Low-speed galloping for rectangular cylinders with side ratios larger than unity

The aeroelastic instability starting at low reduced flow speed due to the interference between vortex-induced vibration (VIV) and transverse galloping, which may be called “low-speed galloping”, was studied in the wind tunnel in the case of an elastically-supported slender rectangular cylinder with a side ratio of 1.5 and the short side face perpendicular to the smooth flow. The tests were carried out in a wide Scruton number range, starting from low values and increasing it in small steps through eddy-current viscous dampers. This study helped understanding the dynamics of the interaction between the excitation mechanisms of VIV and galloping and clearly highlighted the transition through several regimes of interference. It was found that a high value of the mass-damping parameter is required to completely decouple the ranges of excitation due to vortex-induced vibration and galloping and for the quasi-steady theory to correctly predict the galloping critical wind speed. This conclusion is also relevant from the engineering point of view, as it means that structures and structural elements with ordinary mass-damping properties can exhibit sustained vibrations where they are not predicted by classical theories of vortex-induced vibration and galloping. The modeling of this low-speed galloping was attempt by slightly modifying the wake oscillator model proposed in 1987 by Tamura and Shimada to predict the interference of VIV and galloping in the case of a squaresection cylinder. It was found that a proper choice of the coupling parameter between the wake and the mechanical oscillators allows reproducing correctly the main features of the cylinder response. Finally, low-speed galloping can be positively exploited for low-power energy harvesting systems, in view of the low onset flow speed and the weak dependency on the Scruton number up to medium values of it.