Flow-induced vibrations of twin cylinders in the post-critical regime

Cylinder-like structures can be found in various engineering applications. Therefore, fluid flows around a circular cylinder have been extensively studied in the past and correspond to canonical problems in fluid mechanics. In many cases, the structure is in a multiple-body form, which leads to flow interference between the cylinders. Fluid-structure interactions may result from flow interference because of the high fluctuating forces. Hence, a complete understanding of the flow is needed to avoid any critical situation for structural integrity, e.g., failure. Because of the continuous increase in size of the civil engineering structure, the flow around the structure often lies in the post-critical regime. The boundary layers developing around the bodies are fully turbulent before separation in this specific regime.
This thesis investigates the flow around twin cylinders at low incidences and their potential flow-induced vibrations in the post-critical regime. In that sense, extensive experimental studies are performed in the wind tunnel of the University of Liège.
The first part of the work consists in triggering the post-critical flow regime at lower Reynolds numbers because of the limitations in size and flow velocity inside the wind tunnel. It is performed by applying appropriate surface roughness on the cylinders, which promotes the transition from the laminar to turbulent state of flow.
The second part analyses in detail the flow around static twin rough cylinders. The main outcome of this analysis is the sensitivity and complexity of the flow dynamics around the two cylinders. Hence, the aerodynamic quantities are highly dependent on different parameters such as the Reynolds number, the spacing ratio and the flow incidence.
An experimental aeroelastic test campaign is then carried out using springs and elastomers to allow one degree of freedom to each cylinder. The free responses are measured and analysed for different configurations.
Finally, the possibility of mathematically modelling the flow-induced vibrations is assessed. The investigation demonstrates that the classical approaches fail to reproduce the large vibrations observed experimentally and, hence, supports the need for more research on this subject.