| Fishing gear is the main tools for exploitation and utilization of marine living resources,which provides important support for the development of global marine fisheries.Nowadays,developing low energy consumption and Eco-friendly fishing gears is an important task to realize the sustainable development of marine fisheries.The research on hydrodynamic characteristics of basic components of fishing gears is the most basic and indispensable part in the optimization and design of fishing gears.Among them,netting twine is the main unit of fishing gears such as trawl,purse seine and gill net,and its hydrodynamic characteristics directly affects the efficiency of operating fishing gears.Therefore,it is necessary to investigate the hydrodynamic characteristics and flow field of the netting twine to clarify the hydrodynamic performance and mechanism of each section and the full scale of fishing gears.The netting twine is usually composed of three strands,which are twisted in the opposite direction of twist to the cable twisted netting twine.It has the characteristics of compact,stable structure and smooth surface.Unlike other marine structures,netting twine is flexible,easy to deform,small scale and so on,its interaction with flow is a complicated fluid-structure interaction phenomenon.In order to analyze the flow field distribution characteristics of the netting twine,some scholars simplified the netting twine into a smooth cylinder in CFD numerical simulation,and paid little attention to the amount of twist and direction of twist of the netting twine.The results differ greatly from the actual situation and have certain limitations.To investigate the effect of amount of twist and direction of twist on the drag of and the flow field distribution of the netting twine,Solidworks was used to build a three-dimensional physical model of netting twine with different amount of twist,directions of twist and diameters,FLUENT software was used to conduct CFD numerical simulation and the simulation results were compared with the physical test data to verify the validity of the numerical simulation results in this study.In addition,the visualization of flow field around twisted netting twine was accomplished based on the SST k-ω turbulence model,and the vortex structure of the flow field around a physical model of twine and cruciform structure with amount of twist and direction of twist were obtained by the Omega Vortex Identification Method.The main results of the study are as follows:1)The effect of amount of twist and direction of twist on the drag of three-strand twine: Compared with the non-twist(cylindrical)twine,the numerical simulation results of the drag of twine with amount of twist were closer to the model test results and the simulation accuracy was improved by 2.3%.When the diameter of the twine was less than 13.9 mm,the twine drag increased with the increased of amount of twist.As the angle of attack increased,the drag of the twine increases.When the angle of attack ranged from 0° to 60°,the drag increased with the increased of amount of twist;When the angle of attack ranged from 60° to 75°,the drag of twine fluctuated greatly under each amount of twist.For the same amount of twist and diameter of twine,the drag of the twine with S-twist direction was larger than that of Z-twist direction,and the difference in drag between them was the largest,about 8.79% when the angle of attack was 45°.2)The effect of amount of twist and direction of twist on the drag coefficient of three-strand twine: When the twine was perpendicular to the direction of water flow,the drag coefficient of non-twist twine decreased with the increased of Reynolds number,while the drag coefficient of the twine with amount of twist was relatively stable.As the amount of twist increased,the drag coefficient of the twine fluctuated less.Compared with the Z-twist twine,the drag coefficient of the S-twist twine was smaller.3)The effect of amount of twist and direction of twist on the flow field distribution around the three-strand twine: The flow field around the twine with different amount of twist showed that the flow attenuation range of the rear end of the non-twist twine was larger than that of the amount of twist twine,but the turbulence intensity at the tail of the non-twist twine was smaller.When the amount of twist was in the range of 16-24 and 28-40,the flow attenuation area and turbulence intensity at the tail end of the twine increased as the amount of twist increased.However,the flow attenuation area and turbulence intensity at the rear end of the twine with amount of twist of 28 suddenly weaken.With the increased of the angle of attack,the attenuation area and turbulence intensity around the twine tend to increased.When the angle of attack was ranged from0° to 60°,the attenuation ratio of the flow speed at the rear end of the S-twist twine was greater than that of the Z-twist.When the angle of attack was ranged from 75° to 90°,the opposite situation occurs.When the twine was perpendicular to the flow,the flow vortices at the rear end of the twine with Z-twist direction were large and evenly distributed,while the flow vortices at the rear end of the twine with S-twist direction were small and scattered.4)The effect of amount of twist and direction of twist on vortex around the twine and cruciform structure: the vortex features around the twine and cruciform structure were identified based on the Omega Vortex Identification Method.The results showed that the vortex area of the twine and cruciform structure increased with the increased of flow speed.For the twine and cruciform structure with amount of twist,the vortex area of twine and cruciform structure increased first and then decreased with the increasing of amount of twist.For the non-twist twine and cruciform structure,the tail vortex covered the oncoming surface and side of the twine and developed along the direction of the flow.The tail vortices of twine structure with amount of twist were distributed in a spiral pattern with twine and developed along the flow direction.With the increased of amount of twist,the number of vortices of cruciform structure increased,the gap between vortices decreased,and the shape of vortices became smaller.When the amount of twist was 24,the four ends of the cruciform structure were broken.Identification of cruciform structure vortex with different directions of twist showed that the vortex area of the cruciform twines structure vortex with S-twist direction was smaller than that of Z-twist direction at the flow speed ranged of 0.3-0.7 m/s.The situation was opposite when the flow speed ranged from 0.7 to 1.2 m/s.The vortices at the four ends of Z-twist and S-twist were helical and their torsion directions were different.The gap between the vortices at the four ends of the Z-twist cruciform structure was larger and the vortices at the cross position of the direction of S-twist cruciform structure were separated,while there was no obvious separation phenomenon at the Z-twist cruciform structure. |
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