The influence of different unsteady incident flow environments on drag measurements in an open jet wind tunnel

Abstract

Aerodynamic development for road vehicles is usually carried out in a uniform steady-state flow environment, either in the wind tunnel or in Computational Fluid Dynamics (CFD) simulations. However, out on the road, the vehicle experiences unsteady flow with fluctuating angles of incidence 𝛽, caused by natural wind, roadside obstacles, or traffic. In order to simulate such flow fields, the Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart (FKFS) swing® system installed in the quarter scale model wind tunnel can create a variety of time-resolved signals with variable 𝛽. The static pressure gradient in the empty test section, as well as 𝑐𝐷 values of the Society of Automotive Engineers (SAE) body and the DrivAer model, have been measured under these transient conditions. The 𝑐𝐷 measurements have been corrected using the Two-Measurement Correction method in order to decouple the influence of the unsteady flow from that of the static pressure gradient. The investigation has determined that the static pressure gradient in the empty test section varies greatly with different excitation signals. Thus, it is imperative to apply a 𝑐𝐷 correction for unsteady wind tunnel measurements. The corrected 𝑐𝐷 values show that a higher signal amplitude, as in, signals with large 𝛽, lead to higher drag forces. The influence of the signal frequency on drag values varies depending on the vehicle geometry and needs to be investigated further in the future.