The Hydrodynamic Performance And Flow Field Visualization Of Mid-water Otter Board

Trawl is one of the main fishing methods all over the world,with a wide range of fishing objects,including fish,shrimp and other economic aquatic animals in the middle and bottom waters.Trawl belongs to a kind of mobile filter fishing gear.Depending on the power of vessel and the resistance of water flow,the trawling gear is expanded,and the catch in the water area it passes is dragged into the net code.Therefore,the expansion effect of the net opening directly affects the catch in the net gear,and it is also the standard to judge the performance of trawl.According to statistics,in 2018,China's trawl output accounted for 47%of C hina's total marine fishing output,which plays a very important role in China's fishery production,so the research on trawl performance has attracted the attention of C hinese scholars.In trawl system,the proportion of the resistance of net and otter board is the highest,among which net accounts for 40%?65%of the total resistance,and otter board accounts for 13%?27%of the total resistance.The otter board is one of the important attachments in the trawl system,its main function is to expand the net and increase the swept area.The hydrodynamic performance of the otter board directly affects the expansion of trawl mouth and catch performance,and then affects the economic benefit of trawl.Therefore,it is of great significance to study the otter board with excellent hydrodynamic performance.Considering the wide applicability and universality of the otter board,it should have the characteristics of simple structure,easy operation,energy saving and low cost.With the rapid development of trawl fishery in the world,the main types of the otter board are rectangular,oval,V-shaped and vertical V-shaped.However,there are some problems in the actual use of the traditional single wing of the otter board,such as the large angle of obstacles to change the drag,easy to overturn,poor stability and so on.In the 1990s,Fukuda,Japanese scholar,designed a biplane type of otter board composed of two wing plates with double curved surface and all steel structure.Because of its advantages of high lift and good stability,the otter board was widely used in large and medium-sized trawl fishery in northeastern Japan and Hokkaido.Because the traditional trawl operation in C hina's offshore is two boat trawl?except for the South China Sea?,the research on the hydrodynamic performance of the otter board started late,and the structure form of the otter board is single.But in recent years,with the development of ocean fishery and the development of single boat trawl in C hina,the research of otter board and the development of new otter board have been paid mo re and more attention.The starting point of this study is to study the hydrodynamic performance of the biplane type otter board which has been proved to be excellent in practice abroad,to understand its hydrodynamic performance parameters and operation performance,and to provide scientific basis for the C hinese fishery workers to develop the high-performance otter board suitable for the characteristics of the mid-water trawl fishery in C hina.In addition,the research on the hydrodynamic performance of b iplane type otter board can also provide basic data for the subsequent analysis of the feasibility of introducing this type of otter board.The flume tank model test can easily and accurately obtain the hydrodynamic performance parameters of the otter board under controllable conditions,which has become an indispensable standard program in the process of designing and developing the otter board.With the rapid development of computer technology,computational fluid dynamics?CFD?numerical simulation has been widely used in the field of fluid research because of its advantages of time-saving,labor-saving and flexible research content,and has become an important means to solve complex physical problems.The combination of model test and numerical simulation can not only help and verify the results,but also provide technical support for the optimization and development of the otter board in the future.In this paper,study the hydrodynamic performance of the biplane-type model otter boards using the tank model test and numerical simulation?Computational Fluid Dynamics,CFD?with different aspect ratios??2.5?3?3.5?4??camber ratio f/C?10%?15%?20%?and sweepback angle??0°?10°?15°?,obtain the optimal hydrodynamic performance of a certain structural parameter of the otter board,discuss the influence of different inclinations?inward,outward and tilt?on the hydrodynamic performance of the otter board,compare the results of the two methods,and visualize the flow field and surface pressure around the otter board.The results can provide a scientific basis for the optimization of the otter board performance.The main results are as follows:?1?No.2 model otter board??=3.0?f/C=15%??