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.

https://www.math.fsu.edu/~tam/JFM%20vol.%20615,%20pp%20253-292.%202008.pdf

The sources of jet noise: experimental evidence

The air density fluctuations in the plumes of fully expanded, unheated free jets were investigated experimentally using a Rayleigh-scattering-based technique. The point measuring technique used a continuous-wave laser, fibre-optic transmission and photon counting electronics. The radial and centreline profiles of time-averaged density and root-mean-square density fluctuation provided a comparative description of jet growth. To measure density fluctuation spectra a two-photomultiplier-tube (PMT) technique was used. Cross-correlation between the two PMT signals significantly reduced the electronic shot noise contribution. The density fluctuation spectra were found to be remarkably similar for all Mach number jets. A detailed survey in fully expanded Mach 0.95, 1.4 and 1.8 jets further confirmed that the distribution of various Strouhal frequency fluctuations remained similar, except for a spatial stretching with increased Mach number. In spite of this similarity in flow fluctuations the noise sources in these three jets were found to be significantly different. Spark schlieren photographs and near-field microphone measurements confirmed that Mach wave radiation was present in the Mach 1.8 jet, and was absent in the Mach 0.95 jet. Direct correlation measurement between the flow density fluctuation (cause) and far-field sound pressure fluctuation (effect) shed further light on the sound generation process. For this purpose a microphone was kept fixed at a far-field point, mostly at a distance of 50 diameters and 30° to the flow direction, and the laser probe volume was moved from point to point in the flow. In the Mach 1.8 jet, where the convective velocity of Kelvin–Helmholtz instability waves exceeded the ambient sound speed, significant correlation was measured from the peripheral shear layer, while in the Mach 0.95 jet, where the instability waves had subsonic convective speed, no correlation could be measured. Although the same instability waves were present in both Mach 1.8 and 0.95 jets, the peripheral shear layer of the former was found to be an obvious noise source, while that of the latter was not. Further correlation studies along the jet centreline showed that behaviour in the region downstream of the potential core was similar in all Mach number jets tested, 0:6[les ]M[les ]1:8. Good correlation at low Strouhal frequencies was measured from this region, which started from downstream of the potential core and extended many diameters from there.

Experimental investigation of density fluctuations in high-speed jets and correlation with generated noise

Noise generation is investigated in subsonic isothermal round jets at Mach numbers M = 0.6 and M = 0.9, with Reynolds numbers Re D = 1700 and Re D ≥ 10 5 , using causality methods on data provided by large-eddy simulations. The correlations between broadband sound pressure signals and broadband turbulence signals along the jet axis and the shear layer are calculated. The normalized correlations are found to be significant between the pressure emitted in the downstream direction and centreline flow quantities. They are much smaller in the cases involving flow quantities along the shear layer, and fall for large emission angles. The maximum correlations obtained between centreline turbulence and downstream sound pressure are observed just at the end of the potential core for time delays corresponding to the times of propagation evaluated along ray paths. They also appear to be lower as the Mach number is reduced, and to be enhanced as the Reynolds number is decreased. These correlation levels can reasonably be attributed to the noise source which is predominant at small emission angles. This source is therefore located on the jet centreline at the end of the potential core, in a flow region which is shown to be characterized by a dominant Strouhal number over a large axial distance, by a strong level of intermittency, and by a high convection velocity. This supports the contention that the downstream jet-noise component is connected to the periodic and intermittent intrusion of vortical structures into the jet core.

