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 goal of this work is to study some numerical solutions of acoustic propagation problems using linearized Eider's equations. The two-dimensional Euler's equations are linearized around a stationary mean flow. The solution is obtained by using a dispersion-relation-preserving scheme in space, combined with a fourth-order Runge-Kutta algorithm in time. This numerical integration leads to very good results in terms of accuracy, stability and low storage. The radiation of a source hi a subsonic and supersonic uniform mean flow is investigated. The numerical estimates are shown to be in excellent agreement with the analytical solutions. Next, a typical problem in jet noise is considered, the propagation of acoustic waves in a sheared mean flow, and the numerical solution compares favorably with ray tracing. The final goal of this work is to improve and to validate the Stochastic Noise Generation and Radiation (SNGR) model. In this model, the turbulent velocity field is modeled by a sum of random Fourier modes through a source term in the linearized Euler's equations. The implementation of acoustic sources in the linearized Euler's equations is thus an important point. This is discussed with emphasis on the ability of the method to describe correctly the multipolar structure of aeroacoustic sources. Finally, a nonlinear formulation of Euler's equations is solved hi order to limit the growth of instability waves excited by the acoustic source terms.

/pdf/numerical-solution-of-acoustic-propagation-problems-using-38qw9lm8va.pdf

Numerical Solution of Acoustic Propagation Problems Using Linearized Euler Equations

The role of the streamwise vortices on the aeroacoustics of a Mach 0.9 axisymmetric jet is investigated using two different devices to generate streamwise vortices: microjets and chevrons. The resultant acoustic field is mapped by sideline microphones and a microphone phased array. The flow-field characteristics within the first few diameters of the nozzle exit are obtained using stereoscopic particle image velocimetry (PIV). The flow-field measurements reveal that the counter-rotating streamwise vortex pairs generated by microjets are located primarily at the high-speed side of the initial shear layer. In contrast, the chevrons generate vortices of greater strength that reside mostly on the low-speed side. Although the magnitude of the chevron's axial vorticity is initially higher, it decays more rapidly with downstream distance. As a result, their influence is confined to a smaller region of the jet. The axial vorticity generated by both devices produces an increase in local entrainment and mixing, increasing the near-field turbulence levels. It is argued that the increase in high-frequency sound pressure levels (SPL) commonly observed in the far-field noise spectrum is due to the increase in the turbulence levels close to the jet exit on the high-speed side of the shear layer. The greater persistence and lower strength of the streamwise vortices generated by microjets appear to shift the cross-over frequencies to higher values and minimize the high-frequency lift in the far-field spectrum. The measured overall sound pressure level (OASPL) shows that microjet injection provides relatively uniform noise suppression for a wider range of sound radiation angles when compared to that of a chevron nozzle.

The effect of streamwise vortices on the aeroacoustics of a Mach 0.9 jet

The objective of this paper is to present an overview of recent advances in computational aeroacoustics (CAA) During the last decade, CAA has developed quite independent of computational fluid dynamics (CFD) There are computational issues that are unique to CAA and are, generally, not considered in CFD In this paper, these issues are discussed and explained In CAA, there is a great need to resolve high-frequency short waves with the minimum number of mesh points per wavelength There is also a special need to minimize numerical dispersion and dissipation associated with wave propagation computation All these have led to the development of large-stencil high-resolution schemes for CAA A careful examination of dispersion and dissipation errors due to spatial and temporal discretization is provided These errors are quantified and analyzed in wave number space through the use of Fourier-Laplace transforms At this time, some of the original computational challenges to CAA have been resolved satisfactor

Computational Aeroacoustics: An Overview of Computational Challenges and Applications

Gudmundsson and Colonius (J. Fluid Mech., vol. 689, 2011, pp. 97–128) have recently shown that the average evolution of low-frequency, low-azimuthal modal large-scale structures in the near field of subsonic jets are remarkably well predicted as linear instability waves of the turbulent mean flow using parabolized stability equations. In this work, we extend this modelling technique to an isothermal and a moderately heated Mach 1.5 jet for which the mean flow fields are obtained from a high-fidelity large-eddy simulation database. The latter affords a rigourous and extensive validation of the model, which had only been pursued earlier with more limited experimental data. A filter based on proper orthogonal decomposition is applied to the data to extract the most energetic coherent components. These components display a distinct wavepacket character, and agree fairly well with the parabolized stability equations model predictions in terms of near-field pressure and flow velocity. We next apply a Kirchhoff surface acoustic propagation technique to the near-field pressure model and obtain an encouraging match for far-field noise levels in the peak aft direction. The results suggest that linear wavepackets in the turbulence are responsible for the loudest portion of the supersonic jet acoustic field.

/pdf/wavepacket-models-for-supersonic-jet-noise-4x6kkavfk1.pdf

Wavepacket models for supersonic jet noise

Acoustic power output from supersonic jets, considering aerodynamic and acoustic characteristics for supersonic exhaust velocities

H. T. Nagamatsu

Papers

Supersonic jet noise