| The middle trawling net is one of the effective fishing gears for catching mid-level swarm fish.It is an important tool for utilizing and developing aquatic resources.Its advantages are more advantageous than other fisheries.The operation is not restricted by water depth,seafloor sediment and topography,so it can be greatly expand the range of fisheries and the types of fishing.In the trawling system of single boat,the proportion of the net and otter board of total trawling resistance is very high,among which the otter board accounts for 13%~27%of the total resistance.The main function of the otter board is to expand the net and increase the sweep area.Its hydrodynamic performance has a direct impact on the expansion effect of the trawl opening and the catch performance of the trawl.Therefore,it is beneficial to further study the hydrodynamic performance of trawl system to understand the hydrodynamic performance of trawl.However,the middle trawling system is relatively difficult in operation technology.Whether it can effectively control and adjust the water layer where the net is located,so that the net position is always aligned with the water layer where the fish group is located,and the purpose of aiming for fishing is even more crucial.The adjustment measures of the net position are generally changed by changing the length of the warp and the towing speed.In actual production,only the experience of the captain can be used to adjust the net position,which lacks scientific basis.Therefore,taking the East China Sea anchovy trawl system as an example,firstly,this paper use the two methods of tank model test and numerical simulation.The hydrodynamic performance of vertical curved otter board is studied.The changes of hydrodynamic performance of otter board under different angles of attack are analyzed.The differences between the two research results are discussed,and the flow field around the otter board is visualized.Then uses the collected data of the measured trawling nets to study the influence of the length of the warp and the speed of the trawl on the depth of the net,and further analyzes the winch speed and net position when placing and collecting the trawl.The relationship of depth changes provides the scientific basis for the captain to regulate the net position.The authors conducted relevant research on the above questions,and the main results are as follows:(1)The test value and the simulation value of the lift coefficient C_l of the vertical curved otter board increase first and then decrease,and reach the maximum at the critical angle of attack of 25°,respectively 1.45 and 1.59.The test value and the simulation value of the drag coefficient C_d increase with the increase of the angle of attack.The test value and the simulation value of the lift-to-drag ratio both increase first and then decrease,and the values reach the maximum at the angle of attack of 15°,respectively 3.25 and 2.93.The Reynolds number of the hydrodynamic performance of the otter board entering the automatic model area is 5.2×10~4~9.5×10~4.(2)From the distribution of the flow field around the vertical curved otter board,the difference between the inner and outer flow velocity of the otter board causes a pressure difference,and the otter board lift is formed.The increase with the angle of attack increases first and then decreases,and is the largest at the angle of attack of 25°.Due to the formation of vortex at the rear of the otter board,a pressure difference resistance is generated,which increases with the increase of the angle of attack.The boundary layer separation point generated on the back of the otter board gradually moves toward the front end of the otter board with the increase of the angle of attack,and the wing end vortex of the otter board end forms a distinct wing end vortex,which increases with the increase of the angle of attack.The flow velocity attenuation zone at the front end of the otter board gradually increases with the increase of the angle of attack.Due to the boundary layer separation when the water flows through the otter board,a swirling vortex is generated at the rear of the otter board.The pressure distribution on the surface of the otter board increases with the increase of the angle of attack,and the pressure center of the otter board remain unchanged with the increase of the angle of attack.(3)The influence of the length of the warp and the towing speed on the depth of the net is analyzed by the GAM model.The results show that there is a significant correlation between the towing speed and the length of the warp and the depth of the net.The interpretation rate of the model for the total deviation of the net depth is 76.2%,and the length of the drag is positively correlated with the depth of the net(P<0.05).As the length of the drag increases,the net gradually sinks,and vice versa.The towing speed was negatively correlated with the depth of the net(P<0.001).As the towing speed increased,the net was up,and vice versa.The regression analysis of the relationship between the winch speed and the change of the net position shows that the settlement speed of the net is gradually increased with the increase of the winch speed when the tracing is applied(P<0.001).When the winch is warped,the rising speed of the net is increased as the speed of the winding is increased(P<0.001).There was a significant correlation between the changes of the net position and the time under the four kinds of winch speed when the warp was winded(P<0.001),and the change trend of the net position with time was the same,both increased with time.The relationship between the change of the net position and the time under four kinds of winch speeds is fitted by regression.(4)The linear relationship between the net depth and the warp tension was established by using multiple regression.The results showed that wind and fish depth had significant effects on the net depth(P<0.05),but the warp tension and water depth and catch had no significant effect on the depth of the net(P>0.05).Analysis of wind and fish depth through diagnostic charts is the principal component factor affecting the net depth.The regression test shows that the wind and the net depth are quadratic curves,and the fish depth is a curve relationship.The model expressions fitted by the stepwise regression method are D=12.573-0.285W~2+1.015f_d.The measured values and predicted values of the depth changes of the nets were compared and the results of the two studies were found to be high. |
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