Hydrodynamic performance and around flow field of biplane-type otter board with different working positions

Otter board, an important accessory for net expansion in the single trawl, whose stability directly determines the degree of expansion of the trawl mouth and affects the catch efficiency and economic benefits. In this study, the hydrodynamic performance of the biplane-type was investigated by using the flume tank experiment and numerical simulation (Computational fluid dynamics, CFD) at different heel angles (inward, outward and tilt) and angles of attack, and the flow field and surface pressure around otter board was visualized. The results show: 1) At heel angle of 0° and angle of attack of 25°, both the lift coefficients of otter board for flume tank experiment and numerical simulation reached the maximum values (1.69 and 1.88, respectively), while both drag coefficients decreased with increasing heel angle. 2) Both the lift-to-drag ratios of model experiment and numerical simulation decreased with the increase of heel angle, and both reached the maximum values (3.27 and 3.69, respectively) when the heel inward angle was 5°. 3) The pressure center coefficient (C_pb) almost maintained stable with the change of heel angle; C_pc increased and decreased with increasing heel angle when the otter board was set to forward and backward tilting states, respectively. 4) CFD results show that the vortex at the rear of the center surface of otter board decreased with increasing heel angle. When the otter board was in inward and outward inclination states, the front end flow velocity reduction zone increased with increasing heel angle gradually. The pressure center moved toward the end chord of otter board as the heel angle increased in forward tilting state, but moved toward the lower end chord in backward tilting state. The results can provide scientific references for the studies on the stability of otter board and adjustion of fishing strategy.