/pdf/an-analysis-of-the-correlations-between-the-turbulent-flow-4j37wisxs2.pdf

An analysis of the correlations between the turbulent flow and the sound pressure fields of subsonic jets

Using a ‘clean’ jet facility the relationship between the jet flow and its radiation field was studied experimentally in the Mach-number range 0.05 < Mj < 0.20 and a Reynolds-number range 6 × 104 < ReD < 2.3 × 105. The various acoustic source parameters such as strength, frequency and Mach number were varied systematically, and the far-field pressure measured simultaneously. On the basis of these measurements the nature of the sources in the initial shear layer could be characterized. The principal results, equally valid for unexcited and excited jets, are as follows: the acoustic sources are not convected but are located within a confined volume fixed with respect to the nozzle even though they are being generated by moving disturbances in the jet; they are associated with the nonlinear saturation of the unstable wave amplitudes of the shear layer occurring at the vortex-pairing locations; the radiation intensity varies nonlinearly with the source strength and is highly directional, exponential in character.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112083003195

Noise generation by a low-Mach-number jet

To locate noise sources in high-speed jets, the far-field sound pressure fluctuations microphone showed weaker values. By moving the laser probe to various locations in the jet, it was found that the strongest noise source lay downstream of the end of the potential core and extended many diameters beyond. Correlation measurements from turbulent fluctuations along the lip shear layer showed a Mach-number dependency: significant values were measured in supersonic jets, while correlations fell below the noise floor for subsonic jets. Various additional analyses showed that fluctuations from large coherent structures mostly contributed to the measured correlation, while that from small-scale structures fell below the noise floor.

Investigation of noise sources in high-speed jets via correlation measurements

An experiment was conducted on a full‐scale jet engine to investigate the aerodynamic noise generating regions in the free jet. Cross‐correlation measurements were made between the static pressure fluctuations and the farfield radiated sound. These measurements were made for two different static pressure probe positions and a large number of farfield positions (at various angles). In addition, each test geometry was run for four different jet exit velocities. The measured, normalized cross‐correlation functions varied between 0.004 and 0.155. A new Q‐function, based on the above normalized cross correlation is defined and plotted. This function represents the source strength per unit volume within the jet region. This Q‐function shows dependence on the probe position, the angular position of the farfield microphone, and the jet exit Mach number. Third‐octave analyses of both the probe signal and the farfield radiated sound were made. The results show that cross‐correlation techniques are a valuable tool in the investigation of the aerodynamic noise generating regions of an actual jet engine.

Investigation of the aerodynamic noise generating region of a jet engine by means of the simple source fluid dilatation model

An experimental investigation to determine the acoustic transmission from a source in air to a hydrophone in a wind roughened ocean at shallow depression angles is described and the results presented. This experiment was conducted at the NUC Oceanographic Tower in 18‐m‐deep water. Data were obtained for acoustic frequencies of 500–4000 Hz and wind speeds of 3–15 knots. Short pulses of sound were used in order to distinguish the direct path from the bounce paths. The mean energy at the hydrophone was obtained by averaging the direct path energy over many independent pulses. The data show that more energy reaches the hydrophone in the rough ocean than predicted by the smooth surface theory, with the excess increasing for the shallower depression angles. The data also show that the transmission increases with increasing source frequency and ocean roughness and is independent of source altitude for the conditions considered. The experimental results are compared with a geometric rough surface model and show good agreement.Subject Classification: [43]30.20, [43]30.25; [43]20.20.

Experimental investigation of acoustic transmission from air into a rough ocean

Experimental tests were carried out at NASA Ames Research Center. Crosscorrelation measurements were made between the static pressure fluctuations, as measured with a B & K 14‐in. microphone fitted with a nose cone, and the far‐field radiated sound. These measurements were made for two different probe positions and a large number of far‐field positions at various angles. In addition, each test geometry was run for jet exit velocities at four different Mach numbers. The measured, normalized cross‐correlation functions varied between 0.155 and 0.004. These values depended upon the angular position of the far‐field microphone, the jet exit velocity Mach number, and the position of the probe. In addition, the cross‐correlation technique was employed to study the symmetry of the farfield radiated sound. Third‐octave analysis of both the probe signal and the far‐field radiated sound were made. The results of these measurements will be reported.

Patrick M. Hurdle

Papers

Investigation of the aerodynamic noise generating region of a jet engine by means of the simple source fluid dilatation model

Experimental investigation of acoustic transmission from air into a rough ocean

Investigation of the Aerodynamic Noise Generating Region of a General Electric T‐58 Jet Engine by Means of the Simple Source/Fluid Dilatation Model