This paper deals experimentally with the comparison of air bubble effects on the vibration of two circular cylinders installed in tandem at pitch-to-diameter ratios of 1.5 and 3.0. Experiments were done for both the pitch-to-diameter ratios changing the void fraction from 0 to about 20 percent for the reduced velocity ranging from 0.5 to 8; vibrational accelerations were measured in both the lift and drag directions for each cylinder and the results were compared with for each other between the pitch-to-diameter ratios of 1.5 and 3. The comparison revealed that 1) the vibration of the tandem two-cylinder system having a pitch-to-diameter ratio of 1.5 or 3 was excited by air bubbles in the flow for small reduced velocity, but the effect of pitch-to-diameter ratio was found in the response characteristics of the upstream cylinder’s vibration in the lift direction; 2) a small amount of air bubbles equivalent to about 4 percent of the void fraction suppressed the strong, out-of-phase vibration in the downstream cylinder of a large P/D of 3 when the reduced velocity was about 7, but the cylinder arrangement with a small pitch-to-diameter ratio of 1.5 showed a weaker vibration reduction due to adding air bubbles in the flow at high flow velocity; and 3) the vibration suppression was likely due to the entrainment of air bubbles in the wake region behind the upstream cylinder.

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