Experimental Study On The Hydrodynamic Coefficients Of Plane Nettings And Numerical Simulation

Fishing nets are widely used in fishing operation in current world’s fishing industry. Concerning with the fishing gear design and their operating performance, it should be noticed that the loads on the fishing net will directly affect the shape, tension distribution and drag of fishing gears, also to the control and adjustment of gears states; the impacts from the flow state on gears and target species could directly influence fishing efficiency and energy consumption. So, fishing nets design should not consider the behavior and habits of target species only, but also reduce the resistance which closely related with the fishing boat fuel consumption and the fishing efficiency. The appearance of synthetic materials i.e. polyethylene, nylon and the new-type nettings made a great difference to the material and structure of fishing nets.With the rapid development of computer technology, the great progress in numerical computing, finite element method, lumped mass method and the mass-spring system are widely used in mathematics and mechanics modeling and dynamic simulation of fishing gear. Computer numerical calculation is able to solve the relatively difficult engineering mechanics calculations in a relatively short time period and get the reliable results. Apply the numerical simulation method to analyze the forces, displacement and deformation under varied loads can facilitate to understand the operating performance better and save a lot of money and time. The hydrodynamic characteristics of plane nettings are the key factors to determine the mechanical properties of fishing nets. The determination of hydrodynamic coefficients is an important fundamental work for mathematical modeling and numerical calculation of fishing nets, it has an important academic value and practical significance for enriching the design theory, optimizing mathematical and mechanical model, enhancing catch efficiency, improving the safety performance of fishing gears.In2014, a series of experiments had been carried out to measure the hydrodynamic coefficients of the plane netting which fixed on a square frame, in the flume tank of the East China Sea Fisheries Research Institute, and using MATLAB to simulate the performance of netting, the results are as follows:(1) The two sides of the frame are kept perpendicular to the flow direction, while changing the attack angle to another two sides, the relationship between attack angle and the drag coefficient of cylinder is a linear function.+=7958.00089.0 adC 7897.02R= The main force on the framework is the water resistance; when the attack angle is 45°,the ratio of lift to drag approach the maximum, the value of lift coefficient is close to the half of the drag coefficient. Refer to the relationship between the attack angle and resistance, the calculation shows that the CN90 of cylinder is 1.61.(2) The hydrodynamic characteristics of nine kinds of plane nettings were tested in the flume tank. Involved parameters, including the attack angle, flow speed, hanging ratio ET, the lift coefficient CL, the drag coefficient Cd, the ratio of lift-drag coefficient K and so on. The results showed that:①Drag coefficient of nettings increased while the attack angles getting greater, and decreased with the current speed and netting twine diameter increasing;②In general, the values of lift coefficients increased until the attack angle 45 degree, the maximum value reached1.02,and decreased for the attack angle over 45 degree;③The maximum values of the ratio of lift coefficient and drag coefficient(K) occurred at about 20 degree; K values decreased over 30 degree;④when Reynolds number(Re) is less than 1500 and the attack angle is less than 45°, the drag coefficient showed a decreasing trend after increasing; And when the attack angle is greater than 45 °, drag coefficient showed a decreasing trend. When Reynolds number(Re) is greater than 2800, the drag coefficient reaches at slightly stable states.⑤When the mesh size is 60 mm, twine diameters are 1.15 mm, 2.25 mm, 3.5mm, hanging ratios are 0.3, 0.5, 0.707, a draw of the three-dimensional diagram among hydrodynamic coefficients, attack angle and Reynolds number is provided. ⑥The hydrodynamic coefficients firstly increased as d/a increased, then decreased.⑦The hydrodynamic coefficients decreased as the solidity ratio increased.⑧The resistance formula of nettings T0 are as follows:LaaSSCR2=0.865 ⑨When the solidity ratio is large, the drag coefficient of T90 nettings are smaller than T0 nettings, we recommend the designers to use T90 nettings to reduce drag.(3)A piece of netting, consists of the 8×8 meshes, fixed on a square frame, was simulated and the tensions and their distribution, the positions of knots and netting shape were calculated by means of MATLAB in computer. The dynamic mathematic model was developed based on lumped mass method, the netting was treated as spring-mass system, the Runge-Kutta fifth-order and sixth-order method was used to solve the differential equations for every step, then the displacement and tension of each mass point were obtained. Sensitive factors were analyzed in the model, the results are as followed: ①The twine diameters and added mass effect on the plane netting are not negligible, the relationship between twine diameter and the displacement is an inverse function, the relationship between added mass and displacement of mass point is exponential function. ②the relationship between hydrodynamic coefficients and displacement of mass point is linear.