Aerodynamic sound generation by turbulent flow in the vicinity of a scattering half plane

The presence of the edge of a half plane in a turbulent fluid results in a large increase in the noise generated by that fluid at low Mach numbers. The parameter which is important is the product 2kr₀, where r₀ is the distance of the centre of an eddy from the edge. Eddies which satisfy the inequality 2kr₀ 1 have the sound output of the quadrupoles associated with the fluid motion in a plane normal to the edge increased by the factor (kr₀) ^-3. There is no enhancement of the sound from the longitudinal quadrupoles with axes parallel to the edge: the rr, θ r and θθ quadrupoles are the dominant sound sources. The far field sound intensity induced by these sources depends upon the fifth power of a typical fluid velocity. The intensity has a directional dependence on cos 2 ½θ if the half plane is rigid and sin 2 ½θ if it is a pressure release surface, θ = 0 being a direction in the half plane. If the eddies are far from the edge so that (kr₀) ^1{2} 1 then the farfield sound has the same features as would be predicted by geometrical acoustics. The edge does not produce any significant sound amplification.