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The critical change in drag occurring on the Ahmed body when the slanted base has an angle of 30° is due to a transition in the wake structure. In a previous study on flow analysis across the Ahmed body, we investigated the unsteady wake experimentally using hot-wire and particle image velocimetry measurements. However, because the experimental analysis yielded limited data, the spatially unsteady wake behaviour, interaction between the trailing vortex and transverse vortices (up/downwash), and flow mechanism near the body were not discussed sufficiently. In this study, the unsteady wake structures were analysed computationally using computational fluid dynamics to understand these issues, and the hypothesis was tested. The slant angle was 27.5°, which is identical to that in the experiment and corresponds to a high drag condition indicated experimentally.

The critical change in drag occurs in the Ahmed Body at 30° of the slanted base due to the transition in the wake structure. The distinctive feature of this bi-stage phenomenon, which consists of three-dimensional and quasi-axisymmetric separation states, is that the state drastically changes. Because this feature indicates that each state is stable around a critical angle, the transition is believed to be triggered by some instantaneous disturbances. Therefore, in our previous papers, we have paid attention on the unsteady behavior of the wake to determine the trigger that induces the transition. However, the relationship between the spatial transient behavior of the wake structures and the specific frequencies has not been clarified. Then, we tried to control the degree of interaction of the trailing vortices on the downwash by changing the aspect ratio of the slanted base.

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.