol
He spent most of his career at the US Air Force Research Lab, in Dayton, Ohio, where his most recent assignment was as the Technical Advisor of the Aerodynamics Configuration Branch, Air Vehicles Directorate. His work includes topics in experimental low-speed aerodynamics, focusing on topics in low Reynolds number flows, flow separation and unsteadiness, and applications for UAVs. He received his BSE and MSE in Aerospace Engineering from the University of Michigan, and his PhD in Aeronautics from the California Institute of Technology. While at AFRL, he developed a lab-facility featuring an electric motion-rig in a water tunnel, for a range of experiments in dynamic stall, flapping-wing flight, and more basic questions in vortex formation, flow separation, and added-mass. Most recently, this work led to a reconsideration of conventional trailing-edge flaps, somewhat at odds with the more popular trends in fluidic
flow-control. At present, Dr. OL is a private consultant and part-time faculty at the University of Dayton.