A new aerodynamic phenomenon and its effects on the design of ultra-high cylindrical towers

The dissertation addresses the design of ultra-high towers under the wind action and has a special application for Solar Updraft Power Plants. They are a highly sustainable natural resource for electric power generation, based on a combined sun-wind energy solution. The object of the investigation is a 1-km tall solar tower, made of reinforced concrete and stiffened along the height by stiffening rings. They are usually introduced in the design of solar towers in order to reduce the structural vulnerability to the wind action by enhancing a beam-like behaviour. However, the wind tunnel tests performed within this research showed that the presence of rings along the height of the tower modifies the aerodynamics of the flow around the circular cylinder and creates a bi-stable and asymmetric load condition, even at moderately high Reynolds numbers. This phenomenon is new and unknown. Similar effects were observed in the critical range of Re number and around two side-by-side cylinders, but the conditions of occurrence and the physical reasons are profoundly different. The discovery of the existence of such a bistable and asymmetric load condition induced by rings along the height of a finite length circular cylinder, its interpretation, as well as the cross-checked experimental evidence in different wind-tunnels confirmed also by numerical simulations, are the original contributions of this work. Then, the effect is quantified on the structural response. The bistable asymmetric load on the structure did not result to be a prohibitive load condition for solar towers and the magnitude of the effect depends on the number and on the size of the rings. Mitigation strategies are then proposed in the work. Furthermore, the dissertation evaluates the shell response to the stochastic wind loading process and provides to the designer a general unified simple tool to define design wind loads for quasi-static calculations of ultra-high towers in any atmospheric boundary layer flow.