| Summary: | Unsteady aerodynamic forces and flow structures around a road vehicle in dynamic motion were investigated in this study. The special focus was on the aerodynamic rolling moment acting on the vehicle in multiple degrees of freedom motion, namely, roll, yaw and a combination of these two motions. The target model was a simplified hatchback type vehicle in full scale. Unsteady computational fluid dynamic technique with a moving boundary method was applied to predict the transient aerodynamic forces and moments acting on the vehicle, and transient three dimensional flow structures were extracted to explain the flow mechanisms which cause the unsteady aerodynamics. The numerical method was first validated in the stationary case by comparing its results with wind tunnel data. Then the unsteady characteristics of the aerodynamic rolling moment was discussed based on the phase-shift to the vehicle's angular motion as input and its gain. The results indicated that the rolling moment in the combined rolling and yawing motion showed a higher aerodynamic damping effect on the rolling motion than in the cases of monotonic rolling, yawing or the arithmetic sum of these two results. It was confirmed that the pressure changes in the middle and the rear of the underbody contributed to the difference of the aerodynamic damping of the rolling motion. The mechanisms causing the pressure changes on the underbody were explained by the difference of flow structures behind the front wheelhouse and around the rear underbody.
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