Wake-induced vibration of tandem cylinders of different diameters

The wake-induced vibration (WIV) of the downstream cylinder of a tandem pair is investigated for different diameter ratios of D1/D2=1/1, 1/2 and 1/3, where D1 and D2 refer to the upstream and downstream cylinders, respectively. The streamwise separation between the cylinders was L/D1=3.5, 7.0 and 6.5, respectively, measured from the centre of the upstream cylinder to the forward stagnation point of the downstream cylinder. Experiments with low mass-damping cylinders have been conducted in a water channel at around Re=25 000. The dynamic response showed that the downstream cylinder experienced WIV for all diameter ratios investigated, with displacement amplitudes reaching more than 1.5 diameters for higher reduced velocities beyond the vortex resonance range. The frequency response showed a similar behaviour for all three configurations, giving hints that a type of wake-stiffness mechanism might be governing the frequency of oscillation for all diameter ratios. The response was found to be dependent on both D1/D2 and L/D1. In all cases, the static upstream cylinder was found to shed vortices as an isolated cylinder, not influenced by the presence or movement of the downstream body. Lift and drag coefficients as well as measurements of velocity fluctuations in both wakes are presented for all cases.