Flow instability of centre-body diffusers at supersonic speeds

Abstract

This note offers explanations for certain types of flow instability which occur with centre-body diffusers at supersonic speeds. These instabilities manifest themselves as oscillations of the shock pattern ahead of the diffuser and the flow through the diffuser for certain conditions of operation and are likely to affect seriously the performance. Two main types of oscillation have been distinguished : a violent oscillation which occurs when the flow through the diffuser is throttled below a certain value ; a less severe oscillation which occurs when the vortex sheet from the intersection of the shock waves ahead of the diffuser or a separated boundary layer strikes the cowl. The explanations of the oscillations are substantiated by schliere'n photographs of two- and three-dimensional model diffuser tests in a wind tunnel. It seems possible, in the light of the explanation given in this note, to be able to predict the range of instability of any centre-body diffuser configuration by fairly simple model tests.