The interaction of vertical flexible-membrane breakwaters with waves

The interaction of oblique water waves with tensioned, inextensible, vertical flexible membranes wave barriers is investigated in the context of two-dimensional linear wave-body interaction theory. The vertical membrane is hinged at the sea floor and attached to solid cylindrical buoys at its top. Both submerged and surface-piercing buoy/membrane wave barriers of this type are considered, and dual buoy/membrane wave barriers placed in parallel are also considered. A two dimensional multi-domain boundary element method is developed based on a discrete-membrane dynamic model and simple-source distribution (or modified Bessel function in oblique seas) over the entire fluid boundaries. Since the boundary condition on the membrane is not known in advance, the membrane motion and velocity potentials are solved simultaneously. The interaction of the buoy and membrane is taken into consideration through an appropriate boundary condition at the joints. When the buoy is absent, analytic solutions are obtained based on the eigenfunction expansion of the velocity potential in multi-fluid domains and a continuous tensioned-string dynamic model for membrane. The accuracy and convergence of the developed program are checked using the energy-conservation formula. The numerical results are verified by checking several limiting cases and comparisons with analytic solutions, if available. To further verify our theoretical and numerical models, a series of experiments are conducted in the two-dimensional glass-walled wave tank. The experimental results were in reasonable agreement with the computed results. Using the developed computer program, the performance of single or double surface-piercing or submerged buoy/membrane wave barriers is tested with varying buoy radius and draft, water depth, membrane length, mooring-line types and stiffness, submergence depths, and gapped ratios. It is found that the efficiency of various buoy/membrane breakwaters depend critically on the buoy-radius to water depth ratio, mooring conditions, and tuned parameters. From our parametric studies, it can be concluded that buoy/membrane wave barriers can function as very effective breakwaters if they are properly designed.