捕鱼拖网形状与张力特性的数值模拟与水槽试验

为更好地利用数值模拟方法了解拖网的形状与张力变化特性,该文通过有限元计算方法和水槽试验的手段,以日本某四片式中层拖网模型为例,构建了拖网的水动力学特性数学模型,并进行了相应的水槽模型试验。通过计算值与试验值的比较,分析了拖网的形状变化(网口高度、网口宽度以及网身长度)、各部分受力变化(空纲拉力和拖网网衣目脚张力分布变化)以及相关的配线问题,说明仿真模型可以对拖网在水流中的变化进行有效模拟。拖网空纲力的试验值与计算值误差约为10%,网口高度、网身长度符合较好,网口宽度的平均误差小于10%。模型拖网网身中浮沉子纲、力纲及其周边网目受力较大,网囊部分受力较小,随着流速增加,上下网片比左右网片受力更大。同时,数值模拟计算可以为拖网配线提供量化依据,然而由于拖网类型多样,不同设计导致张力分布不同,故此需要具体类型具体分析。该研究结果表明,该文数值计算模型可以有效进行拖网的仿真模拟,为设计和改进拖网提供参考。

Numerical simulation and flume experiment of shape and tension force characteristics of fishing trawl

Abstract: Trawls are funnel-shaped nets that are towed by vessels along different water column. According to different fishing requirement and species behaviors, numerous variations for trawls exist. The performance efficiency and fuel efficiency of trawl are largely influenced by designing, which has direct affinity with net mouth opening, net body shape, netting elongation and twine distribution. Comparing with flume tank tests, the numerical model is more convenient to carry out, and also can easily obtain the tension distribution of mesh bars which otherwise is difficult to measure in water tank experiment. In order to know the shape and tension variation characteristics of trawl, in this study, a nonlinear mixed method was used to determine the equilibrium configuration and tension distribution of a trawl set in a uniform current. The method was based on finite element formulation, and the mesh bars were modeled as straight mesh bar elements which were connected with other elements by a frictionless hinge. Meanwhile, an experimental trawl model for measuring hydrodynamic characteristics was also generated, which took a kind of Japan four-seam mid-water trawl as an example. The model of trawl was made of PA (Polyamide). Measuring speed of current was set between 0.4 to 0.9 m/s. In the experiment, the photos from side view and vertical view were synchronously taken in each current velocity. Besides, the width and height of net mouth, the length of trawl body, and the tension force of leg line were all measured. Then, the corresponding simulation model of the trawl was established and solved using MATLAB based on the mathematical model with the number of mesh bar elements of 2 852 and the number of the knots of 1 399. By comparing the calculation results with measured values from circulating water tank model test, it showed that both of the leg line tension and shape of trawl generally agreed well. The tension distributions of mesh bars of trawl were also presented. The results of shape and tension variation characteristics of trawl were concluded as follows: 1) The drag force of leg line increased by power with the increase of current velocity. Average error between calculation and experimental results was about 10%. As the velocity increased, the angle of same-side upper and lower legs gradually decreased, net body length prolonged, height of net mouth narrowed corresponding with the widening of net mouth. The numerical values of width of net mouth were a little higher than experiment results, but the average error of which was within 10%. 2) The position of float line was behind sinker line in x-axis because of the flow effect, but the distance from each other diminished if current velocity increased. The effect of current velocity to shape change of trawl weakened if current velocity had been comparatively large. 3) The tension of float line, sinker line, lace rope and its surrounding mesh bars were comparatively high, while the force of cod end were small. As increase of the current velocity, the tension of most meshes of net body was increased. Moreover, the tension of up and bottom panels of trawl were larger than left and right panels. 4) Tensions of cod-end were much smaller than other parts. The force on cod-end would be mainly from the weight of catch. 5) Numerical calculation can provide quantitative theory for reasonable twine distribution of trawl. However, owing to the difference of tension distribution caused by different design of each types of trawl, it was difficult to obtain a unified standard among all of the trawls for the twine distribution problem. The mathematical method in this study can be used to effectively simulate the operating status of trawls, which included shape and force variation characteristics. Our results can provide reference for design and improvement of trawl.