Jet noise

About: Jet noise is a research topic. Over the lifetime, 2958 publications have been published within this topic receiving 55018 citations.

Jet Mixing Noise from Fine-Scale Turbulence

Abstract: It is known that turhulent mixing noise from high-speed jets consists of two components. They are the noise from large turbulent structures in the form of Mach wave radiation and the less directional fine-scale turbulence noise. The Mach wave radiation dominates in the downstream direction. The fine-scale turbulence noise dominates in the sideline and upstream directions. A semiempirical theory is developed for the prediction of the spectrum, intensity, and directivity of the fine-scale turhulence noise. The prediction method is self-contained. The turbulence information is supplied by the k-e turhulence model. The theory contains three empirical constants beyond those of the k-e model. These constants are determined by best fit of the calculated noise spectra to experimental measurements. Extensive comparisons between calculated and measured noise spectra over a wide range of directions of radiation,jet velocities, and temperatures have heen carried out. Excellent agreements are found. It is believed that the present theory offers significant improvements over current empirical or semiempirical jet noise prediction methods in use. There is no first principle jet noise theory at the present time.

Instability waves in a subsonic round jet detected using a near-field phased microphone array

Abstract: We propose a diagnostic technique to detect instability waves in a subsonic round jet using a phased microphone array. The detection algorithm is analogous to the beam-forming technique, which is typically used with a far-field microphone array to localize noise sources. By replacing the reference solutions used in the conventional beam-forming with eigenfunctions from linear stability analysis, the amplitudes of instability waves in the axisymmetric and first two azimuthal modes are inferred. Experimental measurements with particle image velocimetry and a database from direct numerical simulation are incorporated to design a conical array that is placed just outside the mixing layer near the nozzle exit. The proposed diagnostic technique is tested in experiments by checking for consistency of the radial decay, streamwise evolution and phase correlation of hydrodynamic pressure. The results demonstrate that in a statistical sense, the pressure field is consistent with instability waves evolving in the turbulent mean flow from the nozzle exit to the end of the potential core, particularly near the most amplified frequency of each azimuthal mode. We apply this technique to study the effects of jet Mach number and temperature ratio on the azimuthal mode balance and evolution of instability waves. We also compare the results from the beam-forming algorithm with the proper orthogonal decomposition and discuss some implications for jet noise.

A generalized acoustic analogy

Abstract: The purpose of this article is to show that the Navier-Stokes equations can be rewritten as a set of linearized inhomogeneous Euler equations (in convective form) with source terms that are exactly the same as those that would result from externally imposed shear stress and energy flux perturbations These results are used to develop a mathematical basis for some existing and potential new jet noise models by appropriately choosing the base flow about which the linearization is carried out

Measurements of subsonic jet noise and comparison with theory

Abstract: Measurements of the noise field from a 25 mm diameter subsonic air jet are presented. These results are analysed in some detail by determining both the jet velocity dependence and the directivity of the intensity of the radiation in 1/3-octave bands at particular values of the frequency parameter, \[ (fD/V_J)(1-M_c\cos\theta). \] This procedure should ensure that a particular source in a geometrically similar position in the jet is always observed, whatever the jet velocity, diameter and emission angle.These results are compared with the predictions of Lighthil's (1952) theory of convected quadrupoles. It is shown that the theory predicts the variation of the intensity with jet velocity and emission angle provided that the observed frequency is below a certain critical value, which depends on jet diameter and emission angle and is independent of jet velocity. Above this critical frequency, the predicted variations overestimate the measurements and it appears that the convective amplification predicted by the theory is much reduced. The variation of this critical frequency is explained by assuming that substantial interaction occurs between the radiated sound and the jet flow when the wavelength of the sound becomes shorter than the sound path length in the jet flow.