Buzz instability in a mixed-compression air intake

Abstract

The buzz instability in an air intake was first observed by Oswatitsch [1]. Ferri and Nucci [2], via detailed experiments on an axisymmetric external compression air intake, attributed the occurrence of buzz instability to the velocity discontinuity across the vortex sheet originating at the intersection of the conical shock and strong shock ahead of the intake entrance. The fluctuations began when this vortex sheet approached the inner surface of the cowling. Dailey [3] attributed the origin of the buzz oscillations to a random pressure pulse from the subsonic diffuser. He related the frequency of oscillations to that of an organ pipe with one end closed and the other end open. Fisher et al. [4] conducted experiments on a rectangular external compression intake with a variable ramp. Two kinds of oscillations were found in their tests: big and little buzz. They had the same frequency of oscillation but different amplitudes. The little buzz was found to be caused by the flow separation below the cowl and is similar to the Ferri type of buzz. The big buzz, on the other hand, was caused by the separation of the boundary layer along the ramp. It is very similar to the Dailey type of buzz. It was also observed that the little and big buzz occur for lower and higher mass flow reduction, respectively. Trapier et al. [5] conducted an experimental as well as a computational study of flowin a rectangular mixed-compression intake for a Mach number range of 1.8?.0. Both little and big buzz were observed. The little buzz was of the Ferri type They related the oscillations to the acoustic resonance of the shear layer instabilities under the cowl lip. The big buzz was of the Dailey type and occurred due to the separated flow on the ramp in a supersonic diffuser that blocks the intake. The oscillations were caused due to the periodic filling and discharging of the intake. The oscillation frequency of the little buzz was found to be higher than that of the big buzz.