A numerical framework for modelling the dynamics of open ocean aquaculture structures in viscous fluids

Abstract

This paper presents a complete numerical framework for modelling open ocean aquaculture structures in waves and current using Computational Fluid Dynamics (CFD). A structural dynamics model is incorporated to account for the motions and deformations of the net. It is based on the lumped mass method, a non-linear material law and implicit time step advancing. The presence of the porous net is considered in the momentum equations of the fluid using a forcing term based on Lagrangian-Eulerian coupling and the acting forces on the net. The proposed framework is suitable for simulating the interaction of nets of arbitrary geometry and under large motion with fluids including complex free surfaces. This is in contrast to existing models which either neglect important non-linearities, the physical interaction with the fluid or are limited to certain net geometries. In addition, the fluid-structure interaction of floating objects with mooring lines, nets and fluid is accounted for in the model. A new floating algorithm is presented for the interaction of the fluid and the rigid structure. It is based on a continuous direct forcing immersed boundary method and a level set representation of the object in the Eulerian fluid domain. This effectively avoids computationally expensive reconstruction processes of existing approaches and enables the application to large three-dimensional structures. The complete numerical framework is first validated against existing measurements for forces on rigid and flexible nets, net deformations and moored-floating structures with and without a net in waves. Then, a semi-submersible and a mobile floating open ocean aquaculture structure are investigated, and the possibilities of the numerical approach are highlighted.