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Study on the Transient Behaviour of the Vortex Structure behind Ahmed Body
Abstract:
On a bluff body which has a slant surface on the rear upper part, it is well known that the drastic change of a wake structure behind the rear body occurs at 30°of the slant angle. Originally, this critical phenomenon was pointed out by L.J. Janssen, W.H. Hucho, and H.J. Emmelmann in the middle of the 1970s. In 1984, S.R. Ahmed conducted systematic measurements by changing the rear slant angle of the bluff body, called the “Ahmed Body”, to find the critical phenomenon. In the 2000s, D.B. Sims-Williams found that the Ahmed Body had vortex structures which had specific frequencies. However, the relationship between the critical phenomenon and the unsteady behaviour has not been clarified yet.
Therefore, as the first step of this study, we measured the unsteady wake behaviour for various slant angles to find the relationship between the Strouhal number and the angle. The characteristics of the fluctuation were captured with two hot-wires. By analysing the coherence and difference of the phases of their signals, the behaviour of the trailing vortices and the vortex shedding of the wake was investigated. PIV measurements were also performed. As a result, two significant Strouhal numbers of the fluctuation of the wake were measured. The Strouhal number corresponding to the fluctuation of the trailing vortex structure was around 0.1 for various slant angles, while that of the up-wash flow varied in the rage of 0.1 to 0.5. Thus, incoherent vortex shedding behaviour was seen in the upper and lower regions of the wake, which was clearly different from the coherent phenomenon behind the body in such a sphere. Furthermore, we captured instantaneous phenomenon when the transition occurred by flow visualisation. Finally, the relationship between those intermittent phenomena and the trigger of the transition was discussed.