Feedback control of a circular cylinder wake

Feedback flow control of a three-dimensional wake behind a circular cylinder at a Reynolds number of Re = 100 was investigated. A combination of numerical simulations, experiments, and control theory was used to understand the flow field and to develop a sensor configuration, a flow state estimator, and a controller. The flow field was analysed by using proper orthogonal decomposition (POD) in two dimensions, and two-dimensional and three-dimensional sensor placements were investigated. The controller input was computed from the POD time coefficients, and actuation was performed by using rigid cylinder motion normal to the free stream. In the two-dimensional computations, feedback was shown to effectively reduce the drag and the fluctuating lift force. When feedback forcing was applied in the three-dimensional wake, both simulations and experiments showed that the vortex shedding could be controlled initially and the amplitude of the fluctuations decreased. However, spanwise phase variations eventually appeared and rendered the controller ineffective.