| Recently,the automobile amount is increasing with traffic jam; at the same time, the environment pollution is increasingly serious. Thus China definitely put forward the call of"Energy Saving and Emission Reduction". In Intelligent Traffic System, adopting platoon travel mode both greatly increases highway capacity, reduces fuel costs and emissions. If appropriate travelling strategy is provided for intelligent vehicle, traffic accidents will be reduced. This paper is supported by the National Natural Science Foundation of China"Influence of multi-vehicle driving turbulent characteristics on traffic safety and its control"presided over by my advisor and is put forward on the background of energetically developing Intelligent Traffic System during national"the Eleventh Five-Year"period.A platoon, as the term is used by the Federal Highway Administration (FHWA) Intelligent Vehicle/Highway Systems (IVHS) program, is a group of multi vehicles travelling in tandem at the same speed with relatively small inter-vehicle spacings. With the attention of Intelligent Traffic System research by the world and the increasing of vehicle automatic control system, this mode of highway travel could be implemented before long. Driving in platoon, since the longitudinal spacing is short, the following car enters the wake of the first car and the flow field around the two cars interfere each other strongly. This aerodynamic interference will make the automotive aerodynamic characteristics parameter change a lot; make its fuel consumption and driving characteristics change and decrease oil consumption and emission. Thus, the research of automobile platoon aerodynamic characteristics in the way of"Energy Saving and Emission Reduction"is very important.At present, automotive aerodynamic characteristics is paid more and more attention. The research of automobile platoon aerodynamic characteristics is a little and most research focus on of single vehicle external flow field. On the foundation of advisor's research on automobile wake structure and its mechanism, this paper focuses on shape's influence on single car and platoon wake structure, contrasts platoon and single car's wake structure in order to reveal the mechanism of platoon drag reduction. Then study the influence law of platoon longitudinal spacing and car number on aerodynamic characteristics and discuss the action mechanism of flow field interference. Firstly make numerical simulation for a single car external flow field. And then make numerical simulation for three single cars with different rear shape and corresponding two-car platoon and make a series of numerical simulation for the influence of longitudinal spacing and car number on automotive aerodynamic characteritics. In order to ensure the correctness of numerical simulation, carry out several wind tunnel experiments with the research object in the first automotive special wind tunnel in China——Jilin University Automotive Wind Tunnel and contrast the numerical simulation result and experiment result for validation. The information and research law obtained by this paper is both very significant for the control system development of automotive automatic driving and intelligent traffic system and make correctness for the research conclusion with the only consideration of single vehicle driving situation. It also can provide theory basis for intelligent system of platoon driving.The automobile external flow field is very complex. The wake flow field related to car shape, head flow field, top flow field and lateral flow field interfere with each other strongly and the typical flow characteristics is three-dimensional, viscous and separating. In platoon driving, since the flow field around each car interfere each other, the whole flow field become more complex and numerical simulation becomes more difficult. From the beginning of basic assumption of automotive external flow field, describe basic theory, including basic control equation, numerical calculation method and numerical simulation method of turbulent and general process of numerical simulation. Then carry out the numerical simulation for a single car, from the building of geometry model to the choice of computational field and mesh strategy, the loading and setting of boundary condition and convergence criterion. The numerical simulation results are concluded.In order to verify the correctness and reliability of single car numerical simulation results, carry out automotive wind tunnel experiment with international standard SAE model as the research object in Jilin University Automotive Wind Tunnel. The aim of automotive wind tunnel experiment is to obtain aerodynamic characteristics data that can reflect automobile driving condition correctly. So certain test guide lines must be met: adequate even flow field, similar geometry shape, similar Renault number and removing the interference of bracket 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 carry out all kinds of wind tunnel experiments according to the wind tunnel test guide lines, including qualitative type and quantitive type. The quantitive research includes balance force measurement test, pressure distribution test and Particle Image Velocimetry (PIV). The qualitative research includes smoke flow method, ribbon and oil-flow method. These test results are obtained under various methods. Finally, compare the numerical simulation and wind tunnel test results. Results indicate that the numerical simulation method adopted in this paper is correct, credible and its numerical simulation parameters can be used in the numerical simulation of platoon.During drving of automobile, complex wake and long trailing vortex dissipation region are formed in the wake region of automobile. Driving in platoon, since the longitudinal spacing is short, the following car enters the wake of the first car and the flow field around the two cars interfere each other strongly. This aerodynamic interference will make the automotive aerodynamic characteristics parameter change a lot. Thus revealing the relation of aerodynamic characteristics and wake structure is important. First study a single car's wake structure and contrast it with platoon vehicle wake structure in order to find the mechanism of platoon drag reduction. Since automobile shape is various on the road, three typical automobile shapes are chosen as study object. Carry out the study on their wake structure and corresponding platoon wake structure to find general law.Firstly make notch-back, fast-back and square-back car as research objects, determine the calculation scheme, for all kinds of research schemes, build geometry models and