Experimental wind tunnel tests and numerical analysis of the aerodynamic behaviour of a high tapered obelisk

Abstract eng:
An 85 metres high tapered obelisk, made of reinforced concrete for the first 9 metres and made of steel with welded plates for the upper 76 metres is the structure studied in this paper. Its bluff cross section consists of two closely spaced triangular sections that are scaled linearly with the height from ground. Having a complex bluff section and being a light structure, aerodynamic instabilities and vortex induced vibrations were investigated using a mixed experimental-numerical approach, in order to define wind loads for the structural design and possible interventions aimed at mitigating the dynamic effects due to wind action. Exploiting the regular tapering of the obelisk section, a rigid sectional model with constant section was initially tested in wind tunnel: static aerodynamic coefficients and vortex-induced response as a function of the Scruton number were measured for several angles of attack. For static force coefficients both external balances and pressure taps around a section were used, while vibrations were measured by means of accelerometers; during vortex induced vibration tests also time varying pressure distribution were measured. Due to its complex cross-section, for some angles of attack two lock-in ranges were identified and studied. Using the power balance principle, the experimental sectional results were successively used to assess numerically the response of the structure considering its tapering, the gradient of wind speed, the modal shapes, and the correlation of wind actions. Even if the tapering drastically reduced aerodynamic issues, the results achieved underlined the pressing need to increase the damping of the structure in order to avoid critical speed of galloping and vortex shedding among the design speeds, and tuned mass dampers were designed.

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