Numerical Simulation Study On Hydrodynamic Characteristics Of Net Fishing Gear

Net fishing gear is made up of soft netting, twine, float, sinker, and so on. Itexpands definite shape when wave, current, floating and gravitational forces act on itduring working process. Therefore, in order to optimize the designing and obtain goodfishing effects, designers expect to predict the expanded shape, suffered force andcharacteristics of motion of fishing gear during designing stage by taking use of tankmodel experiment and mechanical analysis. In recent ten years, owing to progress ofcomputer technology, mathematics and mechanics analysis method, numericalsimulation research about hydrodynamic characteristics of net fishing gear has beenbecoming frontiers gradually in research field of mechanics of fishing gear, causingmore concern of related researchers at home and abroad, and has obtained somenotable fruits.Netting is affected by drag force and inertial force in wave and currentsimultaneously. In calculation analysis of mechanics of fishing gear, the drag equationin engineering is popularly used when calculating the force of flow. However, the keypoint is to investigate and choose reasonable drag coefficient C_dand inertialcoefficient C_m. In previous research, the result on drag coefficient C_dof netting incurrent is comparatively more, but the research of inertial coefficient C_mof netting isclose to vacancy. Therefore, to investigate the behavior and hydrodynamiccharacteristics of supple nets in wave, experiments of forced oscillation wereparticularly designed and conducted in a tank with square net panels firstly. The fourcorners of the net panel were fixed to the oscillation device with a specific pretension,by a specially designed “” like load cell which could be used to test force of twodirections. The hydrodynamic loads acting on the corners of the netting weremeasured. To simulate the experiments, a numerical model based on dynamic finite element analysis was constructed. Comparing the amplitude and phase of measuredloads with the calculated results, the drag coefficient C_dand inertia coefficient C_mwere determined by interpolation and iteration. The effects of various parameters onC_dand C_mwere investigated. The results show that (1) drag force is dominant at lowfrequency, whereas the contribution of inertia force increases with an increase of theKeulegan–Carpenter number or frequency;(2) with increasing frequency, both C_dandC_mshow a decreasing trend, and the trend of C_dagainst the Reynolds number issimilar to that in steady flow;(3) increasing oscillation amplitude makes C_ddecrease,but it has no significant effect on C_m;(4) solidity generally has a positive effect on C_dand C_m.Secondly, the finite element dynamic numerical model of netting is applied inhydrodynamic research of net fishing gear in wave and current. Taking gillnet andstow net as examples, numerical simulation of hydrodynamic characteristics offishing gear are conducted. To simplify calculation, mesh-grouped method is appliedin numerical calculation, namely merge a certain actual meshes into one virtual meshto reduce the computational requirement with condition of keeping similarity ofmechanics and geometry. In tank model experiments of gillnet and stow net, byinstalling light diodes on the netting as tracers, the movement locus of tracer pointwas recorded by CCD. By comparing and analyzing experimental value andcalculated results of load force and motion track in steady current and combinedwave-current, it is found that measured value and calculated results are in goodagreement with each other.Thirdly, numerical simulated method can not only obtain shape, tension distributionand motion characteristics of net fishing gear in wave and current, it can also showshape, tension distribution and motion with3D virtual effect by taking use ofcomputer graphics technique, and further provide theoretical principle for theoreticaldesign of fishing gear by combining with computer aided design.