Experimental and numerical studies on the effect of the reinforced tubes on the drag forces of a gravity cage system

Vertical tubes are proposed to connect the collar and bottom ring to address the significant deformation of gravity cage nets caused by currents and waves. The goal is to minimize the volume loss of the cages, which reduces the habitat available for fish. This study analyses the effect of vertical tubes on drag forces using numerical and experimental methods. Tests are conducted in a towing tank to measure drag forces and validate a numerical model that employs beam and link elements to simulate the gravity cage behaviour. The differences between numerical simulations and experiments are discussed. Cages with thin tubes show larger differences than those with thick tubes due to initial bending. Comparing the results with those of gravity cages without reinforcement, it is found that the vertical tubes increase the drag force and thicker tubes exacerbate this effect. At 0.5 m/s, the cages with reinforcement tilt, leading to a different trend in drag force with wavelength compared to the cages without reinforcement.