Bluff Bodies as a Potential Open Challenge Problem for Active Flow Control

Abstract: This talk was presented at AIAA Flow Control Meeting (June 25-29, 2018 Atlanta) following an invitation by AIAA Flow Control Applications and Impacts Discussion Group. Flows around bluff bodies correspond to locally or massively separated flows and our aim is both to develop techniques for bluff body wake manipulation (actuation methods, Active Flow Control – AFC – approaches) and to understand the origins of wake and drag changes. Drag reduction and driving stability are key problems for terrestrial transportation in order to cope with design, functionality and aerodynamics. For Aeronautics, helicopter or airplane transport may have large, flat loading ramps and poor aerodynamics of the aft body. Fuselage drag reduction and unsteady load alleviation are then key challenges. A fascinating aspect of these turbulent aerodynamics at significant Reynolds number (≈10^6) is the wide range of length and time scales. At the scale of the body, vortex shedding, an absolute instability, is responsible for self-sustained oscillations. Reflexional Symmetry Breaking, leading to random switching with a long time dynamics, is also observed. Specific flow structures, namely turbulent shear layers surrounding the near wake and 3D conical vortices of trailing type for slanted geometries, have very important contributions. Two geometries will be considered in the talk. For a square back wake, our goal is to obtain a robust AFC strategy by driving the wake toward an unsteady symmetric state and by preventing the slow organization dynamic of the asymmetric state. For a slanted and smoothed aft geometry, one has to deal with introduction of longitudinal vorticity in the near wake and to address interactions of flow structures and near wake properties.