A study of wake effects on the drag of the Ahmed’s squareback model at the industrial scaleReport as inadecuate

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1 PSA - PSA Peugeot - Citroën 2 ENSTA ParisTech UME - Unité de Mécanique 3 RENAULT SAS 4 GIE S2A - Groupement d’Intérêt Economique Souffleries Aéroacoustiques Automobiles

Abstract : Experiments are performed at industrial scales over the Ahmed geometry, i.e. at a Reynoldsnumber of Re = 2.5×106 based on the height of the body. The shape of the squareback geometryis first optimised to make an initial substantial drag reduction. The separated flow at the trailingedge is orientated by introducing chamfers at the top and bottom edges. A parametric study basedon both chamfered angles leads to an optimized Ahmed geometry having a drag 5.8% lower thanthe reference squareback model. It is evidenced that this optimized geometry produces 4 intenselongitudinal vortices that still contribute significantly to the drag. The effect of a sideslip yawangle is studied. As expected, it is found that the drag increases with an increase in the yaw angle,but surprisingly the drag remains constant for yaw angles within the interval ±0.5◦ for which theside force displays very large fluctuations. This plateau is explained by recent observation of thebi-stable properties of the squareback Ahmed body Grandemange, Gohlke & Cadot, PhysicalReview E 86, 2012. The suppression of the bi-stable behavior using a passive control techniqueis associated with an additional drag reduction of 1.6%.

Author: Mathieu Grandemange - Olivier Cadot - Courbois, A. - Vincent Herbert - Denis Ricot - Tony Ruiz - Rémi Vigneron -

Source: https://hal.archives-ouvertes.fr/


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