Suppression of vortex induced vibration of a circular cylinder by four control rods

The suppression of vortex induced vibration (VIV) of a circular cylinder by four small control rods at a Reynolds number of 5000 is studied by two-dimensional numerical simulations. The main purpose of this study is to investigate the effects of the vortex shedding of the control rods on the VIV suppression. The four rods are in an axially square arrangement surrounding the cylinder. The rod-to-cylinder diameter ratio is 0.2 and the gap between the rod and the cylinder is the same as the diameter of the rods. A lock-in regime still exists when there are four rods surrounding the cylinder but the maximum vibration amplitude in the lock-in regime is only 19% of that of a bare cylinder without rods. Outside the lock-in regime the vibration has two frequencies, and they are the vortex shedding frequencies of the cylinder and rod, respectively. The vortex shedding from the two upstream rods suppresses the VIV by breaking the coherence of the shear layers from above and below the cylinders. The mechanism of the VIV suppression by the two downstream rods is that the flow through the two downstream cylinder-to-rod gaps attach to the back surface of the cylinder, resulting in a weakening of the vortex shedding from the cylinder.