An experimental study on gravity waves through a floating viscoelastic cover

We report the results of a laboratory study conducted to investigate the dispersion relation and attenuation of surface waves under viscoelastic floating covers. The study uses oil-doped polydimethylsiloxane (PDMS) with different viscoelastic properties quantified in-situ using a rheometer. An additional test is made with a floating PVC film to estimate possible attenuation in the boundary layer under the cover. Within the frequency range tested, the wavelength deviates from open water case negligibly under the PVC film. But for the PDMS covers, depending on the material property, cover thickness and incident wave period, both wave lengthening and shortening are observed. With thin PDMS covers, the measured wavelength matches well with three theoretical predictions based on different viscoelastic rheological models, while the thick cover shows some discrepancy especially for shorter wave periods. In terms of wave attenuation, however, large discrepancies are observed between the measured data and theoretically predicted results for all PDMS covers. Various mechanisms that could contribute to the discrepancies are considered. The most important mechanism is the boundary layer effect, as demonstrated by the measured wave attenuation from the PVC film. The overall results indicate that the attenuation due to the oscillatory boundary layer underneath the floating cover, which has not been included in the viscoelastic theories, contributes significantly to the discrepancies. Direct measurement of the fluid flow under a periodically flexing cover and the related attenuation rate is a challenging but highly important future study.