DOI: 10.1615/ICHMT.2015.THMT-15
ISBN Print: 978-1-56700-427-4
ISBN CD: 978-1-56700-428-8
Vortex breakdown recognition in the turbulent wake of the 25° Ahmed body
ABSTRAKT
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.