The effect of large-scale turbulence on the interference of vortex shedding and galloping: A numerical study

Many bluff bodies, when exposed to an airflow, are prone to both vortex-induced vibration (VIV) and galloping. For low enough values of the mass and damping of the structural system, the two excitation mechanisms are supposed to interfere, promoting a combined instability, which one may call unsteady galloping, with peculiar features and possibly large vibration amplitudes in flow speed ranges where no excitation is predicted by classical theories. A slightly modified version of an existing nonlinear wake-oscillator model was considered here for the case study of a two-dimensional rectangular cylinder with a side ratio of 1.5, having the shorter section side perpendicular to the mean wind flow. This model simply relies on the linear superposition of the unsteady wake force producing VIV excitation and the quasi-steady force that is responsible for galloping. In a realistic wind flow, incoming turbulence not only modifies the aerodynamic properties of the invested bluff body interfering with the shear layer properties, but also acts as parametric and external random excitation. The latter effects were considered in the present work digitally synthesizing two-dimensional homogeneous isotropic turbulent flow velocity fluctuations and including them in the model equations, which were then numerically solved. Two values of the mass-damping parameter (Scruton number) and two levels of turbulence were considered. The first results show that, accounting for parametric and external excitations, the considered nonlinear wake-oscillator model can qualitatively explain the reduction of the root-mean-square of the vibrations due to incoming turbulence, but it is not able to predict the delay of the instability onset flow speed observed during the wind-tunnel experiments.