| Popis: |
Above a critical value of the Reynolds number, flows past bluff bodies are characterized by self-sustained oscillations, due to the formation of the Karman vortex street. This results in the dynamic loading of the body, which, in practical applications, may lead to structural fatigue. In recent work, it was demonstrated that, in flow past a cylinder, implementing surface hydrophobicity results in decreased drag and lift forces, leading to vortex street cancellation at a critical level of control effort. In order to achieve vortex street suppression at lower levels of control effort, utilizing materials with controllable wettability properties appears as a promising possibility. Thus, in the present work, the effectiveness of a proportional feedback control scheme, utilizing the application of time-dependent slip conditions on the cylinder surface, is investigated by means of CFD simulations. Firstly, the feedback control scheme is tested for a problem definition characterized by the application of slip on the entire cylinder surface, for a wide range of the control scheme parameters; it is demonstrated that flow unsteadiness is reduced at substantially lower levels of control effort, in comparison to corresponding passive control schemes. Secondly, in the frame of feedback control, an optimization problem is formulated and solved, aiming at the partial or full suppression of the Karman vortex street at a minimal control effort, using time-dependent slip conditions on a part of the cylinder surface. The results demonstrate a further reduction in control effort. Overall, in comparison to passive flow control, a reduction in control effort by two orders of magnitude is attained with the present optimized feedback control scheme. |
