Anomalous absorption of bulk longitudinal acoustic waves in a layered structure with viscous fluid

It is demonstrated theoretically that the absorptivity of bulk longitudinal waves by an ultra-thin layer of viscous fluid between two different elastic media has a strong maximum (dozens of percent) at an optimal layer thickness. This thickness is usually much smaller than penetration depths and lengths of transverse and longitudinal waves in the fluid. The angular dependence of the absorptivity is demonstrated to have significant structure near critical angles of incidence, where the overall dissipation can be noticeably increased. The effect of non-Newtonian properties of the fluid on the anomalous absorption is also investigated. In particular, it is shown that increase of frequency and/or relaxation times in the fluid results in a noticeable decrease of the absorptivity maximum and optimal layer thickness