calculation field, generate mesh, set boundary conditions and calculation parameters and get simulation results. With numerical simulation and wind tunnel experiment methods, contrast the difference of three shape aerodynamic forces. With streamline figure and velocity vector figure of numerical simulation and smoke method and PIV velocity measurement, analyze the wake structure of automobile with different rear shape and their flow pattern and sum up the characteristics of three shapes'wake structure. Then make two notch-back car platoon, two fast-back car platoon and two square-back car platoon as research objects, study the mechanism of platoon drag reduction. According to the research aim, determine the calculation scheme and carry out numerical simulation. Then through streamline figure and velocity vector figure of two-car platoon, analyze the mechanism of platoon drag reduction. And carry out several wind tunnel expriments, including smoke method, pressure measurement and balance force measurement. Results indicate that the numerical simulation results agree with the wind tunnel experiment results and the numerical simulation is proved to be correct. Through wind tunnel experiment and numerical simulation, the conclusion can be concluded. The wake structure of three shapes is different and they have complex wake structure and long trailing vortex dissipation region. The wake structure of different shape platoon is different, but their drag reduction mechanism is the same. That is, the wake flow field of the first car interferes with the flow field of following car. Existing of following car makes the wake vortex region of the first car reduce. Wake region of first car makes a strong effect on the flow field of following car and make trailing vortex region reduce obviously. So platoon driving can reduce aerodynamic drag.Through the analysis for the research status of platoon vehicle longitudinal control in the intelligent traffic system in the world, it's indicated that in the intelligent traffic system, the onboard control system can control the longitudinal spacing between cars within close range and ensure car driving safely. This provides powerful guarantee for platoon driving in the intelligent traffic system. Then carry out numerical simulation for two-car platoon, with the longitudinal spacing varying from one car length to twenty times car length, totally 12 schemes. Since the computational field of far longitudinal spacing platoon is big and its mesh is huge, these schemes can not be calculated on a single computer and a parallel calculation platform is formed. On the calculation platform, carry out numerical simulation for various schemes, in order to disscuss the influence law of longitudinal spacing on platoon aerodynamic characteristics. Results indicate that for two-car platoon, when the longitudinal spacing is within the range of L to 20L, with the decreasing of longitudinal spacing, the influence of two cars'flow field becomes stronger. The drag coefficients of two cars both decrease and are smaller than the drag coefficient of single car. The drag coefficient of following car is smaller than the first car's. The smaller the longitudinal spacing between cars, the smaller the aerodynamic drag coefficient will be. Then make a deduction of relation of aerodynamic drag coefficient reduction and fuel consumption saving ratio and get the relation of fuel consumption saving ratio and longitudinal spacing. Results indicate that when the longitudinal spacing is within the range of L to 20L, the smaller the longitudinal spacing, the higher the fuel consumption saving ratio will be. From pressure coefficient distribution, static pressure distribution, velocity vector and streamline figure, analyze the reason of the drag coefficient's varying reason with longitudinal spacing. And through wind tunnel balance force measurement, validate the varying law of short longitudinal spacing platoon aerodynamic coefficient and prove the reliability of conclusion.Research of influence of car number on platoon aerodynamic characteristics is of significance. Since the influence law of car number on fuel economy and emission obtained by this research not only can predict the highway capacity increasing situation under different car number platoon schemes and provide theory basic for supporting platoon driving system on the existing highway, but also can design control system correctly according to the force information of each car in the different location of platoon, in order to avoid collision and ensure driving safely. Therefor, calculation scheme is determined. For all the research schemes, carry out numerical simulation. When the fixed spacing is one car length, varying of the drag coefficient of each car with the car number is obtained by this research. Results indicate that for multi-car platoon, the more the car number, the smaller the platoon average drag coefficient will be. The platoon average drag coefficient will be 20%-30% samller than the drag coefficient of single car. When the car number increases upto a big value, the platoon average drag coefficient will approach a limit value. The drag of the first car in the platoon is affected a little by the car number and the drag of following cars is smaller than the first car. With the increasing of car number, the car with the lowest drag is in the center of the platoon. According to the relation of aerodynamic drag coefficient and fuel economy, get the varying law of platoon average fuel consumption reduction with platoon car number. That is, the more the car numer in the platoon, the more the fuel consumption reduction will be. When the car number is short, the fuel consumption reduction ratio is high, and when the car number becomes more, the fuel consumption reduction ratio is slow and approaches a limit. Through comparing the static pressure distribution, pressure coefficient distribution, velocity vector and streamline figure of multi-car plantoon and a single car, clarify the mechanism of multi-car platoon driving drag reduction.In a word, this paper researches the influence law of automobile rear shape, longitudinal spacing and car number on platoon car aerodynamic characteristics and action mechanism of aerodynamic interference between cars with the combined method of numerical simulation and wind tunnel experiment. It can make correctness for the research conclusion with the only consideration of single vehicle driving situation and can also provide reference basis for automobile automotive driving and development of control system in intelligent traffic system. |
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