Effects of Jet Temperature and Nozzle-Lip Thickness on Screech Tones

Abstract

Imperfectly expanded supersonic jets, invariably, generate discrete frequency sound known as screech tones. In the low supersonic jet Mach-number range of 1.0-1.25, the screech phenomenon is axisymmetric. In this investigation the effects of jet temperature and nozzle-lip thickness on the axisymmetric jet screech modes are studied by numerical simulation. At the present time there is no known way to predict the intensities of screech tones, even empirically, except by numerical simulation. Numerical results of screech frequencies and directivities of heated jets will be reported. The computed screech frequencies are found to agree well with experimental measurements. Computed tone intensities indicate a gradual decrease in the screech amplitude with an increase in jet temperature. Screech tones are known to be sensitive to the presence of acoustic reflection surfaces. Sound reflected off such surfaces could sometimes affect the feedback loop that drives the screech tones. The nozzle lip is a good reflection surface. As a part of this work, the effect of nozzle-lip thickness on screech tone frequency and intensity is investigated. There are good agreements between the computed tone frequencies, intensities, and experiments at several nozzle-lip thickness to jet diameter ratios. Thick lip nozzles, as expected, are found to generate the loudest tones. But the change in sound-pressure level between a thin and a thick lip nozzle is found to be quite modest.