Reduction of the performance of a noise screen due to screen-induced wind-speed gradients. Numerical computations and wind-tunnel experiments

Downwind sound propagation over a noise screen is investigated by numerical computations and scale model experiments in a wind tunnel. For the computa tions, the parabolic equation method is used, with a range-dependent sound- speed profile based on wind-speed profiles measured in the wind tunnel and wind-speed profiles computed with computational fluid dynamics (CFD). It is found that large screen-induced wind-speed gradients in the region behind the screen are responsible for a considerable reduction of the performance of the screen, for receivers near the boundary of the shadow region behind the screen. The screen-induced wind-speed gradients cause a considerable re duction of the size of the shadow region. If the screen-induced wind-speed gradients are taken into account, computed sound-pressure levels near the s hadow boundary are in reasonable agreement with levels measured in the wind tunnel. In contrast, computed levels are considerably lower, up to 10 dB, if the screen-induced wind-speed gradients are ignored. This implies that t he performance of a screen can be considerably improved if the screen-induc ed wind-speed gradients can be suppressed, e.g., by the use of "vented" scr eens. (C) 1999 Acoustical Society of America. [S0001-4966(99)02504-7].