浙江舟山桁杆拖虾网的力学计算
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摘要
随着东海区传统经济鱼类资源的衰退,捕捞虾类的作业逐步发展起来,到目前为止,浙江省海洋捕捞的虾类产量已达到37.9万t,占浙江省海洋捕捞总量的13%,其中桁杆拖虾网已发展成为一种主要作业方式。本文的目的是通过对桁杆拖虾网的力学计算,对桁杆拖虾作业作进一步的分析。
     本文根据模型准则制作了模型网,根据实际生产拖曳速度设计模型水槽试验流速,在东京海洋大学动水槽中进行试验,并根据模型试验结果来推算桁杆拖虾网的力学性能。首先,通过对曳纲形状和张力的数值求解,计算出曳纲在拖曳过程中的形状和张力大小;其次,把桁杆等效为一个连续梁,通过求解三矩方程,推出了计算叉纲对桁杆拉力的计算方法,进而计算出桁杆所受到的弯矩和剪力,计算结果显示弯矩是以过桁杆中心点的直线成轴对称,而剪力是以桁杆中心点成点对称;对桁杆拉力最小的叉纲是桁杆两端的叉纲,而以两端到中心的第二根叉纲拉力最大;桁杆所受到的最大拉压应力远小于钢铁的许用应力,因此,我们可以把桁杆改用其他更经济、方便且许用应力允许范围内的材料来制作。最后,对拖虾网的分析,把网片看作充分柔软的薄膜曲面,利用无矩薄壳理论推出网片的形状与张力的基本微分方程,根据网目特殊结构,利用差分法对网片基本微分方程组进行数值求解。从计算拖虾网背网片的第一段网片看,这一段网片基本上呈现为平面网片;形状和受力在z轴方向上没有变化,这也基本符合实际情况。由于网内流态的不确定性,用差分计算方法计算也无法得到精确值,因此利用拖虾网的形状对网身和网囊进行了近似计算。从过网身和网囊的形状近似计算的结果与根据试验计算结果相吻合。
Along with the traditional economic fish resources of the East China Sea declining, beam shrimp trawl gradually developed. So far, the production of the shrimp in Zhejiang province on the East China Sea has reached three hundred and seventy nine thousand t, accounting for 13% of the total marine fish catch in Zhejiang province, the beam shrimp trawl has become a main pattern of fishing operation. The purpose of this Master thesis is to further analysis fishing operation by mechanics calculation of beam shrimp trawl.
     This article is based on the model experiment, which was accomplished in the circulating water channel of Tokyo University of Marine Science and Technology, then according to the model experiments results to calculate the beam shrimp trawl mechanics nature. First, through the numerical solution of warp shape and tension, we can work out the warp shape and tension in the process of drag. Second, the beam is equivalent to a continuous beam. Through solving the three moment equation, we can deduce the method to calculate the tension of cross rope acting on the beam. Then figure out the bending moment and shearing force of the beam. The results show that the bending moment is ax symmetric with the line across the beam center, while the shearing force is point symmetry with the center. The cross ropes whose tension acting on the beam is minimum is the cross ropes on the two endpoints of beam, while the maximum tension cross ropes are the second ropes from the beam endpoints to the center, the maximum tension-compression of the beam under the max experiment trawl speed is far lesser then the allowable stress of steel. So the beam can be made of other material whose allowable stress permits. Last, the analysis of the beam shrimp trawl nets. The nets are regarded as full soft curved film surface. Using nonmomental theory of shell, the basic differential equations about shape and tension of nets were deduced. According to the special structure of mesh, the basic differential equations can be solved with finite difference method. From the results of the first section net solution, this section takes on plane net, the shape and tension in z-axis without change. It complies with the actual situation. Because of the uncertainty of the flow in the net, the precise value can’t be obtained with the finite difference method. Therefore, we can get the approximate value by the beam shrimp trawl net shape and the results match the tension of warp from the experiments results.
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