Hydrodynamic and turbulence flow characteristics of fishing nettings made of three twine materials at small attack angles and low Reynolds numbers

Abstract

The hydrodynamic variation of nettings in fishing applications (e.g., trawl net, purse seine, and fish cage) are centrally important. Their hydrodynamic characteristics and their surrounding flow fields at small attack angles are key because of their operation conditions, especially in flow direction. In this study, a numerical model on the fluid structure interaction in two-way coupling was developed. The developed model is based on the coupling between the porous media model combined with the transitional turbulence kT-kL-ω model and the nonlinear structural finite element model. The effects of twine diameter, mesh size, and twine material on both the hydrodynamic variation and turbulent flow developing around the flexible netting structure were assessed at small attack angles. The hydrodynamic variations of three types of nettings (i.e., polyethylene, polyamide, and polyester) with different solidity ratios at small attack angles were calculated and compared with experimental flume tank data. The numerical results agreed well with the experimental data with average relative errors of 2.82% and 4.82% for the drag and lift coefficients, respectively. Both the numerical and experimental results indicate that decreasing both solidity ratio and Reynolds number and increasing attack angle increases the drag coefficient and decreases the lift coefficient. In addition, increasing the attack angle, twine diameter, and solidity ratio, and decreasing mesh size enlarges both turbulence region and drag. Netting deformation decreases with increasing attack angle and solidity ratio, while the stress and elastic strain increase with increasing attack angle and decreasing solidity ratio.