Wake Galloping Suppression in Two Closely Spaced Cylinders with Surface Modification

Wake galloping is often observed in two closely-spaced circular cylinders. Previous studies have reported large amplitude transverse oscillation (galloping) in the downstream cylinder, where streamwise and transverse center-to-center distances between two cylinders were 3.0D–3.5D and 0.3D–0.5D, respectively (D is the diameter of the cylinders). In this study, the mechanism and countermeasure of wake galloping of two cylinders in these arrangements are discussed. Due to the dynamics between the wake of the upstream cylinder and the oscillation of the downstream cylinder, the unsteady wake was hysteretic and initiated wake galloping in the downstream cylinder. Wake galloping, when occurred, depends on reduced velocity and not Reynolds number because both cylinders were in the subcritical Reynolds number regime at the wake galloping onset. A cylinder attached with twelve spiral protuberances at a 27° winding angle was introduced to suppress wake galloping. The modified cylinder with the spiral protuberances performed most effectively against wake galloping when it was placed on the downstream side. The aerodynamic forces on the cylinder with spiral protuberances did not generate negative damping. Therefore, the surface-modified cylinders successfully suppressed wake galloping.