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A numerical investigation of the asymmetric wake mode of a squareback Ahmed body – effect of a base cavity

Published online by Cambridge University Press:  17 October 2017

J.-M. Lucas ,
O. Cadot Open the ORCID record for O. Cadot [Opens in a new window] ,
V. Herbert ,
S. Parpais  and
J. Délery
J.-M. Lucas
Affiliation:
GANTHA, 12 Boulevard Chasseigne, 86000 Poitiers, France
O. Cadot*
Affiliation:
IMSIA, ENSTA-ParisTech/CNRS/CEA/EDF, Université Paris Saclay, 828 Boulevard des Maréchaux, 91762 Palaiseau CEDEX, France
V. Herbert
Affiliation:
PSA Peugeot Citroën, Route de Gisy, 78140 Vélizy-Villacoublay, France
S. Parpais
Affiliation:
Renault SAS, 13/15 Quai Alphonse le Gallo, 92100 Boulogne-Billancourt, France
J. Délery
Affiliation:
GIE S2A, 2 Avenue Volta, 78180 Montigny-le-Bretonneux, France
*
†Email address for correspondence: cadot@ensta.fr
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Abstract

Numerical simulations of the turbulent flow over the flat backed Ahmed model at Reynolds number $Re\simeq 4\times 10^{5}$ are conducted using a lattice Boltzmann solver to clarify the mean topology of the static symmetry-breaking mode of the wake. It is shown that the recirculation region is occupied by a skewed low pressure torus, whose part closest to the body is responsible for an extra low pressure imprint on the base. Shedding of one-sided vortex loops is also reported, indicating global quasi-periodic dynamics in conformity with the seminal work of Grandemange et al. (J. Fluid Mech., vol. 722, 2013, pp. 51–84). Despite the limited low frequency resolution of the simulation, power spectra of the lateral velocity fluctuations at different locations corroborate the presence of this quasi-periodic mode at a Strouhal number of $St=0.16\pm 0.03$ . A shallow base cavity of $5\,\%$ of the body height reduces the drag coefficient by $3\,\%$ but keeps the recirculating torus and its interaction with the base mostly unchanged. The drag reduction lies in a global constant positive shift of the base pressure distribution. For a deep base cavity of $33\,\%$ of the body height, a drag reduction of $9.5\,\%$ is obtained. It is accompanied by a large elongation of the recirculation inside the cavity that considerably attenuates the low pressure sources therein together with a symmetrization of the low pressure torus. The global quasi-periodic mode is found to be inhibited by the cavity.

JFM classification

Flow Control: Instability control Wakes/Jets: Separated flows Wakes/Jets: Wakes

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Papers
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Journal of Fluid Mechanics , Volume 831 , 25 November 2017 , pp. 675 - 697
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© 2017 Cambridge University Press 

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