Les analysis of incompressible flows around an automobile in motion using ALE method (4th report, unsteady aerodynamic analysis around a simplified car model in pitch motion)

Abstract

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 pitch motions, where an improved discretization scheme in the ALE (Arbitrary Lagrangian-Eulerian) coordinates is employed. The analysis model is the Ahmed model with a rear slant angle of 12.5 degrees. It is forced to oscillate sinusoidally with forcing amplitudes of 0.88 and 3 degrees at two different frequencies of 4 and 8 Hz. The fluctuations of the lift coefficient (C z) are phase-lead to the pitch angle (δθ of the pitching moment coefficient (C My) are phase-lag to δθ. Especially in the cases of the larger amplitude of 3 degrees, the Lissajous curves of C MY form characteristic teardrop shapes. In addition to the three aerodynamic effects observed in the heave motions, such as flow contraction, relative change of inflow angle, and fluid added mass effect, pressure on the upper and under surfaces of the model fluctuates due to the change of incident angle. It is also revealed that the streamwise vortices which induce flow from the underside to the lateral side, are formed close to the side surfaces of the model when the model is in the pitch-up position. These vortices are closely related to the aforementioned teardrop-shaped Lissajous curves of CMY. © 2011 The Japan Society of Mechanical Engineers.