Effect of boundary layer thickness on drag and wake structure of the Ahmed body

A simple car model ‘Ahmed body’ proposed by Ahmed et al [SAE Technical Paper, 840300, 1984] has a slant surface on the rear upper surface, and its aerodynamics characteristics are significantly affected by the slant angle. Since the model is installed near the wall, the influence of the boundary layer is large. We measured the drag acting on the model exposed in boundary layer having a variety of thickness. We also measured velocity distribution around the model with the slant angle 25° and 30° using PIV. Free stream velocity was set at 15 m/s, and Reynolds number based on the square root of the frontal area was 3.13×10⁴. The drag acting on the model tended to increase as boundary layer thickness increased. As increasing boundary layer thickness at the slant angle being 25°, velocity of flow through model undersurface was reduced. At the slant angle being 30°, the increase of the boundary layer thickness exhibited sudden reduction of drag and elongation of the streamwise vortices in wall normal direction.