To perform a similar type of piezoelectric flextensional transducer analysis, consider the following hints and recommendations:
For both the FLUID129 infinite fluid element (2D) and the Robin boundary condition (3D), include at least a quarter-wavelength of fluid in a given direction, where the wavelength is based on the lowest frequency of interest.
For linear (lower-order) fluid elements, use approximately 14-16 elements per wavelength, where the wavelength is based on the highest frequency of interest.