The sources of jet noise: experimental evidence

Abstract

The primary objective of this investigation is to determine experimentally the sources of jet mixing noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyse the characteristics of the far-field sound and then infer back to the characteristics of the sources. The first approach is to make use of the spectral and directional information measured by a single microphone in the far field. A detailed analysis of a large collection of far-field noise data has been carried out. The purpose is to identify special characteristics that can be linked directly to those of the sources. The second approach is to measure the coherence of the sound field using two microphones. The autocorrelations and cross-correlations of these measurements offer not only valuable information on the spatial structure of the noise field in the radial and polar angle directions, but also on the sources inside the jet. The third approach involves measuring the correlation between turbulence fluctuations inside a jet and the radiated noise in the far field. This is the most direct and unambiguous way of identifying the sources of jet noise. In the fourth approach, a mirror microphone is used to measure the noise source distribution along the lengths of high-speed jets. Features and trends observed in noise source strength distributions are expected to shed light on the source mechanisms. It will be shown that all four types of data indicate clearly the existence of two distinct noise sources in jets. One source of noise is the fine-scale turbulence and the other source is the large turbulence structures of the jet flow. Some of the salient features of the sound field associated with the two noise sources are reported in this paper.

Experimental investigation of the transient bleed valve noise

“…It is also reasonable to assume that large turbulent structures will generate larger pulses than small ones. The experimental results obtained in the case of diaphragms are thus in agreement with the existence of the dual source responsible for the mixing noise and are consistent with the work of Tam et al 4 It can also be seen that the width of the correlation peak on the first microphone tends to increase with the diameter of the diaphragm. The increase of the diaphragm size seems also to increase the large turbulent structure size.…”

“…1, 2 In the case of subsonic jets, the radiation of the large coherent structure is attributed to the periodical intrusion of these structures at the end of the potential core of the jet 1, 3 whereas in the case of supersonic jets it is produced by Mach wave radiation generated by the supersonic convection of these structures. 4,5 In the case of supersonic jets, if the ejection pressure of the jet is not equal to the ambient pressure, shock cells are formed in order to balance the two pressures. These shock cells are responsible for the generation of a new acoustic source called shock-associated noise.…”

“…Since radiation from these two sources appears to be very different, the aim is here to identify these two distinct contributions in the acoustic measurements. The following post-processing are strongly inspired by the experimental work of Tam et al 4 A means to identify these two sources is to analyze intercorrelation and autocorrelation on the different microphones. This post-processing tool will allow to obtain information on the spatial structure of the noise field in the radial and angular direction.…”

Understanding jet noise

“…The special role of linear instability waves for jet noise was first noted by Michalke & Fuchs (1975), Laufer et al (1976), Michalke (1977), Moore (1977) and, later, by Tam & Auriault (1999), Tam & Burton (1984), Tam & Chen (1994) and Tam et al (2008). After an extensive analysis of experimental data, Tam et al (1996) empirically constructed two seemingly universal 'similarity sound spectra' that are able to give the best fits to all jet-noise data available to them for small (1982) and (b) the comparison of similarity spectra, FSS (G) and LSS (F ) with the measurements from the JEAN experiment (Power et al 2004).…”

A second golden age of aeroacoustics?

“…(c) Data-driven modelling and uncertainty quantification For jet noise, experimental evidence suggests that supersonic turbulent mixing noise may be composed of two different components, corresponding to the large and small scales of turbulent motion [155,156]. Advanced data-decomposition methods applied to experimental farfield acoustic measurements from high-speed jets failed, however, to separate the data into two different statistically independent components for all radiation angles [157].…”