The Study on the Influence of Gust Wind on Vehicle Stability Chen Wang a, Haibo Huang b*, Shaofang Xu c

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1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 598, pp Accepted: doi:1.428/ Online: Trans Tech Publications, Switzerland The Study on the Influence of Gust Wind on Vehicle Stability Chen Wang a, Haibo Huang b*, Shaofang Xu c Faculty of Mechanical Engineering and Mechanics, Ningbo University, China a wangchen1@163.com, b* huanghaibo2@163.com, c xsf @sina.com Key words: CFD, gust wind, six-component force, vehicle stability Abstract. In this paper, the vehicle driving characteristics are investigated under different Force level of gust wind based on the CFD theory. The distributed pressure acted on the vehicle side surface is equivalent to the six-component force on vehicle gravity center. The six-component force are explored and compared in different Force level of gust wind. The results show that the lateral force Fy and yaw moment Mz vary with v shape and Λ shape under gust wind, respectively. The slopes of the result curves increase with greater Force level of gust wind. Gust wind gives most influence on Fy and Mz while has little effect on other component force. The work will provide a methodology for vehicle stability research under gust wind. Introduction The vehicle dynamics are affected by the gust wind. Gust wind causes transient force on the car body and it is dangerous on vehicle safety, which changes the distribution of the aerodynamic response on the vehicle. The severely gust wind alters the track of vehicle and sometimes cause severe traffic accidents. We can check aerodynamic performance of vehicle through the wind tunnel test or the numerical simulation. The wind tunnel experiment can well represent the influence of wind on the vehicle at the cost of long time and heavy consumption. With the development of numerical and computation method, the numerical method can also provide relative high precise simulations compared to the wind tunnel test and cost less. The position of the air pressure center acted on the vehicle is very important to the vehicle aerodynamics. In this paper we simplify the air pressure center equivalent to the center of mass because the air pressure center and center of mass are both on the symmetrical plane of the car body. The effect of gust wind on vehicle dynamics is equivalent to the six-component force acted on the vehicle[1]. Some simulations were done based on CFD software and the force inspired by gust wind looks on as the dynamic boundary conditions. Then we can investigate the influence of gust wind on vehicle dynamics through checking the six-component forces. And the vehicle stability can be explored under gust wind for different levels. The modeling The vehicle model. A simplified vehicle model is adopted in the simulation. The tire and rear view mirror are omitted on the car body which are allowable in the aerodynamic simulations [2-4]. The vehicle geometry is shown in Fig.1(a). All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-19/2/16,13:4:2)

2 Applied Mechanics and Materials Vol wind speed/ m/s From Force2 to Force 8 Force2 Force3 Force4 Force5 Force6 Force7 Force8 (a) The geometry of vehicle model (b) Computational region and meshed grid Fig.1 Schematic of the simulation system Time/s (c) The speed variation of gust wind The computational geometric field. The simulation is investigated by using finite element commercial code, Fluent. The computational geometric field is a box shape with 2mm in length, 1mm in width and 6mm in height. The distance between the car body and field bottom is 3mm. And the distance between the car body and field top is 46mm. The grid is refined on and near the body surface for computation precision, as shown in Fig.1(b). The vehicle's traveling direction is with negative X axis while the wind's direction is with negative Y axis. The dynamic features of gust wind are very complex. Due to ref [5], the velocity of the wind follows sine function. The wind direction is perpendicular to direction of the vehicle driving. The wind is classified into 7 levels. The maximum values of velocity for various levels are: 2.5m/s, 4.4 m/s, 6.7 m/s, 8.8 m/s, 12.3 m/s, 15.5 m/s, 19. m/s, respectively. The wind lasts 1.8s (from.1s to 1.9s) and the wind velocity reaches the peak at t=1.s, as shown in Fig.1(c). Navier-Stokes equations and the turbulence model. Due to the aerodynamics, the Navier-Stokes equations and a suitable turbulence model are needed. The standard K ε turbulence model, which is usually used in vehicle's aerodynamic simulation, is applied. The detailed description for the equations and the model can be found in Ref[6]. And the parameters in the K ε turbulence model can be found in Ref[7]. The results and conclusion The influence of the gust wind on the vehicle six-component force. The paper takes gust wind of level 4 to investigate the influence of gust wind on vehicle driving stability. The six component force caused by gust wind of level 4 is shown in Fig2 (a) and (b). As shown in Fig.2 (a), the force acts on the vehicle in Y direction that is most influenced by the gust wind compared to the forces in other two directions and varies with "V" shpae. It is due to the sinusoid velocity of gust wind. Force in Y direction reaches the peak at t = 1. s, corresponding to the maximum wind velocity at the same time. The higher is the wind speed, the greater the lateral force Fy is. Fz in the vertical direction shows the shallow "V" shape trend. When the wind speed increases, Fz decreases and when the wind speed decreases, Fz increases. It is mainly because the upper half shape of the vehicle is tilted, especially the windows area. It will cause the pressure in negative Z direction under gust wind and leads the lift force reduced, guaranteeing adhesion between the tire and the ground. The force in longitudinal direction is almost constant and less affected by the gust wind. In Fig.2 (b), the yaw moment Mz varies obviously under the lateral winds, presenting Λ shape trend. It is opposite to the v-shape dynamic characteristics of gust wind. At t = 1. s, the yawing moment values reaches maximum. Yawing moment also corresponds with varies of gust wind. From Fig.2 (c), the car body bears the uneven lateral distribution of the wind pressure. The front body bears significantly larger pressure resulting in the asymmetry between the vehicle front and back, which causes large variations in the yawing moment of vehicles.

