The Effect of Unsteady Incident Flow on Drag Measurements for Different Vehicle Geometries in an Open Jet Wind Tunnel

Automotive engineers use the wind tunnel to improve a vehicle’s aerodynamic properties on the road. However, a car driving on the road does not experience the steady-state, uniform flow characteristic of the wind tunnel. Wind, terrain and traffic all cause the flow experienced by the vehicle to be highly transient. Therefore, it is imperative to understand the effects of forces acting on the vehicle resulting from unsteady flow. To this end, the FKFS swing® installed in the University of Stuttgart’s model scale wind tunnel was used to create 36 different incident flow signals with time-resolved yaw angles. The c_D values of five different 25% vehicle models, each with a notchback and a squareback configuration, were measured while under the influence of the aforementioned signals. The vehicle models were chosen to ensure a variety of different geometries, but at the same time also to enable isolated comparison of specific geometric properties. All drag measurements were corrected using the Two-Measurement Correction method. The study discusses the correction method’s accuracy and suggests possible measures to improve said accuracy for unsteady investigations. Before examining the c_D results in detail, this paper introduces ways to quantify the influence of an unsteady flow environment on aerodynamic drag. The measured and corrected drag values are then investigated using the new methods, which allows a quantitative comparison between the different vehicle geometries. Consequently, this paper offers explanations for the difference in drag characteristics between vehicles by cross-referencing these differences with their geometric properties. This study aims to improve the understanding of aerodynamic drag under unsteady flow and, in the long term, to enable drag optimization tailored to more realistic flow scenarios.