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Vortex breakdown recognition in the turbulent wake of the 25° Ahmed body

DOI: 10.1615/ICHMT.2015.THMT-15.640
pages 335-338

C. Jermann
PSA Peugeot Citroen, Centre Technique de Velizy, Vélizy-Villacoublay Cedex, France; Aix-Marseille Université, CNRS, cole Centrale Marseille, Laboratoire M2P2, Marseille, France

Gregory Pujals
PSA Peugeot-Citroen, 78 943 Vélizy Villacoublay, France

P. Meliga
M2P2, CNRS/Aix–Marseille Université, 13451 Marseilles, France

Francois Gallaire
Laboratory of Fluid Mechanics and Instabilities, EPFL, 1015 Lausanne, Switzerland

Eric Serre
Aix-Marseille Universite, CNRS, Ecole Centrale Marseille, Laboratoire M2P2, Marseille, France

Sinopsis

This paper investigates experimentally and theoretically the near wake of the 25° Ahmed body, considered a suitable test-case to reproduce the two counter-rotating longitudinal vortices widely encountered in automotive aerodynamics. The three-dimensional experimental mean flow is reconstructed at high Reynolds number (Re = 2.8×106) from a series of cross-flow time-averaged planes acquired with a moving automated Stereo-PIV system. We observe a sharp decay of the axial velocity and vorticity in the near-wake, 0.5 times the projected length of the slanted surface downstream the square back, where the streamwise vortices is subjected to a strong adverse pressure gradient and the turbulent kinetic energy exhibits a peak in the vortex core. A stability analysis of the experimental velocity shows that the flow undergoes vortex breakdown roughly at the same position, through a transition from supercritical (x < 0.5) to subcritical (x > 0.5) conditions and the accumulation of upstream propagating axisymmetric waves.

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