WAVE-INDUCED OSCILLATION IN HARBOR WITH POROUS BREAKWATERS

Wave-induced oscillation in a semicircular harbor with porous breakwat ers is studied on the basis of the linear potential wave theory and a newly derived boundary condition for the breakwaters. By separation of variables, general expressions of the velocity potential in terms of unknown constants are obtained in the harbor region and in the open se a. These expressions are matched so that the porous boundary condition and the continuity of mass flux and free surface at the harbor entran ce are satisfied. The velocity potential and, consequently, the free s urface oscillation in the whole domain concerned are thus determined. The frequency response of a harbor with breakwaters of various porous properties is investigated. It is noted that the inertial effect of th e porous structure is mainly to increase the resonant wave number and it does not reduce the amplitude of the resonant oscillation significa ntly. On the other hand, the porous resistance gives rise to little ch ange of the resonant wave number but it reduces the amplitude of the r esonant oscillation effectively. A small but finite permeability of th e breakwater is found to be optimal to diminish the resonant oscillati on.