Aerodynamic phenomena and wind-induced effects in the design of ultra-high towers

The structural behaviour of thin and ultra-high shells - like Solar Updraft Towers - can be optimized by stiffening rings applied along the height, which guarantee a predominant beam-like behaviour. However, too dense or too big stiffening rings may become a double-edged sword, because they modify the aerodynamic of the flow past the circular cylinder and may induce an even more severe load condition. The paper reveals an experimental and numerical evidenced stochastic property of wind-induced pressures, which has been detected by analyzing slender circular cylinders with a free-end in turbulent atmospheric boundary layer flow. Cross-checked wind tunnel tests were performed at WiSt wind tunnel at Ruhr-University Bochum and at CRIACIV wind tunnel at University of Florence. Experimental results are also confirmed by numerical simulations, performed at the Industrial Energy Department, University of Florence. The investigation revealed the existence of a bi-stable and not symmetric flow condition, which is induced by the presence of rings along the height of the tower and does not disappear even at moderately high Reynolds numbers. The effect on the structural response is quantified and mitigation strategies are proposed in the paper.