3 2 Advanced Materials, Mechanics and Industrial Engineering Rolling moment Mx and the longitudinal moment My change relatively small and gentle in amplitude. 4 5 Force/ N Moment/ N.m -5 Mx My Mz -3-4 Fx Fy Fz Times/ s Time/ s (a)the force component (b) The moment component (c) Pressure distribution on vehicle Fig. 2 The vehicle characteristics under Force 4 gust wind Fig.3 shows the six component force on gravity center when t=1s. As shown in Fig.3(a), Fy increases greater and its slope becomes bigger and bigger with the greater Level of the gust wind. Meanwhile, Fx and Fz vary slightly Force/N -5-1 Fx Fy Fz Moment / N.m 5-5 Mx My Mz Gust wind level (a) The force variation with gust wind speed variation Fig.3 Six component force variation at t=1s Gust wind level (b) The moment variation with gust wind speed variation As shown in Fig.3(b), there is obvious rise for Mz with the greater gust wind. Meanwhile, Mx and My decline slightly, especially Mx. The force acted on the vehicle is related to the gust wind speed. Severe yaw moment Mz variation appears due to the violent lateral force variation and the body asymmetry. Longitudinal force Fx is mostly up to the vehicle speed so that Fx has little variation due to the vehicle constant speed. So is the rolling moment My. At the same time, vertical force Fz, is much influenced by the vehicle profile. It is composed of two parts: lift force caused by the vehicle driving speed in longitudinal direction and the vertical down force caused by the gust wind that acted on the slope part of the vehicle body. The force component increases with the wind energy. Conclusions (1) The lateral force Fy and yaw moment Mz change with V and Λ shape under the gust wind. The variation of other component force is slightly compared to Fy and Mz. (2) The lateral wind will be the dominant wind resistance factor when it is over Force 5, and go over the longitudinal wind. (3) The gust wind has great influence on lateral force Fy and yaw moment Mz. The offset distance increases with greater Force level of gust wind. When the Force level of gust wind is beyond 6, the rider will feel uncomfortable due to the greater lateral acceleration.

4 Applied Mechanics and Materials Vol Acknowledgements This research is supported by a grant from the Impact and Safety of Coastal Engineering Initiative, a COE Program of Zhejiang Provincial Government at Ningbo University (Grant No. zj118). References [1] Gu Zhengqi, Zhao Rongyuan, Yang Yi. Application of dynamic modeling of the in the cross-wind stability of automobile gas[j]. Journal of hunan university, 28.9(9): [2] Zhang Yingchao, Fu Limin. Transient Aerodynamic Numerical Simulation of simple shape cars enter process in tunnel[j]. Journal of Jilin University, 26.5(3): [3] Li Yuzhou, Tan Xiamei, Liang MingYing. Analysis of automotive aerodynamics characteristics based on Ansys[J]. Mechanical design and manufacturing, 21.2(2): [4] Lan Tian, Kang Ning, Zheng Hao. Study on start-up phenomenon fastback car boot process[j]. Journal of Aerospace Power, (11): [ 5 ] Chen Xiaodong, Zhan Zhangsong. CFD study of Changan automobile aerodynamics performance[j]. Automobile engineering, 26.9(1): [6] Li Jinliang, Li Chengxi, Hu Renxi. Proficiency in FLUENT6.3 flow field analysis[m]. Chemical Industry Press, [ 7 ] Chen Hongye, Wang Ailing, Zheng Zhizhen. Under different crosswind aerodynamic characteristics of automobile simulation study[j]. Mechanical Management and Development, 21.4(114):81-82.

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