Experimental and numerical analysis of a pivoted cylinder subjected to vortex-induced vibrations

This thesis presents and discusses the results of two investigations. The first involves studying wake dynamics along the span of a pivoted cylinder undergoing vortex-induced vibrations (VIV) and its relationship with the structural response. This experiment used planar Particle Image Velocimetry and image-based tracking techniques to measure the wake and cylinder response at different flow velocities. The second investigation assesses the accuracy of a two-dimensional representation of the first experimental study using RANS models based on the k-w turbulence model. A prior experiment of a bottom-fixed cylinder undergoing VIV was performed to analyse its variable amplitude condition in preparation for the pivoted cylinder case.

The pivoted cylinder results showed maximum amplitudes of approximately half and two times its diameter along and perpendicular to the flow direction, respectively. Similar to the bottom-fixed cylinder, the maximum response was achieved when the cylinder motion and vortex shedding frequencies were equal (i.e., synchronised) to the natural frequency of the structure in water, and when this equivalence was preserved along the cylinder span. At higher flow velocities, a desynchronised region appeared at the water surface, different from the previously observed bottom-up desynchronisation of the bottom-fixed cylinder. Wake measurements closer to the water surface had a broader wake width, higher momentum transference, and higher vortex strength compared to lower water depths. The local response along the cylinder span could not fully explain these differences.

The numerical model reached amplitudes that were 40% lower than the experimental results. Moreover, bistable responses were observed in the upper branch and not in the experimental results. The tested numerical model was insufficient to account for the three-dimensional component of the pivoted cylinder. Additional research is needed to understand the cylinder-wake dynamics of the pivoted cylinder, especially related to its synchronisation-desynchronisation process, to improve the prediction capabilities of two-dimensional numerical models.