Simulation of Active Flow Control on a Stalled Airfoil

Abstract

An Active-Flow-Control (AFC) device, designed to unload the wing of the V-22 tilt-rotor aircraft in hover when the rotors blow down on it, is studied by Detached-Eddy Simulation (DES). The device is a zero-net-flux jet through a slot on the flap shoulder, designed to help the flow turn and follow the highly-deflected flap more closely. The details of the DES are presented, including boundary conditions, grid, slot rendition, and algorithm. Meaningful grid-refinement studies were not possible. As AFC is applied, the flow pattern is altered, the response of the pressure distribution is qualitatively correct, and a drag reduction results. However the agreement on drag is poor if the boundary layers are treated as turbulent in the DES; discrepancies are in the 15–20% range. The boundary layers were tripped in the experiment and therefore expected to be turbulent. However the Reynolds numbers, pressure gradients, and shape of the pressure distributions (experimental and computed) at the leading edge all argue that the tripping failed. If the boundary layers are treated as laminar, the agreement is considerably improved with AFC off, but the response to AFC still differs from that in the wind tunnel, and is not monotonic with respect to the blowing level. Other issues related to the numerical and experimental approaches are discussed, particularly the nature of the simulation in the neighborhood of the slot, transition, and two-dimensionality.