Flow Separation and Wake Dynamics in High Reynolds Number Flows: Implications for Automotive Drag

Abstract

Low pressure wake behind the body of a vehicle is the most important component of its aerodynamic drag at highway speeds, not the viscous friction along the body. In typical scales for vehicles, the Reynolds number is O ( 1 0 6 ) , which means that the boundary layer is turbulent and stays attached for the major part of the front and roof of the vehicle. The major problem is at the back where there is an adverse pressure gradient and the flow has a tendency to separate. So automotive drag is a more complex subject that is more related to a mechanism chain and not necessarily to speed. After separation a detached shear layer rolls up into coherent structures and a recirculating wake is formed. This wake size and structure will influence the mean base pressure and thus the drag. Based on the canonical Ahmed model, boundary-layer theory, separation criteria and wake scaling are used to demonstrate the order of variation in drag relative to relatively small changes in the rear geometry. From this point of view, the effective way of drag reduction is not to avoid separation, but to control the development of the separated wake and to enhance the pressure recovery.