Instability study of the wake behind a discrete roughness element in a hypersonic boundary-layer

Abstract

The linear instability induced by an isolated roughness element in a boundary-layer at Mach 6 has been analysed through spatial BiGlobal and threedimensional parabolised (PSE-3D) stability analyses. It is important to understand transition in this flow regime since the process can be slower than in incompressible flow and is critical to prediction of local heat loads on next-generation flight vehicles. The results show that the roughness element, with a height of the order of the boundary-layer displacement thickness, generates an convectively unstable wake where different instability modes develop. Furthermore, at this high Mach number, boundary-layer modes develop at high frequencies and are also covered here. Important discrepancies are observed between BiGlobal and PSE-3D predictions, mainly for the roughness-induced wake modes. Results are in qualitative agreement with a full Navier-Stokes receptivity study of the same flow.