【作者】 王敏法；

【导师】 许柳雄；

【作者基本信息】 上海海洋大学 ， 捕捞学， 2011， 硕士

【摘要】 金枪鱼围网是捕捞金枪鱼的主要渔具之一,其网具主要由网衣和钢索构成,而作用在网具上的外力直接影响着网具的性能,包括网具最大沉降深度、下纲沉降速度以及网衣在水中的空间形状等,围网的网具性能直接影响到围网捕捞作业的效率,是围网作用成败的关键要素。要使网具具有较高的工作效率,在作业过程中达到设计所要求的特性,就必须了解和掌握网具的空间形状和受力情况。围网网具的作业深度达到上百米的水层,对其进行现场研究具有不可控性,而模型试验至今在模型准则和模型制作上存在一定的困难。因此,运用现代计算机技术进行数值模拟是围网网具研究的重要补充手段。本文以有限元理论为基础,将围网网具系统离散为通过杆单元的质量点的集合,即将网衣的目脚假设为圆柱体的杆单元,目脚与目脚之间的连接处看成质量点,并考虑杆单元为不可受压的。在恒流对网具系统的作用下,将各个杆单元在水流中受到的水动力、张力、重力和浮力均被平均分配到相邻的质量点,根据牛顿第二定律,对各个质量点列出一系列的动力学平衡方程组,从而建立网具系统的三维数学模型(3D Numerical Model,3DNM)。由于杆单元水动力与冲角为非线性关系,网具系统的动力学方程组也为非线性方程组,因此,本文采用改进的隐式欧拉法以及牛顿迭代法对各个质量点的运动方程进行求解,获得稳定的计算时间步长,并利用R语言为模型编写运行速度较快的计算程序,进而得到网具系统各个质量点的位置坐标和杆单元的受力情况。同时,针对不同边界条件,本文利用3DNM分别对顶端固定网片、顶端和底端固定网片、圆柱形网片等进行了动态模拟。模拟显示在稳定水流的环境中,各种状态下的网衣在外力作用下都经历开始阶段剧烈波动到逐渐趋于平衡的运动过程。研究结果表明:(1)以有限元理论为基础,采用集中质量法对网具进行离散,建立了网具三维数学模型;(2)采用改进的隐式欧拉法以及牛顿迭代法,有效地对三维数学模型进行了解析;(3)利用R语言为模型编写计算程序,能够获得稳定的计算时间步长;(4)稳定水流环境中,当水流方向与网衣平面平行时,顶端固定网衣平衡状态下的右侧网衣呈伸展状态,而左侧网衣呈聚拢状态,当水流方向与网衣平面垂直时,顶端固定网衣平衡状态下的呈现“倒三角”展开状态,即越往下网衣越聚拢;(5)稳定水流环境中,当水流方向与网衣平面垂直时,顶端和底端均固定网衣存在两侧向内收缩且变型较大的现象,同时,网衣中间存在“×”字形的两条网线单元绷直的现象,即网衣两对角线所在的网线单元相对其他地方的网线单元呈现绷直状态,而其他地方的网线单元相对松弛;(6)稳定水流环境中,沉子配重不同时,网衣垂直方向的下沉深度不同,同时,网衣水平方向的移动距离也不同;(7)稳定水流环境中,建立的三维数学模型能够有效地应用在围网网具系统的动态数值模拟中,从而为进一步进行围网网具数值模拟研究提供了基础。更多还原

【Abstract】 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.更多还原