Three-dimensional Numerical Simulation Of The Flow Field Inside And Around Net Cages

In recent years, the fishing stock is decreasing because the demand for fish production has grown substantially as the world’s population increase. On the other hand, the ecological environment of near-shore locations becomes worse and worse due to overfishing and excess exploitation. Because of the consideration of fish farming environment and high quality fish food, more and more net cages for aquaculture will be located offshore and the deep-water cage arises at the historic moment. In a deep-water cage, water flow within the net cage is closely related to the fish growth. The flow velocity reduction effect behind the fishing net is a major concern. Thus, the investigation on the flow field around the cage net has become an important subject. The research results will play a positive role in guiding the production practice of fish farming. The fishing net is a kind of small-scale flexible structure, the numerical study on flow field around the net structure is rather complex. At present, the efforts on the flow field around the net cage are mainly concentrated on physical model tests and field measurements and numerical simulation research is relatively rare.A numerical model combined the Realizable k-ε turbulence model and the porous media model is established to simulate the flow field around cage net under current. The unknown porous coefficients are determined from the experimental forces on the net and flow velocities and the angles of attack using the least square method. Therefore, the porous media act the same water-blocking effect as the net. This numerical model is applied for the simulation of the flow field around the plane net and the net cage under current and the numerical results are satisfactory.The contents are mainly composed of five chapters. The first chapter gives a general description on the background of the subject and the development of researches on flow field around net cages. The second chapter is the numerical method of the porous media model including the governing equations, porous media resistance coefficients, mesh grids and boundary conditions, numerical algorithm method. The third chapter is the experimental verifications. Firstly a two dimensional numerical model is applied for modeling the flow field around a net panel and a square cage under current and the numerical results are compared with data obtained by physical model tests. On this basis, a three dimensional numerical model is applied for modeling the flow field around a net panel at different attack angles and velocities, which is verified by physical model tests. In the fourth chapter, the flow field around a plane net with different plane net inclination angles, plane net heights, spacing distances between two plane nets and plane net numbers are presented and the effects of these parameters on the flow field around the plane net are investigated. The fifth chapter is the numerical simulation of the flow field around net cages. Based on the simulation of the flow field around plane net, the flow field inside and around a gravity cage can be simulated by dividing the net cage into many plane nets with different attack angle. Therefore, the simulation of the flow field inside and around a gravity cage is straight forward and the flow fields inside and around single gravity cage and composite-type net cages are investigated.