Coherent Structures in the Wake of an Actively-Controlled Cylinder*

The wake behind a cylinder with circumferential, moving-wall actuators was modified by means of a reinforcement-learning, active-control scheme designed to reduce the net drag force. Experimental measurements of the cylinder system were carried out in a high speed water tunnel facility using particle image velocimetry in the actuation and wake regions in order to identify specific coherent structures associated with the drag modification. The spatial and temporal behavior of these coherent structures was studied using correlation analysis, with an emphasis on the unsteady scale interactions between large- and small-scale motions in the wake, and the inner/outer interactions between the wake and the near-wall region of the cylinder. Phase-locked measurements were used to synchronize the actuation inputs, time-resolved forced measurements, and resulting coherent motions in order to characterize the non-linear dynamics of the system and to identify the most salient coherent motions associated with the drag modification.