Evaluation of Non-Uniform Upstream Flow Effects on Vehicle Aerodynamics

Historically vehicle aerodynamic development has focused on testing under idealised conditions; maintaining measurement repeatability and precision in the assessment of design changes. However, the on-road environment is far from ideal: natural wind is unsteady, roadside obstacles provide additional flow disturbance, as does the presence of other vehicles. On-road measurements indicate that turbulence with amplitudes up to 10% of vehicle speed and dominant length scales spanning typical vehicle sizes (1-10 m) occurs frequently. These non-uniform flow conditions may change vehicle aerodynamic behaviour by interfering with separated turbulent flow structures and increasing local turbulence levels. Incremental improvements made to drag and lift during vehicle development may also be affected by this non-ideal flow environment. On-road measurements show that the shape of the observed turbulence spectrum can be generalised, enabling the definition of representative wind conditions. Here, unsteady Lattice-Boltzmann Method (LBM) simulations are used to evaluate the modification of the aerodynamics of a fast-back saloon by realistic on-road flow conditions. The turbulent conditions are added as fluctuations to the freestream flow, based on a prescribed turbulence spectrum, integral length scale, and turbulence intensity. The impact of these flow conditions on the separated flow structures and the incremental effects of design changes are evaluated. Further, the characteristics of anisotropic turbulence in the unsteady flow field around the vehicle are compared to show how freestream turbulence modifies the local turbulent field. This approach enables the evaluation of design decisions against a broader range of operating conditions than can be achieved in traditional wind-tunnel testing.