Dynamic wind-tunnel tests of a simplified passenger car model were conducted using a two-degree-of-freedom model shaker. Time-resolved aerodynamic loads were derived from a built-in six-component balance and other sensors while the model underwent sinusoidal heaving and pitching motions at frequencies up to 8 Hz. The experimental results showed that frequency-dependent gains and phase differences between the model height/angle and the aerodynamic loads are in close agreement with those predicted by large-eddy simulation (LES) using an arbitrary Lagrangian-Eulerian (ALE) method. Based on these findings, transient aerodynamic loads associated with lateral motions were also estimated by LES analysis.
Based on the above results, a full-unsteady aerodynamic load model was then derived in the form of a linear transfer function. The force and moment fluctuations associated with the vertical and lateral motions are well described by the full-unsteady aerodynamic load model. This load model integrates added mass and moment-of-inertia terms into the quasi-unsteady model that takes into account the effect of the motion rate as an equivalent change in the relative inflow angle. Vertical motion analysis of a nominal passenger car model coupled with the full-unsteady aerodynamic load model showed that the transient aerodynamic loads affect the frequency response in such a way as to reduce its resonance frequency. The added mass and moment-of-inertia terms were eventually confirmed to have little effect on the vehicle dynamics; it should, however, be mentioned that their inclusion is inevitable in the modeling of transient aerodynamic loads, which inherently involves the added mass effect.
