THE EFFECT OF MEAN FLOW ON THE RADIATION EFFICIENCY OF RECTANGULAR-PLATES

The acoustic power radiated by a rectangular plate bounded by a unifor m mean flow is investigated. Asymptotic expressions for the modal radi ation efficiency of a simply supported plate are derived, and confirme d by comparison with numerical results. The asymptotic expressions sho w that the frequency at which a mode becomes an efficient radiator is reduced in the presence of mean flow, and hence that the critical freq uency for a vibrating elastic plate is similarly lowered. The effects of these changes on the radiation damping of an aircraft panel in crui se, and on its radiation efficiency on landing, are investigated. The asymptotic results are then extended to more general plate edge condit ions via analysis of appropriate infinite and semiinfinite problems. H ence the '3 dB rule'-that a plate with clamped edges, vibrating well b elow the critical frequency, radiates twice the power of a plate with simple supports-is shown to apply also in the presence of mean flow.