It is well known that the interference effect reduces the effectiveness of vortex induced vibration (VIV) suppressors such as helical strakes. Usually the major concern about structures fitted with VIV suppressors is its own vibration. In this paper it is shown how an upstream structure fitted with helical strakes can induce higher vibrations than bare cylinders in a structure mounted downstream in a tandem configuration. Experiments were conducted in a water channel facility and demonstrate that the vortex wake generated by an cylinder fitted with strakes can induce higher amplitudes of oscillation than a bare cylinder. Particle image velocimetry was employed to measure the velocity field in the wake behind a smooth cylinder and behind a cylinder fitted with strakes. The Reynolds number for the experiments goes from 1000 up to 10.000. The flow field visualization was conducted for fixed cylinder at Re = 10.000.

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