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

Abstract

An analysis is made of the sound and structural vibrations produced by a large-eddy break-up device (LEBU) in turbulent flow over a flexible wall. The action of the LEBU is to reduce the skin friction over an extensive region downstream of the device. The LEBU consists of a two-dimensional airfoil in the form of a rigid strip set at zero angle of attack to the mean flow, and the wall is modelled by a thin plate. The airfoil chord and its stand-off distance from the wall are typically comparable in magnitude to the thickness of the turbulent boundary layer. Attention is given to underwater applications, where the fluid loading is large. The unsteady pressures developed in the vicinity of the LEBU excite bending waves in the plate, and can lead to a significant transfer of energy from the mean flow to structural vibrations. A substantial increase in the sound generated by the boundary layer turbulence can also occur. For a steel plate in water, it is shown that the acoustic and flexural wave powers developed by the interaction of the turbulence with the LEBU can in practice be several orders of magnitude larger than that produced by the turbulent boundary layer interacting with the whole of the plate in the absence of the break-up device.