Research On Automotive Aerodynamic Characteristics During Overtaking

With the increasing of automobile amount, automobiles'driving conditions on the road are getting more and more complex and overtaking, crossing and platoon often appear. According to the literature, there are about 18,000 traffic accidents caused by improper overtaking in Germany every year. In order to solve increasingly serious transportation problem, America, England, Japan and other developed countries have already invested much manpower and material resources in the research on intelligent vehicles safety control and automobile safety assistant driving technology. In China, some scholars have begun to study overtaking automatic control technology. They focus on front vehicle detection and safety distance control in the view of intelligent vehicle control.This paper studies overtaking process in the view of automotive aerodynamics, in order to provide theory basis for automobile automatic driving system and intelligent transportation system research and guarantee overtaking safely. The research on overtaking in the view of automotive aerodynamics can be traced back to the middle of 1960's. At that time, people noticed that when automobile was driving on the highway, it sometimes drifted the middle line of road and came into collision with coming automobiles. Since people can't explain the reason from vehicle design and mechanical aspect, they were puzzled by this kind of accident. In England, when a driver was changing a tire on a highway shoulder, the car suddenly dropped from the jacking system due to the effect of passing vehicles. From then on, foreign scholars began to research the influence of overtaking aerodynamic flow on vehicle aerodynamics.Since Chinese automobile industry development started late, the research on automotive aerodynamics is a little and most research focus on single vehicle external flow filed. National Natural Science Foundation"Influence of plural vehicle driving turbulent characteristics on traffic safety and its control"presided over by my tutor is the first research on overtaking, crossing and platoon aerodynamic characteristics in China. On the basis of predecessor's research in project team, this paper adopts the combined method of numerical simulation and wind tunnel test, studies the influence law of longitudinal spacing, lateral spacing and other parameters on automotive aerodynamic characteristics during overtaking. With advanced moving mesh technology, this paper implements three-dimensional unsteady numerical simulation of automobile overtaking and simulates six kinds of overtaking conditions with different vehicle speed. The research purpose is to find out the varying law of aerodynamic force and moment of overtaking and overtaken vehicles during overtaking process, and summarize the influence law of relative speed and absolute speed on the vehicle transient aerodynamic characteristics during overtaking process, in order to provide drivers with reference to carry out proper operation and choose reasonable overtaking speed during overtaking process and provide theory basis for automobile intelligent control and traffic safety system research.The external flow field of automobile is very complex and the typical flow feature is three-dimensional, viscoid, separating and unsteady. When one car overtakes another, they affect each other, which makes the flow filed and pressure field around car vary. The entire flow field will become more complex and numerical simulation will become more difficult. According to the numerical simulation flow, present the choice of geometry model, simulation region, mesh model, solver, discretization scheme and turbulent model, near-wall treatment, convergence criterion and parallel computation station built by myself. In order to choose proper turbulent model, carry out single car numerical simulation for Ahmed model and model 1. Then compare their numerical simulation's results with corresponding wind tunnel test results qualitatively and quantitatively and choose Realizablek ?εmodel as the turbulent model of overtaking simulation in the end.In order to verify the correctness and reliability of single car numerical simulation, carry out wind tunnel test for a single car. Present the wind tunnel test model and the first automotive wind tunnel in China——Jilin University Automotive Wind Tunnel. The aim of automotive wind tunnel test is to obtain aerodynamic characteristics data that can reflect automobile driving condition correctly. So certain test guide lines must be met: adequate equal flow field, similar geometry shape, similar Renault number and removing the interference of bracket and wind tunnel wall in wind tunnel test to the best of one's abilities. According to these guide lines, analyze these indexes of this wind tunnel and the analysis indicates that this research meets the requirements of automotive wind tunnel test guide lines. Then introduce all kinds of wind tunnel test methods, such as balance force measurement method, Pressure distribution test, Particle Image Velocimetry (PIV), smoke flow method, oil-flow method and ribbon method. With these methods, carry out tests in Jilin University automotive wind tunnel and obtain the test results under various methods. Finally, compare the numerical simulation and wind tunnel test results quantitatively and qualitatively. Results indicate that the numerical simulation method adopted in this paper is