The effects of symmetric strips on the flow-induced oscillation (FIO) of a cylinder in a uniform flow are investigated numerically. A pair of symmetric smooth strips are attached parallelly to a cylinder at different angles a from the frontal stagnation point. One degree of freedom cylinder movement is only allowed in the direction perpendicular to the incoming flow. The vibration is suppressed for α=30°, and galloping is observed for α=60°. Regarding wake dynamics, the vortex shedding in the galloping range is significantly different from the traditional vortex–induced vibration (VIV) vortex shedding mode, and the number of vortices released per cycle is much greater (at most twelve).
Numerous studies have been dedicated to understanding Flow–induced oscillation (FIO), as a typical problem of fluid–structure interaction (FIS), because it is crucial in a wide range of engineering applications high–rise buildings, cables and marine structures. Vortex–induced vibrations (VIV) and galloping, observed in crossflow are the two most common phenomena in FIO. Leonardo da Vinci first discovered the existence of VIV in the 16th century, and Strouhal mathematically modelled it in 1878. The VIV excitation is caused by the alternating shedding of eddy current from each side of the cylinder. If the frequency of the structural vibration is the same as the vortex frequency, it will cause resonance, referred to as synchronization range (lock–in), and the structure may be exhausted or even catastrophic. Beginning with the VIV experiment conducted by Brooke in 1960 (Brooks, 1960), the elastic support cylinder was used as a typical model of FIS for basic research, since it is relatively simple to establish such a model in experiments and numerical simulations. VIV is discussed in the comprehensive reviews (Bearman, 2011; Gabbai and Benaroya, 2005; Williamson and Govardhan, 2004, 2008). As reduced velocity increase, initial, upper and lower branches have been observed in the oscillation amplitude response for VIV of a smooth cylinder at low values of the mass–damping (Khalak and Williamson, 1996a). Compared to the VIV of the cylinder, less research is devoted to understanding another phenomenon of FIO: galloping, which is the dynamic instability caused by the cross–sectional asymmetry of the bluff fluid immersed in the flow. Galloping is a kind of low-amplitude, high–frequency unstable oscillation, and its motion mechanism is not as complex but more destructive than VIV. The vibration amplitude of galloping is not limited by the flow unlike VIV. An isolated smooth circular cylinder could only undergo VIV does not galloping for an elastically mounted rigid cylinder. However, such as triangle, square non–circular sections may cause large–scale galloping of the structure (Parkinson, 1979). And circular sections with like splitter plate attachments could also galloping (Nakamura et al., 1994). Besides, another cylinder's approach also stimulates galloping of the cylinder (Bokaian and Geoola, 1984).