Effect of upstream turbulence on truck aerodynamics

Abstract: The aerodynamic team at SCANIA has discovered the need to investigate the effect of the upstream turbulence conditions on the aerodynamics of the trucks. This need comes from the fact that there are differences between the drag coefficients obtained using computational fluid dynamics (CFD) and the on-road measurements. This difference can lead to wrong predictions of fuel consumption and emissions, which can cause incorrect evaluation of design changes. In this study the problem of modeling upstream turbulence in CFD simulations is addressed together with its effect on the aerodynamics of the trucks. To achieve this, representative values of turbulence intensity and length scale were found from the work of different researchers, who performed on-road measurements for various conditions. These values were then used in a method by Jakob Mann to generate a synthetic turbulence field. This field was then used to generate time varying velocity components, added to the mean velocity at the inlet of a CFD simulation. After the implementation of the method it was discovered that the conditions at the test section of the virtual wind tunnel were representative of the on-road measurements. The results showed drag increase and wake length decrease, similar to previous studies performed on simple geometries. It also showed that the higher mixing of the flow increases the drag by surface pressure increase of forward facing surfaces and pressure decrease at the base. These conclusions may be extended to other bluff body geometries and it shows the importance of good design around gaps. The comparison between two truck geometries showed that a truck with better aerodynamics in a smooth flow shows less drag increase with introduction of upstream turbulence.