1992-02-01

Unsteady-Wake Analysis of the Aerodynamic Drag of a Notchback Model with Critical Afterbody Geometry 920202

For both notchback-type and fastback-type models, it has been found that critical geometries which increase the aerodynamic drag exist, and the time-averaged wake patterns basically consist of an arch vortex behind the rear window and trailing vortices in the wake.
The unsteady characteristics of the wake seem to be directly related to aerodynamic drag. However, the unsteady characteristics of these wake patterns for notchback and fastback cars were not clear. The purpose of present paper is to clarify these phenomena. We try to analyze experimentally the unsteady characteristics by measuring the velocity fluctuations in the wake, the pressure fluctuations on the trunk deck and the drag-force fluctuations acting on the model. At the same time, the analysis of the numerical simulation was made by using the same numerical model as the experimental model.
The computed flow visualization behind the rear window showed a fluctuating arch vortex. The velocity-fluctuation phenomena in the wake with critical geometries corresponded to that of the drag fluctuation in the experiment. As a result, it was found that the critical geometries have the peculiar frequency of the arch vortex and its fluctuation is related to the increase in aerodynamic drag.

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