This article provides a critical review of computational techniques for flow-noise prediction and the underlying theories. Hybrid approaches, in which the turbulent noise source field is computed and/or modeled separately from the far-field calculation, are afforded particular attention. Numerical methods and modern flow simulation techniques are discussed in terms of their suitability and accuracy for flow-noise calculations. Other topics highlighted include some important formulation and computational issues in the application of aeroacoustic theories, generalized acoustic analogies with better accounts of flow-sound interaction, and recent computational investigations of noise-control strategies. The review ends with an analysis of major challenges and key areas for improvement in order to advance the state of the art of computational aeroacoustics.

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Computational prediction of flow-generated sound

A high-order compact-differencing and filtering algorithm, coupled with the classical fourth-order Runge–Kutta scheme, is developed and implemented to simulate aeroacoustic phenomena on curvilinear geometries. Several issues pertinent to the use of such schemes are addressed. The impact of mesh stretching in the generation of high-frequency spurious modes is examined and the need for a discriminating higher-order filter procedure is established and resolved. The incorporation of these filtering techniques also permits a robust treatment of outflow radiation condition by taking advantage of energy transfer to high-frequencies caused by rapid mesh stretching. For conditions on the scatterer, higher-order one-sided filter treatments are shown to be superior in terms of accuracy and stability compared to standard explicit variations. Computations demonstrate that these algorithmic components are also crucial to the success of interface treatments created in multi-domain and domain-decomposition strategies. Fo...

Very high-order spatially implicit schemes for computational acoustics on curvilinear meshes

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

Navier–Stokes solvers in European aircraft design

This paper analyzes a number of high-order and optimized finite-difference methods for numerically simulating the propagation and scattering of linear waves, such as electromagnetic, acoustic, and elastic waves The spatial operators analyzed include compact schemes, noncompact schemes, schemes on staggered grids, and schemes which are optimized to produce specific characteristics The time-marching methods include Runge--Kutta methods, Adams--Bashforth methods, and the leapfrog method In addition, the following fully-discrete finite-difference methods are studied: a one-step implicit scheme with a three-point spatial stencil, a one-step explicit scheme with a five-point spatial stencil, and a two-step explicit scheme with a five-point spatial stencil For each method, the number of grid points per wavelength required for accurate simulation of wave propagation over large distances is presented The results provide a clear understanding of the relative merits of the methods compared, especially the trade-offs associated with the use of optimized methods A numerical example is given which shows that the benefits of an optimized scheme can be small if the waveform has broad spectral content

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Comparison of High-Accuracy Finite-Difference Methods for Linear Wave Propagation

▪ Abstract 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.

Valana L. Wells

Papers

Computing aerodynamically generated noise