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Turbulent wake past a three-dimensional blunt body. Part 1. Global modes and bi-stability

Published online by Cambridge University Press:  28 March 2013

M. Grandemange ,
M. Gohlke  and
O. Cadot
M. Grandemange*
Affiliation:
Unité de Mécanique, Ecole Nationale Supérieure de Techniques Avancées, ParisTech, Chemin de la Hunière, 91761 Palaiseau CEDEX, France PSA Peugeot Citroën, Centre Technique de Velizy, Route de Gisy, 78943 Vélizy-Villacoublay CEDEX, France
M. Gohlke
Affiliation:
PSA Peugeot Citroën, Centre Technique de Velizy, Route de Gisy, 78943 Vélizy-Villacoublay CEDEX, France
O. Cadot
Affiliation:
Unité de Mécanique, Ecole Nationale Supérieure de Techniques Avancées, ParisTech, Chemin de la Hunière, 91761 Palaiseau CEDEX, France
*
†Email address for correspondence: mathieu.grandemange@ensta-paristech.fr
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Abstract

The flow around the three-dimensional blunt geometry presented in the work of Ahmed, Ramm & Faitin (Tech. Rep., 1984) is investigated experimentally at $\mathit{Re}= {U}_{0} H/ \nu = 9. 2\times 1{0}^{4} $ (where ${U}_{0} $ is free-stream velocity, $H$ the height of the body and $\nu $ viscosity). The very large recirculation on the base responsible for the dominant part of the drag is characterized. The analyses of the coherent dynamics of the wake reveal the presence of two very distinctive time scales. At long time scales ${T}_{l} \sim 1{0}^{3} H/ {U}_{0} $ , the recirculation region shifts between two preferred reflectional-symmetry-breaking positions leading to a statistically symmetric wake; the sequence of these asymmetric states is random. This bi-stable behaviour is independent of the Reynolds number but occurs only above a critical value of ground clearance. At short time scales ${T}_{s} \sim 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 motivate further drag reduction strategies.

JFM classification

Wakes/Jets: Separated flows Wakes/Jets: Shear layers Wakes/Jets: Wakes/Jets

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Type
Papers
Information
Journal of Fluid Mechanics , Volume 722 , 10 May 2013 , pp. 51 - 84
Copyright
©2013 Cambridge University Press

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