The influence of length–width ratio of an unconventional mobile finfish structure on the drag force and dissolved oxygen: Laboratory experimentation and Computational Fluid Dynamics (CFD)

Abstract

Open ocean aquaculture (OOA) offers a number of benefits from inshore aquaculture, such as stronger currents and waves enhancing the water quality. However, conventional static aquaculture systems deployed in the open ocean do not resolve environmental issues such as the effects of heatwaves, or physical challenges such as extreme wave action. Farming fish in a mobile or semi-mobile system would have the advantage of being able to move fish to the best conditions for the fish year-round. The current study proposes an unconventional fin fish structure that could be deployed in the open ocean on either a single-point mooring (fixed), or as a fully mobile (mobile) system. The body of the structure is a square cross-section cylinder shape, covered by water-proof woven fabric cloth, and nets placed at the front and back of the body. This study considers how adjusting the length–width ratio, influences drag forces (important for a mobile scenario) as well as the dissolved oxygen (DO) inside the structure (important for a fixed scenario). This was achieved using flume experiments and Computational Fluid Dynamics (CFD). The volume of the structure is kept at 10,000 m³, current speed at 0.1 m/s and fish stocking density is 30 kg/m³ with the length–width ratio varying from 1 to 6. We found that when the length–width ratio increased, this resulted in the decrease of drag force and an increase of the unhealthy volume fraction (i.e. DO < 70 % of ambient DO) inside the structure, and vice versa. We also found that speed reduction caused by bio-fouling results in an increase of drag force and unhealthy volume fraction. Therefore, it is recommended that the design of mobile structures should balance the demands of the fish as well as the fish farmers which will be dependent on the operation (fixed or mobile).