Flow/acoustic interaction in duct inflow

Abstract

Sound generation in the convection of turbulence into a flow intake duct is discussed by examination of an idealized problem. Ideal fluid is in motion with uniform low, subsonic velocity above and parallel to a plane rigid wall. A thin, rigid, semi-infinite plate is parallel to the wall and the fluid flows past its leading edge. The radiated sound is calculated for a turbulent eddy modeled by a weak line vortex that is allowed to convect passively past the leading edge of the plate. Account is taken of the contribution to the radiated sound from the disturbance produced by the convecting vortex in the boundary layers on each side of the plate by means of Howe's (1981) theory of displacement thickness fluctuations (Proc. R. Soc. London Ser. A 374, 543–568), the strength of such disturbances being fixed by a leading edge Kutta condition. It is concluded that the predicted level of the radiated sound is substantially reduced by these boundary layer disturbances relative to when they are neglected. The case of the convection of a frozen two-dimensional gust is also considered. Examination of the analogous problem of plane-wave radiation from the duct shows that the presence of displacement. waves enhanced both the far-field intensity in the ambient fluid and the reflected field within the duct. [Work supported by ARE Teddington, UK.]