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.
Highlights
Wind tunnel testing is employed during automobile development, in order to optimize a car’s aerodynamic properties, as well as during certification, to determine a car’s fuel economy
The finite geometry of the wind tunnel creates a flow field in the test section that differs from the one a vehicle experiences on the road
The quasi-steady cD values are always 2–3 counts higher than all unsteady drag values. These results prove that the signal frequency does affect the aerodynamic drag, not for all, but for some vehicles
Summary
Wind tunnel testing is employed during automobile development, in order to optimize a car’s aerodynamic properties, as well as during certification, to determine a car’s fuel economy. In both cases, the test engineer operates under the assumption that the measured aerodynamic coefficients, especially aerodynamic drag, are equal to the ones the vehicle experiences on the road. There are two main reasons why this is not the case: One is the finite geometry of the wind tunnel and the other is the transient flow experienced by the on-road vehicle. The finite geometry of the wind tunnel creates a flow field in the test section that differs from the one a vehicle experiences on the road. For the open jet wind tunnel, which is ubiquitous in the auto industry, the current state of the art cD correction method is the Two-Measurement
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