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Biofouling is an inevitable problem in marine aquaculture and it is prone to cause many negative effects on the hydrodynamic behavior of the aquaculture facility. In this study, the dominant biofouling on the netting of fish farms in the Yellow Sea of China was determined by field sampling. Plane nets with various levels of biofouling were obtained by submerging nets at different depths and for different durations. Drag acting on the biofouled nets in currents were investigated using laboratory experiment. The results indicate that drag acting on the net increases with increasing level of biofouling. Compared to the clean net, the accumulation of biofouling can lead to over 10 times more hydrodynamic load on nets. Flow fields around the biofouled nets were simulated using a porous-media fluid model which was validated by corresponding laboratory experiment. As the level of biofouling increases, the flow velocity downstream from the net decreases continuously. The net with the greatest level of biofouling produces the most serious shielding effect with 21.4% reduction in flow velocity. Using the proposed empirical equations, both the drag on and flow velocity downstream from a hydroid-fouled net can be predicted. [ABSTRACT FROM AUTHOR]

Gravity-type fish cages are playing an increasingly important role in fishery aquaculture. As the main load-bearing component of gravity-type fish cages, the floating collar supports the whole cage and undergoes large deformations. In this paper, a numerical method is developed to study the motions and elastic deformations of an HDPE floating collar in waves. The governing equations of motions are established by Newton׳s second law, and those governing the deformations are obtained according to the curved beam theory and the modal superposition method. Damping of the elastic deformations is introduced and analysed in detail to improve the numerical model. To validate the numerical model, a series of physical model tests are conducted. Good agreement between the numerical and experimental data is obtained. The numerical results indicate that the in-plane and out-of-plane deformations calculated using the first four modes yield satisfactory precision. The positions of the maximum deformations in different wave propagation directions and the effect of the forms of the mooring line arrangement have also been investigated. The numerical results show that the four-point-mooring cage is more reliable than the single-point-mooring cage. The endpoints of the diameters that are parallel or perpendicular to the wave direction have the largest deformation. [ABSTRACT FROM AUTHOR]

Abstract: A three-dimensional (3D) numerical model is established to simulate the flow field around fishing plane nets in a current. The realizable k–ε turbulence model was chosen to describe the flow, and the governing equations are solved by using the finite volume method. In this model, the net is modeled using the porous media model. The unknown porous coefficients are determined from the hydrodynamic forces on the net and from the flow velocities and the attack angles using the least squares method. In order to validate the numerical model, the numerical results were compared with the data obtained from two physical model tests. The comparisons show that the numerical results are in good agreement with the experimental data. This numerical model is applicable for the simulation of the flow field around a fishing net. Using the proposed method, this paper presents 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. This study provides information about the flow field around the fishing net cage. [Copyright &y& Elsevier]

Abstract: Recent work in the area of open-ocean aquaculture-system dynamics has focused either on the response of fish cages in waves or the steady drag response from ocean currents, not on them combined. In reality, however, the forces bearing on these open-ocean structures are a nonlinear, multidirectional combination of both waves and current profiles. In this paper, a numerical model has been developed to simulate the dynamic response of the gravity cage to waves combined with currents. When current flows are combined with regular waves, gravity-cage motion response (including heave, surge, and pitch) and mooring-line forces have been calculated. To examine the validity of simulated results, a series of physical model tests have been carried out. The results of our numerical simulation are all in close agreement with the experimental data. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to investigate the shape and tension distribution of fishing nets in current. A numerical model is developed, based on lumped mass method to simplify the net. The motion equation is set up for each lumped mass. The Runge–Kutta–Verner fifth-order and sixth-order method is used to solve these simultaneous equations, and then the displacement and tension of each lumped mass are obtained. In order to verify the validity of the numerical method, model tests have been carried out. The results by the numerical simulation agree well with the experimental data. [Copyright &y& Elsevier]

Abstract: Based on the lumped-mass method and rigid-body kinematics theory, a mathematical model of a gravity cage system attacked by irregular waves is developed to simulate the hydrodynamic response of cage system, including the maximum tension of mooring lines and the motion of float collar. The normalized response amplitudes (response amplitude operators) are calculated for the cage motion response in heave and surge, and the mooring line tension response, in regular waves. In addition, a statistical approach is taken to determine the motion and tension transfer functions in irregular waves. In order to validate the numerical model of a gravity cage attacked by irregular waves, numerical predictions have been compared with the experimental observations in the time and frequency domain. The effect of wave incident angle on the float collar motion, mooring line tension and net volume reduction of the gravity cage system in irregular waves is also investigated. The results show that at high frequencies, the cage system has no significant heave motion. It tends to contour itself to longer waves. The variation amplitude of mooring line forces decreases as the wave frequency increases. With the increasing of wave incident angle, the horizontal displacement of the float collar increases. [Copyright &y& Elsevier]

In the present work, the hydrodynamic characteristics of full-scale rectangular otter board were studied by numerical simulation, including the effects of aspect ratios (AR), flow distribution around the otter board, induced velocity, induced angle of attack (AOA), effective AOA and induced drag coefficient. It was demonstrated that the otter board had a critical AOA of 40° (when the stall appeared) and induced AOA of 3~10°. The maximum induced drag coefficient was 0.185, which is a great contribution to the total drag coefficient. The otter board exhibited better performance when the AR was set to 0.5, and higher AR had a smaller critical AOA (the stall happened earlier). The maximum lift coefficient decreased with an increasing AR, while the maximum lift-to-drag ratio increased. The induced drag coefficients showed similar trends across ARs, initially rising with an increase of the AOA, and then dropping. The flow distribution around the otter board showed that the wing-tip vortex was fully developed at the AOA of 40°, and the AR can affect the development of the wing-tip vortex. [ABSTRACT FROM AUTHOR]