Large-Scale Separated Vortex Generated in a Wake Flow of Ahmed’s Body

Abstract

Unsteady wake structures around a bluff body, which has a slanted rear surface, are investigated experimentally with measuring the velocity and the pressure fields. Velocity distribution obtained by Particle Image Velocimetry showed that vortices are intermittently generated and occasionally it grows to a large-scale vortex which separates the flow from the slanted surface. From the pressure distribution, which is simultaneously measured with the velocity field, it is clarified that the large-scale separated vortex has considerable extent in the spanwise direction. This fact suggests that the re-attachment line of the downwash is completely separated from the slanted surface when the large-scale vortex is generated. From the time-frequency analysis, which is applied to the velocity fluctuation measured by hot-wires, the flow observed at points downstream of the body exhibits the periodic fluctuation, whereas the flow near the slanted surface does not involve the periodic fluctuations. Instead, the intermittent fluctuation is observed for the case when the downwash of the flow structure is separated from and re-attaches to the slanted surface. The intermittent fluctuation on the slanted surface is observed regardless of the existence of upwash, whereas the periodic fluctuation at the point downstream vanished when the upwash is absent.