Study On The Flow Field Characteristics Of Vehicles' Transient Passing Process

The aerodynamic characteristics is one of the most importantparameters for road vehicles (especially the passenger cars), whichhas a great effect on the performance, fuel economy and handlingquality. When vehicles are running in a high speed, the vehicles andflows around them interfere each other, which definitely influencevehicles' handling stability, driving safety and riding comfort.During multi-vehicle running, the turbulent characteristics andaerodynamic phenomena are especially complex. Such aphenomenon is identified as aerodynamic interference betweenvehicles. High-speed vehicles encounter drastic gust loads whendriven in close proximity to each other. Such phenomenon is identifiedas aerodynamic interference between road vehicles. Whenhigh-speed vehicles, especially light vehicle are overtaking orovertaken by heavy vehicle, vehicles will be subject to drift because ofstrong aerodynamic interference, This will cause loss of driversteering or control and arose traffic accident. The traffic accidentsrelated to overtaking is a part of collision accidents on highway everyyear, which brings severe loss to the life and properties of nation andpeople. Due to the lack of research on multi-vehicle aerodynamicinterference characteristics, it's hard to control the abrupt trafficaccidents during multi-vehicle overtaking. Therefore, this researchhas an important realistic significance.Multiple-vehicle running has been attached importance to recentresearch consideration from worldwide range. Several universitiesand research centers have begun to search in this area and someresearch results have proved that the total drag coefficient of vehicleswhich are running in a line is much smaller that the sum of dragcoefficient of these vehicles which are running single in road.In this paper, a commercial CFD (Computational Fluid Dynamics)solver Fluent is used to study the aerodynamic interference actionmechanism during multi-vehicle transient passing process. Andseveral discussions are made in this paper, such as dispersionmethod of transient term in turbulence models, numerical modelsusing in CFD transient simulation computation. Meanwhile, threedifferent passing routine are under consideration, and significantpositions in each routine are chosen to analyze the relationship andchanges of drag coefficient, lift coefficient and side force coefficient ofboth the passing vehicle and the passed vehicle. Other analysis suchas static pressure field, dynamic pressure field, velocity field andsurface pressure fluid in the section of flow fluid are under discussion.Through analysis several conclusion are made:1) In the whole passing process, drag, lift and side force aredistribute like sinusoid shape and changed dramatically as side spacedecreased.2) In the passing process, the aerodynamic force varies with thelongitudinal relative position. When the front end of overtaking vehiclereaches the rear of overtaken vehicle, the side force coefficient anddrag coefficient reach the maximum. When the front end of overtakingvehicle reaches the middle of overtaken vehicle, the side forcecoefficient and drag coefficient reach the maximum.3) As side space decreased, influence between models increased.For the wake shape, as side space decreased, the shape of wakes arechanged more largely which make drag coefficient increased.4) For whole passing processes, the side distance between twocars is the major reason which determines drag coefficient. For apassing process, the changes of drag coefficient are determined bythe distortion of wakes.5) In the whole passing processes, side forces are changing notonly in amount but also in direction. These changes lead to vehiclesstatue changed, from repulsion to attraction.6) The moment started to change from repulsion to attraction inboth vehicles is not the same. That is to say, the passing vehiclechanges from repulsion to attraction firstly and the passed vehiclelags a little. This phenomenon happened because of passing vehicle'swake influence.7) As the distance between vehicles decreased, the structure ofwakes of passing car is changing and that is also why the dragcoefficient is increased.8)In the passing processions, wakes have asymmetry distributionand the center of wakes at interior side of two vehicles are a littlehigher than those at exterior side.