Control of cellular separation using adaptive surfaces

Abstract

We report results from an experimental investigation of the three-dimensional separation produced by a high-lift aerofoil at moderate incidence, with constant section, where the separation is controlled by the implementation of an adaptive surface. Mean and time-resolved measurements are made using a NASA GA(W)-1 aerofoil with AR=6 at Rec=3.5×105. Surface oil visualisation and stereo Particle Image Velocimetry (PIV) are used to explore the flow field. The mean topology of the flow identifies characteristic spanwise periodic behaviour, “stall cells”, along the surface of the model. Analysis of the time-dependent surface pressure shows two distinct frequencies within the flow field. The higher frequency appears at a Strouhal number, St≈0.2, representative of vortex shedding, and the typical von Kármán vortex street. The lower frequency appears at St≈0.02, observed as a global fluctuation in stall-cell extent. This lower frequency is apparent in many separated flows, but in the present context, appears to have received only little attention. It correlates with widely observed low-frequency unsteadiness in the wing loading around stall. While this mode is analogous to that observed in other types of separation, here the streamwise extent of the separation varies because the flow is separating from a curved surface rather than from a sharp edge; the width of the separated region also varies. We show that fully-reversible point actuations of an actuated surface with auxetic structure, introduced immediately upstream of the saddle point at the leading edge of the stall cell, reduce the extent of the separated region.