A Research Into The Vortex Structures Around Vehicles And The Optimization Of The Aerodynamical Performance During The Overtaking Maneuvers

Along with the developments of the highway and the vehicle engineering,the safe velocity of the vehicle gradually increases.And the stability and the safety of vehicles are paid more attentions.The aerodynamical coefficients are positive correspond to the velocity of vehicles.In other words,all the aerodynamical coefficients increase with the increasing velocity.The increasing aerodynamical coefficients,such as drag force and lift force,will cause the increasement of the fuel consumption and the reduction of the adhesion force,which affects the stability and the safety of the vehicle.When a vehicle travels in high speed,the outer flow fields are closely connected to the stability.A little variation of the outer flow fields sometimes can arouse wide variation of the aerodynamical coefficients and the stability.As a result,it is valueable to research into the connection between the outer flow fields and the stability.Moreover,it is an effective method to optimize the shape of the vehicles by this connection.In this paper,the overtaking maneuvers are maily discussed,because it is the most important driving condition to influence the aerodynamical coefficients.When vehicles overtakes each other in high speed,the outer flow fields around them change dramatically,which causes the aerodynamical coefficients fluectuate rapidly.In this situation,the vehicles may close to each other or leave the road.More seriously,because the influences between the vehicles,the safe velocity will decreases,and the adhesion force will decrease to zero at a lower vehicle speed,which decreases the responses of the steering system.The contents in this paper are shown as follows:(1)The informations of vortex structures of different rear shape vehicles are collected.(2)The researches into the computing method are conducted.There are two traditional method to calculate the aerodynamics.One is the wind-tunnel test or road test which are accurate but expensive and dangerous.The other one is the numerical simulation test which is quite time-consuming and requires advanced computer equipment.In this paper,a semi-analytical method which is often adopted in calculating the lift force of the airplanes is applied in the calculation of the aerodynamics during the overtaking maneuver in crosswind.This method can quickly predict the risky positions and provide comparatively accurate results of the aerodynamics magnitude.The results of this algorithm do provide effective guidance to the methods,improving the efficiency of wind-tunnel test or numerical simulation test and the degree of safety of road test.(3)A simulation model of overtaking maneuvers is established to research into the flow and vortex structures around two identical Ahmed vehicles.The simulation demonstrates how the flow and vortex structures generate and change,which influences the aerodynamical coefficients of the vehicles most.In this study,a transient solver is used on the platform of Ansys/FLUENT,and the sliding mesh method is adopted to simulate the motion of the overtaking vehicle,which realizes a more actual recovery of the road conditions.Based on the fact that the primary factor that influences the aerodynamical coefficients of a vehicle is the change of the outer flow around it,a research into the variation of the outer flow field is conducive to in-depth analysis of the aerodynamical characteristics of the vehicle.Most previous studies only focused on the lateral aerodynamical coefficients during overtaking maneuvers.This paper establishes a comprehensive model that takes the lift force into consideration to more accurately simulate the aerodynamical characteristics Moreover,it is aimed at figuring out the key factors that cause the change of the aerodynamical coefficients at vortex level in this study,not just at pressure level as most previous studies did.(4)When a passenger car overtakes a truck or a bus at high speed in highway,the driver can feel obvious change of external flow field of the car which may possibly causes the car to deviate or lift up away from the road.However,this dangerous situation is seldom researched into before.Previous aerodynamic studies concerning overtaking maneuvers were mainly focused on the situation of two similar-size vehicles.In fact,the flow field changes more dramatically during the overtaking maneuvers of two different-size vehicles,which means more severe influence on the driving stability.This paper takes a passenger car and a bus as a representation to research into the overtaking maneuvers of two different-size vehicles.The target of this paper is to figure out the mechanism how vehicle size influences the aerodynamics of the vehicles during overtaking maneuvers,which will provide theoretical guideline for shape-optimizing in order to improve driving stability of the vehicles.(5)The wake flow of a vehicle significantly influences its aerodynamic performance and the stability during high-speed drive.Therefore,optimization of the vehicle wake flow is an effective way to improve its aerodynamic performance,and further improve the handling stability and fuel economy.In this paper,a new method—the leading edge rotating cylinder—is used to optimize the wake flow of a vehicle.According to the results of simulations,this method can reduce the pressure drag,increase the negative lift force,and strengthen the stability of the vehicle under cross-wind.Furthermore,this method optimizes not only the wake flow of the vehicle with rotating cylinders,but also the interactive vehicles in the driving route in overtaking maneuvers or platoon driving.In conclusion,this method effectively optimizes the flow fields around the vehicles,and it significantly helps to improve the handling stability and fuel economy of the vehicle.