Experimental characterization of ﬂow unsteadiness
in the centerline plane of an Ahmed body rear slant
Received: 28 August 2012 / Revised: 5 February 2013 / Accepted: 7 February 2013 / Published online: 20 February 2013
Ó Springer-Verlag Berlin Heidelberg 2013
Abstract This study presents the results of an experi-
mental analysis of the unsteady features of the ﬂow around
the rear part of an Ahmed body with rear slant angle of 25°.
This analysis focuses on the 3D separated and reattaching
zone that develops on the rear slanted surface and provides
new information, improving the understanding of the ﬂow
unsteadiness. Flow investigations were performed using
particle image velocimetry, hot wire anemometry, and
unsteady ﬂush-mounted pressure transducers in the plane
of symmetry above the rear slanted surface. Spectral
analysis and proper orthogonal decomposition of the output
signals show the emergence of low frequency unsteadiness
and high frequencies activity. Characteristic timescales of
both instabilities are provided and the physical effect of the
low frequency unsteadiness is related to a ﬂapping motion
of the separated shear layer, while the high frequency
activity is associated with a large-scale vortex emission.
The results focus on the centerline plane of the rear slant
and more speciﬁcally demonstrate relevant similarities
with the unsteady mechanisms of 2D recirculated ﬂow.
Since the pioneering experimental studies by Onorato et al.
(1984) and Ahmed et al. (1984), a large amount of infor-
mation in relation to the complex three-dimensional
topology and aerodynamic features of road vehicles has
been published using the Ahmed body (Ahmed et al.
1984), enabling the identiﬁcation of the main pressure drag
sources and their contribution to the overall aerodynamics.
Consequently, considerable effort has been put into
developing ﬂow control devices and strategies, promising
efﬁcient drag reduction (Gillie
ron 2002). Unfortunately,
current knowledge of ground vehicle aerodynamics is no
longer sufﬁcient since it is often limited to time-averaged
information. Apart from a few experimental (Sims-Wil-
liams and Duncan 2003; Vino et al. 2005; Gilhome et al.
2001; Duell and George 1999) or numerical (Howard and
Pourquie 2002; Krajnovic
and Davidson 2005; Minguez
et al. 2008; Serre et al. 2011) studies, the physical mech-
anisms related to the unsteady process involved in the ﬂow
dynamics are rarely characterized, whereas they are of
signiﬁcant interest especially for ﬂow control applications
(e.g., closed-loop control).
The Ahmed body simpliﬁed car geometry constitutes a
reference model. It is a parallelepiped with a streamlined
front part and a slanted surface at the rear that reproduces
the basic ﬂow features of the rear part of ground vehicles.
Ahmed et al. (1984) pointed out that the ﬂow is mainly
controlled by the slant angle (for a given Reynolds number)
and suggested a classiﬁcation of the averaged ﬂow topol-
ogies. The case of a slant angle of 25° is particularly
interesting, since this conﬁguration presents a complex
organization of the ﬂow with a high drag level and is
representative of the ﬂow topology at the rear part of
common fastback cars (rear window and rear part). In that
case, the ﬂow is characterized by a complex combination
of three structures: two longitudinal vortical structures, a
3D separation bubble on the rear window, and a 3D wake
located at the rear base.
From an unsteady point of view, the 3D wake that
develops behind the rear base is commonly related to 3D
vortex shedding advected downstream in the wake (Sims-
Williams and Duncan 2003; Vino et al. 2005), while the
A. Thacker Á S. Aubrun (&) Á A. Leroy Á P. Devinant
Laboratoire PRISME, Universite
ans, 8 rue Le
de Vinci, 45072 Orle
ans Cedex 2, France
Exp Fluids (2013) 54:1479