牧區(qū)道路行車(chē)安全臨界側(cè)風(fēng)風(fēng)速分析.doc

Threshold of Natural Critical Wind Speed for Driving Safety in Pastoral AreaPeng Hu 12, Xiaodong Pan 1, and Zhen Yang 11 Transportation Engineering School, Tongji University, Shanghai, 201804, China. PH (0086) email: 2 Lecturer, Civil Engineering Department, Shan Dong Jiao Tong University, Jinan 250023, China; PH (0086) Abstract: Crosswind has a great influence on traffic safety in pastoral area because of its special geology conditions. Finite element model is created to analyze wind speed field of embankment. With no sideslip and overturning as targets, threshold of natural crosswind speed is analyzed under the influence of the multi factors including crosswind, traffic speed, friction coefficient and alignment in static vehicle model. The results will supply guidance for driving safety management and road design in pastoral area.Key words: Traffic safety; Roads in pastoral area; Threshold of wind speed; Radius of curves; Friction coefficient; Driving speed; Super elevationRoads in pastoral in Inner Mongolia have low-volume traffic, second or third class criteria is taken when they are designed with minor radius, high driving speed; because of its special geology conditions wind speed is high usually, and it is higher on the top of embankment than other places as embankment has great influence on wind speed field; Drift sand usually cover road surface, snow and ice usually do in the same way in winter, lowering road surface friction coefficient; all these adverse factors contribute to traffic accidents.Traffic safety caused by multi-factors has been paid much more attention to. Considering multi factors including crosswind speed, driving speed, friction coefficient and super elevation, Snbjrnsson et al (2007) and Sigbjrnsson et al(1998) assessed traffic accidents probability by accident rates. Considering multi influence factors, Jiabin et al (2006) create equivalent wind speed model of bridge deck to analyze traffic safety for Hangzhou Bay Bridge and Sutong Bridge.Starting with embankment wind speed field analysis, utilizing vehicle static model, threshold of crosswind speed for driving safety is computed considering coactions of crosswind force, centrifugal force, aerodynamic lift force, also with friction coefficient.1 Wind speed analysis for pastoral roads 1.1 Finite element models for pastoral roads Two dimension finite element models are created according to actual roads in pastoral area, taking the embankment for example whose height is 2m, width is 10m, slope grade is 1:1.5, seen in figure 1.1.2 Boundary conditionsBoundary conditions is shown in figure 2. The left and top are applied with wind speed Vx=10.0m/s; the bottom is applied with Vx=0，Vy=0; the right is applied with Pressure DOF=0. Figure 1. Finite Element Model Figure 2. Boundary Conditionsof Embankment 1.3 Wind Speed Field AnalysisAs standard of pastoral roads is low, its width of subgrade is 8m, 10m or 12m. Computing results show that when width is changed from 8m to 12m, it has little effect on wind speed field, so influence from width is ignored. The 1m, 2m, 3m, 4m, 5m height subgrade wind speed field is analyzed, taking the 2m height, 1:1.5 slope grade subgrade for example, seen in figure3.Figure 3. Subgrade Wind Speed FieldFrom the figure above, it is found wind speed field changes greatly causing vehicles on left lanes bear the greatest crosswind force. According to Technical Standard of Highway Engineering (Ministry of Communications of the Peoples Republic of China 2004), height of van and lorry is 4m; ground clearance for most van and lorry is more than 50cm, so average wind speed is calculated from 0.5m to 4.0m on the left lane and it is the basis to analyze crosswind forces, the growth rate of wind speed is shown in table 1.From table 1 it is found the wind speed on left lane increases variously, the higher the embankment is, the sharper the wind speed field changes.2 Threshold of crosswind speed for driving safety in target of no sideslip and no overturning.2.1 Influence factors and parameters（1）Crosswind side force: （1）（2）Aerodynamic lift force： （2）Where: , vehicle reference area (frontal area); , side force coefficients; , lift force coefficient; , the density of air; , true wind velocity; , vehicle speed; , crosswind speed.Table 1. Average Growth Rate of Wind SpeedSubgrade height Average wind speed Growth rate(m) (m/s) (%)1 10.5 52 11.7 173 12.5 254 13.5 355 14.4 44Aerodynamic force is the key to analyze traffic safety in windy environment. Related study (Baker and Humphreys, 1996; Krnke and Sockel, 1994)manifests that side force coefficient and lift force coefficient are in proportion with wind direction angle, that is ，, and are coefficient which are related with vehicle shape. Van is research object in this article, then, .（3）Centrifugal force: ，: Radius of curve.