Oscillation of Oblique Shock Waves Generated in a Two Dimensional Asymmetric Nozzle

The present work is aimed at experimentally investigating both the development and the oscillation phenomena of pseudo shock waves in a two dimensional asymmetric nozzle. A shock tube was used to generate supersonic flows in nozzles which consisted of both short and long walls diverging in the flow direction. Two kinds of nozzles were tested; one with both side walls of the nozzle diverging (Model A) and the other with only one side wall diverging (Model B). Nozzles were connected to a duct with constant cross sectional area. Initial pressures in the test section were 25, 50, and 100 kPa and the initial flow Mach numbers, before the diverging section were about 1.2 for model A and 1.4 for model B. The flowfield in the supersonic nozzle was visualized sequentially by schlieren photographs to observe the development of a pseudo shock wave and also the unsteady motion of this shock wave oscillating in the flow direction. The oscillation frequencies were determined from pressure measurements by means of a piezo-pressure transducer fixed at different loci. It was found that an oblique shock wave appeared near the turning corner of the diverging section owing to a separation vortex and then it developed into a pseudo shock wave. The supersonic flow that issued from the duct was not much expanded in the flow direction and behaved like an underexpanded jet which was deflected towards the shorter wall of the nozzle. The oscillation frequency was not affected by the initial pressure. However when the initial pressure was decreased, the oscillation amplitude was increased.