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 tension force characteristics. Our results can provide reference for design and improvement of trawl. © 2017, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.
