Study On The Influence Of Roof Box On Aerodynamic Drag Reduction Of Vehicle

With the prosperity and development of China's economy,the increasing number of car ownership has caused China to face severe energy problems.Reducing the aerodynamic drag of automobiles is one of the important methods to solve such problems.In recent years,more and more cars have been installed with roof boxes.Although this method can effectively solve the problem of affecting the cleanliness of the car or the storage of articles that are not allowed to be placed in the car,it also increases the aerodynamic drag of the car.The factors affecting the aerodynamic drag of the roof box include two aspects,the relative spatial position of the roof box and the body and the non-smooth surface.The contents of this paper are as follows:First,the simulation method is verified by wind tunnel experiments.Based on the DrivAer model of the box back type,the external flow field simulation parameters are analyzed.Finally,the appropriate calculation domain size,boundary condition setting,turbulence model,boundary layer setting and grid layout scheme are determined.The comparison error between the simulation result and the wind tunnel experiment result is less than 2 %,confirmed the reliability of the simulation method,and used the fluid mechanics software STAR-CCM+ to simulate the external flow field of the whole vehicle.The results show that the roof box has a great influence on the external flow field of the whole vehicle,which increases the aerodynamic drag of the whole vehicle.20%,indicating that there is great potential for studying the influence of the roof box on the aerodynamic drag reduction of the whole vehicle.Then,based on the Isight optimization platform,the relative position of the roof box and the body is optimized.The optimization parameters include the roof clearance h(mm),the roof box angle of attack ?(°),and the roof box relative distance L(mm)along the X-axis direction.Through the orthogonal test design and the range analysis method,it is found that the parameters from the high to the low on the aerodynamic drag of the vehicle are the angle of attack ?>height>the relative distance L of the horizontal.Based on the sample point data designed by the full factor experiment,the Kriging approximation model is established,the influence trend of each optimization parameter on the aerodynamic drag is analyzed,and the improvement of the external flow field after optimization is analyzed.The multi-island genetic algorithm is used to reduce the aerodynamic drag of the vehicle.The goal is optimized to eventually reduce resistance by 7%.Finally,apply a non-smooth surface to reduce drag.By studying the influence of the type,arrangement and arrangement position of the non-smooth unit on the drag reduction effect of the whole vehicle,it is found that the non-smooth surface exhibits two kinds of drag reduction methods,one is due to the local rotating air flow generated in the non-smooth surface depression.The sliding friction can be changed into rolling friction to reduce the frictional resistance.This phenomenon is generally called “air cushion effect”;the other drag reduction method is that the non-smooth surface can increase the boundary layer turbulence and reduce the back pressure gradient of the boundary layer.In addition,the rear flow field is affected,and the non-smooth surface is arranged in the sensitive field of the wake field,which can improve the negative pressure region of the wake field and the turbulent energy dissipation state,thereby reducing the differential pressure resistance.The difference is arranged,and the different arrangement results in different inheritance of the “air cushion effect” along the air flow direction,so the rectangular arrangement is superior to the diamond arrangement;when non-light When the surface normal direction is perpendicular to the air flow direction,the drag reduction effect is better,because the plane normal direction of the bottom of the roof box is perpendicular to the air flow direction,so the non-smooth surface is arranged here to reduce the resistance better,and the top of the vehicle body is not smooth.The surface is combined with the non-smooth surface at the bottom of the roof box to achieve the best drag reduction effect,and the vehicle has a drag reduction of 5.8%.In summary,this paper provides an effective research method for the aerodynamic drag reduction of the whole vehicle based on the roof box,and also makes a useful attempt to reduce the resistance of the non-smooth surface on the roof box.