Flow Structures above the Trunk Deck of Sedan-Type Vehicles and Their Influence on High-Speed Vehicle Stability 1st Report: On-Road and Wind-Tunnel Studies on Unsteady Flow Characteristics that Stabilize Vehicle Behavior

In vehicle development, it is generally said that vehicle with lower aerodynamic lift coefficient has better straight-ahead stability at high speed. However, in some cases, straight-ahead stability differs between two vehicles with similar low aerodynamic lift coefficient. It is natural to think that this variation is caused by the difference of suspension characteristics or vehicle body rigidity. But from our experiences, different straight-ahead stability was observed between two vehicles having same suspension characteristics, same vehicle body rigidity and almost similar aerodynamic lift coefficient, but different vehicle configurations. A moving vehicle shows complex behaviors due to the unsteady aerodynamic forces caused by unsteady flow around the vehicle and unsteady vibration induced by undulated road. As aerodynamic lift coefficient simply indicates time-average, it could be considered that aerodynamic lift coefficient may not fully describe such complex vehicle behaviors during high speed driving.

Therefore, in this study, an attempt was made to clarify the relationship between unsteady flow around vehicle and straight-ahead stability, by investigating aerodynamic forces generated from unsteady flow and its effects on vehicle behavior. Wind tunnel experiments and on-road driving experiments showed that vehicle straight-ahead stability is greatly influenced by a distinctive unsteady vortex structure above the trunk deck, and that a comprehensive aerodynamic technique based on unsteady characteristics of flow field is in need to evaluate vehicle's straight-ahead stability in addition to the conventional steady aerodynamic approach.