Effects of jet temperature on broadband shock-associated noise

This paper describes an experimental study of the effects of jet heating on broadband shock-associated noise. The noise measurements are supplemented with schlieren flow visualizations that show that the appearance of a Machdisk in the shock cell structure is consistent with the saturation of the broadband shock-associated noise levels for both highly under- and overexpanded jets. The effects of jet heating are then described for the design Mach-number-1.5 nozzle operating at four Mach numbers: two overexpanded (Mj = 1.2 and 1.4), and two underexpanded (Mj = 1.7 and 1.9). Total temperature ratios in the range of 1.0 to 2.2 are considered in increments of 0.2. These values are chosen to supplement heated air measurements at higher total temperature ratios (between 1.8 to 3.2) and to examine the effects of low levels of heating. It is shown that the peak broadband shock-associated noise rapidly approaches a saturation value in all cases. In the underexpanded cases, the peak broadband shock-associated noise level decreases with increasing total temperature ratio and the reverse is true for the overexpanded cases. Although the changes are very small, a systematic trend can be observed. The spectral shape of fundamental broadband shock-associated noise is found to be very similar for all the cases considered.An empirical prediction model for the fundamental broadband shock-associated noise is proposed.