A Comparison Study On Hook Depth Of Pelagic Tuna Logline Based On Theoretical Calculation,Measurement At Sea,Numerical Simulation And Model Test

Tuna（Thunnini）belongs to the class Osteichthae,order Scombriformes,family Scombridae.The tuna fishery,known as the"Golden Fishery",is one of the most important fisheries in the world.As one of the important fishing gears in tuna fishery,longline fishing gear has the advantages of catching adult fish,keeping catch fresh,wide operating range and so on,which is of great value to the exploitation and utilization of tuna resources.The catch rate of longline fishing gear is related to the habitat water layer of the fishing object.Controlling the depth of the hook in longline fishing plays a key role in increasing the catch of the target species and reducing the bycatch of non-target species.The study on the hook depth of longline fishing can provide help for the improvement of the fishing gear of longline fishing and provide reference for the operation of longline fishing,which is of great significance to the improvement of the catch performance of longline fishing gear and the development and utilization of tuna resources.Longline fishing gear is composed of float,float line,main line,branch line,hook and other parts.The depth of the hook is related to the operation mode,the structure of fishing gear,the material of fishing gear and the operating sea conditions.At present,most of the domestic and foreign researches on the hook depth of longline fishing adopt the methods of ocean measurement,numerical simulation and model test.It is difficult to control variables to analyze the influence of various factors on the depth of the hook due to high cost,long sampling period and the common influence of many factors on the depth of the hook.With the development of computer technology,related research is based on finite element theory to carry out numerical simulation of longline fishing,and study its hook depth and hydrodynamics.However,most of the relevant researches use Matlab and R to discrete longline fishing gear into two-node rod element,assuming that it is a rigid rod structure,so the calculation accuracy still needs to be further improved.With the development of simulation technology,simulation software ANSYS Workbench in the simulation of flexible body has good application.ANSYS Workbench has better convergence and higher computational efficiency,more in line with the advantages of flexible body numerical calculation based on the discrete finite element theory to flexible body for more nodes unit,using separate method to solve the fluid domain and structure domain respectively.ANSYS Workbench is one of the most commonly used numerical simulation software for engineering analysis.In this paper,the depth of the hook in longline fishing is studied by sea measurement,numerical simulation and model test.Based on the hook depth data,operation parameters and the sea condition information collected from January 28,2016to August 8,2016,on board of the tuna longliner"Feng Hui 18"in the high seas,west of Polynesian,the multiple linear regression was used to analysis the relationships between hook depth and hook position,shortening ratio,the wind angle,leeway and drift angle and the current shear.Catenary formula was used to calculate the theoretical depth of the hook and analyzed the difference between it and the measured hook depth.In this paper,the numerical simulation of longline fishing was carried out based on ANSYS Workbench.The Design Modeler module was used to establish the geometric model of longline fishing gear,and the fluid volume model was used to construct the layered fluid model.Based on the finite element theory,the Mesh module was used to discretize the longline fishing gear into 10-node tetrahedral solid units,and the surrounding fluid model was discretized by automatically dividing the Mesh.The equilibrium equation of the unit was established by the principle of minimum potential energy to control the motion information of the structure,and the Navier-Stokes equations were solved by the Reynolds time-mean method.The fluid motion information was controlled by the RNG k-εturbulence model,and the coupling surface governing equation was used to control the coupling surface motion information.The coupling surface information was transmitted by the unidirectional fluid-solid coupling method.Separate solution method was adopted for numerical solution,FLUENT module was used for SIMPLE algorithm for fluid domain information.Static Structural module was used for Newton iterative method for Structural domain information,and the convergence number was set as 10^-3.The measured sea condition information and actual operation parameters were input into ANSYS Workbench simulation,and the simulation depth of hook was calculated.SPSS 26.0 sample T test was used to analyze the difference between the measured depth of hook and the simulated depth of hook,and the applicability of numerical simulation in the study of hook depth of long line fishing gear was verified.The actual longline fishing hook depth is mainly affected by fluid viscous force.The Reynolds number range is 10³～4×10³,which agrees to the premise of Tauti criterion.In this paper,the model of longline fishing gear was built based on the Tauti criterion,and the change of hook depth in longline fishing under different shortening ratio,attack angle and flow velocity was discussed by numerical simulation and flume tank model test,and the relationship between hook depth and shortening ratio,attack angle and flow velocity was discussed.The accuracy of ANSYS Workbench simulation results was verified by analyzing the difference of SPSS sample t-test between the model test hook depth and the simulation hook depth when the model parameters and test conditions were consistent.The conversion method of the Tauti criterion was used to convert the hook depth in the model test,and the difference between the model test hook depth and the measured hook depth at sea was analyzed to discuss the applicability of the Tauti criterion in the study of hook depth in the longline fishing.The results show that:（1）The depth of hook is mainly affected by hook position,current shear,leeway and drift angle,shortening ratio,wind speed and wind angle,and the explanatory degree is 0.767（R²=0.767）.The hook position accounted for 30.51%of R²,current shear accounted for 21.61%of R²,leeway and drift angle accounted for 20.76%of R²,and shortening ratio accounted for 15.25%of R².（2）Compared with the theoretical hook depth calculated based on catenary theory,the actual operating hook depth is relatively shallow（8%～33%）.The shoaling degree is related to the hook position.The closer the hook position is to the middle section,the shallower the actual depth is.（3）There is no significant difference between the measured hook depth at sea and the simulated hook depth based on numerical simulation（P=0.499>0.05）,and the simulated hook depth based on numerical simulation has a good applicability in the study of the hook depth of longline fishing.（4）Longline hook depth is greatly affected by its surrounding fluid,but the flow pattern change around the main line and branch line of longline fishing is small.In the actual operation of longline fishing,the influence on the surrounding fluid is small,and the change of flow pattern can be ignored in the numerical calculation.（5）The results of simulation calculation and model test show that the hook depth is positively correlated with the position of the hook,negatively correlated with the shortening ratio,negatively correlated with the flow velocity,and negatively correlated with the attack angle.（6）When the flow velocity is small,the shape of main line is approximate to the catenary when the longline gear is stable.When the flow velocity is larger,the shape of main line changes greatly,and the hook is shoaling,and the degree of shoaling is different according to the position of the hook.The farther the hook from the float,the greater the degree of shoaling.（7）The depth of the hook obtained by the model test of longline gear based on the Tauti criterion is not significantly different from the actual depth after conversion by the similarity criterion（P>0.05）.The Tauti criterion is more suitable for the study of the hook depth of the longline.（8）Theoretical calculation,measurement at sea,numerical simulation and model test have their own advantages and disadvantages in the study of hook depth of longline fishing gear.The advantage of theoretical calculation is that the calculation is simple,but the disadvantage is that the error between theoretical calculation and actual depth is large,and the correction coefficient is affected by the environment such as current speed.The advantage of sea measurement is that the depth of the hook is accurate,but the disadvantage is that the period of data acquisition is long,the influence of various factors at sea is complex,and the test conditions cannot be controlled.By contrast,numerical simulation and model test,as design tests,can control the test conditions,and the calculated results are more accurate.Therefore,this study suggests that the measurement at sea,numerical simulation and model test are more suitable for the study of hook depth of longline fishing gear.