Study of Unsteady Aerodynamics of a Car Model in Dynamic Pitching Motion

The unsteady aerodynamic loads produced due to vehicle dynamic motions affect vehicle dynamic performance attributes such as straight-line stability or handling characteristics. To improve these dynamic performances, understanding the detailed mechanisms by which unsteady aerodynamic loads are caused during dynamic motions and the effects of unsteady aerodynamic loads on vehicle dynamic performance are needed. This paper describes the numerical study of unsteady aerodynamics of a 1/4 scale car model in dynamic pitching motion to clarify the detailed mechanisms by which unsteady aerodynamic loads are caused during the motion. Vortical structures around front wheelhouse and front under side of the body are analyzed by introducing schematic views to understand the mechanisms of unsteady flow fields. Furthermore, effects of aerodynamic devices devised based on the analyses on unsteady aerodynamics are discussed. Large-Eddy simulation with ALE method was conducted to analyze the transient flow fields around the car model in dynamic motion. As a result of this study, it was revealed that the flow field under the front and middle under side of the body has a dominant effect on the unsteady front and rear lift in dynamic pitching motion. The behavior of the unsteady vortex induced by the interference of the flow leaving the engine compartment toward the underbody with the external flow greatly affects the unsteadiness of the lift expressed by a pronounced phase shift. Furthermore, it was proved that the phase shift in the front lift and also rear lift could be reduced by decaying or preventing the unsteady vortex.