Unsteady Vehicle Aerodynamics during a Dynamic Steering Action: 2nd Report, Numerical Analysis 2012-01-0448

Unsteady aerodynamic forces acting on vehicles during a dynamic steering action were investigated by numerical simulation, with a special focus on the vehicles' yaw and lateral motions. Two sedan-type vehicles with slightly different geometries at the front pillar, side skirt, under cover, and around the front wheel were adopted for comparison. In the first report, surface pressure on the body and total pressure behind the front wheel were measured in an on-road experiment. Then the relationships between the vehicles' lateral dynamic motion and unsteady aerodynamic characteristics during cornering motions were discussed. In this second report, the vehicles' meandering motions observed in on-road measurements were modeled numerically, and sinusoidal motions of lateral, yaw, and slip angles were imposed. The responding yaw moment was phase averaged, and its phase shift against the imposed slip angle was measured to assess the aerodynamic damping. It was found that the vehicle model with a higher sensory rating of high speed stability in the on-road test showed higher aerodynamic damping for the slip-angle change. The difference in flow structures on the sides of the vehicles and their contributions to the yaw and lateral motions were also investigated by visualizing the transient numerical results. Observations revealed that the vehicle with higher stability had thinner side flow structures, and flows around the front wheel house and rear-end corner contributed to restraining the rotating motion during the meandering motion. As a result, the discussions and analogy included in the previous report based on the restricted on-road measurements were confirmed in the present numerical analysis.