Experimental Investigation and Numerical Modeling of Hydrodynamic Force Characteristics of a Heaving Net

With the increasing world demand for seafood and environmental problems in coastal aquaculture, offshore area has been increasingly expected to be utilized for aquaculture. The offshore aquaculture system has fewer effects on the surrounding marine environment through the rapid diffusion of organic wastes from the cultured fish than the coastal one. The offshore area then provides clean waters for cultured fish. On the other hand, the offshore aquaculture system is subject to the severe natural condition such as typhoon attack. Actually, in the current aquaculture system, the offshore sea cages are always submerged around 10m below the sea surface to avoid the effects of high waves and strong currents. However, the safety of the sea cage against the incident wave has seldom been examined, while that against the water current has been analyzed by model tests in tank and numerical simulation. We investigated the hydrodynamic force properties of a heaving sea cage in the previous work and its results proved that the deformation of net much affects on sea cage’s motion characteristics. To study the most fundamental problem, we investigated the hydrodynamic force properties of an undeformable net in the present study. Nine net models are arranged. These models have square-shaped frame and its inner dimensions are 216 × 216 mm. In these net models, three different twine diameters and three different twine intervals are arranged. The porosities of these models are in the range of 0.39 to 0.91. In addition, an impermeable plate model is also arranged. Forced heaving tests and wave exciting force tests are carried out on each net and plate model. Added mass and damping coefficients, as obtained by forced heaving tests, differ according to porosity of net and oscillation frequency and amplitude. Wave exciting forces, as obtained by wave exciting force tests, also differ according to porosity of net and wave slope and the forces increase with wave amplitude in higher order than one. To compare with experimental results, we theoretically calculate the hydrodynamic forces from a linearized potential flow approach supplementing the effect of porosity. In the boundary condition on the net surface, the vertical velocity to the surface is not equal to zero, but determined by the permeate coefficient of the surface.