PHASE-LAG-INDUCED FLUCTUATING LIFT FORCES ON TWO TANDEM BLUFF BODIES

摘要

A numerical simulation at a Reynolds number Re = 200 is conducted to find how flow-induced forces on two tandem circular cylinders is connected to the phase lag between vortex sheddings from the cylinders. The spacing ratio L~* (= L/D) is varied from 2 to 9, where L is the cylinder center-to-center spacing and D is the cylinder diameter. Here we mainly focus on fluctuating lift coefficient C_(Lf) of the upstream cylinder, vortex dynamics in the gap between cylinders, and phase lag Φ between the vortex sheddings from the two cylinders for L~* larger than the critical where the co-shedding flow prevails. Φ is indeed nonlinear function of L~*, Strouhal number (St) and convection velocity of vortices in the gap between the cylinders. We unearth that the upstream cylinder C_(Lf) is affected by both L~* and Φ. While the contribution of L~* to C_(Lf) diminishes rapidly with L~*, that of Φ makes the L~*-dependent C_(Lf) variation damped-sinusoidal, persisting in the L~* range examined. The inphase and antiphase flows respectively correspond to a local maximum and minimum C_(Lf). How C_(Lf) is correlated with L~* and Φ can be deduced as, C_(Lf) = Ae~(-αL~*) + Be~(-βL~*) sin(Φ + π/2) + C, where A, α, B, β and C are constants. The physics behind the damped-sinusoidal variation in C_(Lf) is discussed.