ATTENUATED LEAKY RAYLEIGH-WAVES

The propagation of leaky Rayleigh waves under the influence of viscous damping and heat conduction in boundary layers is studied using the m atched asymptotic method. Viscosity of the fluid is considered unimpor tant except in a thin viscous boundary layer at the interface. A new c haracteristic equation is obtained in which the effect of boundary lay er is shown by terms associated with R(-1/2) = (omega nu)(1/2)/c(t), w here R is the Reynolds number, omega is the frequency, nu is the kinem atic viscosity of the fluid, and c(t) is the shear velocity of the sol id substrate. One of the numerically obtained solutions gives the leak y Rayleigh wave speed and the attenuation coefficient. It is shown tha t, together with radiation, viscosity and heat conduction in the bound ary layer also affect the attenuation of the leaky Rayleigh waves. Fur thermore, it is shown that, because of the effect of the viscous bound ary layer, the attenuated leaky Rayleigh wave speed can be smaller tha n the Rayleigh wave speed at the interface of a vacuum and a solid sub strate. A critical Reynolds number of about 2500 is found beyond which a viscous boundary layer stops influencing leaky Rayleigh wave propag ation. Finally, a new wave mode sustained by the viscous boundary laye r alone is found in the limit of a small fluid-solid density ratio. Th is mode exists for appropriate frequency and layer thickness combinati ons. For air, the corresponding propagation speed is shown to be highe r than the sound speed and the corresponding attenuation is significan t. These results may be used to improve our interpretation of acoustic signature of materials.