Research On Numerical Modeling Of Hydroelastic Characteristics Of Pelagic Trawl Gear System

The ocean is an important food source for humans.In marine fishery,trawl is one of the most widely used fishing techniques,it accounts for more than 40%of the global catching,its importance is self-evident.Trawl gear,which including trawl net,bridle,trawl door and warp etc.,is the key component in trawl operation.It has a great effect on trawling performance and efficiency as well as trawler energy consumption.Therefore,the design and optimization of trawl gear have become a hotspot in marine fishery.Three design and optimization methods for trawl gear are commonly used:sea trial,model test and numerical simulation.Compared with the other two methods,numerical simulation has the advantages in low cost,short develop cycle,easy to adjust parameters and good data consistency,so it plays a more and more important role in trawl gear design and optimization.However,existing numerical simulation methods of trawl gear still have some problems,the biggest one is their low computational efficiency.Usually,it requires a couple of days to simulate one net,this severely limits its application in trawl gear design and optimization.On the other hand,due to its low computational efficiency,the trawl gear model can hardly become more complex as well as more accurate.In order to overcome the existing problems and further improve the accuracy of trawl gear numerical model,a research on trawl gear numerical modelling method is carried out.This dissertation firstly presents the fundamental numerical modeling method of trawl gear.Based on the established numerical model,the numerical stability criterion of trawl gear model was derived.According to the numerical stability criterion,an optimization method is presented to improve the computational efficiency.After optimization,the computational efficiency of trawl gear model can be improved about 40 times,and the distortion caused by the optimization is less than 2%.This dissertation futher developed a numerical modeling method for the interaction between trawl gear and surrounding water,the simulation results were compared with the experimental data.The differences between the simulated results and the experimental values in flow field and net's hydrodynamic force are about 2%-6%,and the simulation speed can be hundreds of times faster than existing methods.This dissertation also proposed a 6 DOF trawl door model to make the trawl gear model more accurate when trawling condition varies in a large range.At last,based on the numerical model established above,numerical experiments were carried out to summarizes the load characteristics of trawl gear.The dissertation is organized as follows:Chapter 1 introduced the composition of trawl system and trawl gear performance investigation methods,stated the research background,significance as well as the main research contents.Chapter 2 introduced the details of trawl gear modelling using lumped mass method.It presented the hydrodynamic force calculation formula of net panel,mesh bar and knot.The motion equation of lumped mass points was also derived based on the calculated hydrodynamic force.The accuracy of mesh combination method was also investigated.Chapter 3 firstly investigated the calculation divergence process of a trawl gear lumped mass model,then performed a numerical stability analysis based on the basic element of the lumped mass model.The numerical stability criterion of trawl gear model was derived by the numerical stability analysis.Then an optimization method was put forward to improve the computational efficiency.Chapter 4 established the coupling model between trawl net and surrounding water.The flow field is modeled by the finite volume method.A novel hybrid volume approach is used to add the resistance force of the net into the flow field for coupling the fluid and net.The net resistance to the flow is calculated directly by the net's current load using Newton's Third Law.The resistance force is discretized in the hybrid volume and represented in the source term of the Navier-Stokes equation.The final results were derived by the segregated iterative calculation of net shape and flow field.The simulation results were compared with the existing experiments.Chapter 5 introduced the six degree of freedom modeling method of trawl door.Firstly,the trawl door geometric parameters and the reference coordinate systems were defined.The variation of trawl door's hydrodynamic parameters with different yaw,pitch and roll angles was also taken into consideration.First-order approximation is used to simplify the relationship between the trawl door's hydrodynamic parameters and its working angle.The coefficients included in the trawl door model were then estimated by the sea trail data.The movement of trawl door and its influence on trawl gear performance was also analyzed based on the proposed 6 DOF trawl door model.Chapter 6 performed 600 numerical experiments with different trawling conditions and difference wave frequencies to get the load characteristics of trawl gear based on the established numerical model.Using the results of numerical experiments,curve fitting was employed to parameterized the trawl gear load model.The warp tension calculated by both the load model and physical model was compared to verify the accuracy of obtained trawl gear load model.Chapter 7 summarized the work of this dissertation,main conclusions and innovations of this dissertation were concluded,and prospects for the next stage research were also presented.