| Pacific saury is an important pelagic fish species in the distant water fisheries of China,and one of the priority species managed by the North Pacific Fisheries Commission.The stick-held dip(SHD)net is the main fishing gear for saury fishery and consists of netting,floater,and cables.The sinking characteristic,equilibrium configuration,and hauling performance of net are important factors that affect the success of catch and productivity effect.To investigate the hydrodynamics and catch performance of SHD net in current,physical model test and computational fluid dynamics(CFD)are used to study the basic hydrodynamic characteristics of the prototype netting panel and floater of the SHD net,to provide basic parameters for the numerical modeling;the coupled model of SHD net was developed by combing the lumped mass method with principle of rigid body kinematics,and verificated by the model tests and field measurements at sea,the key operation of shooting and hauling process as well as the motion of SHD net under different conditions(current velocity and winch speed)were studied.Then,the influence of net structure(weight of foot line,mesh size,netting material),operating technology(length of haul rope)and sea condition(wind speed)on performance of motion,shape,and tension of cable were analyzed,finally,the optimization proposals for the SHD net was put forward.The main results are as follows:(1)The physical model test in flume tank and CFD simulation were carried out to analysis the the characteristics of hydrodynamics and surrounding flow field of the prototype panel of SHD net,and the effect of mesh opening angle on hydrodynamics and flow field distribution were explored.The results showed that the hydrodynamic coefficient of the netting was related to the Reynolds number(Re),attack angle(θ)and solidity ratio(α).The normal drag coefficient was positively related to the solidity ratio when the panel was perpendicular to the current,however,the parallel drag coefficient had a negative correlation with the solidity ratio when the net was parallel to the current.When the netting panel was inclined to current,the drag coefficient increased with the increase of the attack angle,and the lift coefficient increased first and then decreased;the solidity ratio had a dual effect on the drag coefficient,the lift,drag coefficient and the solidity ratio were negatively correlated when the attack angle was less than 50 °,while the opposite results were found when the attack angle was greater than 50 °.The result of the porous medium model showed that the current velocity through the netting panel was reduced,and the shielding effect of the netting on the flow was related to the difficulty in separating the vessel and the SHD net at the initial stage of shooting operation.The mesh opening angle could affect the hydrodynamic force of the panel and its surrounding flow field distribution.When the netting panel was perpendicular to the current,the drag coefficient decreased with the increasing mesh opening angle and gradually stabilizes when it was greater than 60 °;the drag coefficient increased with the increase of opening angle when the panel was parallel to the current;when the panel was inclined to the incoming flow,the drag coefficient increased with the increase of the opening angle and the lift coefficient decreased when the attack angle was in the range of 10° to 50°,the drag coefficient and lift coefficient tended to be flat when the attack angle was greater than 50 °.The smaller the opening angle,the more obvious the attenuation of the flow velocity.When the opening angle was greater than 60°,the influence of the opening angle on the flow field distribution was reduced.(2)The hydrodynamic characteristics of the floater under different experimental conditions were obtained by analysing the drag,stress on surface,and its surrounding flow field at different immersion depths and current velocities using model test in flume tank and CFD simulation.The results showed that the drag coefficient of the floater,range from 0.12 to 0.63,was related to its initial immersion depth and current velocity.The greater the immersion depth was,the greater the drag coefficient was,and the relationship with the velocity was affected by whether the flow jumps and the state of the submerged floater.The flow patterns on the upstream side and the downstream side of the floater were different,the water accumulates on the upstream side of the floater and appeared the phenomenon of "climbs" on the surface of the floater,and the flow on the downstream side was chaotic and eddies appear.The surface stress of the floater induced by fluid was mainly concentrated at the both ends and middle part of the upstream side of floater,and the magnitude of the stress was affected by the submerged depth of the floater and flow velocity.(3)The mathematical model of SHD net was developed by combining the masslumped method with the principle of rigid body kinematics,the performance of the numerical model was verified by the cable tension of the model test and the sinking data collected at sea.Thus,the shooting and hauling process,netting shape,and cable tension of SHD net at different current velocities were analyzed.The results showed that the mean relative error of tension force of cable and sinking depth in numerical simulation,physical model test and field measurement was 20 %,and the variance analysis showed that there were no significant differences among each other(P >0.05),indicating the numerical model had a good performance on the simulation of the operation of SHD net.The current velocity affected the net shape and sinking characteristics of the SHD net,the deformation of the net was intensified and the sinking depth of the foot line decreased with the increasing velocity.While the flow velocity at sea was greater than 0.8 m/s,the volume of SHD net was decreased by62.9 %.The tension of the cables increased gradually during the hauling and reached the maximum tension at the end,the tension on the haul rope was greater than that of the purse line.The winch speed of the cables affected the hauling time,the shape of the net and the tension of the cable.With the increasing winch speed,the hauling time was shortened,and the maximum tension of the cable increased.When the winch speed of haul rope and purse line of the model net was 0.36 m/s and 0.29 m/s,the maximum tension was 24.37 N and 44.75 N,respectively.(4)The effects of weight of foot line,length of haul rope,net material,mesh size and wind speed on the catch performance of SHD net were analyzed by numerical simulation.The results showed that the weight increase of foot line was beneficial to improve the sinking depth and velocity of the fishing gear,however,the increase was limited after reaching a critical value(23.5 kg/m).For every 10% increase in the weight,the tension of the cable increased by about 4.43 % to 5.30 %;With the increase of the haul rope’s length,the sinking depth and mouth expansion of the net increased,and the sinking depth would increase 9.29 % to 13.53 % for every 10%increase in the length of the haul rope;The sinking depth and speed of SHD net made by the polyester(PES)material was slightly higher than that of polyamide(PA)material,and the tension on cable of the former was 3.47 % higher than that of the latter;With the increase of the mesh size,the sinking depth and rapidity of the nets increased,and the resistance of the net could be reduced by 9.02 % and 12.10 %by modifing the mesh size from 24 mm to 30 mm and 35 mm;The wind spead could affect the speed of movement of the fishing gear,but did not affect its final release distance in the horizontal direction;The resistance of the porotype SHD net in equilibrium was about 10600.5 N at the current velocity of 0.4 m/s.In summary,it is suggested that the mesh size of the main netting of SHD net can be increased to 30 mm;The working immersion depth of the floater should not great than 75% of its cross section;Two environmental factors,including flow velocity and wind condition,are firstly recommended to be considered before operation,and the length of the haul rope can be increased in months in the poor sea condition,especially during October and November. |
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