| Nowadays,tuna longline fishery is an important part of pelagic fishery.The purpose of improving the fishing performance of fishing gear is to improve the catch rate of target species and reduce or avoid bycatch of marine animals.The actual measurement of fishing gear at sea has the disadvantages of high cost,long peorid and low accuracy.The theoretical analysis of fishing gear also has some disadvantages,such as unable to quantify expression,unable to comprehensively consider the impact of complex marine environment.The model test can only be carried out in a reduced scale,and the size,quality and stress of the test model are difficult to be reduced in a unified scale,which affects the accuracy of the model test.Due to the complex underwater force on fishing gear,the measurement at sea and mechanical analysis can not meet the needs of high-precision research.With the progress and development of science and technology,the computer simulation of the hydrodynamic performance of fishing gear has been widely used.The digitization of fishing gear and the automation and scientization of its design have become the forefront of the world’s fishery development research.Numerical simulation has more advantages than model test,so numerical simulation has more advantages.The proposed method is introduced into the research of tuna longline fishing gear,and can be compared with the results of measurement at sea and model test.The water layer of tuna inhabiting shows a certain rule,so it is very important to study the underwater shape of tuna longline fishing gear and the working depth of the hook.In the process of operation,the shape of tuna longline fishing gear and the working depth of the hook are not only affected by the complex sea conditions,but also directly affected by the structure and operation parameters of the fishing gear itself.In this paper,the longline fishing vessel "Fenghui 18" with Thunnus alalunga as the target species in Zhejiang Fenghui pelagic fishery Co.,Ltd.from January 28,2016 to August 8,2016 was used to conduct a field survey at sea,including fishing gear parameters,operation parameters and sea conditions.According to the criteria that vertical current is smaller than 0.1m/s,the three-dimensional current data obtained from the survey are screened and fitted to the three-dimensional current data needed in the follow-up study and not obtained from the survey;the hook depth data obtained from the survey in the stable(the fishing gear settlement is stable when the depth fluctuation is not more than 10%)is determined as the measured depth of the hook.The model test was carried out based on the physical parameters of the fishing gear used in the field.The data of fishing gear depth and main line tension were measured at 7 velocities,7 angles of attack and 6 shortening ratioes.By geometric analysis of the position relationship in the side view and the top view,the node depth data were obtained by using the different refractive indexes of glass,water and air,and the corresponding results were obtained by using the Tauti criterion conversion,which were compared with the measured results at sea.The finite element method was used to analyze the structure of the fishing gear,and the lumped mass method was used to decompose the whole force to the corresponding nodes,and the dynamic equations of each node were established.The seven level and six order Runge Kutta optimization was used to solve the equations,and the model was solved by Matlab software.By the numerical simulation method,the changes of hydrodynamic force,node tension,node space position and node movement speed of longline fishing gear in the whole operation process of line shooting,settlement and stability could be obtained.There were some difficulties to simulate the tuna longline fishing process by numerical model.On the premise of being consistent with the actual operation state as far as possible,according to the finite element theory and Newton’s law of motion,this paper made a series of assumptions on the numerical model:1)The setting process proceeded from left to right.2)The float always floated on the water surface(Z coordinate was zero).The float provided buoyancy and dynamic balance with the gravity and vertical hydrodynamic force of the whole fishing gear.3)In longline fishing,the stress state of fishing gear between two floats was the same,which was represented by the state of fishing gear between two floats.4)The three-dimensional currents of different water layers(0m,50 m,100m,150 m,200m,250 m,300m and 350m)of each station were respectively input into the model as the three-dimensional currents acted on fishing gear nodes of corresponding water layers.If the velocity of 0 m,50 m and 100 m can not be found in the fitted threedimensional current curve,or there were two velocities in the corresponding depth,the velocity was selected according to the measured current value.5)In the process of numerical simulation,there was no bending moment,torque and pressure in the float rope,main line and branch line,but only axial tension in the internal tension of float rope,main line and branch line.Finally,t-test was conducted to compare the measured hook depth with the hook depth obtained from model test,to compare the measured hook depth with the hook depth obtained from numerical simulation,to compare the hook depth obtained from model test with the hook depth obtained from numerical simulation,to compare the measured settlement velocity with the numerical simulation settlement velocity,to compare the model test main line tension with the numerical simulation main line tension.It was used to evaluate the reliability of model test and numerical simulation.The results show that:(1)The float will move with the action of three-dimensional current and wind;(2)The shape of fishing gear is not completely symmetrical catenary when it is stable;(3)In the process of fishing gear settlement,the velocity of main node(1,3)and main node(1,28),branch node(2,3)and branch node(2,28)in Z-axis direction is very small,and the velocity of the node is within ± 0.1m/s when there is no main line tension at the beginning of settlement,and the velocity in Z-axis direction is basically 0 after the settlement is stable.However,in the XY axis direction,the fluctuation range of the motion velocity is relatively large,which basically remains in the range of 0.4-0.8 m /s;(4)With the continuation of the settlement process,the hydrodynamic force is gradually stable;(5)With the settlement process,the tensile force on the main node increases gradually;(6)Tension range of model test is 0-260N;(7)There is no significant difference between the hook depth of model test and the actual hook depth,so model test can be used to study longline hook depth;(8)T-test was conducted between the measured hook depth and the hook depth obtained from the model test,between the measured hook depth and the hook depth obtained from the numerical simulation,between the hook depth obtained from the model test and the hook depth obtained from the numerical simulation,between the measured settlement velocity and the numerical simulation settlement velocity,between the model test main line tension and the numerical simulation main line tension to analyze whether there is any difference between them.The results show that the p value of the above test is > 0.05,and there is no significant difference between each of them.The model experiment and numerical simulation are reliable.By the comparative analysis of the results of model test,numerical simulation and measurement at sea,it is fully verified that the model test and numerical simulation can be used to study longline fishing gear performance.The numerical model established in this study can effectively obtain the hydrodynamic force,tension,spatial shape and velocity of tuna longline fishing gear under the action of different three-dimensional currents,and visualize them;the model test designed in this study can also effectively obtain the model main line tension and node position.This paper can provide a reference for the further study of tuna fishing line performance. |
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