Coupled dynamics of the moving Antarctic krill trawl structure and its hydrodynamics behavior using various catch sizes and door spreads based on wavelet-based and Fourier analysis

Abstract

Antarctic krill is a key element of the complex ecology of the Antarctic Ocean and a target species for commercial fisheries for aquaculture supply, krill oil production, and as a major food source for various natural predators. Therefore, understanding the interaction between the fluid structures of the Antarctic krill trawl structure is the key factor in solving problems such as selectivity, energy efficiency, catchability, and ecological sustainability in this fishery. Thus, this study experimentally investigated the influence of catch sizes, door spread, and inflow velocities on the bridle tension, trawl motions, trawl deformation, and flow field inside and around a scaled trawl model in flume tank based on the electromagnetic current velocity meter measurements. Fourier analysis using power spectrum density and the continuous wavelet transform is used to analyze the time-frequency contents of the instantaneous flow velocities, bridle tension, and trawl motions; and in the interaction between the unstable turbulent flows and the fluttering trawl motions, wavelet coherency analysis was employed to detect coherent structures. Results indicated that the bridle tension and trawl motions increased as the catch size, door spread, and inflow velocity increased. Owing to the significant trawl deformation, a complex fluid–structure interaction occurred and a strong positive correlation between the bridle tension and trawl motions was obtained. Furthermore, a significant decrease in the flow field occurred inside and outside different parts of the trawl. Fourier and wavelet analysis results showed that the trawl motions and bridle tension are mainly of a low-frequency activity and of another component related to the unsteady turbulent flow street, and decreased with the increasing catch size and door spread. A complex fluid–structure interaction is then demonstrated where the hydrodynamics of the moving Antarctic krill trawl structure are an intricate interplay between trawl fluttering motions and unsteady turbulent flow influenced by catch size and door spread. The knowledge of such trawl hydrodynamic behavior is of great importance to understand and design the optimal structure of trawl nets to maintain the sustainability of the Antarctic krill fishery.