Unsteady Fluid Force Acting on Two Tandem Circular Cylinders Subjected to Two-Phase Cross-Flow

Abstract

To understand the excitation and suppression effects of air bubbles on the vibration of two tandem cylinders, we experimentally investigate the characteristics of unsteady fluid force in the lift direction acting on the downstream cylinder having a pitch-to-diameter ratio of 3.0 and subjected to two-phase bubble cross-flow. The average diameter of bubbles is about 3 to 4 mm. Experiments to measure the unsteady fluid force are performed, changing the void fraction at three different reduced velocities (2.15, 4.88, and 7.05) for oscillation amplitudes ranging from 5.0 to 15.0 mm. The downstream cylinder is forced to oscillate sinusoidally at the natural frequency of the upstream cylinder in water. Experiments reveal the following major characteristics of fluid force. (1) At low reduced velocity, almost no difference is observed in the characteristics with respect to void fraction and oscillation amplitude ; (2) air bubbles reduce the fluid force in the range of the void fraction a≤5% and at a smalloscillation amplitude for the lock-in state reduced velocity ; and (3) at high reduced velocity, unsteady fluid force increases with void fraction.