Transient Aerodynamic Characteristics Of Vans Under Different Motion Conditions In Crosswinds

With both the rapid development of highway construction and the continuous increase of car ownership,there are more and more cars on the road.As a result,it is more frequent than ever to observe behaviors such as overtaking,passing and parallel driving between vehicles.During various complicated driving processes,the flow field around vehicles may generate strong mutual interference,and additional aerodynamic forces and moments are exerted on the vehicles,which causes vehicle's aerodynamic characteristics to change and affects the dynamic and safety stability of vehicles.When there is crosswind while driving,more complicated changes would happen on the aerodynamic characteristics of vehicles.In addition,the overtaking vehicle often needs to accelerate to complete the overtaking,which leads to the change on the aerodynamic characteristics of both vehicles.Such a change is more obvious when there is crosswind.When the vehicle overtakes on a highway in crosswinds,the central isolation belt has also great impact on the aerodynamic characteristics of both the overtaking and overtaken vehicles.Therefore,it is of great practical importance to investigate how the aerodynamic characteristics of vehicles vary under different motion conditions in crosswinds.In recent years,more and more vans are on the roads due to the fast development of the logistics industry.Because vans have larger body sizes than normal vehicles,their mutual influence is immense resulting severer change on vehicle's aerodynamic characteristics.We,therefore,conduct our research on vans in the current study.The results presented in this study provide both general guidelines for drivers to achieve safety while driving with crosswind and theoretical foundations for other studies on the vehicle intelligent control and traffic safety system.This study adopted both numerical simulations and wind tunnel experiments to investigate complex driving processes of a single van,and overtaking,passing,and parallel driving in crosswinds.We obtained the transient pattern of vehicle's aerodynamic characteristics under different motion conditions.We also got inference on the main causes of such changes.Through the numerical simulation study,we found that both the aerodynamic coefficients and aerodynamic moment coefficients of a single van increase as the crosswind increase,but the aerodynamic drag force coefficient increases first and then decreases.We also conducted simulations using the relative motion principle on the uniform overtaking process under conditions with no wind.The results show that the overtaking process has little effect on the aerodynamic characteristics of the overtaking van,but great impact on that of the overtaken van.Furthermore,we did experiments to obtain the aerodynamic coefficients and aerodynamic moment coefficients both of the single van and the overtaken van in the overtaking process in a wind tunnel.The test results agree with those of the simulation.We thus would claim that the numerical simulation method provides trustable results.We conducted simulations on the uniform overtaking process in crosswinds using the dynamic mesh technology and slip interface technology.We found that crosswinds have large impact on the aerodynamic characteristics of both the overtaking and overtaken vans.As the crosswind increases,the aerodynamic coefficients of the overtaking van increase,while the aerodynamic coefficients of the overtaken van have change in stages.The magnitude of such changes of the overtaken van is relatively larger than the overtaking van,which means the crosswind influences the overtaken van more.We interpreted the main reasons for the change of aerodynamic coefficients of the vans using the pressure contours and the velocity path lines.The crosswinds changed the flow and pressure fields around both vans,which in turn increase the airflow diversion around them.The range of pressure changes around the van increase,and the pressure difference increase,causing a significant change in the aerodynamic coefficients and aerodynamic moment coefficients of the vans.When comparing the uniform overtaking process against the accelerated overtaking process,we found that the speed variation of the overtaking van would influence the aerodynamic characteristics of both vans.However,the influence was lightly larger on the overtaken van than on the overtaking van.In the simulation,we manipulated three different accelerated overtaking processes and found that the greater the acceleration of the overtaking van,the larger the aerodynamic coefficients of the overtaken van.When the acceleration of the overtaking van was increased by 1 m/s2,the maximum drag force,side force and yawing moment coefficients of the overtaken van all increased more than 6%.Such dramatic change would seriously affect the performance of power and the stability of vehicles.Through the study on the accelerated overtaking process under different crosswind conditions,we found that the accelerated overtaking process increased the side force coefficient of the overtaking van but decreased the side force coefficient of the overtaken van.In addition,the increase range of the overtaking van is greater than the decrease range of the overtaken van.Due to the accelerated overtaking process,the crosswind stability of the overtaking van becomes worse,however,that of the overtaken van is slightly better.During the whole overtaking process,the side force coefficient and the yawing moment coefficient of the overtaken van vary tremendously,and its crosswind stability is thus still poor.We studied the effects of the isolation belt on the aerodynamic characteristics of vans during the overtaking process in crosswinds.The results showed that,the isolation belt has little effect on the aerodynamic characteristics of vans without crosswinds,but significant effects with crosswinds.The isolation belt diminishes the impact of crosswinds and improves the aerodynamic characteristics of vans.The higher the crosswind speed is,the more obvious the effect of the isolation belt would be.In addition,we also found that the denser the isolation belt,the more obvious improvement of van's aerodynamic characteristics.Through the numerical simulation of two vans passing in crosswinds,we found that the trend and amplitude of change on aerodynamic coefficients of two vans are the same without crosswind.However aerodynamic coefficients of both vans increase in the crosswind environment.In addition,the change patterns of the two vans are no longer the same.The aerodynamic coefficients of the windward side van simply increase as the crosswind increase,while the aerodynamic coefficients of the leeward side van vary in stages.Under the same crosswind conditions,the amplitudes of aerodynamic coefficients of the leeward side van are larger than that of the windward side van.Therefore,the crosswind has a greater influence on the aerodynamic characteristics of the leeward side van during the passing process.When studying the parallel driving process of two identical vans,we found that without crosswind,the drag force coefficient and the lift coefficient of the two vans were the same,and the side force coefficients were equal in magnitude and opposite in direction.Under the crosswind conditions,the aerodynamic coefficients of the windward side van increase as the crosswind increase.Meanwhile,the drag force coefficient of the leeward side van first increases and then decreases as the crosswind increase.Its side force coefficient and lift coefficient increase as the crosswind increase.When the crosswind speed changes,the aerodynamic coefficients of the windward side van change more than the leeward side van.Under the same crosswind condition,the aerodynamic coefficients of the windward side van are larger than those of the leeward side van.Such difference is more obvious under strong crosswind conditions.Therefore,crosswinds have a larger influence on the aerodynamic coefficients of the windward side van during the parallel driving.Especially under the action of strong crosswind,the side force coefficient and the lift coefficient of the windward side van would increase greatly,which is extremely detrimental to the safe and stable operation of the van.When driving side by side in the crosswind environment,the driver of the windward side van should be more careful to prevent accidents.