The unsteady wake of the Ahmed model with 25 degree backlight has been investigated experimentally using time-accurate 5-hole and hot-wire probes. The unsteady reconstruction technique described by Sims-Williams and Dominy was used to visualize the unsteady wake. Exa PowerFLOW simulations were performed using the same model geometry tested in the wind tunnel. The 25 degree backlight angle was selected to be slightly below the critical angle observed computationally by Duncan et al. The CFD results are similar to experimental results for backlight angles slightly below the critical angle (eg: 27.5 and 30 degree) indicating that the proximity of the backlight angle to the critical angle is more important than the absolute angle. Unsteady flow-field reconstruction of the experimental and CFD results both revealed a vortex-shedding type structure from the bottom of the model base on the wake centreline. The large ratio of model width to ground clearance (15.6:1) produces a region of relative two-dimensionality in this area which fosters this nominally two-dimensional phenomenon. The production of transverse vortices from under the model results in a semi-periodic build up and collapse of the near wake. As the near wake collapses packets of low total pressure are released and these then convect along the ground boundary into the far wake. The nominally two-dimensional behaviour near the centreline produces a symmetric oscillation in the strength and position of the rear pillar vortices. The period of this phenomenon observed both experimentally and computationally corresponds to a Strouhal number of approximately 0.5 based on free-stream velocity and the square root of the model frontal area.
The Ahmed model unsteady wake: experimental and computational analyses
Experimentelle und virtuelle Analyse des instationären Nachlaufs beim Ahmed-Körper
2003
12 Seiten, 17 Bilder, 24 Quellen
Conference paper
English
The Ahmed Model Unsteady Wake: Experimental and Computational Analyses
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