Energy transfer mechanism of supersonic jet noise through rectangular nozzles

A series of experiments were conducted with nozzle pressure ratios (NPRs) from 2.2 to 8.0 to explore energy transfer mechanisms of under-expanded supersonic jet noise through rectangular nozzles. The results from these experiments resulted in the discovery of a fundamental-frequency-moving phenomenon. The fundamental frequencies were extracted from sound pressure signals for different nozzles, and then by switching modes, energy flows and flow fields' Schlieren structures were compared and analyzed. As a result, two key structural parameters, cross section area and aspect ratio (AR) of the exit, were found to have a contributing effect in jet noise. Single screech tone and the turbulent mixing noise switched, dominated and then disappeared when jet pressure was varied. Besides that, the AR has a more significant effect on the screech tone than another parameter according to the analysis results. Specifically, for AR4 nozzle with cross-section area 20 by 5 mm2, an interesting phenomenon was noticed. As jet pressure decreased after the switching of modes energy moved from the high-frequency tone screech to the low-frequency turbulent noise, and finally gathered in a lower-frequency region, the so called fundamental-frequency-moving phenomenon. Explanations for this fundamental-frequency-moving phenomenon were given based on Powell's acoustic feedback theory and Tam's weakest link theory, as well as analysis on spectra by Schlieren photos.