Experimental investigation into the effect of diameter ratio on the flow-induced vibration of tandem flexible cylinders

Flow-induced vibration of tandem unequal-diameter flexible cylinders has, to date, received scant attention, particularly the effect of the diameter ratio on the cylinder's flow-induced vibration is understudied. An experiment is therefore conducted to investigate the flow-induced vibration of two tandem flexible cylinders with different diameter ratios. While the diameter of the upstream cylinder, d, is fixed at 16 mm, the diameter of the downstream cylinder, D, evenly varies from d to 2d. The center-to-center distance is kept constant at 6d. The total cylinder length is 2.7 m, of which 1.2 m is submerged in water. Cylinders can oscillate freely in the cross-flow and in-line directions. Based on the strains measured by the Fiber Bragg Grating sensors, extensive and detailed analyses are performed to investigate the influence of the diameter ratio (d/D) on the cylinder's flow-induced vibration. Decreasing the diameter ratio weakens the high-frequency, high-mode response of the upstream cylinder, especially in the in-line direction. The typical wake-induced vibration (WIV) response is more likely to occur for the downstream cylinder in small-diameter-ratio cases. As the diameter ratio decreases, the downstream cylinder sequentially develops the weak WIV response, the atypical WIV response, and the typical WIV response.