The effect of blockage due to the presence of the wind tunnel walls has been known since the early days of wind tunnel testing. Today there are several blockage correction methods available for correcting the measured aerodynamic drag. Due to the shape of the test object, test conditions and wind tunnel dimensions the effect on the flow may be different for two cab variants. This will result in a difference in the drag delta between so-called open-road conditions and the wind tunnel. This makes it more difficult to evaluate the performance of two different test objects when they are both tested in a wind tunnel and simulated in CFD.
A numerical study where two different cab shapes were compared in both open road condition, and in a digital wind tunnel environment was performed. The main focus of the study was to investigate how the flow field was influenced by the blockage for the different cab shapes, and how different blockage correction methods for ground vehicles would influence the delta between the different test objects.
The constricted flow conditions in the digital wind tunnel increased both the acceleration around the cab corners and influenced the shape and size of the wake structures. When applying blockage corrections on the predicted drag coefficient it was observed that there was a significant spread in the delta value depending on the method utilised. However, cabs of similar shape showed better agreement between the blockage corrected results and open-road conditions.