Dynamics and wake structure of a near-wall flexible cylinder

Abstract

Interactions between the structural dynamics and wake of a vibrating flexible cylinder close to a wall are investigated using three-dimensional direct numerical simulation at reduced velocity Ur = 5-25, gap-to-diameter ratio G/D = 0.8, aspect ratio L/D = 50, and Reynolds number Re = 500. The 1st to 4th cross-flow (CF) vibration modes and the 1st to 9th in-line (IL) vibration modes are excited, and four types of modal groups for the ratio of IL to CF modes are identified, including groups ‘n:n’, ‘2n-1:n’, ‘2n:n’, and ‘2n+1:n’, where n refers to the excited mode in the CF direction. With the increase of Ur, the vibration responses are first dominated by one specific mode and then co-dominated by multi-modes with comparable intensities. Within the same modal group, the maximum CF and IL amplitudes and mean drag/lift coefficients decrease with Ur under smaller Ur conditions, whereas they increase with Ur under larger Ur conditions. The cylinder oscillation exhibits a standing-wave behavior and a mixture of standing and traveling behavior at the lower and higher Ur cases, respectively. Compared to the isolated case, the reduction of the IL vibration frequency by half and the enhanced IL amplitude are caused by the vortex shedding suppression from the lower side of the cylinder and by the corresponding IL lock-in state along the span. The vortex sheddings at the antinode and node of the CF displacement are in ‘2S’ and weak ‘2S ’ modes, with the energy input and output from the wake, respectively; whereas, for the isolated case, they are in ‘2P’ and ‘2S’ modes, respectively.