Experimental Analysis of the Influence of Gear Design and Catch Weight on the Fluid–Structure Interaction of a Flexible Codend Structure Used in Trawl Fisheries

This study evaluated the behavior of different codend designs to provide the basic information that is relevant for improving the gear selectivity, energy efficiency, to better understand the fish behavior inside the codend, and prevent the probability of the fish escaping. Three different codends were designed from the standard codend commonly used in the Antarctic krill fisheries based on modified Tauti’s law and evaluated. The first and the third codends were designed with four-panel and two-panel nettings, respectively, both made of diamond meshes. While, the second one was a four-panel diamond mesh design with cutting ratio 4:1(N [NBNBN]16). We measured the drag force, codend shape, fluttering codend motions, and the flow field inside and behind the different codends composed of different catch weights under various flow velocities in flume tank. The power spectra density was undertaken to analyze the time evolution of measured parameters. The results showed that the drag force and the codend motion increased and decreased, respectively, with the number of net panels and the cutting ratio. Due to the catch weight and flow velocity, which caused significant codend motions and deformation, a complex interaction occurred between the fluid and the structure, and there was a strong correlation between the codend drag, the codend motions, and the turbulent flow inside and behind the codend. The study showed that the use of the four-panel codend with cutting ratio and the two-panel codend resulted in drag reductions of 6.07% and 6.41%, respectively, compared to the standard codend. The velocity reduction and turbulent kinetic energy were lower inside and behind the four-panel codend than inside and behind the two-panel codend, indicating that turbulent flow through the two-panel codend is more important than through the four-panel codend. The results of the power spectral density analysis showed that the drag and codend motions were mainly low frequency in all codends, with another component related to turbulent flow street. In addition, the two-panel codend showed more unstable behavior with more pendulum motion compared to the four-panel codends, resulting in a smaller mesh size in this codend that could affect swimming energy and thus influence fish escape, making it the least selective codend. The results of this study provide fundamental insights useful for understanding and improving the hydrodynamic performance and selectivity of trawls in the Antarctic krill fishery, especially to reveal the masking effects of the number of net panels on codend design.