A Study on Underexpanded Radial Jet Impinging on Cylindrical Inner Wall

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. Visualization by the shadowgraph method captured some ring-shaped shock waves. These shock rings indicate cell nodes of the underexpanded jet and a detached shock wave formed in front of the wall surface. The diameter of the shock ring increased as the nozzle pressure ratio increased, and the ring of the outermost node approached gradually that of the detached shock. At a certain nozzle pressure ratio, the detached shock ring jumped in the upstream cell. These rings oscillated and amplitude of the oscillation varied with nozzle pressure ratio. Dynamic strain of the inner wall generated by jet impingement was measured, and the strain histories at stations arranged in circumferential direction were analyzed. The most dominant frequency and the phase difference between neighboring stations was obtained. As a result, the cylindrical wall was found to vibrate at an integer multiple of the most dominant frequency at the nozzle pressure ratio = 2.8, 3.6 and 4.8.