A Study on Screech Tone Emitted from Underexpanded Radial Jet

Abstract

An underexpanded jet has typical shock-cell structure and strongly oscillates and its behavior is known to cause many industrial problems. An underexpanded jet radially issues from intake and exhaust valves of an internal combustion engine, a pressure control valve, and so on. When a supersonic jet exhausted from a circular nozzle impinges on a flat plate, the wall jet formed on the plate often becomes underexpanded and spreads out radially. Such underexpanded impinging jet is one of the models of supersonic jets on laser cutting process and glass tempering process. In this study, an underexpanded jet radially discharged from a circular slit nozzle, which consists of two circular tubes, is experimentally examined for different nozzle pressure ratios and for different diameters of tube. Jet structure is analyzed by means of visualization, e.g. Schlieren method. A noise emitted from the jet is measured and the frequency of screech tone is analyzed. The experimental results are compared with those of a two-dimensional jet issuing from a rectangular nozzle. As a result, in the radial underexpanded jet, multiple nodes of cell structure are visualized as ring-shaped shocks and collapse of the cellular structure of radial jet is found to occur at the upstream location in comparison with the case of rectangular jet. Furthermore, a comparison of visualized sound waves with the screech tone frequency reveals that the sound source of noise measured is in the vicinity of the end of the second cell and that the length of the second or third cell is one of the most important parameter of the frequency of the emitted screech tone.