A Study on Vortex Occurring in Jet Boundary of Underexpanded Jet

This study discusses an advective velocity of vortex occurring in the field near jet boundary of an underexpanded jet. The underexpanded jet is well known as one of the supersonic jet and it is formed when the pressure ratio across a convergent nozzle, from which the jet is exhausted, is more than the critical value. Owning to outstanding physical characteristics of the underexpanded jet, it has been used in many fields, such as laser cutting, exhaust jet of rocket and cooling process of tempered glass. However, the jet is not uniform and has typical cell structure because the expansion wave, the compression wave and the shock wave are periodically formed in it. Furthermore, vortices are induced in the flow field due to shearing stress generated along the jet boundary. And they interact with shock wave at a cell node of jet, which is closely related to a noise radiating from the jet. In the experiment, a convergent nozzle was used. The nozzle pressure ratio was changed, and the flow field was visualized using optical techniques such as Schlieren method. A number of photographs were taken at random under each condition and a structure of the flow field was examined. And the acoustic noise emitted from the flow field is measured using a microphone. Furthermore, the motion of vortex near the jet boundary was measured using a device composed of a laser and a photo-electric sensor such as a simple Schlieren system. Finally, from these results, the advective velocity of vortex was investigated. The results showed that the oscillation mode of the jet is changed from axisymmetric to lateral as the pressure ratio increases and the characteristic of advective velocity of vortex changes correspondingly. The advective velocity of vortex conspicuously increases with increase in the pressure ratio at axisymmetric mode.