Change of Oscillation Modes of Circular Underexpanded Jet by Impingement on a Small Plate

Abstract

The frequency characteristics of the discrete tones generated by the impingement of a circular underexpanded jet on a small circular plate were studied experimentally. When the plate was moved along the jet axis at fixed pressure ratios, it was found that the frequencies could basically be divided into two groups. These groups belong to the categories of the impinging tones and the screech tones. Furthermore, it was observed that three types of frequency changes of the screech tones, (sawtooth, stepwise and intermittent ones), are realized periodically with the increasing nozzle-to-plate distance. Therefore, it is considered that the frequency change of the screech tone by the insertion of a small plate into the jet is associated with the self-sustained oscillation of the circular underexpanded free jet. The most interesting phenomenon discovered is the stepwise change of the frequencies of discrete tones. The pressure ratio range of the stepwise change of the frequencies overlaps that for the helical oscillation mode of the free jet and that for the radiation of the strong hole tone.