Diffraction radiation produced by turbulent boundary layer excitation of a panel

Abstract

An analysis is made of the mechanisms by which sound is produced during low Mach number turbulent boundary layer excitation of a finite panel in a thin plate. The principal source of radiated noise is diffraction at the joints, where the panel is attached to the supporting side-plates, of flexural vibrations excited either directly by the low wavenumber, sub-convective components of the wall pressure fluctuations, or indirectly by diffraction at joints of the very much more energetic, high wavenumber boundary layer pressures. Fluid loading modifies the mechanical properties of the joints. Whereas in vacuo a joint may be sufficiently massive, say, to produce total reflexion of flexural modes and effect the establishment of a strong reverberant field when the panel is subject to external forcing, fluid loading provides a channel by means of which flexural waves can be efficiently transmitted across a joint and thereby reduce reverberant intensities. Numerical results are given for a steel plate in water, with either clamped or pinned joints, and also for the case of a “floating” panel, whose edges are free to vibrate without restraint.