Experimental investigation of in-line flow-induced vibration of a rotating circular cylinder

This study experimentally investigates the in-line flow-induced vibration (FIV) of an elastically mounted circular cylinder under forced axial rotation in a free stream. The present experiments characterise the structural vibration, fluid forces and wake structure of the fluid-structure system at a low mass ratio (the ratio of the total mass to the displaced fluid mass) over a wide parameter space spanning the reduced velocity range 5 <= U* <= 32 and the rotation rate range 0 <= alpha <= 3.5, where U* = U/(f(nw), D) and alpha = vertical bar Omega vertical bar D/(2U), with U the free-stream velocity, D the cylinder outer diameter, f(nw) the natural frequency of the system in quiescent water and vertical bar Omega vertical bar the angular velocity of the cylinder rotation. The corresponding Reynolds number (defined by Re = UD/v, with v the kinematic viscosity of the fluid) was varied over the interval 1349 <= Re <= 8624, where it is expected that the FIV response is likely to be relatively insensitive to the Reynolds number. The fluid-structure system was modelled using a low-friction air-bearing system in conjunction with a free-surface water-channel facility. Three vibration regions that exhibited vortex-induced vibration (VIV) synchronisation, rotation-induced galloping and desynchronised responses were observed. In both the VIV synchronisation and rotation-induced galloping regions, significant cylinder vibration was found to be correlated with wake-body synchronisation within the rotation rate range 2.20 less than or similar to alpha less than or similar to 3.15. Of significant interest, the frequency of the streamwise fluid force could be modulated by the imposed rotation to match that of the transverse lift force, resulting in harmonic synchronisation. Measurements using the particle image velocimetry (PIV) technique were performed to identify the wake structure. Interestingly, the imposed rotation can cause regular vortex shedding in in-line FIV at rotation rates that see suppression of the Benard-von-Kaman vortex shedding in the case of a rigidly mounted cylinder (alpha greater than or similar to 1.75). There is a monotonic increase in the drag coefficient with rotation rate beyond alpha = 2 for a non-oscillating rotating cylinder. This suggests that the mechanism for sustaining the large rotation-induced galloping oscillations at higher alpha is due to a combination of aerodynamic forcing from the locked induced vortex shedding associated with the oscillations, assisted by aerodynamic forcing, evaluated using quasi-steady theory.