Experimental and numerical study of nonlinear galloping oscillations interfering with vortex-induced excitation

This paper deals with the fascinating phenomenon of interference of two nonlinear fluid-elastic phenomena, viz. vortex-induced vibration (VIV) and transverse galloping. The resulting phenomenon, which one may call unsteady galloping, occurs in the case of low-damped rectangular cylinders with a side ratio comprised between about 0.75 and 3. The results of a wide experimental campaign carried out in the wind tunnel on a rectangular 3:2 cylinder (having the short side perpendicular to the flow) are discussed here, with a special focus on the role played by the mass-damping parameter of the oscillating body. Moreover, the problem is analytically addressed through an existing nonlinear wake-oscillator model, which has been modified in the present work, especially concerning the setting of a key parameter, obtaining previously unexplored solutions of the equations. The model was found to reproduce correctly several features of the complicated phenomenon, shedding some light on the mechanism of unsteady galloping. Finally, the nonlinear behavior of the wake has also been discussed.