| With the continuous development of China’s fishing industry,the improvement and stability of fishing gear has become a priority and need to be solved.As an important equipment in the trawl system,the hydrodynamic performance of bottom otter board directly affects the horizontal and vertical expansibility of otter board and changes the expansion degree of trawl network port.Thus,the trawling performance,fishing efficiency and economic benefits of fishing boats are affected.The bottom otter board will be affected by the bottom effect and sand curtain effect of flow field in the operation process,which will increase the power consumption of fishing boats and increase the cost of trawling,but these effects are often ignored.Therefore,the research on the hydrodynamic performance of bottom otter board based on the bottom effect of flow field and sand curtain effect is of far-reaching significance to the protection and sustainable utilization of fishery resources.CFD numerical simulation is an important method of calculation sheet hydrodynamic performance,but the present stage of research for halftone hydrodynamic performance mostly ignored the bottom effect and sand curtain effect,the study on rectangular surface sheet and elliptical surface otter board as the research object,using the CFD numerical simulation method to calculate the hydrodynamic performance of the rectangle,oval,surface mesh,The visualization analysis of flow field around two otter boards is carried out by using post-processing software,which provides scientific and effective reference for improving and optimizing the otter boards.This paper consists of six chapters: The first chapter is the introduction,which mainly introduces the research background and significance of the subject,the research status at home and abroad and the research content of this paper.The second chapter introduces the material and method,numerical calculation method,calculation condition setting,two kinds of screen model and calculation model parameter setting including parameter definition,turbulence model selection,etc.In addition,the experimental research methods are introduced.The third chapter introduces the validity verification of the numerical model,and verifies the validity of CFD numerical simulation of the hydrodynamic characteristics of rectangular and elliptical curved mesh plates,which provides a method for the subsequent research.In chapter4 and chapter 5,the influence of bottom effect and sand curtain effect on the hydrodynamic performance of bottom otter board is studied.The hydrodynamic performance of bottom otter board is calculated by changing the flow attack Angle,the height from bottom surface and the concentration of solid particles on bottom surface of flow field.According to the calculated results and the visualized flow field diagram obtained after processing,the influence of the bottom boundary effect of the flow field and the surrounding fluid media on the hydrodynamic performance of the screen is studied.The main results are as follows:(1)The lift and drag coefficients of bottom trawl board are always greater than those of middle trawl board.With the increase of the Angle of attack,the lift coefficient increases first and then decreases.The maximum Angle of attack of lift coefficient is about 40°.The drag coefficient increases with the increase of the Angle of impact.With the increase of the Angle of impact,the lift-drag ratio increases first and then decreases,and the best lift-drag ratio of the Angle of attack is about 10°.When the Angle of attack is small(0°～30°),the lift-drag ratio of bottom trawl net is always larger than that of middle trawl net,and tends to be the same when the Angle of attack is greater than 30°.Therefore,the bottom effect reduces the hydrodynamic performance of the screen in the range of small Angle of attack(0°～20°).Combined with the pressure cloud diagram and flow diagram,it is found that a large number of vortices are formed in the case of high Angle of attack.The main reason that affects the hydrodynamic performance of the screen is vortices,and the large and complex vortices make the bottom effect less obvious.Therefore,the influence of bottom boundary effect of flow field on hydrodynamic performance of bottom trawl net plate can not be ignored within the range of impact Angle of normal operation.The hydrodynamic performance of rectangular and elliptical curved net plate at bottom is greater than that at middle,and the expansion performance of bottom trawl net plate can be maintained higher.(2)When the distance from the bottom of the mesh plate to the bottom of the flow field is more than 1000 m,the lift-drag ratio tends to be stable.The optimal lift-drag ratio tends to be stable when the dimensionless height between the bottom of the screen and the surface boundary of the flow field exceeds 0.4,and the influence of the bottom boundary effect on the hydrodynamic performance of the screen can be ignored.Dimensionless height less than 0.4,the bottom boundary effect will affect bottom trawl halftone hydrodynamic performance and floor effect of halftone hydrodynamic performance with the decrease of the dimensionless ratio and the influence of the more significant,the more verified the rectangle,oval,surface mesh plate in the underlying hydrodynamic performance is greater than its hydrodynamic performance at the middle,it is more suitable for bottom trawling.At the same time,the optimal Angle of attack does not change with the change of height,which also indicates that the bottom effect has no significant effect on the optimal Angle of attack.(3)The optimal Angle of impact and the maximum lift coefficient of the two kinds of otter boards do not change with the change of the height from the bottom and the concentration of water and sand mixture,which also indicates that the bottom effect and sand curtain effect have no significant influence on the optimal Angle of impact.(4)When the Angle of attack is large(30°～50°),the effect of sand curtain will increase the lift and drag coefficient of bottom otter board,but when the Angle of attack is small(0°～30°),the effect of sand curtain on lift and drag coefficient is not obvious.The effect of sand curtain effect on lift-drag ratio always exists.When the Angle of impact is small(0°～30°),the lift-drag ratio of bottom otter board decreases,but when the Angle of impact is large(30°～50°),the effect of sand curtain effect increases.Therefore,the sand curtain effect reduces the hydrodynamic performance of the bottom otter board in the range of normal Angle of attack,but keeps the expansion efficiency of the bottom otter board in the range of larger Angle of attack.(5)When the screen is in the water and sand mixture,the optimal lift-drag ratio of the screen decreases obviously compared with the pure water medium.When the concentration of the water and sand mixture is low,the sand curtain effect begins to appear,but its influence on the lift-drag ratio of the screen is relatively small.With the increase of the concentration of water and sand mixture,the lift-drag ratio of the screen begins to decrease,until the concentration reaches 0.02,the trend of lift-drag ratio decline is reduced.When the concentration is greater than 0.02,the increasing of the water-sand mixture concentration does not have a great influence on the lift-drag ratio.In general,the sand curtain effect significantly reduces the hydrodynamic performance of bottom otter board,and the lift-drag ratio tends to be stable when the concentration of water and sand mixture reaches 0.02. |
PDF Full Text Request