Hydrodynamic characteristics of frustum-type depressor for sampling midwater trawl

Accurate and quantitative sampling of larval and juvenile fish is crucial for assessing dynamic changes in fishery resources and estimating the abundance of recruitment. The depressor plays a vital role in the sampling trawl, providing essential sinking force to ensure consistent trawling depth within a specific water layer. This study investigated the hydrodynamic characteristics of the frustum-type depressor at different angles of attack based on flume experiments and Computational Fluid Dynamics (CFD) simulation. The investigation unveils the flow field distribution on the center plane of the depressor and a gradual intensification of the wingtip vortex as the angles of attack increased. The turbulence kinetic energy and Reynolds stress correlations in the wake region of the depressor both increases with the increase of angle of attack. The parameters of the frustum-type depressor were optimized using a Response Surface Method (RSM), through the simulation of the hydrodynamic performances of twenty-five candidate models. The aspect ratio, camber ratio, and dihedral angle were selected as independent variables, enabling the determination of the optimal depressor from the proposed solutions. The optimal parameters, which included an aspect ratio of 4.1, a camber ratio of 17.1 %, and a dihedral angle of 20.6, resulted in 7 % improvement in the maximum sinking force coefficient compared to the initial depressor. The newly optimized frustum-type depressor demonstrates its reliability as an essential component and exhibits the potential to transform midwater sampling trawls into highly effective multi-layer sampling trawl systems.