Large eddy simulation of flow past road vehicles subjected to pitching oscillation

Abstract

We examine the unsteady aerodynamic effect on pitching stability of road vehicle by large eddy simulation. To probe the vehicle's dynamic response, a forced-sinusoidal-pitching oscillation was imposed on the vehicles during the simulation. For validation purpose, we compare the numerical result with wind tunnel measurement. The simulation result shows the strong influence of unsteady flow structures around the rear end of the vehicles on their pitching stability. These flow structures include the front and rear pillar vortices and cross flow component above the trunk deck. The configuration of curved front pillar coupled with sharp-edged rear pillar produce a trend for the unsteady aerodynamic force to restrain the pitching motion, while the presence of front pillar vortices, which are associated with sharpedge front pillar configuration, resulted in a tendency for enhancing the pitching motion. I. Introduction ONVENTIONALLY, development of road vehicle aerodynamics was mainly focused on the steady-state component, particularly the drag coefficient, which was obtained through wind tunnel measurement or CFD computation. This trend was first carryover from aeronautical practice in the early 20 century. At that time, low engine power and bad road condition did not permit high driving speed, and thus aerodynamic was not playing any important roles. Hence, beside the attempts to reduce drag for achieving higher driving speeds, not much attention has been paid in regard of other aspects of vehicle aerodynamics. Today however, vehicles can run at much higher speed, and thus aerodynamic influences on driving stability become significant. Consequently, the mean drag coefficient alone can no longer adequately reflect the vehicle’s performance in real life. To improve realism in vehicle’s aerodynamic assessment, it is important to take into consideration the dynamic aspect of the aerodynamic forces and moments. In this study, we conducted Large Eddy Simulation (LES) on flow past two vehicle models to investigate their pitching stability characteristics. During the LES, sinusoidal-forced-pitching oscillation was imposed on the vehicle