=10°?.The lift coefficient is maximum,when the angle of attack is 25°,the model test value and the numerical test value are 1.70 and 1.88 respectively,which have the same trend with the angle of attack.The lift coefficient increase gradually when the angle of attack is smaller than the critical angle of attack,and decrease gradually when the angle of attack is larger than the critical angle of attack.The drag coefficient increases with the increase of angle of attack.The model test value and simulation value of the lift drag ratio are the maximum,3.35 and 3.24,respectively,when the angle of attack angle is10°.The average deviation of the simulated and measured values of the flow velocity at the back is 4.4±2.3%,and the results obtained by the two methods are highly consistent?P<0.01?.When the Reynolds number is between 3×10⁴?6.1×10⁴,the lift coefficient,drag coefficient and moment coefficient of the otter board remain unchanged.?2?Hydrodynamic performance of the otter board under different inward and outward inclinations.Both the lift coefficients of otter board for model test and numerical simulation are decrease with increasing heel angle and reach the maximum values,1.70 and 1.88,respectively,at heel angle of 0°.Both the lift coefficients of otter board for model test and numerical simulation are increases first and then decreases with the increase of angle of attack,and are the maximum when the critical angle of attack is 25°.Both the drag coefficients of otter board for model test and numerical simulation are decrease slightly with the increase of the heel angle,and increase gradually with the increase of the angle of attack.Both the lift-to-drag ratios of model experiment and numerical simulation decreases with the increase of the heel angle,and the model test value and numerical simulation value of the lift to drag ratio are the maximum,3.27 and 3.69,respectively,when the heel inward angle is 5°.The model test value of the pressure center coefficient C_pb increases with the increase of the heel angle,while the simulation value basically does not change with the change of the heel angle.Under the condition of outward,the pressure center coefficient C_pc decreases gradually with the increase of the heel angle;under the condition of inward,the simulated value of pressure center coefficient C_pc keep basically unchanged with the increase of the heel angle,but the test value of pressure center coefficient C_pc decreases first and then increases with the increase of the heel angle.The results of two methods of pressure center coefficients C_pband C_pc are almost unchanged with the increase of angle of attack.?3?Hydrodynamic performance of the otter board under different tilt inclinations.Both the lift coefficients of otter board for model test and numerical simulation are decrease with increasing heel angle and reach the maximum values at heel angle of 0°.The model test value of the drag coefficient decreases slightly with the increase of the heel angle,while the simulation value remains basically unchanged with the increase of the heel angle;both of the results of the drag coefficient increase with the increase of the angle of attack.Both the lift-to-drag ratios of model experiment and numerical simulation decreases with the increase of the heel angle and angle of attack,and the maximum are 3.27 and 3.61,respectively.The pressure center coefficient C_pb can be basically considered to maintain invariable with the change of the heel angle.The pressure center coefficient C_pc decreases with increasing of the heel angle when the otter board is set to the backward tilting state;C_pc increases with increasing of the heel angle when the otter board is set to the forward tilting state.Both the pressure center coefficient C_pb and C_pc of model experiment and numerical simulation are basically unchanged.?4?Visualize the flow field and surface pressure around the No.2 otter boardIn the condition of no inclination,the separation point of the boundary layer of the No.2 otter board moved with the increase of the angle of attack and gradually moved to the leading edge of the fore wing,the rear vortex of the central plane increase with the increase of the angle of attack,the left side low pressure zone increases first and then decreases with the increase of the angle of attack,the tail of the otter board increases with the angle to form a distinct wing tip vortex.In the condition of different inclinatio n,the vortex at the rear of the center surface of the otter board decreases with increasing heel angle.When the otter board is in the inward and outward inclination state,the front end flow velocity reduction zone gradually increases with increasing of the heel angle,but a positive relationship for the otter board is in the tilt state.The pressure center moves toward the end chord of the otter board as the heel angle increases when the otter board is in the forward tilting state,but the pressure center moves toward the lower end chord of the otter board as the heel angle increases when the otter board is in the backward tilting state.The distance between the pressure center and the wing span is basically unchanged with the change of the heel angle.?5?Comparison of hydrodynamic performance between the biplane type otter board and the current Antarctic krill trawl otter boardThe maximum lift coefficient of the otter board of Antarctic krill trawl is 1.35,when the angle of attack is 22°,the critical angle of attack is 30°and the suitable working angle of attack is 21°?28°.The drag coefficient increases with the increase of angle of attack.The maximum lift-to-drag ratios is 3.25,when the angle of attack is 15°.The maximum lifting coefficient of biplane type otter board is 1.3 times that of Antarctic krill otter board,and its stability is also higher than that of Antarctic krill otter board.If the biplane type otter board is used in Antarctic krill trawl fishery,the estimated otter board area under the same trawl system is 9.6 m²,which is 26%less than that of Antarctic krill otter board,making the otter board smaller and easier to operate.