Effects of Supersonic Jet Conditions on Broadband Shock-Associated Noise

Abstract

This paper describes an experimental study of the effects of jet heating on broadband shockassociated noise (BBSAN). Far field noise measurements are made for three nozzle geometries – a convergent nozzle, and smooth convergent-divergent nozzles with design Mach numbers 1.5 and 1.76. The effect of jet heating is simulated with the use of helium-air mixtures. It is shown that the effects of simulated heating on the BBSAN give good agreement with measurements in jets using heated air. Measurements for unheated jets are presented and are consistent with previous BBSAN experiments. The noise measurements are supplemented with schlieren flow visualizations that show that the appearance of a Mach disk in the shock cell structure is consistent with the saturation of the BBSAN levels for both highly under- and over-expanded jets. The effects of jet heating are then described for the design Mach number 1.5 nozzle operating at four Mach numbers – two over-expanded (1 .2,1.4) j M = and two under-expanded (1 .7,1.9) j M = . Total temperatures in the range 1.0 to 2.2 are considered in increments of 0.2. These values are chosen to supplement existing measurements at higher total temperature ratios and to examine the effects of low levels of heating. It is shown that the peak BBSAN rapidly approaches a saturation value in all cases. In the under-expanded cases, the peak BBSAN level decreases with increasing total temperature ratio and the reverse is true for the over-expanded cases. However, these changes are always small, having maximum values of 4 ± dB. The Strouhal number of the primary BBSAN peak also varies only slightly, being within 20% of the unheated jet value. Again, there is a measureable difference between the under-and over-expanded cases with the peak frequency increasing in the former case and falling in the latter. It is also shown that the characteristic spectral shape of BBSAN is very similar for all the cases considered (with and without heating), and is described very well by the theory developed by Tam [13,14]. The variation of the peak frequency, peak sound pressure level and spectral width with jet Mach number and total temperature ratio are described. This is used as the basis for a proposed empirical prediction model for BBSAN.