UNSTEADY FLUID FORCE ACTING ON 2 TANDEM CIRCULAR-CYLINDERS SUBJECTED TO 2-PHASE CROSS-FLOW

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 acti ng 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 red uced velocities (2.15, 4.88, and 7.05) for oscillation amplitudes rang ing from 5.0 to 15.0 mm. The downstream cylinder is forced to oscillat e sinusoidally at the natural frequency of the upstream cylinder in wa ter. Experiments reveal the following major characteristics of fluid f orce. (1) At low reduced velocity, almost no difference is observed in the characteristics with respect to void fraction and oscillation amp litude; (2) air bubbles reduce the fluid force in the range of the Voi d fraction alpha less than or equal to 5% and at a small oscillation a mplitude for the lock-in state reduced velocity; and (3) at high reduc ed velocity, unsteady fluid force increases with void fraction.