Aerodynamic control using distributed active bleed

Abstract

A novel approach for controlling the global aerodynamic loads on lifting surfaces using regulated distributed active bleed that is driven by pressure differences across the surfaces is investigated in wind tunnel experiments. The experiments focus on the flow mechanisms that govern the interaction between the bleed and the local cross flow over a wide range of pre- and post-stall angles of attack. Particle image velocimetry (PIV) and load cell measurements indicate that low momentum quasi-steady and time-periodic bleed [momentum coefficients of O(10-4)] lead to bi-directional deflections of the time-averaged vorticity layer and effect increases or decreases in lift, drag, and pitching moment. High-speed PIV and proper orthogonal decomposition (POD) of the vorticity flux show that the bleed engenders trains of discrete vortices that advect along the surface and are associated with a local instability that is manifested by a time-averaged bifurcation of the vorticity layer near the bleed outlets. The vortices advect over several convective times and alter the vorticity flux over the airfoil and thereby the aerodynamic loads. Active bleed is also investigated on a dynamically pitching airfoil (reduced frequencies up to k = 0.42) to study the effects of modulating the evolution of vorticity concentrations during dynamic stall. Time-periodic bleed mitigates adverse pitching moment behavior (“negative damping”) that can precipitate structural instabilities while maintaining the cycle-average loads to within 5% of the base levels by segmenting the vorticity layer during upstroke and promoting early flow attachment during downstroke. The present investigations demonstrate that active bleed can be implemented for direct lift control, maneuvering, or stabilization of flexible aerostructures (airframes, flexible wings, and rotor blades).