| As one of the main ways for the inter-city transportation, heavy duty vehicles have gotrapid development in its market and large increase in population since reforming andopening. According to statistics, the heavy duty truck population had accounted for13.9%ofthe total vehicle population by the end of2011.Meanwhile, the safety, stability and ridingcomfort of a vehicle is receiving increasing attention. Long distance driving and poor drivingconditions lead a higher requirement to the safety and stability of a heavy duty vehicle.As one of the most important vehicle performances, the aerodynamic characteristic isclosely related to driving safety and stability. When a vehicle is running, the flow statearound it is turbulence, which means magnitude and direction of the flow will change overtime and has transient characteristics. If this transient characteristics is too intense, thedriving vehicles will be affected then lose part of the riding and handling stability, leadingto a threat to driving safety. There is usually crosswind in natural wind conditions. When thevehicle encounters a crosswind gust boundary, this effect can be more obviously, even willcause sideslip or side tumbling. Heavy commercial vehicles are always traveling on differentforms of road, such as urban roads and mountain roads, and complex road conditions areeasy to form a natural or artificial gust. To avoid security problems caused by those factors,the heavy commercial vehicles should have good crosswind aerodynamic characteristics.Therefore, the research on transient aerodynamic characteristics of heavy commercialvehicles traveling on different forms of road under crosswind conditions becomes verysignificant.In this paper, general purpose CFD code FLUENT is used to simulate the situation of aheavy duty vehicle running on different configurations of roads under cross-wind which isbased on previous research of vehicle transient aerodynamic characteristics and the impact of obstacles such as bridges. The impacts of three different road conditions on the transientaerodynamic characteristics of the heavy commercial vehicle are mainly focused, includingthe buildings near urban roads, highway subgrades of plain area and mountains aroundmountain roads. Specifically, the urban road conditions are including these three situations:the vehicle goes by a single building located in either leeward side or windward side of thecar body, and two buildings located in the windward side; the plain road conditions aredivided into two situations: the vehicle moving on the lane which is near or far from thewind source; in the mountain road condition, mountains are simplified into mountain-shapedmodels and placed in the windward side of the vehicle.To study the transient aerodynamic characteristics, straight driving of heavycommercial vehicles need to be simulated. Therefore, basing on the dynamic meshingmodule of the FLUENT software, two strategies of node displacement method are putforward: global mesh node displacement and local node displacement. Simulation of heavycommercial vehicle driving on open road proves the latter is feasible and calculable. On thisbasis, transient numerical cases of heavy commercial vehicles driving within three roadconfigurations are established and solved. The simulation results of all cases mentionedabove are analyzed, including the open road condition, the variations of resistancecoefficient, lateral force coefficient and yawing moment coefficient of different cases arecompared. Moreover, the flow field characteristics around the truck and pressure coefficientdistribution of body surfaces in different cases are also compared. The results show that theeffects of buildings and mountain-shaped models on the transient aerodynamiccharacteristics of heavy commercial vehicles are similar and obvious. In some cases, thechange of lateral force and yawing moment coefficient can be up to5.0in one second, andthe corresponding change values of lateral force and yawing moment are about12kN and30kN m, which make the change rates of lateral force and yawing moment reach-12kN·s-1and35kN·m·s-1. In addition, the direction of these two parameters have changedtwice during the driving process. In the other hand, the fluid fields around truck driving onplain road are relatively stable. However, due to the influence of subgrades, both the lateral force coefficient and yawing moment coefficient in these two cases are about0.5highercompared to the open road, which is detrimental to driving stability. |
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