Turbulent wake past a three-dimensional blunt body. Part 1. Global modes and bi-stability
Résumé
The flow around the three-dimensional blunt geometry presented in the work of Ahmed et al. (1984) is investigated experimentally at Re = U 0 H/ν = 9.2 10 4. The massive recirculation on the base responsible for a dominant part of the drag is characterized. The analyses of the coherent dynamics of the wake reveal the presence of two very distinctive timescales. At long timescales T l ∼ 10 3 H/U 0 , the recirculation region shifts between two preferred reflectional symmetry breaking positions leading to a statistical symmetric wake; the succession of these asymmetric states is random. This bi-stable behavior is independent of the Reynolds number but occurs only over a critical value of ground clearance. At short timescales T s ∼ 5H/U 0 , the wake presents weak coherent oscillations in the vertical and lateral directions. They are respectively associated with the interaction of the top / bottom and lateral shear layers; when normalized by the height and width of the body, the Strouhal numbers are close to 0.17. These results suggest an alternate shedding associated with the vertical oscillation and a one-sided vortex shedding in the lateral direction with an orientation linked to the current asymmetric position. Finally, the impact of these coherent wake motions on the base pressure is discussed to orient further drag reduction strategies.
Origine | Fichiers produits par l'(les) auteur(s) |
---|
Loading...