Abstract
Keywords Ground effect, Rotating wheels, CFD, Aerodynamic loads, Relative drag (lift) increment
Abstract - As aerodynamic phenomena, ground effect represent an increase of the lift for a wing (in aviation), or the increase of the downforce on a car spoiler (in design of vehicles) which is placed closer to the ground. There are various possibilities for representing of road in the wind tunnels, but most used are following: the mirror method using an image model of the tested one (this method affects in a negative way the blockage ratio) or using a flat surface (moving belt), with or without rotations of the wheels (the main inconvenient derive from the limited ability of belt to support the weight of a full-scale vehicle; in the case without wheels in motion, this ability increase). But, the aerodynamic evaluation of the vehicles in wind tunnels is subject to various negative influence parameters a.g. flow ratio, blockage ratio, test section geometry and position of the car within that geometry a.o. All these limitations of wind tunnels have led to CFD analyses, which have been successfully applied to study the aerodynamics of cars in recent years. The primary reason of using numerical methods in design of vehicles is that they can generate information before a testable model even exists. In addition, CFD analyses are not necessarily burdened with the limitation of size and geometry of the test section of the wind tunnels. In this sense, computational space can be made large enough to eliminate blockage effects, according with hard resources. Also, simulation of relative motion between vehicle and road and rotations of the wheels are comparatively easy to accommodate. On the other hand, once the equations of mathematical model have been solved, there is much more information available than from a routine experiment. According with previously related, using a prototype of terrain vehicle as an example, a process for the simulation of flow around the vehicle is described with the aid of the ANSYS-Flotran, professional software, CFD module. The main steps involved in the simulation process, with a special emphasis on the influence of ground effect, considering also the motion of wheels, are presented. The conclusions are demonstrated by the results of relative drag (lift) increment D c ∆( L c ∆) and by the computer-graphics visualization. The references for the computing of D c ∆( L c ∆) are the values obtained in the analyses with relative motion between vehicle and ground, but without motion of wheels. The obtained results show that for studied case, taking into consideration of the relative motion between vehicle and ground, combined with the motion of the wheels has as result a decrease of D c ∆accompanied by an increase of L c ∆. In real terms, for medium Reynolds number D c are increasing ( D c ∆is positive), but for higher velocities D c are decreasing ( D c ∆become negative). Also, the effect of rotation of wheels produces a decrease of L c .