In order to clarify the relations between vehicle motions and flow fields, LES (Large Eddy Simulation) is conducted for flows around a simplified car model in heave motions, where an improved discretization scheme is employed in the ALE (Arbitrary Lagrangian-Eulerian) coordinates. The analysis object is the Ahmed model with a rear slant angle of 12.5 degrees. It is forced to oscillate sinusoidally with forcing amplitudes of 4 and 8 mm at two different frequencies of 4 and 8 Hz. The inflow velocity is set at 40 m/s, which yields the Reynolds number of 2.8 million based on the model length of 1 044 mm. In the stationary calculations, where the ride height is statically changed, the lift coefficients and the pitching moment coefficients become smaller with the decrease of the vertical displacement. In the oscillated states, they form hysteresis loops with the same positive inclinations as the stationary cases, and their rotating directions are counter-clockwise, which means that the aerodynamic forces are phase-lag to the heave displacement. From the visualization of flow fields and the frequency response of pressure fluctuations, it is found that three dominant aerodynamic effects appear in the oscillated states, such as flow contraction, relative change of inflow angle, and fluid added mass effect. These effects are dependent on heave displacement, velocity, and acceleration, respectively.
