Research On Automotive Aerodynamic Characteristics In Crosswinds

Automotive aerodynamic characteristics are important performance of a vehicle, which has significant effects on the driving stability, safety and comfort performance of a vehicle. When a vehicle is sujected to crosswinds with high speed, the driving stability and comfort performance will decrease, and even lead to safety problems for extreme instance. As reported in literature, there were accidents in Sinkiang province that the gales blew vehicles to turn over for 13 times, total 79 vehicles were turned over and the economy loss was over 42 million RMB. In addition, for the consideration of environment protection, to improve the fuel economy, the trend towards lightweight and low drag cars will exacerbate the sensitivity to crosswinds. So it is very important for the driving safety control to research the automotive aerodynamic characteristics under crosswind effects.There are 3 methods to research the automotive aerodynamics: wind tunnel experiment method, road experiment method and numerical simulation method. Wind tunnel experiment method is generally used and is a reliable method. The crosswind effects research using wind tunnel experiment method is usually limited in steady conditions, i.e. the car body is fixed in the test section of the wind tunnel with a yawing angleβand the aerodynamic loads are taken at various yawing angles. With this method only the aerodynamic characteristics under certain vehicle velocity and crosswind velocity can be taken, it is impossible to catch the transient effects of the varying vehicle velocity and crosswind vehicle. Another method that can be applied in wind tunnel experiment is to introduce a secondary gust source with a relative angle to the main stream and, by using a shutter mechanism to simulate a crosswind. The velocity of the secondary gust can be controlled to be varying to simulate unsteady crosswind. This method still uses the relative motion of the stream to simulate the motion of the vehicle, and there are problems associated both with the mixing of the two jets and with the separation around the shutter mechanism. In addition, there is a third method can be used in wind tunnel, in which the vehicle model is driven along a track to traverse the test section. This method can represent the real motion condition of the vehicle and the crosswind, but it also brings the difficulties such as transient aerodynamic load measurement and the signal contamination of the inertial loads or track vibration. Further more, the cost of this kind of mechanism is very high and it is very difficult to achieve the unsteady vehicle and crosswind velocity because the testing space of the wind tunnel is very limited and it is hard to control the crosswind velocity varying.The road experiment method costs very high and it is very dangerous sometimes, and the result can be affected by subjective factors, such as the driver response time, and the objective factors, such as the road conditions and the weather. So the applicability of this method is limited.Compared with the wind tunnel experiment and road experiment methods, computational simulation method (CFD) is more efficient, low costing and more applicable, it can simulate the flow conditions around the vehicles in crosswinds, and it is helpful to get more flow field information, which is the important reference to research the crosswind stability of the vehicle together with the experimental research.The automotive aerodynamic characteristics under crosswind effects have been carried out a lot for nearly a half century time. However, most of these researches were limited in steady vehicle velocity and crosswind velocity conditions, and the researches regarding unsteady vehicle velocity and crosswind velocity were rarely referred in literatures. However, both the vehicle velocity and the crosswind velocity are frequently varying when the vehicle is being driven on the road. In order to research the crosswind effects of the unsteady vehicle velocity and the crosswind velocity on the vehicle aerodynamic characteristics, it is the key point to represent the unsteady vehicle velocity and crosswind velocity conditions, and it is also very difficult to achieve.As the research of the automotive aerodynamic characteristics under unsteady vehicle velocity and crosswind velocity effects is very limited, this thesis researched the automotive aerodynamic characteristics under steady and unsteady velocities of vehicle and crosswind with the wind tunnel experimental method and CFD, developed the dynamic numerical simulation strategy to simulate the unsteady vehicle velocity and crosswind velocity and abtained the automotive aerodynamic characteristics under the effects of vehicle velocity, crosswind velocity, vehicle accelarations and the varying rates of the crosswind velocity to time.The external flow fields of a domestic car model with 11 yawing angles were researched with steady numerical simulation method based on the RANS method, and then the aerodynamic characteristics of the car model under the effects of different crosswind velocities were obtained. The wind tunnel experiments with a scaled car model for all of the corresponding yawing angles were carried out, the visual flow field around the car body was obtained with the qualitative wind tunnel experiments including smoke flow method and surface tufts method, the static pressure values of 71 points on the model surface were measured with the PSI 9816 intelligent electronic pressure scanning system under 11 model yawing angles and the pressure coefficients were computed, the aerodynamic loads of the car body were measured also using the wind tunnel balance. The qualitative and quantitative comparison between wind tunnel experiments data and numerical simulation results were carried out, and it showed that they were in good agreement with each other, so the numerical simulation methods