EXPERIMENTAL VARIABLE GAIN FEEDBACK CONTROL OF A CIRCULAR CYLINDER WAKE

The effect of feedback flow control on the wake of a circular cylinder at a Reynolds number of 100 is investigated in a water tunnel experiment. Our control approach uses a low dimensional model based on proper orthogonal decomposition (POD). The mode amplitudes are estimated in real time using Linear Stochastic Estimation (LSE) and an array of 35 sensors distributed in a streamwise plane in the near wake. The controller applies linear proportional and differential (PD) feedback to the estimate of the first POD mode. In the context of this experiment, actuation is implemented as displacement of the cylinder normal to the flow. We find the Karman Vortex Street to be either weakened or strengthened depending on the phase shift applied by the PD controller. For all cases with a strengthening in vortex shedding, the flow becomes two-dimensional and phase locked across the entire span of the model. For all cases with a reduction in vortex shedding strength, a strong spanwise phase variation develops which ultimately leads to a loss of control even at the sensor plane location. This suggests that for reduction of vortex shedding a threedimensional sensing and / or actuation approach is needed.