Noise emission and structure of underexpanded radial jet and rectangular jet

Abstract

An underexpanded radial jet is generated downstream of intake and exhaust valves of an internal combustion engine, a pressure control valve and so on. In addition, when a supersonic jet issuing from a circular nozzle impinges on a flat plate, the wall jet on the plate often becomes underexpanded and spreads out radially. The underexpanded jet has typical shock-cell structure and strongly oscillates and its behavior is known to cause many industrial problems. In this study, a jet structure and an emitted noise from an underexpanded jet radially discharged from a circular slit nozzle, which consists of two circular tubes, are examined experimentally for different nozzle pressure ratios and different diameters of tube, and the experimental results are compared with those of a two dimensional jet issuing from a rectangular nozzle. As a result, multiple ring-shaped shocks are visualized in the radial underexpanded jet, and the sound source of noise measured is found to be in the vicinity of the end of the second cell. Furthermore, collapse of the cellular structure of radial jet occurs upstream in comparison with the case of rectangular jet and the length of second cell is shown to be one of the most important parameter of a frequency of the emitted screech tone.