ABSOLUTE AND CONVECTIVE INSTABILITIES OF A SEPARATED BOUNDARY LAYER NEAR THE LEADING EDGE OF AN AEROFOIL

Present work describes the stability analysis of a separated boundary layer over a constant thickness aerofoil subjected to an impulse at the boundary for varying angles of attack (ad and tail flap deflections (beta(1)), where, the Reynolds number (Res) based on chord is 1.6 x 10(5) and the inlet free-stream turbulence (fst) being 1.2%. The features of the boundary layer are investigated through flow field measurement by a planar particle image velocimetry (PIV) and hotwire anemometer. The response to the impulse can grow either in space and time. The Orr-Sommerfield (O-S) equation has been solved, where the mean velocity field is obtained from the experiment. Criteria following Huerre and Monkewitz [1] are used here to decide the type of instability. It has been observed that the separated boundary layer near the leading edge is absolutely unstable for low angles of attack, whereas, it is convectively unstable for higher angles of attack, when the Kelvin-Helmholtz instability is bypassed. Further, it shows a convective instability for flap angles of +/- 30 deg.