Acoustics of turbulent flows: a report on Euromech 142 (original) (raw)

Progress in Computational Aeroacoustics in Predicting the Noise Radiated from Turbulent Flows

The International Journal of Acoustics and Vibration, 1997

In recent years a number of simple unsteady flows involving the interaction between vortices have been studied using computational fluid dynamics. These have been extended to include the sound radiated to the far field either by Direct Numerical Simulation, by the use of acoustic analogies, or by the use of Kirchoff methods. For more complex flows results have been obtained using methods based on solving the time dependent large scale flow structures using the unsteady Reynolds Averaged Navier-Stokes equations and then using acoustic analogies to derive the noise in the radiation field. Some success has been made with the latter methods in the predictions of the noise radiated from the flow over cavities at supersonic speeds, where the noise characteristics are dominated by large scale events associated with self-excited flow oscillations. Similar methods are being applied to other self-excited flows, and ultimately to turbulent flows such as jets. The paper describes these methods and results together with some limited preliminary comparisons with experimental data. In an Appendix an extension of Lighthill's equation for aerodynamic noise is presented covering the effects of flow-acoustic interaction.

Large Eddy Simulation of Sound Generation by Turbulent Reacting and Nonreacting Shear Flows

The objective of the present study was to investigate the mechanisms of sound generation by subsonic jets. Large eddy simulations were performed along with bandpass filtering of the flow and sound in order to gain further insight into the role of coherent structures in subsonic jet noise generation. A sixth-order compact scheme was used for spatial discretization of the fully compressible Navier-Stokes equations. Time integration was performed through the use of the standard fourth-order, explicit Runge-Kutta scheme. An implicit low dispersion, low dissipation Runge-Kutta (ILDDRK) method was developed and implemented for simulations involving sources of stiffness such as flows near solid boundaries, or combustion. A surface integral acoustic analogy formulation, called Formulation 1C, was developed for farfield sound pressure calculations. Formulation 1C was derived based on the convective wave equation in order to take into account the presence of a mean flow. The formulation was derived to be easy to implement as a numerical post-processing tool for CFD codes. Sound radiation from an unheated, Mach 0.9 jet at ReD = 400, 000 was considered. The effect of mesh size on the accuracy of the nearfield flow and farfield sound results was studied. It was observed that insufficient grid resolution in the shear layer results in unphysical laminar vortex pairing, and increased sound pressure levels in the farfield. Careful examination of the bandpass filtered pressure field suggested that there are two mechanisms of sound radiation in unheated subsonic jets that can occur in all scales of turbulence. The first mechanism is the stretching and the distortion of coherent vortical structures, especially close to the termination of the potential core. As eddies are bent or stretched, a portion of their kinetic energy is radiated. This mechanism is quadrupolar in nature, and is responsible for strong sound radiation at aft angles. The second sound generation mechanism appears to be associated with the transverse vibration of the shear-layer interface within the ambient quiescent flow, and has dipolar characteristics. This mechanism is believed to be responsible for sound radiation along the sideline directions. Jet noise suppression through the use of microjets was studied. The microjet injection induced secondary instabilities in the shear layer which triggered the transition to turbulence, and suppressed laminar vortex pairing. This in turn resulted in a reduction of OASPL at almost all observer locations. In all cases, the bandpass filtering of the nearfield flow and the associated sound provides revealing details of the sound radiation process. The results suggest that circumferential modes are significant and need to be included in future wavepacket models for jet noise prediction. Numerical simulations of sound radiation from nonpremixed flames were also performed. The simulations featured the solution of the fully compressible Navier-Stokes equations. Therefore, sound generation and radiation were directly captured in the simulations. A thickened flamelet model was proposed for nonpremixed flames. The model yields artificially thickened flames which can be better resolved on the computational grid, while retaining the physically currect values of the total heat released into the flow. Combustion noise has monopolar characteristics for low frequencies. For high frequencies, the sound field is no longer omni-directional. Major sources of sound appear to be located in the jet shear layer within one potential core length from the jet nozzle.

Aero-Acoustics

CFD Open Series, 2023

In developing numerical methods for sound generation and propagation problems, it is natural to try to adapt methods used generally in CFD. To reliably do so, however, we must first examine those characteristics of sound generation and propagation problems that are likely to pose a challenge to traditional methods. The generation of acoustic waves by fluid motion is, by its nature, an unsteady process; steady flows generate no sound. Turbulence modeling, leading to RANS, unsteady RANS, and LES, filters small spatial and high frequency fluctuations from the solution; the impact of such filtering on sound generation has not yet been characterized in any systematic way. Most computational results for sound generation, therefore, have used DNS, where all relevant scales of motion are resolved. Use of LES for aeroacoustics is under active development. Acoustic waves may propagate coherently, with very low attenuation due to viscous effects, over long distances in the flow. Artificial dissipation and dispersion at a level that may be tolerable for hydrodynamic fluctuations can lead to unacceptable attenuation of acoustic waves.

Basic principles of aerodynamic noise generation

Progress in Aerospace Sciences, 1975

This paper gives a simple, unified, analytical description of a wide range of mechanisms associated with the generation of sound by unsteady fluid motion. Topics treated include radiation from compact and non-compact multipole sources, Lighthill's theory of sound emission from free turbulence, effects of source convection, sound generation from flow interaction with solid surfaces and inhomogeneities of the medium, and singular perturbation aspects of the aerodynamic sound problem. The concluding section discusses several areas of current interest and importance, including noise generation by supersonic shear layers, shallow water wave simulation of flow noise, the excess-noise problem, and the general issues bound up with shear layer and jet instability and the orderly structure of turbulent jets.

Aerodynamic sound generation by turbulence in shear flows

2009

The nonlinear aerodynamic sound generation by turbulence has been long analyzed since the foundation of the theory of aerodynamic sound in pioneering paper by Lighthill [1]. Also, it was Lighthill [2] who noted that velocity shear can increase the acoustic wave emission in the aerodynamic situation due to the existence of linear terms in the inhomogeneous part of the analogy equations (second derivative of the Reynolds stress). In [3] it was disclosed and described a linear aerodynamic sound generation mechanism. Specifically, it was shown that the flow non-normality induced linear phenomenon of the conversion of vortex mode into the acoustic wave mode is the only contributor to the acoustic wave production of the unbounded shear flows in the linear regime. From the physical point of view the potential vorticity was identified as the linear source of acoustic waves in shear flows.