Dynamic Simulation On Aerodynamic Characteristics During Lane-changed Overtaking Based On Overlapping Grid

At the end of 2010, the actual mileage of expressway of China has reached 74,000 kilometers, the second longest in the world. In 2009, automobile industry of China for the first time surpassed the United States, made China as the country with largest production and sales of automobile in the world, and the car possession in China reached 199 million. As the continuous development of high-grade expressways, the number of car possession and vehicle speed are also increasing, and the driving conditions are more and more complex. At the same time, the operational stability, dynamic, safety, comfort and economy of the vehicle are more and more important. These performance characteristics are related to vehicle aerodynamics, so the researches of the air flows around cars, the flow pattern and the interaction between the vehicle and air become very important now.As a important direction of aerodynamics research, aerodynamic interference between vehicles, including platoon, overtaking and crossing, attracted a lot of attention in recent years, and its main subjects are the aerodynamic forces produced by gusts load on vehicles when two vehicles driving at a close range. On the expressway, overtaking situations are often occurred by vehicles with different shapes and speeds, and When a vehicle overtakes another vehicle especially a sedan overtakes a truck or the contrary, the air flow and pressure field around the car will changing constantly, and seriously affect the aerodynamic characteristics of both vehicles, then affect the handling stability and security of driving, sometimes even cause severe traffic accidents. According to statistics, overtaking take a large proportion in traffic accident occurred on the highway, and that causes serous losses to our nation.The study was the numerical simulation of changing lanes to overtake transient, on the basis of the previous steady overtaking and transient overtaking straight. In this paper,fluid software SC / Tetra from Japan's CRADLE company was used to analysis and calculation, which was developed from the basis of unstructured mesh and control volume method (FVM).We could get the aerodynamic characteristics of the vehicle by calculating. The study chose two domestic car models,called overtaking vehicle and overtaken vehicle respectively. The paper studied the turbulent flow characteristics of blunt body wake structure and vehicle interaction law between the turbulent flow during the process of changing lanes to overtake. The study conducted for the reasonable traffic flow management and provided a reasonable basis for automatic driving. At the same time the paper can provide a scientific basis for traffic safety research and simulate the state of emergency.In order to select the correct turbulence model, boundary conditions, mesh type and difference schemes for numerical simulation of changing lanes to overtake, first choose Ahmed model alone which was aerodynamic reference model in the international for numerical simulation. The numerical simulation and wind tunnel test Results were compared to verify correctness and accuracy of SC / Tetra from CRADLE company's CFD software. Then I chose overtaken vehicle alone for numerical simulation of flow field, which could provide the basis and guarantee for the subsequent transient simulation of changing lanes to overtake. Through the velocity and pressure distribution analysis and the flow mechanism analysis of the flow field, the work In advance lay a solid foundation for the research of numerical simulation of changing lanes to overtake.At last, in this paper overlapping grid was used in the research of the vehicle aerodynamic characteristics in the process of change-lane overtaking, with the use of transient simulation, it is more realistic to simulate the actual movement of cars on the road. In this simulation, according to the road traffic regulations in China, the overtaken speed is 70km/h (19.44m/s) and the overtaking speed is 120km/h (33.33m/s), the overtaking car which was in the same lane with the overtaken car should drive through passing lanes to achieve a change-lane overtaking. In the process of overtaking, airflow around these two cars had an interaction, the flow field is transient, due to the development of overlapping grid technology, the simulation of change-lane overtaking can be achieved. This paper analyzed the relationship among drag coefficient, side force coefficient and yaw moment coefficient in different locations of overtaking and overtaken cars during the overtaking process, and the static pressure on profiles of flow field. Throughout the process of overtaking, aerodynamic drag of overtaking and overtaken cars changed with the relative position between them. Yaw moment had a large influence on the driving stability and safety of overtaking, at the vertical relative position x/L=0.5 and x/L=1.5, where the front of overtaking car is at the center of overtaken car and half vehicle length beyond the overtaken car, yaw moment has a maximum, so they are most dangerous positions in a overtaking process. Drivers should be noted that to assure a safe distance between the two vehicles during a overtaking.This paper had do a research on transient simulation of change-lane overtaking by the application of overlapping grid technology, and focused on the mechanism of air movement in flow field and aerodynamic characteristics of aerodynamic interference through the process of overtaking. The work can be used as the basis of research on transient characteristics of aerodynamic interference for further research.