（4）Friction Coefficient:After rain, snow or some attachments fall on the road surface, friction coefficient degreases greatly having bad effect on driving safety. While drifting sand covers road surface, friction coefficient is about 0.3, snow covers, it is about 0.15, and ice covers, it is just 0.07(Qunli et al, 2008).2.2 Driving safety analysis under the influence of multi factors targeting of no sideslipWhen no side slip happens to vehicles, side force must be less than maximum friction force, and force diagram is shown in figure 4. （3）Where: super elevation. When the vehicle is in the critical state of side slip, equation is established, ，and are substituted into equation, it becomes （4）2.3 Driving safety analysis under the influence of multi factors targeting of no overturningWhen no overturning happens to vehicles, overturning moment must be less than stabilizing moment, and force diagram is shown in figure 4. （5）When the vehicle is in the critical state of overturning, equation is established, ，and are substituted into equation, it becomes: （6）Equation (4) and equation (6) are nonlinear, Newton iteration method is employed to solve threshold of crosswind speed, the results are named computed critical crosswind speed, and then it is converted into natural wind speed which are named side slip natural critical wind speed (SNCWS) and overturning natural critical wind speed (ONCWS).3 Examples and analysisRelated research (Airong et al, 2006) shows van, lorry and container are affected by crosswind easily, especially when they are empty. Some van is analyzed in this article, whose size is m, gross weight is 5500kg. In order to lower yaw velocity, aerodynamic center and gravity center usually lie in the same position (Zhengqi et al, 2005), and it is supposed that they do in this article. The main factors which influence vehicle side slip and overturning are super elevation, radius of curves, traffic speed and friction coefficient. Natural critical wind speed is computed according to the practical situation of pastoral roads, and the results are shown in the example of 2m height embankment.3.1 Side slip natural critical wind speed (SNCWS) analysisParameters: R=200m,400m, 600m, 800m, 1000m; driving speed 60km/h，80km/h，100km/h，120km/h；Friction coefficient 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7; super elevation 4%，5%，6%，7%，8%.SNCWS is computed according to the parameters and the relationship between super elevation and SNCWS is shown in figure 5 (radius is 400m, friction coefficient is 0.5); Relationship between radius and SNCWS is shown in figure 6 (friction coefficient is 0.5 and super elevation is 6%）；Relationship between friction coefficient and SNCWS is shown in figure 7 (R=600m, super elevation is 6%). Figure 4. Force Diagram of Vehicle Figure 5. Relationship between Super Elevation and SNCWS Figure 6. Relationship between Figure 7. Relationship between Friction Radius and SNCWS Coefficient and SNCWS From figure 5 it is found super elevation have little influence on SNCWS；From figure 6 it is found when radius is more than 400m, influence on SNCWS decreases, while radius is less than 400m, it has great influence on it, and the higher the speed is, the greater the influence is, so the radius of pastoral roads shall be no less than 400m for driving safety；From figure 7 it is found friction coefficient has great influence on SNCWS, and they are almost in proportion.3.2 Overturning natural critical wind speed (ONCWS) analysis.parameters: R=200m, 400m, 600m, 800m, 1000m; driving speed 60km/h，80km/h, 100km/h, 120km/h; super elevation 4%, 5%, 6%, 7%, 8%.ONCWS is computed according to the parameters and the relationship between super elevation and ONCWS is shown in figure 8 (R=400m); Relationship between radius and ONCWS is shown in figure 9 (super elevation is 6%).From figure 8 it is found super elevation have little influence on ONCWS; From figure 9 it is found when radius is more than 400m, influence on it decreases, while radius is less than 400m it has great influence on it, and the higher the driving speed is, the greater the influence is, so the radius of pastoral roads shall be no less than 400m. Figure 8. Relationship between Figure 9. Relationship between super elevation and ONCWS Radius and ONCWS3.3 Relationship between SNCWS and ONCWS From figure5 to figure 9 it is found vehicle speed has great influence on natural critical wind speed of overturning and side slip, but the relationship between them is still unknown, so it is analyzed with different friction coefficient, traffic speeds and radius, taking the R=400m for example shown in figure 10(negative value means side slip having occurred already). It is found that when fiction coefficient is less than 0.7, critical wind speed of overturning is always larger than side slip. With the increase of friction coefficient, the difference between them diminishes, and while friction coefficient is 0.7, they are almost same. Figure 10. Relationship between SNCWS and ONCWS4 ConclusionsConsidering side force, centrifugal force, aerodynamic lift force, traffic speed, friction coefficient, super elevation and radius of curves, targeting of no side slip and overturning, natural critical wind speed is computed based on embankment wind speed field analysis. The results taking the van driving on 2m height embankment as example manifest: (1)Friction coefficient has great influence on side slip natural critical wind speed; the larger the friction coefficient is, the higher the side slip wind speed and traffic safety are, they are almost in proportion. (2)Traffic speed has great influence on critical wind speed, the higher the speed is, the lower the critical wind speed and traffic safety are. (3)When radius of curves is less than 400m, it has great influence on critical wind speed, but when it is more than 400m, the influence is not obvious, so the radius of pastoral road shall be more than 400m. (4) Super elevation has little effect on critical wind speed, so it may not be required strictly in pastoral area roads.References:Airong, C., Dalei, W., and Jiabin, P. (2006). Study of Wind-Related Riding Safety on Long Sea-Crossing Bridge. Bridge construction, 3, 1-4.Baker, C. J., and Humphreys, N. D. (1996). Assessment of the adequacy of various wind tunnel techniques to obtain aerodynamic data for ground vehicles in crosswinds. Journal of Wind Engineering and Industrial Aerodynamics, 60, 49-68.Jiabin, P., Dalei, W., and Airong, C. (2006). Probability Evaluating Method of Bridge Deck Side Wind E