Direct Numerical Simulation of a Square-Notched Trailing Edge for Jet-Noise Reduction (original) (raw)
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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.
Mechanisms and Active Control of Jet-Induced Noise
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2009
Fundamental mechanisms of jet noise are investigated by means of direct numerical simulation. In the mixing layer, subharmonics of the respective vortex pairing are found to be responsible for the main part of the generated noise which is directed in downstream direction. By modifying the phase shift between introduced disturbances it is possible to diminish or enhance relevant portions of the emitted sound. Optimal control has been applied successfully to a plane mixing layer. In the far field, the mean noise level could be reduced. Depending on the measurement line, some distributed control or anti-noise is generated by the control. A more realistic configuration is achieved by adding a splitter plate representing the nozzle end. Rectangular serrations lead to a breakdown of the large coherent spanwise vortical structures and thus provide a noise reduction of 9dB. mixing behind the trailing edge of the nozzle. However the underlying physical mechanisms are not yet fully understood.
Computing Aerodynamically Generated Noise
Annual Review of Fluid Mechanics, 1997
In contrast to computational aerodynamics, which has advanced to a fairly mature state, computational aeroacoustics (CAA) has only recently emerged as a separate area of study. Following a discussion of the classical field of aeroacoustics as introduced by Lighthill, the paper provides an overview and analysis of the problems associated with utilizing standard computational aerodynamics procedures for acoustic computations. Numerical aspects of computing sound-wave propagation are considered, including assessments of several schemes for spatial and temporal differencing. Issues of particular concern in computing aerodynamically generated noise, such as implementing surface and radiation boundary conditions and algorithms for predicting nonlinear steepening and shocks, are discussed. In addition, the paper briefly reviews alternatives to the conventional finite-difference schemes, such as boundary-element and spectral methods and the uncommon lattice-gas method. MODERN AEROACOUSTICS Sir James Lighthill (1993b) has stated that the aeronautical community has recently entered into the second Golden Age of Aeroacoustics. The first of these eras, as defined by Lighthill, focused on the problems of jet noise and jet engine noise, and lasted from the late 1940s until the mid 1970s. By that time, high-bypass-ratio turbine engines had become the standard for all new transport aircraft, and the initially excessive noise produced by turbojets had been
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The program includes~study of the physics of compressible turbulence, shock-turbulence interactions, reacting flows with heat release, and aerodynamic sound generation in shear flows. The objective of the work in aerodynamic sound generation is to use direct numerical simulations as a tool to study the noise generation processes directlyj 'pecifically,..-wier-t answer the following questions:
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.