Influence of grazing flow and dissipation effects on the acoustic boundary conditions at a lined wall

Aurégan, Yves; Starobinski, Rudolf; Pagneux, Vincent

doi:10.1121/1.1331678

Cited by 114 publications

(122 citation statements)

References 16 publications

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“…Another modified Myers condition was proposed by Auré gan et al [17] where an additional parameter is introduced to account for the effects of the viscous boundary layer. This impedance condition has not been considered here since the other impedance conditions and the exact solution do not include viscous effects.…”

Section: Discussionmentioning

confidence: 99%

A comparison of impedance boundary conditions for flow acoustics

Gabard

2013

Journal of Sound and Vibration

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a b s t r a c tAcoustic liners remain a key technology for reducing community noise from aircraft engines. The choice of optimal impedance relies heavily on the modeling of sound absorption by liners under grazing flows. The Myers condition assumes an infinitely thin boundary layer, but several impedance conditions have recently been proposed to include a small but finite boundary layer thickness. This paper presents a comparison of these impedance conditions against an exact solution for a simple benchmark problem and for parameters representative of inlet and bypass ducts on turbofan engines. The boundary layer thickness can have a significant impact on sound absorption, although its actual influence depends strongly on the details of the incident sound field. The impedance condition proposed by Brambley seems to provide some improvements in predicting sound absorption compared to the Myers condition. The boundary layer profile is found to have little influence on sound absorption.

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Section: Discussionmentioning

confidence: 99%

A comparison of impedance boundary conditions for flow acoustics

Gabard

2013

Journal of Sound and Vibration

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“…In addition, the analysis here assumes a flat impedance surface, where as the Myers boundary condition [8] allows for arbitrary curvature. The boundary layer was also considered to be inviscid, and other boundary conditions that include viscosity [34,35,38,42,43] could also be investigated. Numerically, we assume a finite difference numerical scheme with a constant grid spacing ∆x, and either a variable grid spacing or indeed a different scheme such as a finite element simulation could be considered.…”

Section: Resultsmentioning

confidence: 99%

“…While we are concerned here with only inviscid flows, the techniques described here will be equally applicable to modified boundary conditions that incorporate viscosity [34][35][36], provided the modified boundary conditions remain well-posed. It is worth noting that viscosity by itself does not regularize the illposedness due to the assumption of an infinitely thin boundary layer [35], but viscosity is likely to be important for accuracy in certain situations [37,38], and for stabilizing the well-posed inviscid instabilities [36].…”

Section: Introductionmentioning

confidence: 99%

Time-domain implementation of an impedance boundary condition with boundary layer correction

Gabard

2016

Journal of Computational Physics

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A time-domain boundary condition is derived that accounts for the acoustic impedance of a thin boundary layer over an impedance boundary, based on the asymptotic frequency-domain boundary condition of Brambley [ , AIAA J. 49(6), pp. 1272[ -1282. A finite-difference reference implementation of this condition is presented and carefully validated against both an analytic solution and a discrete dispersion analysis for a simple test case. The discrete dispersion analysis enables the distinction between real physical instabilities and artificial numerical instabilities. The cause of the latter is suggested to be a combination of the real physical instabilities present and the aliasing and artificial zero group velocity of finite-difference schemes. It is suggested that these are general properties of any numerical discretization of an unstable system. Existing numerical filters are found to be inadequate to remove these artificial instabilities as they have a too wide pass band. The properties of numerical filters required to address this issue are discussed and a number of selective filters are presented that may prove useful in general. These filters are capable of removing only the artificial numerical instabilities, allowing the reference implementation to correctly reproduce the stability properties of the analytic solution.

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“…Viscosity has been included in a number of studies [15][16][17] aimed at deriving an effective impedance boundary condition that accounts for viscosity and shear in the boundary layer and may be applied at the wall of a uniform inviscid flow. In order to arrive at closed-form analytical solutions, these studies all make simplifying assumptions.…”

Section: Introductionmentioning

confidence: 99%

Acoustics in a Two-Deck Viscothermal Boundary Layer over an Impedance Surface

Khamis

2017

AIAA Journal

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The acoustics of a mean flow boundary layer over an impedance surface or acoustic lining are considered. By considering a thick mean flow boundary layer (possibly due to turbulence), the boundary layer structure is separated asymptotically into two decks, with a thin weakly viscous mean flow boundary layer and an even thinner strongly viscous acoustic sublayer, without requiring a high-frequency. Using this, analytic solutions are found for the acoustic modes in a cylindrical lined duct. The mode shapes in each region compare well with numerical solutions of the linearised compressible Navier-Stokes equations, as does a uniform composite asymptotic solution. A closed-form effective impedance boundary condition is derived which can be applied to acoustics in inviscid slipping flow to account for both shear and viscosity in the boundary layer. The importance of the boundary layer is demonstrated in the frequency domain, and the new boundary condition is found to correctly predict the attenuation of upstream-propagating cuton modes, which are poorly predicted by existing inviscid boundary conditions. Stability is also investigated, and the new boundary condition is found to yield good results away from the critical layer. A time-domain formulation of a simplified version of the new impedance boundary condition is proposed.

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Influence of grazing flow and dissipation effects on the acoustic boundary conditions at a lined wall

Cited by 114 publications

References 16 publications

A comparison of impedance boundary conditions for flow acoustics

A comparison of impedance boundary conditions for flow acoustics

Time-domain implementation of an impedance boundary condition with boundary layer correction

Acoustics in a Two-Deck Viscothermal Boundary Layer over an Impedance Surface

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