Sensivity of Aerodynamic Force Coefficients to Changes in Onflow Velocity Components for a 1/4 Scale Automobile Model

Abstract

When driving under normal road conditions a vehicle will regularly encounter the wakes of other vehicles, either in the execution of drive behind or passing manoeuvres. Complex aerodynamic interactions occur when two vehicles are involved in a passing manouvre. Experimental studies have shown that, when vehicles cross or overtake, their respective flow fields become coupled and that the flow has a multi-physics character with a wide range of relevant flow and time scales. Consequently an understanding of the wake effects on vehicle aerodynamic loading is of interest to the automotive industry. On-road measurements are expensive and repeatability of test conditions can by difficult to achieve in practice. Existing experimental and computational methods for predicting the effects of a wake on car aerodynamics are costly- simplified approaches are being developed, for example Schröck et al. developed a transfer function model for the aerodynamic forces acting on a vehicle as a function of side wind conditions. The current work aims to investigate the influence of a time dependent on-flow conditions on a 1:4 scale model vehicle. Outcomes of this study will assist the development of accurate and cost effective prediction methods. The aerodynamic forces acting on a 1:1 scaled generic car model downstream of a moving flap system are studied in this work. Four upstream NACA airfoils are used to provide controlled inflow disturbances to the model. The oscillation amplitude b (0° ≤ b ≤ 20°) and the oscillation frequency f (0 ≤ f ≤ 50) Hz characterize both the onflow length and time scales of the upstream disturbances which, in combination of the measured force coefficients, could be used to assess the validity of the Schröck transfer function approach to onflow variations. The numerical simulations are carried out using the open source OpenFOAM software. The CFD problem contains three principal components : a) wind tunnel, b) flapped system, and c) model vehicle. Mesh refinement studies and comparison against published experimental data are undertaken in assessing the numerical methods used. The numerical approach is then applied to a model geometry under both steady and unsteady conditions. Comparative studies of the force coefficient under steady and unsteady conditions are then assessed.