The effect of large-eddy breakup devices on flow noise

Abstract

The installation of large-eddy breakup devices (LEBUs) or ‘flow manipulators’ in a turbulent boundary layer over a rigid plane surface is known to lead to reductions in skin-friction coefficient, turbulence intensity and fluctuating Reynolds stress. We investigate the effect of such devices on the surface pressure spectrum and the far-field sound radiation. A model problem, in which a two-dimensional elliptical vortex is convected past a LEBU, is solved analytically in the low-Mach-number limit. The main noise source mechanisms are identified in this idealized problem and we go on to obtain scaling laws for the sound produced by a turbulent boundary-layer flow over a LEBU. The introduction of a LEBU reduces the strength of the Lighthill quadrupole source terms, but it produces an additional dipole source. However, the pressure fluctuations in this dipole field decay rapidly with distance from a LEBU, and we find that an array of LEBU's could have a beneficial effect on the flow noise for radian frequencies which are large in comparison with c/30Δ, where c is the sound speed and Δ denotes the boundary-layer thickness. At lower frequencies the LEBUs are predicted to increase the flow noise.

Structural and acoustic noise generated by a large-eddy break-up device

“…Indeed, in the absence of the lebu the fluctuating Reynolds stresses constitute a distribution of weak quadrupole sources generating sound whose intensity varies as p0 U3Mb per unit area of the wall ( density and U the velocity of the main stream), whereas unsteady forces exerted on the lebu by the flow are equivalent to a distribution of dipoles that radiate more efficiently when M < 4 1, with radiation intensity per unit p0U3]\P. Dowling (1989) has investigated analytically the acoustic radiation from a lebu consisting of a two-dimensional strip at zero mean angle of attack in the special case in which the wall is rigid. The unsteady lift dipole (whose amplitude is proportional to p0uU, where u is the turbulence velocity) generates sound that is cancelled by destructive interference with the radiation from an image dipole in the wall.…”