correct, credible and it can be used in the numerical simulation of overtaking.With the numerical simulation method validated by wind tunnel test, carry out three-dimension flow field simulation of vehicle overtaking process, in order to discuss the influence law of longitudinal relative position and lateral relative position on vehicle aerodynamic characteristics and the influence of aerodynamic force on vehicle driving stability. Firstly adopt"limited increasing step"to build steady overtaking model. According to the research purpose, determine nine longitudinal relative positions and six lateral relative positions and study on the steady overtaking under different conditions. This paper presents the numerical simulation process of steady overtaking in detail, such as the building of computational domain, mesh generation, solution setting, magnitude of y+ and convergence situation. Then according to the steady numerical simulation results study the influence law of longitudinal relative position on overtaking process. Results indicate that during overtaking, the flow filed and pressure filed will vary with longitudinal relative position between vehicles, which will result in the varying of drag, side force and yawing moment coefficient. During overtaking, the most dangerous three longitudinal relative positions respectively are where the front end of overtaking vehicle is in the middle of overtaken vehicle, the front end of overtaking vehicle is parallel with front end of overtaken vehicle and the front end of overtaking vehicle overtakes the front end of overtaken vehicle by half vehicle length. Through analyzing the flow field varying situation, explain the varying reason of aerodynamic force and moment coefficient. According to the above analysis, put forward some advice for traffic control. Similarly, study the influence of lateral relative position on overtaking process with steady numerical simulation results. Results indicate that in order to ensure overtaking safely, safe lateral spacing must be guaranteed between two vehicles. In particular, there must be enough lateral safety spacing from overtaking vehicle approaching overtaken vehicle to overtaking vehicle leaving overtaken vehicle. When vehicle is driving on highway, it should drive in the middle of road strictly, which can decrease the influence of aerodynamic interference during overtaking and be beneficial to drive safely. Finally, in order to verify the correctness and reliability of steady overtaking numerical simulation, carry out steady overtaking wind tunnel test with Jilin University automotive wind tunnel. The test method is smoke flow method. The streamline of various relative position obtained by numerical simulation is nearly the same with the streamline obtained by wind tunnel test, which verifies the correctness and reliability of numerical simulation of steady overtaking.It's obvious that overtaking is an unsteady process. The unsteady overtaking test and numerical simulation will be close to realistic overtaking process. During unsteady overtaking simulation, the longitudinal relative position between two vehicles varies with time and the two vehicles have different speed. Since the unsteady overtaking test is difficult, this paper only carries out the numerical simulation of unsteady overtaking. First build the simulation model of unsteady overtaking, and then adopt moving mesh technology to complement the three-dimension and unsteady numerical simulation of overtaking process. Comparing the aerodynamic force coefficient and flow field distribution obtained by steady and unsteady overtaking simulation, we can see that there is much difference between steady and unsteady simulation. The reason of difference is that unsteady simulation can catch the transient effect during overtaking. Carry out unsteady numerical simulation for five kinds of relative vehicle speed respectively and compare their aerodynamic force coefficients. Research indicates that relative vehicle speed has a little influence on overtaking vehicle during overtaking, but it has much influence on overtaken vehicle. The higher relative vehicle speed, the more dangerous overtaken vehicle will be. The driver of overtaken vehicle should drive carefully. In order to decrease the influence of transient aerodynamic interference caused by overtaking, the driver of overtaking vehicle should not choose high relative vehicle speed to overtake vehicle in the front.The unsteady overtaking research on two conditions with the same relative vehicle speed and different absolute vehicle speed indicates that when the relative vehicle speed is the same, two vehicles with high absolute vehicle speed will produce much transient aerodynamic interference during overtaking. Thus the suggestion for traffic control is that on the highway, overtaking is inadvisable when the speed of overtaken vehicle is very high.In a word, this paper makes a systemic and scientific research on overtaking process in the view of automotive aerodynamics. On the one hand, the research results can provide reference for driver to make proper operation and choose reasonable overtaking speed during overtaking. On the other hand, it can provide theory basis and technology reservation for research and development of Chinese intelligent vehicle technology. This paper has important significance for choosing proper overtaking spacing and overtaking speed, decreasing traffic accidents and ensuring traffic safety.