Adaptive control of the dynamics of a fully turbulent bimodal wake using real-time PIV

In this study, we focus on the control of the dynamics of the three-dimensional turbulent wake downstream a square-back Ahmed body (\({\textit{Re}}_\mathrm{H}=3.9\times 10^5\)) with a perturbed underbody using a home made real-time particle image velocimetry (PIV) system. After presenting this system, the peculiar dynamics of such a wake are first characterized through the trajectories of the pressure center over the rear part of the model as well as the recirculation barycenter in the wake. These dynamics allow in particular the detection of the two so-called reflectional symmetry-breaking (RSB) modes and a preferred transient symmetric configuration. It was shown recently that the time-fluctuations of the pressure center could be characterized as a stochastic strange attractor (Varon et al. in Phys Rev Fluids 2:034604, 2017). Using tangential continuous or pulsed blowing in three different regions along the upper edge of the model rear part in an open-loop control, the dynamics of the bimodal wake can then be forced into one of the stable asymmetric modes or into the unstable symmetric state. Finally, a simple closed-loop adaptive control based on real-time identification of the wake barycenter in the PIV fields is used to force the 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 multi-modal behavior. We show that this new mode also exhibits a peculiar dynamics with up-down oscillations. 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, especially when the natural shedding frequency is chosen as the jet frequency.