Research On Vehicle's Transient Aerodynamic Characteristics At Special Running Situations

With the rapid growth of car numbers as well as a large number of construction of road tunnels and other transportation facilities, traffic increases in density. Driving car on the road faces more and more complex environment because complex traffic situations like vehicles overtaking, crossing, going through tunnels occur frequently. In these complex traffic situations, the vehicles'out flow fields interfere each other, or they are influenced by road tunnels or other facilities. So severe gust loads can be generated under certain conditions. These loads act as additional forces in the vehicle which can alter its road holding and thus result in safety problems. Therefore, aided by National Natural Science Project "investigation on traffic flow characteristics and fuel consumption reducing of vehicles in cities", this thesis investigates the vehicles' transient aerodynamic characteristics at two complex situations of overtaking, and entering the road tunnel. The research will benefit the traffic system and is of great practical significance.In this paper, the vehicle's transient aerodynamic characteristics were investigated during the both processes of overtaking and entering the road tunnel by the two methods of wind tunnel experiments and numerical simulation. The results were obtained from the analysis of the flow phenomena involved. The following researches were done.Wind tunnel experiments were carried out to investigate the relationship of a single van's aerodynamic drag coefficient Cx and the Re number. The two van's overtaking process was also experimentally investigated by separating into several time segments. The variation of the overtaken van's aerodynamic drag coefficient Cx and side force coefficient C(?)is obtained in the overtaking process. The overtaking process was also steadily simulated using the finite volume mothed and the RNG k-εtwo-eaquation tunbulence model.A mesh updating method is put forward which is based on dynamic layering technique and is suitable for moving objects on the ground like vehicles. The basic idea is as follows:with the vehicle's movement, the unstructured grids of the complex flow field near the van body translate rigidly, the deformation of the calculation domain caused by vehicle is transferred to the structured grids area where is far from van. Then the mesh is updated using the dynamic layering technique. The grids on the ground is also updating successfully although it is difficult to keep the quality of grid near the vehicle tires meet the ground. Based on dynamic mesh technique and sliding interface technique, three-dimensional transient numerical simulation of vehicle's aerodynamic characteristics in complex running situations is realized.The vehicles'aerodynamic characteristics were compared which are obtained by the three methods of wind tunnel experiments, steady numerical simulation and transient numerical simulation. The results show that all the three methods are available for the flow phenomena which are not continuously changing with time, like a single vehicle running in a uniform velocity. But in the study of overtaking process, the results from the three methods are apparently different. The overtaken vehicles' aerodynamic force coefficients from the wind tunnel experiments are quite coincidence with that obtained from the steady numerical simulation, and overtaking process has the same influence on both the overtaking and overtaken vans' aerodynamic characteristics. But the results obtained from the transient numerical simulation method are much different. In transient simulations, the vans'aerodynamic force coefficients are changing greater than that of wind tunnel experiments and steady simulations. And the overtaking van's aerodynamic characteristics change greater than overtaken van in the transient numerical overtaking process. Further analysis of the flow field shows that it is research method that causes these differences. In wind tunnel experiments and steady simulation which are both based on steady time segments method, the flow field depends only on the relative positions of the two vehicles at each time segment, and it neither reflects the effect of two van's relative speed nor inherits the previous flow field characteristics. While transient numerical methods do. So for the flow phenomena which are continuously changing with time like overtaking process, more accurate conclusions can be obtained by the transient numerical method.The basic structure of the flow field is analyzed when a single van running in uniform velocity. The pressure distribution on the van surface, as well as the reason, is discussed. With the van running forward, the upwind surface of van impacts air and a big positive pressure area is formed. When the van runs forward, negative pressure is generated in the back of the van, and it induces a large number of high-speed air flow following the van, thus a long tail is formed. There is a pair of vortex in the tail region. The vortex develops and breaks off gradually as it is far away from the van.The process of a van overtaking another van was simulated using transient numerical calculation. The variation rules of vans'transient aerodynamic characteristics are summed up. And the changing mechanism of vans'aerodynamic characteristics is also revealed by analysis of flow field. In the overtaking process, the aerodynamic characteristics of both vans change obviously and regularly. The aerodynamic characteristics of the overtaken van change greater than that of overtaking van in the overtaking process. What leads to this is the interference of vans'out flow fields. For overtaken van, its body is passed by the overtaking van's front positive pressure, negative pressure of side area and tail area from back to front in turn, and thus pressure distribution on overtaken van surface changes. This is the main reason why the aerodynamic characteristics of the overtaken van change greatly.The effect of vehicle velocity, style and road conditions on aerodynamic characteristics in the overtaking process is also investigated by transient numerical simulation. Velocity has a big influence on the overtaken vehicle but has a little influence on the overtaking one during the overtaking process. In the conditions of one vehicle running in the same velocity, with the increase of the relative velocity, the variations of the overtaken vehicle's aerodynamic force coefficients increase, while the variations of the overtaking vehicle decrease. Under the same relative velocity, the variations of the overtaken vehicle's aerodynamic force coefficients increase in the overtaking process with slower absolute velocity, while the variations of the overtaking vehicle decrease. The vehicle style has great influence on overtaken vehicle's aerodynamic characteristics too. With the increase of overtaking vehicle's volume or with the decrease of the overtaken vehicle's volume, the variations of the overtaken vehicle's aerodynamic force coefficients increase in the overtaking process.If the overtaking process is taking place in the road tunnel, both vehicles' aerodynamic characteristics vary in the same trend as that taking place in the open road, but vary in a greater range.The process of a van running into road tunnels was simulated by transient numerical calculation, and the effects of the velocity and blockage ratio are investigated. Because of the influence of tunnel wall, van's aerodynamic characteristics are changing during the process of van running into a tunnel. The drag coefficient increases sharply near the tunnel entrance, then decreases gradually after the van entering tunnel. After the van entering tunnel, lift force increases, and side force is generated that is pointing to the tunnel wall closer to the van. Blockage ratio has significant influence on van's aerodynamic characteristics as the van running into tunnels. When van-tunnel dimension ratio increases, changing extent of all the three aerodynamic force coefficients increase.Multiple vans running into road tunnel are also numerical simulated. Two situations of two vans running into tunnel side-by-side and in platoon are investigated. When two vans run side-by-side, the drag coefficient of both vans increase and the side force is formed that push the two vans closer to each other. When they run into the tunnel, the Cx increase greatly near the tunnel entrance, meanwhile, the side force vary sharply and their direction changes twice. When two vans run in platoon, the drag coefficient Cx of both vans decrease and the latter van has a much higher reducing extent. When running into the tunnel, the former van'aerodynamic characteristics is similar to that of the single van, but the aerodynamic forces of the latter van don't have obvious change.If there are multiple vehicles on the road, driving into tunnel side-by-side should be avoided for safety. From the aspect of aerodynamics, driving in platoon is recommended, and the longitudinal spacing between vehicles should be adequately long.