Study On High-speed Aerodynamic Stability Of Vehicle Based On Coupled Of Vehicle-Wind-Driver

With the continuous increasing of the speed of vehicles,the aerodynamic stability of vehicle driving at high speeds has become a critical problem which is affecting the vehicle safety.So far the research on the high-speed aerodynamic stability of vehicle is mainly based on the one-way coupled study of vehicle aerodynamics or vehicle system dynamics.However,the change of posture of the vehicle body and path during actual driving changs the air flow around the vehicle body,thus it leads to the interaction of vehicle movement,air flow and driver control feedback,which will cause the vehicle-wind-driver system in a specific coupling state.But the aerodynamic load of the vehicle,the posture of the vehicle and the change of the path is treated as known conditions in one-way coupling method,which is not able to truly reflect the transient change process when the vehicle is running at high-speed,and cannot really reflect the mutual influence of vehicle-wind-driver coupled system.For this purpose,the conventional method,such as the one-waycoupling method,have limitations,including the mutual influence of the vehicle-wind-driver at high-speed.In this research,the real－time transmission mode of the data and different step size control strategy is adopted and the two-way coupling method is established to study the high-speed aerodynamic stability of vehicle based on ALE method.Furthermore,the high-speed aerodynamic stability of vehicle and the risk of aerodynamic instability of the vehicle are investigated.It provides a reliable theoretical basis for solving engineering problems related to vehicle aerodynamic,which will have broad impact on theoretical aspects and application in real product design and analysis.The main contents of this paper are as follows,1.The numerical simulation of transient aerodynamic characteristics of vehicle and wind tunnel test is studied.It is known that the transient flow field around the vehicle body is generated when the vehicle is moving at high-speed,which provides important features for the analysis of the aerodynamic stability.Considering the transient characteristics of the flow field around the vehicle body when the vehicle is moving or body posture is changed,the basic requirements of computational domain and the scheme of mesh generation for high precision transient computation are determined.The Smagorinsky-Lilly model in LES is utilized for transient simulation,and the Cs in the model is optimized by Enumeration method.The accuracy of the Cs is validated by wind tunnel experiments and verified by numerical simulations of the Mira model,as well as two real vehicle models.2.The study on transient aerodynamic characteristics of vehicle at high-speed is conducted when the posture of vehicle body is changed.According to the actual driving condition of a vehicle,the pitching and rolling model of the vehicle body is established with determined parameters such as amplitude,frequency and so on.The coefficient of aerodynamic load of the vehicle in different postures are obtained by wind tunnel experiment,and the results of the transient simulation,the quasi-static simulation and the experimental are compared and analyzed.Then,the transient flow of complex vortices around the vehicle body is studied in both space and time by the visualization of flow field Q-criterion.It then analyzes the characteristics of transient flow field and its change mechanism,as well as the hysteresis effect of flow field and its influence.3.The evaluation principle of high-speed aerodynamic stability and the risk analysis model of aerodynamic instability are developed.To deal with the instability problem of the vehicle moving at high speed,the major aspects on causing high-speed aerodynamic instability are analyzed,and the general formulation with basic criteria of the high-speed aerodynamic instability of vehicle are summarized.The steady-state equilibrium equation of vehicle in crosswind condition is established by theoretical analysis.Then the theoretical wind speed is calculated for vehicle moving safety under crosswind condition.Considering the risk of vehicle aerodynamic instability,a comprehensive analytical risk model and its computational approach of the vehicle aerodynamic instability is established.4.It proposed a wo-way coupling method and associated computational model of vehicle aerodynamics and vehicle dynamics based on the ALE approach.Compared to one-way coupling method which does not consider the interaction between the aerodynamic and the vehicle motion,the two-way coupling method based on ALE method is eatablised via data real-time transfer mode and different step size calculation strategy.The two-way coupling analysis model of two and five degrees of freedom is presented.The nonlinear interpolation method is employed to improve the computational accuracy during the process of data transfer.In order to consider the effects of driver intervention in a crosswind condition,an applicable driver model under crosswind conditions is suggested with the consideration of the moving characteristics of the vehicle under crosswind condition,which is inspired from the driver model proposed by Auflage.The robustness of the two-way coupling model is verified through adjusting the location of the vehicle centroid.5.The high-speed aerodynamic stability of vehicle under steady crosswind condition is analyzed.The influence of steady crosswind on the aerodynamic characteristics of the vehicle is detailed analyzed based on the changes of the coherent structure,surface pressure and the streamline around the vehicle and so on.The aerodynamic stability of a vehicle under steady crosswind is compared and analyzed by steady one-way coupling method,quasi-steady one-way coupling method and two-way coupling method,which is based on the two degree of freedom vehicle model.The high-speed aerodynamic stability of a vehicle under different speeds and different wind speeds is analyzed and evaluated,which is based on two-way coupling model of the five degree of freedom.It also analyzed the transient aerodynamic characteristics and motion characteristics of the vehicle in detail.6.The high-speed aerodynamic stability of a vehicle under transient crosswind condition is analyzed.A transient crosswind model is developed with the consideration of the transient characteristics of crosswind in nature.In order to save the computational resources and improve the computation efficiency,the moving boundary method is performed to realize the vehicle moving in the transient crosswind condition.Based on the coupling analysis model of two and five degrees of freedom,the two-way coupling method and moving boundary method are employed to simulate the moving process of the vehicle under the single peak transient crosswind condition.Finally,the high-speed aerodynamic stability of the vehicle at different speeds in single peak transient crosswind condition is analyzed and evaluated,and the transient aerodynamic characteristics and motion characteristics at different speeds are thoroughly investigated.In summary,the two-way coupling method of vehicle aerodynamics and vehicle dynamics which is based on ALE method provides an effective numerical method for the study of high-speed aerodynamic stability.In this work,the high-speed aerodynamic stability of vehicle is comprehensively investigated,which aims to provide an effective methodology and a practical tool for the aerodynamic design of vehicles.