Hydrodynamic behaviours of elastic net sheets in waves

Abstract

From previous experimental investigations on a heaving sea cage, it has been observed that the hydrodynamic forces and motions of the sea cage are strongly affected by the deformation of the cage net. As a fundamental study, the hydrodynamic behaviours of elastic net sheets with a circular or a square shape are studied in the present work. The net sheet is regarded as a piece of elastic porous membrane. The deformation of the net sheet is expanded into a series of natural modes deduced from the free vibration of the membrane (dry mode). The boundary value problem of fluid motion is formulated on the basis of the linear wave theory. A Darcy-type law is applied to the porous boundary. The problem is solved either by the eigenfunction expansion (for the circular shape only) or by the hybrid method with the Rankine source as the Green’s function. To validate the theoretical work and the numerical calculation, experiments are carried out in a wave basin. The models used in the tests are square shaped with a metal frame. The net part of the models is made of elastic cords with pre-tension. Both diffraction tests and forced heaving tests are performed. In addition to the vertical force, the deformation of the net surface is recorded by video cameras and analysed with a video image analysis system.