Further results on the flow-induced streamwise vibration of cylinders

Abstract

Flow-induced streamwise vibrations of elastically mounted rectangular cylinders and cut-out square cylinders are investigated in the reduced velocity range 1 < U ND′ < 13 (where D′ is the dimension of the model perpendicular to the flow) at angles of incidence, α, in the range from 0 to 90°. The Scruton number, Sc, of the models used for the investigation ranges from 0·68 to 714·4 and the structural damping is δ s ≈ 0·0017. Here, Sc = 2Mδ s (ϱD 2L) , where M is the equivalent mass of the vibrating system, ϱ is the fluid density, and D and L are cylinder diameter and length respectively. Sustained streamwise oscillation can occur at about one or two times the natural vortex-shedding frequency. When a line of symmetry of the cylinder cross-section is parallel to the approach flow and the separated shear layers do not reattach to the cylinder, sustained streamwise oscillation occurs at about twice the natural vortex-shedding frequency but only when there is an interaction between the shear layers and the afterbody. When the afterbody is small and there is little or no possibility of shear layer/trailing corner interaction, sustained streamwise oscillation can still occur, but over a very limited range of reduced velocity. It is suggested that these resonant vibrations are sustained either by vortex shedding on its own or by vortex shedding in combination with wake breathing. Beside the response to turbulence buffeting, if the flow around a rectangular cylinder is bi-stable, there can be an additional nonresonant vibration whose source is thought to be the highly unsteady wake and the change in the cylinder drag force accompanying the change in the flow pattern.