Effects of Jet Temperature on Broadband Shock-Associated Noise

Abstract

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 Mach disk 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 (M j 1.2 and 1.4), and two underexpanded (M j 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. Nomenclature a ∞ = ambient acoustic velocity D = exit diameter of the nozzle D j = fully expanded diameter of the jet plume f = frequency f c = characteristic frequency; U j ∕D j I = intensity of shock noise L = shock cell spacing M a = acoustic Mach number; U j ∕a ∞ M c = convective Mach number; U c ∕a ∞ M d = design Mach number of the nozzle M j = average Mach number of the fully expanded jet R = physical distance of the microphones from the jet exit R e = Reynolds number R prop = propagation distance for the acoustic measurements T o = jet stagnation temperature T ∞ = ambient temperature St = Strouhal number; f∕f c U c

Inclusion of Broadband Shock-Associated Noise in Spectral Decomposition of Noise from High-performance Military Aircraft

“…The F-35B BBSAN spectral shape and trend in peak frequency hold for all four engine conditions. The expected variations in peak frequency across engine condition also agree with prior laboratory-scale studies in which the temperature and Mach number were varied [29]. The variation in peak level and width do not agree with laboratoryscale studies nor the F/A-18E study.…”

“…The use of % is employed here because the jet parameters are not available for high-performance military aircraft engines. This abbreviated form is also convenient because it ties more directly with the subsequent simplifications by Kuo et al [29]. The Sr U in the denominator is the Strouhal number (Sr = % / ) if St > 0.35, otherwise Sr U = 0.35, and is measured relative to the jet centerline, and V is the first order Bessel function of the first kind.…”

“…After additional experimental results that explored the shock-turbulence interactions [21], evaluated the BBSAN spectral shape [24], and investigated the role of shock structures and jet mixing layer development [25 ], Tam and colleagues [ 26,27,28] proposed a more sophisticated BBSAN spectral model. More recently Kuo et al [29] provided a simplification of Tam's model that works well for the first spectral peak.…”

“…Because of these consideration, the Tam et al empirical similarity spectra for turbulent mixing noise [10] and Kuo et al's BBSAN model [29] are used to identify the contributions of the different noise sources. These two spectral models are described in this section.…”

Summary of “Supersonic Jet and Rocket Noise”

“…For non-ideally expanded jet flow, BBSAN contributions to the spectral shape need to be included in spectral decompositions in the sideline and forward directions. A model proposed by Tam et al 22 and later simplified by Kuo et al 23 provides a spectral function that models the BBSAN spectral shape. The ability of the BBSAN and similarity spectra shapes to account for the measured spectra is evaluated for ground-based microphones that covered a spatial aperture from 35 to 152 degrees.…”