There has recently been resurgence of activity around 3D wakes of bluff bodies. These have specific attributes particular to3D flows. A new type of dynamics has been recently discovered involving the random switching between two steady asymmetric mirror states of the wake. These symmetry breaking states are referred to as static states because their resting time is about two to three orders of magnitude larger than the natural convective time of the flow. It is a generic instability, observable for bodies having a reflectional symmetry in their geometries and for Reynolds numbers covering a wide range from a few hundred to a few millions. The asymmetric states are reminiscent of a steady pitchfork bifurcation at low Reynolds number from the trivial symmetric state. Each asymmetric state breaks the reflectional symmetry and it is associated with a large cross flow force, which consequently produces a substantial additional drag on the bluff body compared to the symmetric state. These observations have been generalized to axisymmetric geometries where the only two asymmetric states are replaced by an infinite number of asymmetric states due to the multiplicity of reflectional symmetries of the axisymmetric geometry. For large Reynolds numbers, the turbulent fluctuations allow the system to explore randomly the static asymmetric states.These features differ from the common approach adopted in flow control of 2Dwakes and based on the unsteady, periodic modes originating from the Bénard-von Kármán instability.
This SIG aims at furthering the understanding of these basic concepts as well as exploiting them for drag reduction. These applications are of direct relevance to the transport sector, such as generated by road vehicles, landing platform docks, fighters at large angle of attack, to name but a few. There are other potential applications, such as civil engineering for the structural integrity of buildings. The challenge is to stabilize the flow on the symmetric state using flow control. In addition to the reduction of force fluctuations that the stabilization will produce, potential gains as much as10% in pressure drag reduction can reasonably be expected for flat backed bodies.
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