Hydrodynamic Performance And Hook Depth Numerical Simulation Of Tuna Longline Gear

Tuna longline gear is composed of floats,float ropes,main lines,branch lines,hooks and other components.The depth of hook determines the fishing efficiency.The main influencing factors include operating parameters,fishing gear structure,fishing gear materials,and operating sea conditions.The performance testing methods of tuna longline gear mainly include actual measurement at sea,theoretical analysis,model test and numerical simulation.Compared with actual measurements at sea and model experiments,the advantages of numerical simulation methods such as low cost,easy parameter adjustment,and strong operability have become more prominent,and their importance has also increased.At present,there are still some deficiencies in the research work on the numerical simulation of tuna longline gear,including:(1)When establishing the longline fishing simulation model,the fishing gear structures such as main line,branch line,and float rope are simplified into smooth cylinders,but in reality,the first section of the buoy rope and the branch line is a twisted structure with a certain roughness,so it is oversimplified at this time.It may affect the calculation accuracy.At the same time,due to the particularity of the rope itself,the hydrodynamic performance of the rope under different materials and diameters cannot be simply considered to be the same;(2)When establishing the longline gear simulation model,the metal components in the fishing gear are ignored.However,since the physical properties of metal components are different from flexible ropes,their own weight and hydrodynamic characteristics will affect the force and shape of fishing gear,ignoring metal components may lead to a low accuracy of calculations.In order to improve the deficiencies in the simulation of tuna longline hook depth,this paper calculates the resistance coefficient by measuring the water resistance of flexible ropes and metal structures commonly used in tuna longline fishery.Substituting the resistance coefficient into the numerical modeling,combining the relevant operating parameters and sea state factors measured at sea in the Solomon Islands waters,the simulation calculation of the hook depth is carried out,and the calculation results are verified by the data measured at sea to determine the model reliability.The results of the research will be helpful for the calculation of hook depth in the Solomon Islands waters,combined with the hook number of the catch,the catch per unit fishing effort in different water layers can be calculated,and it can also provide guidance for the optimization of the structure of fishing gear and operating parameters.In this paper,the water resistance coefficient of 5 kinds of polypropylene twisted ropes(diameter 4.0,5.0,5.5,6.0,6.5 mm)commonly used in tuna longline fishery is studied,and the flow state of equal-diameter juxtaposed cylinders in water is simulated by ANSYS software.The longitudinal optimal spacing is measured by using the streamlined frame.In the flume tank,the water resistance of multiple polypropylene twisted ropes at 90°to the water flow is measured,and the water resistance coefficient CN90 under 7 flow velocities(0.2-0.8 m/s)is obtained by calculation.Conducting research on the metal components used in the tuna longline fishery,the automatic buckle attachment rings,45 g lead swivel rings and 14/0 round hooks are chosen representatively to measure their water resistance coefficients.The image processing software Image J is used to analyze the area of metal components and to calculate the force-bearing area in water.The flow state of parallel cylinders with equal diameters and parallel cylinders with unequal diameters in water is simulated by ANSYS software to determine the optimal horizontal and vertical spacing.The water resistance of multiple metal components under the attack angle of 90°,and 7 kinds of flow vilocity(0.2-0.8 m/s)is measured by using streamlined frames in the flume tank,and resistance coefficients of the automatic buckle attachment ring,45 g lead swivel ring and 14/0circle hook are calculated.Based on the low-temperature tuna longline fishing vessel "Zhongshui 757" of CNFC Overseas Fisheries Co.,Ltd,during the period from May 6,2022 to July 19,2022,in the exclusive economic zone of the Solomon Islands at 11°10~14 °33′S,158°9~169°33′E,actual measurements at sea are conducted.The fishing gear structure and the operating parameters are record.The marine environment data,actual hook depth and three-dimensional ocean current data are measured.After processing,the daily hook depth data and current fitting data curve are obtained.The numerical simulation and verification of longline fishing gear are carried out.The finite element method of numerical simulation is used to discretize the structure of the fishing gear,and the lumped mass method is used to decompose its force into the corresponding nodes.The dynamic equation according to Newton’s second law is established.By using the method of lumped mass and Newton’s second law,the numerical simulation model of longline fishing gear is established,and the kinetic equation is optimized and solved by the six-level five-order Runge-Kutta method.The above process is realized by MATLAB programming.The hydrodynamic coefficients of each component of the longline fishing gear used in the model are obtained by the actual measurement in the flume tank.The results show that:(1)For parallel diameter cylinders and parallel cylinders with unequal diameters,when the spacing is small,the influence of the two cylinders on the surrounding flow state is superimposed,so that the midpoint flow velocity between the two cylinders increases significantly.As the distance between the two cylinders increases,the flow velocity at the midpoint decreases gradually.Compared with parallel cylinders of equal diameter,at the same spacing,the increase in the diameter of a single cylinder leads to an increase in the midpoint flow velocity,and the optimum spacing becomes larger.When the diameter of a single cylinder decreases,the midpoint flow velocity decreases,and the optimum spacing increases.(2)The resistance coefficient CN90 of the common polypropylene twisted rope in longline fishing gear has little change at the flow velocity of 0.2-0.8 m/s.The drag coefficients of the 4.0 mm,5.0 mm,5.5 mm,6.0 mm,6.5 mm polypropylene twisted rope are about 1.225,1.230,1.275,1.281,and 1.321,which can be regarded as constant values.When the Reynolds number is low,the drag coefficient increases slightly with the increase of the Reynolds number.As the drag coefficient reaches the peak value,the drag coefficient decreases with the increase of Reynolds number.(3)The image processing software Image J is used to calculate the area of the irregular objects.By determining the pixel area of the irregular objects on the image,it can be used to calculate the area of metal components of the tuna longline fishery.After calculation,the projection areas of the automatic buckle attachment ring,45 g lead swivel and 14/0 circle hook are 20.618 cm2,8.055 cm2,6.359 cm2 respectively.(4)The resistance coefficient CN90 of the metal component may be related to the asymmetry of the metal.The greater the asymmetry,the greater the resistance coefficient of the metal component.The resistance coefficient of the buckle is about0.976,and the resistance coefficient of the 45 g lead swivel is about 0.901,and the average value of the 14/0 circle hook resistance coefficient is about 1.796.And the change law of the drag coefficient is the same as that of the flow around the cylinder,and decreases with the increase of the flow velocity.At the same time,the asymmetry has a great influence on the drag coefficient.(5)The actual measurement results at sea show that the daily horizontal X and Y direction velocities in the relevant sea area are relatively close,within a range of ±0.3m/s and the hook depth ranges from 65 m to 280 m.(6)The MATLAB program based on the explicit sixth-level and fifth-order Runge-Kutta method can stably and effectively calculate the hook depth under different three-dimensional ocean currents.There is no significant difference between the simulation hook depth and the measured hook depth,and the numerical simulation method can be used to study the hook depth in tuna longline fishing.