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.
Highlights
The demand for aquatic food products is expected to increase by thirty million tonnes until 2050 (Ferreira et al, 2014), and aquaculture production has to play a significant role in satisfying the demand to avoid overfishing
Computational fluid dynamics (CFD) offers a two-way coupled numerical approach that can be applied to understand the environmental loading and structural response by modelling the hydrodynamic forces affecting the dynamics of the floating structure, the flexible net and mooring system, and their effect on the surrounding fluid
Based on the review of existing approaches, this paper presents itself as the first attempt to establish a numerical framework using Computational Fluid Dynamics (CFD) for Open ocean aquacul ture (OOA) structures
Summary
The demand for aquatic food products is expected to increase by thirty million tonnes until 2050 (Ferreira et al, 2014), and aquaculture production has to play a significant role in satisfying the demand to avoid overfishing. Computational fluid dynamics (CFD) offers a two-way coupled numerical approach that can be applied to understand the environmental loading and structural response by modelling the hydrodynamic forces affecting the dynamics of the floating structure, the flexible net and mooring system, and their effect on the surrounding fluid. The momentum equation is solved in the whole domain including a forcing term This additional term is calculated from the rigid body velocities at each grid point and smeared over the fluid-solid interface using a smoothed Heaviside step function. A forcing term is calculated from the hydrodynamic loads on the net and distributed on the fluid domain using an interpolation kernel This method was successfully validated for rigid (Martin et al, 2020) and deforming nets (Martin and Bihs, 2021) and will be utilised in the pre sent paper.
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