Spanwise wake development of a bottom-fixed cylinder subjected to vortex-induced vibrations

This study analyses the spanwise wake dynamics and structural response of a bottom-fixed cylinder subjected to a range of open-channel fully developed turbulent flows. The experiments were performed with a Reynolds number ranging between 4.5 x 10 2 and 1 x 10 3. The cylinder free end response and wake dynamics were measured using Particle Image Velocimetry and image-based tracking techniques. The cylinder had a significant modulated response, from which a Proper Orthogonal Decomposition revealed a clockwise elliptical-type trajectory at the maximum cylinder response. Wake dynamic analysis showed that the maximum response is achieved when the cylinder motion and vortex shedding frequencies are equal (i.e. synchronised) to the natural frequency of the structure measured in still water, and when this equivalence is preserved along the span of the cylinder. As the flow velocity increases, a spanwise bottom-up desynchronisation process develops moving towards the water surface. This process decreases the vortex-formation region strength along the span of the cylinder, reducing the maximum structural response and eventually changing the vortex shedding pattern. Despite the highly three-dimensional experimental conditions under significant turbulent incoming flows, the findings of previous studies based on simpler experimental models can still be used to broadly explain the observed desynchronisation process.