Evaluation of acoustic sources in an excited unstable laminar shear layer

Hu, Z.W., Morfey, C.L. and Sandham, N.D. (2006) Evaluation of acoustic sources in an excited unstable laminar shear layer. International Journal of Aeroacoustics, 5 (1), 91-108. (doi:10.1260/147547206775220416).

Abstract

The radiation of sound from artificial sources in a developing shear layer is studied numerically, in order to address an issue that arises in acoustic analogy models of jet noise: namely whether the unstable response of the mean-flow shear layer has a significant effect on sound radiation. Direct numerical simulation of a forced two-dimensional compressible laminar mixing layer has been carried out at a Reynolds number of 250, based on the mixing layer initial vorticity thickness and the upper free-stream velocity. The free-stream Mach numbers of the mixing layer are 0.9 and 0.45. The flow is excited with a single-frequency body force field that is acoustically compact and is derived from an applied-stress distribution. Sound radiation from the mixing layer is calculated at the forcing frequency, and compared with radiation from a uniform flow under the same forcing. Comparisons are shown for the most-unstable forcing frequency over a wide amplitude range. The pressure radiated on either side of the mixing layer differs very little from that radiated into a uniform flow of the same Mach number under the same forcing, although the higher forcing amplitudes used are sufficient to trigger the non-linear process of vortex roll-up in the case of the mixing layer. The dominant source position for the radiated pressure at the forcing frequency is estimated via a wavenumber–frequency domain analysis. It is found to be close to the location of the applied forcing, with little contribution from mixing-layer vortical structures that develop downstream.

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Contributors

Author: N.D. Sandham

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