Dynamic Response and Active Control of Flexible Cylindrical Structures Undergoing Vortex-Induced Vibrations
This dissertation primarily focuses on understanding the canonical problem of vortex-induced vibration (VIV), a self-excited vibration of bluff bodies caused by the instability of the bluff-body wake. In this work, dynamic response and active control of low mode number flexible cylinders undergoing vortex-induced vibrations are addressed which has potential to significantly impact the development of predictive models for flow-induced-vibrations, a topic of critical importance to the offshore oil and gas industry and important to the cost-effective development of new ocean structures, such as floating offshore wind platforms and offshore wind energy systems.
In the tests, first, dynamic response of a tensioned flexible cylinder is investigated in a recirculating flow channel. Different than hysteresis in the amplitude response, the idea of mode hysteresis is introduced. A transition in the amplitude of the response of a flexible cylinder undergoing vortex-induced vibrations is shown to be related to the transition between fluid coupled structural modes excited by the flow. Also, due to the symmetric drag loading across cylinder's span, the hypothesis of being unable to sustain a asymmetric excitation (even mode excitation) in in-line is discussed. To understand more about this fluid-structure interactions, tensioned flexible cylinder data is used for multivariate analysis. It is shown that traditional reduced order models such as proper orthogonal decomposition and recently introduced smooth orthogonal decomposition methods help to identify nonlinear mode interactions in the flexible cylinder's response. Later, mode shape effect in VIV and the idea of being unable to sustain asymmetric modes in a uniform flow is considered. In the tests, three bending-dominated cylinders are tested with varying stiffness in the cross-flow and in-line directions of the cylinder in order to produce varying structural mode shapes associated with a fixed 2:1 (in-line:cross-flow) natural frequency ratio. Then, the structural mode excitation of bending-dominated flexible cylinders undergoing vortex-induced vibrations is investigated using multivariate analysis of excited empirical modes. Both the analytic and experimental results show that for excitation of low mode numbers, the cylinder is unlikely to oscillate with an even mode shape in the in-line direction due to symmetric drag loading, even when the system is tuned to have an even mode at the expected frequency of vortex shedding. Later, to understand the effect of three-dimensional wake on an oscillating flexible cylinder in VIV, a novel experimental method is introduced. Finally, the idea of active control of flexible cylinders in VIV using piezo stripe actuators is discussed. Piezo stripe actuators are bonded at the anti-nodes of a flexible cylinder in the in-line direction to control low vibration modes (i.e. first, second and third). Experiments show that upto 75% of amplitude reduction is possible in cross-flow where large vibrations occur. In addition to vibration suppression, vibration enhancement is also possible if piezos are activated before an apparent amplitude jump.