An approximation finite element surface impedance representation for energy absorbing layers

A procedure for determining the scattered pressure field due to a monochromatic harmonic wave that is normally incident upon an energy absorbing submerged structure is treated. The case where the structure is modeled with finite elements and the surrounding fluid is represented with either acoustic finite elements or a boundary integral approach is considered. Finite element modeling problems arise when the details of the submerged structure in the neighborhood of the fluid-structure interface are not macroscopically homogeneous and in particular when the inhomogeneities are small relative to the acoustic wavelength. An approximate procedure is presented for replacing the detailed microscopic representation of the layered surface configuration with an equivalent simple surface impedance finite element which is specially designed to work at the frequencies of interest.