Transient phenomena induced by thunderstorm outflows on slender structures

The climatology at mid-latitudes (for instance, Europe) is dominated by both extra-tropical depressions at the synoptic scale and by mesoscale thunderstorm outflows (also called downbursts). Thunderstorm outflows are non-stationary phenomena, complex and potentially devastating, which strongly differ from synoptic winds under many points of view (genesis, scale, duration above all). Consequently, the induced wind fields are highly different. Modern codes and guidelines are mainly based on the cyclonic model, because of the persistent lack of knowledge about thunderstorm outflows, in particular concerning full-scale measurements. On the other hand, severe wind damage is often induced by downbursts, especially concerning low- and medium- rise structures (e.g., cranes, small turbines, light poles, low-canopies). The present PhD Thesis is collocated within the framework of the ERC THUNDERR Project. It investigates aspects connected with the aerodynamic loading of structures subjected to thunderstorm outflows, particularly focusing on the transient aerodynamics and transient aeroelasticity. This is firstly pursued through the definition of analytical formulations which, starting from compatible vertical wind fields, permit to evaluate the aerodynamic wind loading by using the strip and quasi-steady theory. The application of the procedures on selected slender test structures shows that a crucial role is played by thunderstorm-induced variations of the wind angle of attack, which may increase or reduce the structure response. The second part of the Thesis is devoted to an extensive experimental campaign carried out at the multiple-fan wind tunnel of the Tamkang University, Taipei, which is able to simulate unsteady flows. The sectional model of a sharp-edged square cylinder, equipped with 94 pressure taps, is investigated and numerous configurations of the flow parameters are considered in order to study the effects of acceleration on the aerodynamic loads and on the vortex-shedding from the body. The drag coefficients and the fluctuating cross-flow force coefficients connected with vortex shedding are found to be either comparable or definitely lower than their corresponding values for steady flows. Furthermore, discontinuities of the shedding frequency are present during the transients and their number and magnitude appear to be connected with the acceleration of the flow.