Damping of Water Waves by Grid Geometry

Sparse grid geometries may serve for protection of floating solar energy systems, other sensitive geometries located in coastal areas, breakwaters of harbors, and damp by dissipation strong water waves. Wave reduction and amount of energy dissipation by grid geometries are investigated by modeling and laboratory experiments. The wave transmission at the laboratory scale is found to depend on the Keulegan–Carpenter number, $$KC=2 \pi a/D$$ , Stokes number, $$\beta =f D^2/\nu $$ , Reynolds number $$Re=2\pi a f/\nu $$ , and the wave steepness parameter ka (a amplitude, D rod diameter, f wave frequency, $$\nu $$ kinematic viscosity, k wavenumber). Laboratory experiments with KC up to 13, Stokes number up to 230, and Reynolds number up to 3000, show that the wave transmission coefficient reduces when the amplitude is increased. The theoretical modeling, based on a quadratic drag law on the grid members and a drag coefficient of unity, is found to compare favorably to the measured dissipation and the transmission coefficient. Model calculations beyond the experimental range suggest that a sparse grid geometry may almost totally dissipate the incoming wave energy. Tests with a second geometry of very thin members of very small rod diameter have KC up to 80, Stokes number up to 6, and Reynolds number up to 500, exhibit a wave transmission coefficient that is insensitive to the amplitude, however. The energy loss is quadratic in the amplitude in this case. A linear drag law is valid in this parameter range.