A comparative study of separated boundary layer transition on a flat plate with a blunt/semi-circular leading edge

Abstract

Boundary layer may separate due to an adverse pressure gradient or due to flow geometry and when a laminar boundary layer separates the free shear layer formed is very unstable even at low Reynolds number, undergoing a transition process to turbulence. This paper presents a comparative numerical study of the transition process in a separated boundary layer induced by changes of curvature of the surface. The geometry is a flat plate with two different leading edges: a blunt and a semi-circular. One of the main purpose of the study is to identify how similar or how different the transition process is with two different leading edges. It is evident that for both cases (blunt and semi-circular leading edges) the primary two-dimensional instability originates from the free shear layer of the separation bubble via the Kelvin-Helmholtz mechanism. Three-dimensional motions develop under any small spanwise disturbances and similar coherent structures have been observed in both cases, strongly indicating that the whole transition process is very similar.