Investigations on the porous resistance coefficients for fishing net structures

Abstract

The porous media model has been successfully applied to numerical simulation of current and wave interaction with traditional permeable coastal structures such as breakwaters. Recently this model was employed to simulate flow through and around fishing net structures, where the unknown porous resistance coefficients were adjusted by fitting the available experimental data. In the present paper, a new approach was proposed to calculate the porous resistance coefficients based on the transformation of Morison type load model. The transformation follows the principle that the total forces acting on a net panel from Morison type load model should be equal to the forces obtained from the porous media model. In order to account for the interaction effects in-between the twines, two coefficients were introduced, and they were calibrated by minimizing the least square error function. Extensive validation cases were carried out to examine the performance of the numerical model. This includes steady current flow through plane net panels and circular fish cages, and wave interaction with plane net panels. A variety of fishing nets with different solidity ratios were used in the validation cases, from which it was seen that the overall agreement between the numerical and experimental results is fair.

Simulating the hydrodynamic response of a floater–net system in current and waves

“…The porous media model was applied to describe flow through the net structure, where it was represented by a sheet of porous media zone. Jensen et al (2014) revised the formulation for porous media model based on the volume averaged Navier-Stokes equations, which has been applied in our previous works in Chen and Christensen (2016) and Chen and Christensen (2017). However, in the present work, again due to mesh motion, the convective velocity u c should be applied:…”

“…The formula proposed in Chen and Christensen (2016) was applied in the present work to relate the magnitude of C 1 , C 2 and C 3 with the physical parameters of the net cage. Regarding the detailed derivation process, readers are referred to that paper.…”

“…Therefore, the geometry of the net cage at pore scale was not resolved, but the effects of the net were represented by a resistance term. In Chen and Christensen (2016), this term was formulated based on the Morison type force model. In addition, due to the flexibility of the net, the effects of the structural deformation should also be taken into account in the numerical model.…”

“…Therefore, in the present work we will further develop our CFD model presented in Chen and Christensen (2016) and Chen and Christensen (2017). The capability of the numerical model will be extended, where a six degree of freedom motion solver is incorporated in the numerical model.…”

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“…Cornejo et al (2014) nu merically studied the flow-velocity distribution downstream fro m a salmon farm in semidiu rnal current. Chen and Christensen (2016) developed a numerical model based on OpenFOAM to study steady current flow through plane net panels and circular fish cages. While, the above studies relate to flow through the net cage or fish farm, there is paucity of research taking the biofouling problem into account.…”