Suppressing vortex-induced vibrations via passive means

We investigate numerically the hydrodynamic effect of a slit in circular cylinders with the goal of finding a geometric modification that minimizes vortex-induced vibrations (VIV) without any energy consumption. A slit parallel to the incoming flow is found to be very effective in suppressing VIV by either weakening or detuning vortex shedding. Through a series of simulations, the optimal size of the slit to suppress VIV is found at Reynolds number 500; this size is smaller at a higher Re=1000Re=1000. The effectiveness of the slit in suppressing VIV is confirmed over a wide range of values of the reduced velocity. Modified cylinders with a slit of sufficiently large width cause a strong jet flow into the wake that changes the vortex shedding pattern dramatically. For slits larger than the critical width, the instantaneous flow fields do not show periodic vortex shedding. While unsteadiness of the flow persists in these cases, it is the effective detuning process of the jet flow interacting with the wake flow that leads to VIV suppression. A linear stability analysis shows that jet flow through a slit changes the stability in the wake of cylinder by inducing two small pockets of absolute instability at the back of the cylinder followed by bands of convectively and absolutely unstable regions. We also present results with alternative geometry modifications using perforated thin shells.