Evaluation AC1-05
Ahmed body
Application Challenge 1-05 © copyright ERCOFTAC 2004
Comparison of Test data and CFD
Experiments provide very detailed data that offer a particularly difficult challenge to CFD. They showed that the drag crisis experienced by the body around 25°-30° is related to a dramatic change of the structure of the wake. The low-drag configuration consists in a massively separated wake, which is quasi-2D, while the high-drag configuration consists in a very complex, 3D wake structure, with a reattachment of the flow on the slant part and a strong interaction of the bubble with intense corner vortices, which are very energy-consuming.
EXP1 shows that fixing a splitter plate in the wake of the body, in the symmetry plane, forces the flow to turn back to the low drag configuration (massively separated wake). The mechanism underlying these phenomena is not clear, but it could be due to the fact that the splitter plate counteracts a flapping of the wake in the span-wise direction. Therefore, there are some evidences that large-scale unsteadiness of the wake could play a crucial role in the wake structure transition. It could also explain high levels of turbulent stresses above the slant part that are very difficult to predict with steady-state RANS calculations.
It appears from all the CFD results that the wake structure of the low drag configurations is correctly reproduced by all the turbulence models tested. The correct trend of the drag coefficient with the slant angle is correctly reproduced (CFD1), but the correct level is not found. In general, since the wake structure is correct, the pressure levels on the slant part are realistic, but the exact pressure repartition on the slant part and the vertical base are hardly reproduced.
Concerning the high-drag configuration, the great majority of the CFD computations were not able to reproduce the complex, 3D structure of the wake: a massively separated wake is obtained, which shows that the wake structure transition is missed. The number of computation and the variety of numerical schemes and meshes give many indications that the main issue is not numerical, but linked to the physical modeling: turbulence model and steady-state strategy. It appears that only two types of modeling are able to reproduce the structure of the wake: LES (CFD9) and low-Reynolds number Reynolds stress model (CFD13). It should indicate that the large-scale unsteadiness of the wake must be resolved (the potential of URANS has not been investigated extensively yet) or, alternatively, the absence of large-scale unsteadiness resolution must be compensated by a very refined turbulence modeling (Reynolds stress transport equations and integration down to the wall). However, these partial conclusions are only based on one LES and one low-Re RSM computation. Additional studies are necessary to confirm these favorable conclusions.
The paper of Florian Menter extracted from the Proceedings of the 10th ERCOFTAC IAHR Workshop (http://www.ercoftac.nl/workshop10/index.html) with permission, gives a further comparison of experimental and CFD results, including various figures. This paper can be obtained by clicking [Image:Ahmed.florian.menter.pdf here].
© copyright ERCOFTAC 2004
Contributors: Remi Manceau; Jean-Paul Bonnet - Université de Poitiers
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