EXCESS ATTENUATION OF LEAKY LAMB WAVES DUE TO VISCOUS-FLUID LOADING

In two recent papers [J. Acoust. Soc. Am. 97, 3191-3193 (1995) and 98, 1057-1064 (1995)], Zhu and Wu presented an analytical technique to as sess the effect of viscous fluid loading on the propagation properties of Rayleigh and Lamb waves in fluid-loaded solids. They modeled the v iscous fluid as a hypothetical isotropic solid having rigidity C-55=-i omega eta, where eta denotes the viscosity of the fluid and omega is the angular frequency. In this way, the vorticity mode associated with the viscosity of the fluid is formally described as the shear-mode in the fictitious solid. In this paper this technique is further develop ed by removing certain inconsistencies that unnecessarily reduce the a ccuracy and the range of validity of Zhu and Wu's results. By properly accounting for viscous effects on the bulk compressional wave in the fluid and applying a rigorous treatment of the held equations and boun dary conditions, the exact dispersion equations that are not limited t o low frequencies and viscosities are derived. Examples of these resul ts are presented to illustrate the effect of fluid viscosity on the lo west-order symmetric and antisymmetric Lamb modes. One interesting fea ture revealed by these calculations is the presence of a sharp minimum in the viscosity induced attenuation of the lowest-order symmetric mo de of thin plates either immersed in or coated with a viscous fluid. T his minimum occurs at a particular frequency where the otherwise ellip tical polarization of the surface vibration becomes linearly polarized in the normal direction. (C) 1997 Acoustical Society of America.