The Blunt Body Paradox — A Case for Transient Growth

Abstract

The ‘blunt body paradox’ refers to the early transition on spherical forebodies (even those that are highly polished) observed at supersonic and hypersonic freestream speeds both in flight and in wind tunnels. This transition occurs usually in the subsonic portion of the flow behind the bow shock wave (see for example Buglia 1961), a region of highly favorable pressure gradient that is stable to T-S waves. Surface cooling leads to even earlier transition. This phenomenon, identified in the mid-1950’s, has defied clear explanation. It has always been prominent on Morkovin’s list of unsolved problems. The tentative suggestions are generally roughness related since stagnation point boundary layers are very thin. But no connection has been made between the microscopic roughness on the surface and the features of the observed early transition such as location, sensitivity to surface temperature level, etc. This has led the present authors to seek an explanation through transient growth.