1Numerical Simulations of Pitching Airfoil Flowfields for MAV Applications

Flowfields of interest for micro air vehicle (MAV) applications such as wings undergoing pitching/plunging motion at high rate are essentially unsteady and can involve large flow separation and vortex shedding. Examples in the recent literature show that the pure-plunging airfoil flowfields can be captured with acceptable accuracy by a variety of computational techniques, but the pitching problem is more complex. Possible reasons for computational-experimental discrepancy for pitching airfoils include installation and blockage effects in the experiments, and modeling of turbulence, of fine structures and 3D effects in the computations. To further study such discrepancies, we compare a set of RANS computations with phase-averaged and instantaneous particle image velocimetry, at reduced frequency k=3.93, and amplitude of 21.5 degrees. Flows at Re=10,000 and Re=40,000 are considered. Laminar, transitional, and fully turbulent computations are performed. Additionally, to investigate 3D effects, a 3D laminar computation for Re=10,000 is performed. All computations broadly capture the experimentally observed flow structures, provided that one compares with the experimental flowfield that exists after many periods from motion onset. Indeed, the experimental evidence is for two time scales en route to the flowfield relaxing to periodicity: a symmetric reverse Karman-type vortex street, and a skewed trailing vortex system. The relative size of the two time scales depends on the geometric orientation of the airfoil at motion onset, but the eventual wake structure is always skewed, and this is flowfield captured by the computation. Nomenclature CL = airfoil lift coefficient Re = Reynolds Number, based on c and U c = airfoil chord length (=152.4mm) f = airfoil pitch physical frequency k = reduced frequency=c/2U = fc/U U = freestream velocity xp = pitch pivot point t/T = dimensionless time, in fractions of one oscillation period = phase at which pitch motion I