| The frictional resistance of the surface is widespread in the fields of aviation,navigation and transportation.It is the major source of energy consumption for objects that contact with viscous fluids,such as aircraft,ships,high-speed trains,submarines and torpedoes.Exploring new methods and developing new technologies for reducing resistance have always been a priority field for governments.It is important engineering practical significance.In recent years,various drag reduction methods have been emerging.The bionic flow control method to achieve drag reduction by controlling the quasi-sequence structure of turbulent boundary layer has attracted extensive attention from academia and military application because of its convenience,environmental protection and high efficiency.Researchers at home and abroad have carried out large number of bionic micro-groove drag reduction,which can achieve good drag reduction effect through the model,but it is not significant enough in practical application.In this dissertation,the shortfin mako shark was used as a biomimetic model.A design strategy is provided for the drag reduction of turbulent components by exploring the relationship between the scale parameters and the surrounding flow field at the characteristic parts of sharkskin.On this basis,the numerical simulation method of turbulent drag reduction was constructed to study the laws of drag reduction on the bionic groove surface and reveal the mechanism of drag reduction.The surface of the micro-dimple or micro-groove with drag reduction effect is quickly manufactured by spraying on the suction surface of the fan blade,and the aerodynamic performance test is carried out.The research contents of the dissertation are as follows:(1)Quantitative analysis of the distribution of scales in the shortfin mako sharkThe shortfin mako shark was studied as the biomimetic model,and the scales in characteristic parts of shark skin were sampled.The morphology,size and distribution of scales were characterized by digital microscope and scanning electron microscopy.There are differences in the parameters of the scales.The surface of the scales is smooth and flattened,the ratio of riblet depth to width RD/RS is about 0,and the scale density is smaller in the area facing the water.The surface of the scales has a groove structure composed of three longitudinal riblets in areas with less disturbance of water flow.The RD/RS of the scale riblets at the posterior edge of the fins and the posterior region of the body is larger and the scale density is greater.The riblets RD/RS of the shark body increased gradually from the anterior zone,the middle zone,and to the posterior zone,with values of 0.05～0.17,0.08～0.23,0.32～0.33,respectively.(2)Analysis of the relationship between the surrounding flow field and scale parameters of the shortfin mako sharkBased on the distributions of scales in typical parts of shark skin,the relationship between the parameters of scales and the surrounding flow field of sharks was discussed.A 3D model of a shortfin mako shark in a cruising attitude was obtained using reverse engineering techniques.The flow field at 0°in the smooth shark model was calculated using Ansys Fluent software.The results show that the surface has a higher pressure,the flow field is laminar flow,the scale density is low,and the surface is smoother in the area facing the water.However,the pressure on the surface is relatively low,the flow field is transitional or turbulent,the scale density is high,and the surface has a microgroove structure in the area with less influence of water flow.The movement of the riblets of the scales on the shark’s body follows streamlined direction.The riblet RD/RS and turbulence intensity of the first dorsal fin,caudal fin,pectoral fin and the body of shark gradually increased along the anterior,middle and posterior edges.(3)Establishing a numerical simulation method for turbulent drag reduction of grooves and platesBased on the microgrooved scales of the sharkskin’surface are mostly in turbulent areas,models of triangular grooves and flat plate inspired by shark skin are constructed.Numerical simulation methods for turbulent flow in groove and flat channel flows were established using Open FOAM open source software.It is verified that the microgroove structure increases resistance in the laminar flow state and reduces the resistance in the turbulent flow state.Due to the small scale of the micro-groove structure and the large number of meshes of the computational model,the hierarchical refinement method is used to discretize the computational model mesh,which reduces the demand for computing resources.The influence of the size,arrangement law of the staggered cubic columns and flow velocity at the inlet on the internal flow field of the channel flow was analyzed.The cubic column with two rows of staggered distribution(row distance is3.0mm),and its length,width and height and the distance between columns are 0.50mm,which meets the calculation requirements of the flow field.To obtain the stable flow field of the computational model,the region between x=0.04m and x=0.08m of cross-section is selected for post-processing to ensure the accuracy of drag reduction performance analysis.(4)Research on the law of drag reduction and mechanism of drag reduction on biomimetic microgrooved surfaceAccording to the function of reducing the drag of the microgroove structure in the turbulent state,the calculation scheme of the numerical model of channel flow is established using the quadratic orthogonal rotation design method.The relationship between the ration of drag reduction(DR)and the microgrooved height h,width s and incoming flow velocity U is given.The microgrooved scale with drag reduction effect varies at different flow velocities.The incoming flow velocity is larger,the height and width scale of the groove is smaller in the velocity range of 5～100m/s.The surface of the groove structure has the function of reducing drag when the dimensionless height and dimensionless width of the groove are 8.50≤h~+≤29.75,and 8.50≤s~+≤29.75.The drag reduction rate of groove is the largest at h~+=s~+=25.29.The internal field of the channel flow was studied to reveal the mechanism of drag reduction on the microgrooved surface,when the friction Reynolds number was 85,258,443,599,695.Compared to smooth plate,the interior of the groove and its vicinity have less shear stress,which means less viscous resistance.The values of pulsating velocity in the three directions are reduced,indicating that the turbulence near the wall is suppressed and the turbulence intensity is reduced.(5)Experimental study of aerodynamic properties of bionic fan bladesBased on the drag reduction law of the microgroove surface,the microstructural size of the bionic fan blade is determined.The high-precision and rapidly spraying test platform is designed and manufactured,and the preparation test of the microstructure on the suction surface of blade is carried out.The coating of the biomimetic microstructure was characterized,the depth h of the micro-dimple was 0.089～0.105mm,the width S was 0.119～0.298mm,and the height and width of the micro-groove was0.30mm.Aerodynamic tests have shown that bionic fan blades with microstructure improve aerodynamic performance,reduce drag and increase efficiency. |
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