The Impact of Unsteady Flow on Drag Measurements in Automotive Wind Tunnels

The CFD simulations have shown that while absolute drag results agree between wind tunnel and on-road flow, there are still discrepancies in the flow fields owing to coherent structures caused by vortices in the wind tunnel shear layer.

The airflow that cars experience on the road is inherently unsteady, but aerodynamic development mainly occurs in steady-state environments. Xiao Fei aims to deepen the understanding of unsteady drag and the corresponding simulation methods. Wind tunnel measurements were carried out in the University of Stuttgart Model Scale Wind Tunnel using the active turbulence system. Selected measurements were recreated using CFD to explain the differences in drag measured in the wind tunnel. As a result of the investigations, new quantifiers were introduced that measured the magnitude of the signal’s effect on vehicle drag, as well as the feasibility of approximating unsteady drag with steady-state methods. The CFD simulations have shown that while absolute drag results agree between wind tunnel and on-road flow, there are still discrepancies in the flow fields owing to coherent structures caused by vortices in the wind tunnel shear layer. This work adds to the knowledge base of unsteady aerodynamics and turbulence systems in wind tunnels. It serves as a starting point for efforts to establish unsteady aerodynamics as the future industry standard.