Shock-Induced Large Separation Bubbles Near the Leading Edge of a Flat Plate at Hypersonic Mach Numbers

Abstract

The interaction between a shock wave and boundary layer occurs commonly in supersonic/hypersonic flows and is called shock boundary layer interactions. They are of particular importance in scramjet intakes at hypersonic speeds. At off-design (higher than the design) Mach numbers, the ramp shock hits the cowl plate boundary layer; the possible separation of the boundary layer, which occurs near the leading edge, significantly affects the performance of the system [1]. In such a case, the separation length is comparable to the distance of shock impingement location from the leading edge, and separation bubbles of such length scales are termed large separation bubbles [2]. At high impinging shock strengths (typical of hypersonic flows), the large separation bubbles near the leading edge are found to follow an inviscid similarity law independent of Reynolds number: the scaled separation length was proportional to the ratio of reattachment pressure to the free stream pressure and inversely proportional to the free stream Mach number. While the similarity law is proposed based on shock tunnel experiments on impinging shock interactions near sharp leading edge, the role of leading edge bluntness on such interactions presents a curious case especially as the flow separation is expectedly near the leading edge. There are very few investigations on the effect of leading edge bluntness on shock boundary layer interactions [3, 4] in general; however, it is an interesting case when the separation occurs close to the leading edge, which is not systematically studied. Experimental studies carried out by Borovoy [5] for the impinging shock boundary layer interaction in the presence of bluntness reveal a saturation of separation zone length, focusing only on the heat transfer near the regions of flow interaction for small leading edge bluntness. It is with this backdrop that shock tunnel experiments complemented by numerical studies are initiated in order to understand hypersonic impinging shock boundary layer interactions near blunt leading edges.