Dynamic Response of an Inverted flag Subjected to a Cylinder Wake

The behavior of an inverted flag immersed in the wake of a circular cylinder is investigated to determine how the oncoming shedding vortices affect the vibration response of the downstream flag. The flag motion is observed by water channel experiments. Numerical simulations are also performed by using a two-dimensional incompressible smoothed particle hydrodynamics (SPH) method and fluid-structure interaction model to examine the vortex-induced vibration mechanism. A flag subjected to the wake exhibits three states, straight, flapping and deflected modes, as well as that in a uniform flow. However, the mutual interaction of the flag with the cylinder strongly depends on its distance and bending rigidity. When the flag is placed just behind the cylinder, a quasi-straight state is only observed irrespective of the bending rigidity, because periodic vortex shedding is interrupted by the flag and at the same time distorting force acting on the flag becomes weakened in such a low-velocity environment. As the separation distance increases, the flag still remains stationary though the vortex shedding takes. The flag further away from the cylinder exhibits a different behavior depending on the original states without the cylinder. The straight and flapping modes are free of the influence of the cylinder wake. On the other hand, for the deflected mode, the flag flaps with large amplitude in synchronization with oncoming shedding vortices. Its oscillation frequency is given by a multiple of the vortex shedding frequency, not to the natural frequency.