Flow-induced vibration (FIV) suppression of two tandem long flexible cylinders attached with helical strakes

The characteristics of the flow-induced vibration (FIV) of two tandem flexible cylinders with or without helical strakes were experimentally investigated in a towing tank. The two cylinder models with a center-to-center spacing of 8.0D (where D is the smooth cylinder diameter) were allowed to oscillate in both the cross-flow (CF) and in-line (IL) directions. The uniform flow was simulated by towing the cylinder models along the tank. The corresponding Reynolds numbers, defined by the towing carriage velocity and the smooth cylinder diameter, ranged approximately from 800 to 16000. The strake with a pitch of 17.5D and a height of 0.25D, which is generally believed to be the most effective configuration for reducing vortex-induced vibration (VIV) of an isolated flexible cylinder in water, was adopted in the experiment. The experimental results, including the displacement responses, frequency responses and mean drag force coefficients, were analyzed by comparing with those of the isolated smooth and straked cylinders. The FIV of the upstream cylinder with helical strakes is in reasonable agreement with that of the isolated straked cylinder, which means that the performance of the helical strakes for suppressing the FIV of the upstream cylinder is almost independent of the downstream cylinder at the selected spacing T = 8.0D in the experiment. The FIV suppression efficiency of the helical strakes of the downstream cylinder was observed to decrease in both the CF and IL directions due to the unsteady wake from the upstream cylinder. Under certain conditions, the displacement amplitude of the downstream cylinder can even be enhanced by the helical strakes. Moreover, the dynamic responses of a smooth cylinder placed upstream or downstream of a straked cylinder were also investigated for improving the understanding of the FIV behaviors of a two-tandem-cylinder system.