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.

Aeroacoustics of volcanic jets: Acoustic power estimation and jet velocity dependence

“…Broadband shock-associated noise (2) and screech tones (3) both require well-formed and steady internal shock structures, which are difficult to achieve in volcanic systems because of, e.g., nozzle irregularity, lithics, or tephra larger than fine ash, wind, and evolution of vent shape. In addition, jet noise research indicates that strong broadband shock-associated noise is not [Tam et al, 1996;Tam, 1998;Tam et al, 2008]. The two jet mixing noise sources are the fine-scale turbulence (FST) and large-scale turbulence (LST).…”

“…(b) In the case of volcano acoustics field experiments, sampling of the hypothesized volcanic jet noise as a function of angle from the jet axis (arc with arrowheads) is not usually possible; we are typically restricted to observations at a limited angular range, e.g., the infrasound sensor indicated as a black triangle (see also Figure 3). The figure is modified from Tam et al [2008]. seen for military jet or rocket noise [Neilsen et al, 2013a].…”

“…Previous studies in the field of volcano acoustics have used results from the classical aeroacoustics literature to link acoustic signals with volcanic gas exit velocity [e.g., Woulff and McGetchin, 1976;Vergniolle and Caplan-Auerbach, 2006;Caplan-Auerbach et al, 2010;Ripepe et al, 2013]. However, some of the key results used in these studies have largely been abandoned in modern aeroacoustics research [e.g., Tam et al, 2008;Viswanathan, 2009]. In this paper, we highlight results from modern jet noise research and discuss their implications for volcano acoustics.…”

A technique of flux reconstruction at the interfaces of nonconforming grids for aeroacoustic simulations

“…The spectra at the three locations display broadband shapes, which is typical of subsonic turbulent jet noise. 36 Upstream of the grid interface, at x = 13.4r 0 , the spectra from the two simulations are very similar, which is expected since the two meshes are identical for x ≤ 14r 0 . Downstream of the interface, at x = 14.7r 0 and x = 20r 0 , the acoustic spectra predicted by the two simulations are in good agreement for Strouhal numbers St < 2.…”

“…That monoblock simulation is carried out using the same computational domain with mesh spacings Δx = Δy = Δ. By comparing the pressure p interface with p no-interface instead of with the analytical vortex solution (36), the error thus obtained only results from the effects of the nonconforming grid and not from discretization errors. In addition, the pressure field differences Δp = p interface − p no-interface are only computed at the left-hand side of the block interface where the mesh is similar in the two computations.…”