Effects of Jet Noise Source Distribution on Acoustic Far-Field Measurements

Abstract

Jet noise production is well known to be of a distributed nature along the jet, with high frequency noise components radiating from locations close to the nozzle exit and low frequency noise being produced farther downstream, around the end of the potential core. Such a distributed source implies that measurements need to be made at a significant distance from the source in order to be in the true geometric (acoustic) far field. The current study presents measurements of fully expanded M j = 1.5 jets operating with cold air and heat simulated at TTR = 2.2 made at various positions in the acoustic field, some short of the minimum distance required to be in the true geometric far field. A close look is taken at the details of the noise generation region in order to better understand the mismatch between spectra measured at various acoustic field radial locations. A processing method is presented to correct for near-field effects and efficiently compare near and far field spectra with unprecedented accuracy. This technique is then further used to clarify the nonlinear propagation effects that can be observed at the high frequency end of high speed jets noise.

Toward Active Control of Noise from Hot Supersonic Jets

“…Spectra that are filtered around higher center frequencies (figure 13 d-/) appear to shift their origins upstream, but never originate from the nozzle exit plane. These findings are in agreement with Kuo et al (2010) who observed that the angular orientation of the lobe of peak intensity remained mostly unchanged with frequency for an unheated Mach 1.5 jet. Contrary to the current result, Kuo et al (2010) observed a broadening of the lobe at higher frequencies and concluded that this was more consistent with the notion that the noise pattern produced by the fine-scale turbulence was omni-directional.…”

“…This is caused by a considerable drop in axial phase velocity of the instability waves for low frequencies (St < 0.2) (Troutt & McLaughlin 1982); shallow propagation of low frequency noise was observed in a similar fashion by Kuo et al (2010). For frequencies centered around Stpj = 0.2 in figure 13 (c), the peak radiation angle follows along the Mach wave radiation angle.…”

“…Applications can be found in the literature and include the work of Mclnerny & Ölcmen (2005), Petitjean et al (2006), Gee et al (2007a) and Kuo et al (2010). Originally derived by Morfey & Howell (1981), this indicator starts with the general from of the Burgers equation for spherically outgoing waves.…”

Acoustic waveforms produced by a laboratory scale supersonic jet

“…This is especially problematic close to the hydrodynamic region where pressure waves comprise a superposition of both acoustic and evanescent signatures [37]. Choosing the wrong path artificially triggers the appearance of nonlinear distortion in the signal that one might interpret as a sign of wave steepening; this problem has been addressed in past studies [2,20]. Here we attempted to place the microphones as close to the jet as possible without them being endangered, and without contaminating the signals with evanescent signatures.…”

Toward Active Control of Noise from Hot Supersonic Jets

“…At low frequencies, figure ' 'a, the peak intensity direction is shallower than <p = 45° and is related to a considerable drop in the axial phase velocity of the instability waves for low frequencies (St < 0.2). 9 Shallow propagation of low frequency noise was also observed by Kuo et al (2010).…”