Numerical model of an aquaculture structure under oscillatory flow

Marine aquaculture is widely distributed in coastal areas. The aquaculture farms generate drag resistance to fluid motion and alter ambient hydrodynamics. Meanwhile, aquacultural structures are subjected to complex flow conditions including waves and currents. With the expansion to more open areas with severe flow field conditions, marine aquacultural structures face greater challenges and risks of damage. Culture unit is an important component of aquacultural structures and shows flexibility in both field observations and laboratory measurements. Underestimating or overestimating the drag resistance of culture units under the action of fluid flow can lead to damage risks or overdesign of the structure. A dynamic model is developed to estimate the deflection of flexible culture units and is incorporated into an aquacultural structure numerical model in this paper. Critical factors for safety as well as routine operation of aquacultural structures are considered including structural responses and mooring line forces. A suspended mussel long line system is taken as an example, and the results show that the calculated value (9.2 kN) of the maximum tension of the north mooring line is in good agreement with the measured data (9.8 kN) under the action of tide flow. The influence of different flow field parameters on structural dynamic responses is investigated. The numerical results indicate that decreasing wave height can reduce maximum mooring line tension and longitudinal and vertical motion amplitude of the main line. The maximum tension of the mooring line generally decreases with the increase of the angle between the main line and the inflow direction under the action of waves and tide flows. In structural design, the arrangement angle of structures can be determined according to the force calculation of mooring lines based on the numerical model. The spacing of culture units and the distance between adjacent long lines can be determined by referring to the motion calculation of structures to avoid damage due to intertwinement of structural components.