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A cavity at the base of the squareback Ahmed model at Re 4 × 10 5 is able to reduce the base suction by 18% and the drag coefficient by 9%, while the geometry at the separation remains unaffected. Instantaneous pressure measurements at the body base, fluid force measurements and wake velocity measurements are investigated varying the cavity depth from 0 to 35% of the base height. Due to the reflectional symmetry of the rectangular base, there are two Reflectional Symmetry Breaking (RSB) mirror modes present in the natural wake that switch from one to the other randomly in accordance with the recent findings of Grandemange et al. (2013b). It is shown that these modes exhibit an energetic 3D static vortex system close to the base of the body. A sufficiently deep cavity is able to stabilize the wake toward a symmetry preserved wake, thus suppressing the RSB modes and leading to a weaker elliptical toric recirculation. The stabilization can be modelled with a Langevin equation. The plausible mechanism for drag reduction with the base cavity is based on the interaction of the static 3D vortex system of the RSB modes with the base and their suppression by stabilization. There are some strong evidences that this mechanism may be generalized to axisymmetric bodies with base cavity.

▪ Abstract  The description and the physical understanding of three-dimensional separated flows are challenging problems mainly because of the use of inappropriate terms linked to the consideration of two-dimensional flows. This fact was realized in the early 1950s by Robert Legendre, who introduced the basic concepts of the Critical Point Theory to provide a rational definition of separation in three-dimensional flows. In parallel, demonstrative experiments were executed by Henri Werlé in the Onera water tunnel laboratory. From the close cooperation between these two scientists resulted the construction of a powerful theoretical tool allowing the elucidation of the structure of largely separated three-dimensional fields. The importance of their contribution to fluid mechanics is illustrated here by the consideration of basic configurations: flow past wings or elongated bodies, in front of obstacles, and behind a base. For each case, the flow organization is discussed by considering representative water tunnel visualizations and corresponding topological interpretations.

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