FSI modeling of fish nets for offshore aquaculture. A two-way coupled fluid-structure interaction model for indus trial scale problem

Abstract

In the development of offshore aquaculture for salmon farming, a number of challenges that are not present in the design of traditional nearshore aquaculture systems arise. On offshore locations, the harsher environment that in a sense is what makes it beneficial to move aquaculture offshore is also what increases the complexity of system analysis. Greater current speeds, wave heights and risk of storms all induce a greater demand for accuracy in numerical analysis of loads acting on systems and the resulting dynamic be havior. To answer this demand, computational fluid dynamics (CFD) is a great tool that can increase the fidelity of system analysis and the understanding of local flow effects affecting the system. Due to the large range of scales involved, it is however not compu tationally feasible to include high fidelity resolution of net structures in analysis of full scale aquaculture systems. This gives rise to the need of including simplified models used to analyze net structures in otherwise high fidelity CFD models. In the present master’s thesis work, a two-way coupled fluid-structure interaction model based in commercial software Star-CCM+ and Abaqus is developed. The model is based on analytic force models developed for fish nets, and includes the net motion and large scale flow effects resulting from said force models. The forces and deflections calculated by the model are validated towards experiments on a circular net cage exposed to steady currents. In the validation it is shown that the use of simplified net models within a CFD framework is a viable method for consistent prediction of the forces acting on a net. For large levels of net deflection however, the structural model used fails to accurately predict the deformed shape of the net cage, causing force measurements to deviate from experimental data. In addition to model validation, the importance of certain model parameters and components is tested. In this process it is shown that for the force model to remain accurate, it is important to include dependence on net solidity and inflow angle as well as an appropriate reduction of velocity downstream of the net.