Open- and closed-loop control of flow-structure interaction on square cylinders in a cross flow

Abstract

The thesis deals with active control of fluid-structure interaction. Five interrelated subtopics are addressed. 1). A novel perturbation technique, creating a local perturbation on a cylinder surface using piezo-ceramic actuators, was presented to perturb fluid-structure interaction on the cylinder and subsequently to simultaneously control both vortex shedding and its induced vibration. 2). The open-loop control of resonant fluid-structure interaction was experimentally studied when the vortex shedding frequency fs synchronized with the natural frequency, f'n, of the dynamic system. A square cylinder, flexibly supported at both ends, was allowed to vibrate only in the lift direction. Three actuators were embedded underneath one side, parallel to the flow, of the cylinder. They were simultaneously activated by a sinusoidal wave, thus causing the cylinder surface to oscillate. As the normalized perturbation frequency f*p lied outside the possible synchronization range (f*p= 0.11~ 0.26), structural vibration Y, vortex circulation (T) and mean drag coefficient (CD) were reduce effectively, especially at f*p of 0.1 while as f*p fell within the synchronization range; the three quantities were enhanced, especially at f*p of 0.13. 3). The same technique was extended to include closed-loop control. The control action of the actuators was controlled by a Proportional-Integral-Derivative (PID) controller. Three control schemes were investigated using different feedback signals, including flow velocity u, Y and a combination of both. It was observed that the control scheme based on the feedback of the coupled Y and u led to the almost complete destruction of the Karman vortex street and a significant reduction in Y, T and CD, outperforming by far an open-loop control as well as the other two closed-loop schemes. The alteration in the phase shift between Y and u (撜u) from in phased to anti-phased, the impaired spectral coherence (Coh Yu) and effective damping might be responsible for this. 4). Vortex shedding from a fix-supported rigid cylinder was manipulated by the perturbation technique, based on a feedback flow signal via a PID controller. It was observed that T, u, lift and drag coefficients and CD may be effectively decreased or increased, corresponding to anti-phased or in-phased perturbation and flow, respectively. Similar effects were obtained as the Reynolds number varied. The relationship between the perturbation and force acting on fluid was also examined. 5). Experiments on the closed-loop control of the vortex-induced flexible cylinder vibration was finally conducted under resonance and non-resonance case. Five control schemes were investigated based on the feedback from either individual or combined responses of Y and u. For resonance case, fs coincided with the first-mode natural frequency of the fluid-structure system. The best performance was achieved using the feedback signal from a combination of u and Y. Vortex shedding was almost completely destroyed, resulting in a great reduction in T, Y and CD. Non-resonant cases were also briefly discussed, leading to similar observations in terms of control effects.