Unsteady galloping of a rectangular cylinder in turbulent flow

Slender structures with a bluff non-axisymmetric cross-section are prone to both vortex-induced vibration (VIV) and transverse galloping. When the mass-damping parameter of the system is low, the two phenomena can interfere giving rise to a peculiar type of instability, for which the quasi-steady theory does not apply, and therefore which one may call "unsteady galloping". Since for large structures (such as high-rise towers or bridge pylons) this phenomenon seems to be potentially an issue, rather than the quasi-steady galloping, it is particularly important to verify whether the former also occurs in realistic turbulent wind flows, and to understand its specific features. With this aim in mind, static and dynamic wind tunnel tests have been carried out on a two-dimensional rectangular cylinder with a side ratio of 1.5 (having the short side perpendicular to the flow) immersed in various grid-induced homogeneous isotropic turbulent flows. From a quasi-steady perspective, the static tests showed the proneness to galloping instability of the considered cross-section even in highly turbulent flow, though strongly dependent on the integral length scale of turbulence. In addition, they revealed an attenuation of the strength of vortex shedding, but suggested an increased tendency of VIV and galloping to interfere as compared to the smooth-flow case. The dynamic tests confirmed this tendency and highlighted a complicated behavior of the model in turbulent flow, with some features that still remain unexplained. In particular, even larger values of the mass-damping parameter of the system are necessary for the quasi-steady theory to be able to predict correctly the galloping instability threshold. Another important result is that the integral scale of turbulence was found to play a key role also in the unsteady galloping behavior of the considered rectangular cylinder.