were proved to be correct and reliable. It showed that the drag coefficient CD varied not too much with the crosswind velocity and the side force coefficient CS was affected very notably by the crosswind velocity, its increasing trend was nearly a line along with the crosswind velocity increasing. For the lift coefficient CL, it increased very slightly when the yawing angleβwas less than 12°, and it increased notably whenβexceeded 12°and the increasing trend was nearly a line. At the same time, the car body was affected by a increasing yawing moment and its stability was degraded notably.To model the dynamic boundary during the flow field computation and to make the car body move, various methods for modeling the dynamic boundary were researched and compared. The dynamic layering method was chosen with the consideration of the computational mesh limitation and applicability of various dynamic mesh techniques and the very large computational space of the crosswind research.The dynamic numerical simulations were carried out using the dynamic layering method for the external flow fields of the car model under various constant crosswind velocities, the car motion and the crosswind motion were represented in a more real condition, and the aerodynamic characteristics of the moving car model at 30m/s velocity under crosswind effects with various velocities were obtained. The dynamic numerical simulation results were compared with the steady numerical simulation results. The comparison also showed that there were large differences for the side force coefficient CS, the rolling moment coefficient CRM and yawing moment CYM between steady numerical simulation and dynamic numerical simulation when the yawing angleβwas more than 15°. The wake vortex outlines of the car model obtained by the steady numerical simulation and the dynamic numerical simulation were very similar to each other, but the velocity distribution was reverse to each other. The static pressure distributions between steady numerical simulation and dynamic numerical simulation were in good agreement with each other.To model the unsteady vehicle velocity, C programs were compiled based on the User-Defined Functions to make the car body wall boundary to move at different accelerations during the transient numerical simulation, and the varying aerodynamic characteristics of the car model with different accelerating motions were researched. The results showed that during the car velocity was accelerated from 10m/s to 30m/s, the drag coefficient CD varied slightly, and the side force coefficient CS, lift coefficient CL, rolling moment coefficient CRM and yawing moment coefficient CYM decreased with the car velocity increasing, and this trend was in agreement with the trend of the yawing angleβ, but the corresponding aerodynamic loads were increasing notably. The car body was affected by an increasing yawing moment with negative direction, which had disadvantageous effects on the driving stability. The pitching moment coefficient CPM and pitching moment PM varied slightly during this process, but the directions of them changed from positive direction to negative direction at the car velocity of 26m/s, so it could be concluded that the acting point of the lift L was moving backwards of the car body during this process. For the same car velocity and crosswind velocity, the drag coefficient CD and side force coefficient CS of the car with large acceleration were larger than the CD and CS with small acceleration; however the lift coefficient CL had a just reverse trend.To model the unsteady crosswind velocity, C programs were compiled based on the User-Defined Functions to improve the capacity of the numerical simulation software, and the unsteady external flow field simulations of the car model were carried out under the effects of the crosswind whose velocity was increasing with time at different rates. The results showed that all the aerodynamic force coefficients and aerodynamic moment coefficients increased with the increasing crosswind velocity except pitching moment coefficient CPM. The side force coefficient CS, lift coefficient CL and yawing moment CYM had the relative large varying amplitude, which could affect the driving stability notably. The pitching moment coefficient CPM was changing smaller but slightly. The side force coefficient CS, lift coefficient CL and rolling moment CRM were notably affected by the varying rates of the crosswind velocity. For the same car velocity and the crosswind velocity, the higher varying rate of the crosswind velocity, the larger drag coefficient CD, side force coefficient CS and rolling moment CRM, but smaller lift coefficient CL.In summary, the numerical simulation methods applied in this thesis were proved to be correct through the wind tunnel experiments; the dynamic mesh technique was used to make the car body move during the transient numerical simulation to model car motion and the crosswind motion in more real conditions. The capacity of the numerical simulation software was improved by compiling C programs based on the User-Defined Functions to make the car body accelerate in the computational domain and to make the crosswind velocity vary with time, so the aerodynamic characteristics of the car model under unsteady car velocities and crosswind velocities were researched, and the aerodynamic loads varied with the unsteady car velocities and crosswind velocities were obtained; the mechanism of the varying aerodynamic characteristics of the car were explained with the distribution of the velocity and the pressure of the flow field. This research developed research strategies to research the effects of unsteady vehicle velocities and crosswind velocities, provided important data for the previous research results under steady vehicle velocity and crosswind velocity, and the research results could be applied as important reference for the vehicle crosswind stability research.