Experimental and Numerical Investigations of Vortex-Induced Vibration for a Single Cylinder

Vortex-induced vibration (VIV) of cylindrical structures in fluid flow is a well-explored topic that is relevant to many fields of engineering. In particular, it is a classical topic within fluid-structure interaction (FSI). In offshore engineering, VIV often causes fatigue in slender structures, such as risers, mooring lines, and pipelines. Recently, VIV has been a key issue in wind turbine tower design. Turbine towers can be subject to VIV when exposed to stream flow because the shedding of vortices downstream of the structure induces forces on the structure that may cause vibrations. A detailed understanding of this FSI phenomenon and efficient estimation of such self-excited and self-sustained oscillations are required for the reliable prediction of the fatigue damage and the development of VIV suppression techniques. The study of this problem is hampered by a lack of high-quality measurements and a lack of reliable models to predict the response and the fluid loading on towers undergoing VIV.


 In the present study, experimental and numerical investigations on a cantilever cylinder with free end boundary conditions have been studied. Methods of eliminating or reducing vortex-induced oscillations are by varying structural or aero/hydro-dynamics behavior.  Structural methods, such as those that involve altering damping, mass, or natural frequency, however, aerodynamic/hydrodynamic techniques, such as those that change the flow pattern using alternative methods.

A series of experiments were performed in an enclosed towing tank to investigate the response amplitudes, hydrodynamic forces, lock-in region, Strouhal number, and frequency response while varying the locations of different newly developed angular-position fin plates. The experiments were conducted in a uniform current flow with cylinder models below the critical mass value and allowed to oscillate in two degrees of freedom in the inline and transverse directions. The results demonstrated the disappearance of the synchronization features when the models approached their natural frequencies, which led to a significant reduction in the response amplitudes. Interestingly, strong suppression of over 90% and 75% was observed for the inline response and transverse response, respectively. This trend is also apparent for the lift and drag forces when compared with the measuring data of the smooth cylinder, demonstrating a potential solution for these mitigation tools.

A further experimental investigation was performed in a wind tunnel to examine VIV behavior in an attempt to understand the free end effect with a high Reynolds number and a rigid installation without adding storing elements. Wind tunnel experiments were conducted at Reynolds numbers ranging from 3.6 × 104 to 3.26 × 105. The current study produced several important results. The study showed that the response amplitude with vortex shedding was finite, even at the synchronization point. It increased with flow velocity and affected synchronization behavior. The lock-in region was reduced to a certain point. Results suggested that nonlinearities occurred when vortices were shed due to the influence of the free end conditions and the higher Reynolds numbers. The present study focused on aspects that have not been fully addressed by previous studies, such as end-cell induced vibration. The end-cell-induced vibration that occurred at a high wind speed was not influenced by the high damping ratio of the model, while the amplitude response of the ordinary VIV decreased significantly.

Generally, the use of computational fluid dynamics (CFD) has added value in supporting decisions surrounding the risk management of operating assets. Regarding the numerical investigations of the VIV of a cantilever cylinder, the new models for suppressing VIV were examined. The effect of these tools was tested numerically using CFD simulation, which is an attractive and cost-effective alternative to model tests. Numerical simulations were carried out for a low mass ratio cylinder model subjected to uniform flow.

The present numerical model was first verified for different grid resolutions and validated by comparison with published experimental data, and the results indicate that the outcome was satisfactory. The supercritical to upper transition flow regime around a 3D smooth circular cylinder at a Reynolds number range of  4,000 to 14,000 was examined numerically using standard 3D detached-eddy simulation approaches. The objective of the present study was to evaluate whether the models are applicable for further experiments and engineering design within these flow regimes. The simulation results also demonstrated the feasibility of these devices and encourage further experimental studies.

Over the past decades, VIV has been extensively studied, and the majority of literature consists of experiments or semi-empirical modeling. In contrast, FSI simulations, by combining CFD and computational structural dynamics solvers, have received less attention. One of the objectives of this thesis was to investigate the VIV of elastically mounted rigid cylinders and flexible cylinders using fully 3D FSI simulations. The results from 3D simulations were close to previous experimental results. The solid stress tool coupled with a morphed mesh was utilized within StarCCM+ to test their ability to simulate such cases. Finally, further experimental investigations and CFD simulations regarding the effect of group arrangements of the towers and the effectiveness of suppression measures are proposed for future research


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