Change of oscillation modes of circular underexpanded jet by impingement on a small plate

Results are presented on measurements of frequency characteristics of the discrete tones radiated out when a circular underexpanded jet impinged on a small circular plate, that were carried out when the plate was moved along the jet axis at eleven fixed pressure ratios of the jet. Results showed that the discrete tones can be divided into two groups, belonging to the categories of the screech tones and the impinging tones. It was found 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, indicating 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.