  In this study we focus on the control of the dynamics of 3D turbulent wake
downstream a square-back Ahmed body ($Re_H=3.9\times10^5$). The peculiar
dynamics of such a wake are first characterized through the trajectories of the
pressure barycenter over the rear part of the model as well as the
recirculation barycenter in the wake. In particular it is shown that these
dynamics allow the definition of three different states: the two so-called
reflectional symmetry-breaking (RSB) modes and the transient symmetric (TS)
mode. It was shown recently that the time-fluctuations of the pressure
barycenter could be characterized as a weak chaotic system with a well-defined
attractor. We show that the dynamics of the bimodal wake can then be forced
into a stable asymmetric or symmetric state in open loop control, using
tangential continuous or pulsed blowing in three different regions along the
upper edge of the rear part of the model. Finally, a simple closed-loop
opposition control, based on real-time identification of the wake barycenter in
the PIV fields, is used to force the chaotic dynamics of the wake into a
regular oscillatory motion at a well-controlled frequency. Depending on the
actuation parameters, the wake dynamics can also be switched from bimodal to a
new multimodal behavior. We show that this new mode also exhibits a peculiar
dynamics with an up-down instead of left-right chaotic oscillations.
Interestingly, the recirculation area (size of the recirculation bubble) is
much more reduced for the closed-loop experiments when the jets are pulsed
rather than continuous. For the pulsed jets, the reduction is also increased
when the proper frequency is chosen.

Fluid Dynamics