Absorption of sound at a slot in a splitter plate in a mean-flow duct

The mechanism of sound dissipation caused by vorticity in perforated screens used to attenuate aerodynamic sound in mean flow ducts is investigated theoretically. Using the Howe (1981) linearized wave theory of unsteady shearing flow over a slot, the mean-flow/acoustic energy exchanges are examined for a thin rigid plate lying along the centerline of a duct and having a single slot. Predictions are made, based on the Strouhal numbers, of the conditions necessary for the acoustic energy dissipation. It is shown that accurate energy is absorbed when the Strouhal number is sufficiently small; at higher frequencies, there exists an infinite set of discrete frequency intervals within which there is a net production of acoustic energy at the slot at the expense of the kinetic energy of the mean flow.