The Effect Of Grid-bycatch Reduction Device On Hydrodynamic And Flow Field Characteristics Of Midwater Trawl Codend

Bycatch and discard problems exist in many commercial fisheries around the world.The UN Food and Agriculture Organization(FAO)estimates that 9.1 million tons of bycatch were discarded each year from 2010 to 2014,equivalent to about 10 percent of the global marine catch.The problem of bycatch and discard in shrimp trawl fisheries is particularly serious,accounting for 1/3 of the world’s total by-catch.In order to solve the problem of bycatch and discard in trawling fishery,many countries and fishery organizations have improved the trawling gear selection by improving the codend structure and developing and designing bycatch reduction devices(BRD).According to the different objects of interaction between the device and Marine animals,the BRD can be divided into many types,such as the separator panel,flexible separation grid,fisheye.Among them,the most common application is to install a Grid type BRD at the front end of the codend.However,in the practical application of this device,the catch separation effect will be greatly different due to the change of grid materials,specifications,grid angle,bar spacing and other factors.Because the changes of the above factors may cause changes in the hydrodynamics,shape and flow field distribution of the BRD and the codend structure,thus affecting the behavioral response of fish entering the net,resulting in a certain difference in separation efficiency.Therefore,it is necessary to carry out the study on the influence of the Grid-BRD on the hydrodynamic characteristics of the codend structure under different structural parameters,and to clarify the influence mechanism of the Grid-BRD on the hydrodynamic characteristics of the codend structure,so as to determine the optimal design and installation of the BRD,improve the trawl selectivity,and achieve the purpose of reducing the bycatch.To this end,the SST K-ω turbulence model and computational fluid dynamics of large deformation nonlinear structure model was used in this study to conduct numerical simulation,and the variation rules of surrounding flow field and hydrodynamic characteristics of grids with different structure types(bar spacing and grid angle)at different flow rates were explored.At the same time,the hydrodynamic performance,deformation and flow field distribution of grid structure,Grid-BRD and codend were studied by physical model test.Finally,the numerical simulation results were compared with the physical test data to obtain the best structure and installation layout of the Grid-BRD.The main results are as follows:(1)Three physical models of grid with bar spacing of d-5mm,d-11 mm and d-15 mm were constructed by using two methods of physical model tests and CFD numerical simulation.physical model tests were carried out under seven grid angle(A-30°,A-35°,A-40°,A-45°,A-50°,A-55°,A-60°).The results show that the drag of d-5mm,d-11 mm and d-15 mm grid in physical test are 0.8%,8.15% and 5.39% higher than those in numerical simulation.The drag and velocity increase in a quadratic curve,and decrease with the increase of the bar spacing and the decrease of the grid angle.(2)Test the codend without grid and three different specifications grid(Grid-0.3,Grid-0.4 and Grid-0.5)in grid angle of 30°,45° and 60°,three states of the codend mesh opening degree,shape and drag changes.The results show that: The results showed that:(1)The average mesh opening angle of Grid-free codend is 22°±5°.Compared with Grid-free codend,the average mesh opening angle of Grid-0.4 codend and Grid-0.5 codend is increased by 3.3% and 18.8%,respectively.The average mesh opening angle of Grid-0.3 codend is reduced by 9.2%.(2)Grid-0.4 has the least influence on the shape of the codend,and it is streamlined to fit the codend;Grid-0.3and Grid-0.5 make the codend distorted.(3)The difference drags of Grid-0.3 codend under different grid angle is small,the average drags of Grid-0.3 codend is similar to the drag of Grid-free codend.The average drags of codend with Grid-0.4 codend and Grid-0.5 codend at different grid angles is 12.7%,24.4% higher than that of Grid-free codend.Except Grid-0.3-A-30° codend,the drag of codend increased with the increase of grid angle and size.(3)Solid Works was used to build the Grid-BRD and codend model,and CFD numerical simulation was used to analyze the influence of nine different structural parameters(three bar spacing d-5mm、d-10 mm and d-15 mm,and three grid angles A-30°,A-45° and A-60°)on the hydrodynamic characteristics and flow field of the codend.The results show that: a.The numerical simulation of Grid-BRD is in agreement with the physical test results.The average relative errors of drag and flow field are 5% and 4.7% respectively,indicating that the numerical model can fully reproduce the experimental results.b.The drag of codend and Grid-BRD combination increases with the increase of grid angle and the decrease of bar spacing.c.The flow velocity inside the codend gradually decreases with the increase of the grid angle and the decrease of the bar spacing between the bars.D.The total deformation of the codend increases with the increase of gird angle and the decrease of bar spacing.According to the research results of this paper,when the grid angle is 45-60°,it is conducive to the escape of juvenile fish.