| Trawling is a form of fishing that relies on filtering gear towed by the fishing vessel to haul the target catch into the net for the purpose of target fishing.The trawl system consists of a trawl net and its gear,of which the otter board is an important component of the trawl system,mainly to increase the horizontal expansion of the net opening and to drive the fish into the net.The drag generated by the otter board during the trawling operation accounts for about 30%of the drag of the trawling system,which in effect increases the economic cost of the trawling operation.The development of efficient,low-carbon and energy-saving fishing gear and methods is the direction for optimizing the development of fishing gear in the new era.In order to improve the operational performance of the otter board to meet the development direction of today’s fishing gear,this study is based on the study of the hydrodynamic and flow field characteristics of the otter board and the optimization of the structural parameters of the V-shaped otter board in order to improve the operational performance of the otter board.The main methods used in the study of nets are sea trials,model tests and CFD(Computational Fluid Dynamics)numerical simulations,but before applying the experimental methods to optimize the performance of nets,1)how to select the appropriate scale of otter board when constructing the otter board model in model tests and simulations?2)What is the effect of the simulation conditions on the simulation results and 3)what is the effect of different working postures on the hydrodynamic performance and stability of the otter board?These are some of the main questions that need to be clarified and addressed in order to optimize the performance of the otter board.To this end,this paper uses CFD numerical simulations and flume model tests to analyses the effect of different scale ratios,model thicknesses,aspect ratios,dihedral angle and tilt states on the variation of the hydrodynamic performance of the V-shaped otter board,compare the experimental results of the two research methods,assess the effect of the simulation conditions on the simulation accuracy,and use the line method and the Fluent method to realize the visualization of the flow field distribution around the otter board at different positions,scales and structural parameters,respectively.The visualization of the surrounding flow field distribution under different attitude,scale and structural parameters of the otter board was achieved using the line method and fluent method respectively.The main results are as follows:(1)The flume test results show that when the aspect ratio of V-shaped otter board is 0.5 and the dihedral angle is 10°,and the Reynolds number is 2.1×10~5～4.2×10~5,the drag coefficient C_d increases with the increase of the Angle of attack,and the lift coefficient C_L increases first and then decreases with the increase of the Angle of attack.The drag coefficient and lift coefficient basically remain unchanged with the increase of Reynolds number,that is,they have entered the automatic model region.On the premise of following The Reynolds criterion,the Realizable K-εturbulence model of RANS model was used for numerical simulation to calculate the hydrodynamic performance of V-shaped otter board with different scale ratios and thicknesses.The error of maximum lift coefficient decreases with the increase of model scale,which are 12.62%(1:4),6.03%(1:3)and 1.13%(1:2),respectively.The error of the maximum resistance coefficient decreases gradually,which are15.07%(1:4),7.99%(1:3)and 1.48%(1:2)respectively.The numerical difference of hydrodynamic coefficient between models with different thickness is less than2.6%.It is found that the maximum lift coefficient error of the 5 mm thick model is the smallest(3.39%),and that of the 2 mm thick model is the largest(6.03%).The error of the maximum resistance coefficient of the 10 mm thick model is the smallest(7.24%),and the error of the maximum resistance coefficient of the 5 mm thick model is the largest(8.37%).(2)Different simulation conditions,including the number of grids,the thickness of inflation layer,the type of grids and the ratio of inlet and outlet,were used to simulate the V-shaped otter board with aspect ratio of 0.5,the dihedral angle of 10°and the scale ratio of 1:4,and compared with the model test results.The simulation results are affected to some extent by changing the simulation conditions.It is found that the number of 4×10~6 grids is the best mesh density of the model,and the maximum lift coefficient and maximum drag coefficient of the model are 4.89%and9.87%,respectively.The error of the maximum lift coefficient of the 0.088 mm thick model is the smallest(4.19%),and that of the 0.099 mm thick model is the largest(5.6%).The error of the maximum resistance coefficient of the model with the thickness of 0.099 mm was the smallest(10.11%),and that of the model with the thickness of 0.11 mm was the largest(10.5%).The maximum lift coefficient error and maximum drag coefficient error of tetrahedral mesh model is 5.6%and 10.11%,respectively.The error of the maximum lift coefficient of the 1:2 ratio models was the smallest(5.6%),and the error of the maximum lift coefficient of the 2:1 ratio model was the largest(9.51%).The error of the maximum resistance coefficient of the 1:2scale models was the smallest(10.11%),and that of the 2:3 scale models was the largest(14.25%).(3)When the aspect ratio of V-shaped otter board is 0.4,0.5,0.6,0.7,and the dihedral angle of the otter board is 16°,the lift coefficient increases first and then decreases with the increase of the Angle of attack and the optimal Angle of attack is between 37.5°and 42.5°.With the increase of aspect ratio,the optimal attack competition gradually decreases.The maximum lift coefficients were 1.26,1.28,1.32and 1.48,respectively.The lift coefficients of different aspect ratios vary with the Angle of attack.When the dihedral angle of the otter board is 16°,19°,22°,25°and aspect ratio is 0.4,the lift coefficient increases first and then decreases with the increase of the Angle of attack,and the best Angle of attack is about 40°.The maximum lift coefficients were 1.26,1.21,1.19 and 1.18,respectively.The lift coefficients of different dihedral angles vary with the Angle of attack.The V-shaped otter board(No.13)with the aspect ratio ofλ=0.7 and the dihedral Angle of otter boardγ=16°has the best hydrodynamic performance.The lift coefficient is 1.482when the Angle of attack is 37.5°,and the expansion efficiency is the highest when the Angle of attack is 10°.With the increase of the Angle of attack,the pressure on the flow surface of no.13 otter board increases gradually,and the velocity difference between the flow surface and the flow surface also increases.(4)The lift coefficient decreases and the drag coefficient decreases with the increase of the tilt Angle.When the otter board tilts inside and outside,the pressure center on the flow surface moves to the front of the otter board with the increase of the Angle,and the change of separation vortex behind the center surface is not obvious.When the otter board tilts forward and backward,the pressure distribution on the flow surface of the otter board changes obviously,that is,with the increase of the inclination Angle,the pressure center gradually moves forward and down,and the vortex behind the center surface of the otter board gradually decreases.When the Angle of attack increases to about 35°,there are generally two separate vortices on the upper and lower sides behind the otter board.The size of the two separate vortices is different,and the separation vortices on the lower side are far away from the relative position of the otter board.The model test results show that the distance between the lower vortex and the otter board is about 1.7 times that of the upper vortex.The relative position of the separation vortex and the otter board is affected by the scale ratio of the otter board,the structure of the otter board and the attitude of the otter board.The number of separation vortexes behind the otter board is reduced to 1 when the otter board tilts in and out(in the case of inward tilting,in the case of upward tilting),and the relative distance between the separation vortexes and the otter board is affected by the tilt degree of the otter board. |
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