In this paper, the oscillation phenomena of underexpanded jet and the motion of the vortex induced by the jet are discussed. The jet becomes underexpanded when the nozzle pressure ratio of convergent nozzle discharging it exceeds the critical value. The underexpanded jet is not uniform and has a typical cell structure because the expansion wave, the compression wave and the shock wave are periodically formed in it. A vortex is induced one after another in the vicinity of the jet boundary due to shear stress and interacts with the jet as it moves downstream. The interaction of vortex and jet is closely related to the strong noise radiating from such flow field. This flow field is often observed as the assist gas of laser cutting, the exhaust jet of VSTOL aircraft and rocket, and so on. In this paper the flow field is experimentally visualized in order to analyse the behavior of cell structure of jet. Furthermore, a simple system for measuring the advective velocity and the frequency of vortex generation is proposed.
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