On the sub-critical stability of variable ramp intakes at mach numbers around 2

Abstract

The need to minimise the pre-entry drag of supersonic intakes demands an understanding of the link between the shock pattern in the region of the cowl lip, and aerodynamic instability consequent on shear planes or zones generated in the supersonic compression field entering the subsonic diffuser. The present Report aims to illuminate this link. It describes flow instabilities observed in tests with a range of model variable ramp intakes, and by invoking observations made during earlier experiments develops a consistent picture of instability in variable ramp intakes at Mach numbers of about 2. In general two forms of instability were observed in the tests described. These, of similar frequency but appreciably different amplitude, are termed 'big' and 'little' buzz. Experimental observations suggest that the latter involves a flow separation from the internal surface of the cowl, whilst 'big' buzz is thought to be associated with separation of the ramp boundary layer. Flow instability in the Concorde intake is discussed in the light of these results. Although, because of the complexity of the phenomena investigated much work remains to be done, a design technique is suggested which offers the prospect of achieving minimum pre-entry drag with freedom from instabilities induced by shear planes or zones.