| Tuna purse-seine tuna gear is one of the main fishing gear, which are mostly constructed by the nettings and ropes, and the external loads acting on the gear directly impact on the performance of gear, including maximum sinking depth, sinking speed and shape of netting in the water. Performance of purse seine gear directly impacts the efficiency of purse seining, which also is a key element of fishing. In other words, if the gear has to appoach the states as expected and has its catch efficency as designed, it is important to understand and master the loads distribution and the shape of netting. But, purse seine gear are mostly operated in deep water, hundred meters with strong current, it is incontrollably to conduct field studies, and also there are still some difficuties in model rule and model-making during model test. Therefore, the use of modern computer simulation study is an very important supplementary means for purse-seine gear study.In this paper, based on the finite element theory, the purse-seine gear is dispersed as aggregation of mass points connecting with rigid bars. In other words, in our gear system a kont with a mesh is regarded as a mass point and a bar as rigid bar, which is assumed only able to behave under tension. In a uniform water current, we equally concentrate forces such as the hydrodynamic force, tension, sinking force and buoyancy on the adjacent points. According to Newton's second law, the dynamics eaquations of all mass points are set up, then, all of these equation are presented as a three-dimensional mathematical model (3D Numerical Model, 3DNM ). The relationship between hydrodynamics and the attack angle of mesh bar is nonlinear, so dynamics equations for the gear system is also nonlinear, we use the improved implicit Euler method and Newton iteration method to solve equations for the mass points each, and acquire the stable calculation time-step, and we also use R language to write calculation program for system model, which has advantage of running time. By this model the tension and coordinates of all mass points can be calculated. Meanwhile, Based on their different boundary conditions, this paper use 3DNM to conduct dynamic simulation for analysis of the force and deformation of cylindrical netting, plane netting with the top boundary of the netting fixed on, and plane netting with the top and bottom boundary of the netting both fixed on. The results of simulations show that, the netting under external force have experienced duration with the beginning of the violent movement to the process of becoming more balanced.The results show that: (1) Based on finite element theory, lumped mass method has been successfully used to discrete gear system, and establish three-dimensional mathematical model for gear system; (2)By using modified implicit Euler method and Newton iteration, three-dimensional mathematical model has been effectively solved; (3) By using of R language to write calculation program for the model, the model has obtained a stable calculation time-step; (4) In a uniform water current, when the flow direction parallel to the plane of top fixed neeting, the right-hand of netting is extended well, but, the left-hand of netting is gathered at equilibrium state. when the flow direction perpendicular to the plane of top fixed neeting, the equilibrium configuration of the plane net is unfold as "inverted triangle" state, in another word, more down to gather more netting; (5) In a uniform water current, when the flow direction perpendicular to the plane of top and bottom fixed netting, both sides of the netting exists the phenomenon of the inward contraction of the larger variant, while , on the plane of netting there is "×"-shaped netting cord, namely, compared to other parts of the netting cord, netting cord where the two diagonal lines present tight state, while other parts of the netting cord present relative relaxation; (6) In a uniform water current, when the weight is not different, sinking depth of netting is different accordingly, meanwhile, the movement of the horizontal distance of netting is also different; (7) In a uniform water current, the establishment of three-dimensional mathematical model can be applied effectively in the numerical simulation of dynamic systems for purse-seine gear, so as to provide a basis for further numerical simulation for purse-seine gera. |
