POWER-FLOW ANALYSIS OF SURFACE-WAVES ON A CYLINDRICAL ELASTIC SHELL IN AN ACOUSTIC FLUID

It is known that an elastic cylindrical lossless shell immersed in an acoustic fluid can guide acoustic energy longitudinally without attenu ation. In fact, for the physical and geometric parameters within suita ble ranges, different surface wave modes, with different angular depen dence around the cylinder, can be supported by the structure. These mo des propagate longitudinally with subsonic phase velocities, and are e vanescent in the transverse direction. The concomitant dispersion rela tionships, which provide the phase velocity versus frequency, are here established by using a thick shell model, in the form of a transverse resonance equation. Numerical results indicate that for frequencies g reater than ka greater-than-or-equal-to approximately 5 the surface wa ve propagation tends to become isotropic. This effect is manifested by the change of the shape of the dispersion curve, relating longitudina l and circumferential wave numbers which turns into a circle from the ''figure 8'' characteristic of the low-frequency regime. The intensity vector field in the fluid and in the shell are studied for different surface wave modes, at different frequencies with the aim of understan ding the mechanism of the power flow. For the axisymmetric case it is found that in the lower portion of the frequency region of existence o f the mode, the power flowing in the fluid is substantially larger tha n in the shell, whereas at higher frequency the situation is reversed. For nonaxisymmetric modes the power flow within the shell is higher t han in the fluid with the exception of the n = 1, mode for which in a limited frequency region the power transported within the fluid is sli ghtly higher. Also for the intermediate frequency range, i.e., approxi mately 2 < ka < approximately 4, the power is transported via a shear and bending mechanism.