A study on underexpanded radial jet impinging on inner wall of cylinder

Abstract

A jet radially spreading like a disc from a small gap between two circular tubes placed face to face with each other was experimentally analysed in this study. The jet is underexpanded and impinges on a cylindrical inner wall concentric with the circular tubes. Typical cellular structure appears in the jet and a detached shock wave forms near the wall. As this flow pattern is distributed over the circumferential direction, visualization by the shadowgraph method captured them as some ring-shaped shock waves. They indicate the nodes of cell sstructure and a detached shock. The diameter of the shock ring increased as the nozzle pressure ratio increased, and the ring of the node gradually approached that of the detached shock. At a certain nozzle pressure ratio, the detached shock ring jumped in the upstream cell. These shock rings oscillated and amplitude of the oscillation was varied with the pressure ratio. Strain of the wall surface was measured at nozzle pressure ratios ranging from 2.0 to 4.8. The wall surface is deformed by the pressure acting on the wall. The strain histories were analyzed using FFT, and multiple dominant frequencies were observed. These were classified by frequency as high, middle and low bands. The middle-band frequency increased gradually and dropped suddenly at certain nozzle pressure ratios, where the jump of the detached shock occurred. Furthermore, the middle-band frequency was in good agreement with the integer multiple of the dominant frequency in the low band. This means the low-band frequency is the fundamental frequency of the pressure acting on the wall. Therefore, the pressure oscillation on the wall has multiple modes and the mode change synchronizes with the flow pattern change such as the detached shock jump.