ON THE BEHAVIOUR OF FLUID-LOADED SANDWICH PANELS WITH MEAN FLOW

This paper considers a fluid-loaded sandwich panel, made up of layers of relatively rigid skin material surrounding a layer of less dense core material, with an unsteady point driver in steady mean flow. For a simple plate Brazier-Smith and Scott, and Crighton and Oswell predicted a number of unusual phenomena, including absolute instability, a neutral mode with its group velocity pointing towards the driver, and negative energy waves. The possible occurrence of these phenomena for realistic parameter values is considered. It is shown that absolute instability is unlikely to be found at normal speeds, but in contrast the anomalous propagation mode is present, although over a limited frequency range. It is shown that the negative energy waves could also well occur in practice. Study of both the full dispersion relation and asymptotic analysis demonstrate that at very low frequencies it is the ratio of core to skin stiffnesses and the flow speed which control the dynamics, but at higher frequencies shear coupling and rotational effects become important as well. Consideration of the relative magnitudes of the energy fluxes leads to the unexpected conclusion that for predominantly shear waves more energy is carried in the fluid than is carried in the plate.