Suppression of vortex-induced vibrations by fairings: A numerical study

Fairings are nearly neutrally buoyant devices, fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIV) and possibly reduce the drag force. Here we study numerically how VIV can be practically eliminated by using free-to-rotate fairings. Since the rotational inertia is low for the fairings, direct numerical simulations based on standard fluid-structure interaction algorithms may fail because of the so-called added mass effect. To resolve this problem we introduce fictitious methods and successfully stabilize the simulations. We then investigate the effect of rotational friction Cf on the stabilization effect of the fairings. In particular through two-dimensional (2D) simulations we find that when the Reynolds number is low (Re=100), Cf=0 is the most effective choice in suppressing VIV. Moreover, at this low Reynolds number there exists a critical value of Cf around which large oscillations and non-symmetric trajectories are observed. On the other hand, at higher Reynolds number (Re=500) a different behavior emerges, i.e. VIV are suppressed continuously as Cf increases. At Re=1000, we perform 3D simulations to investigate the effects of three-dimensionality of the flow on the vibration and rotation responses. In this work we quantify numerically for the first time various salient features of free-to-rotate devices for VIV suppression and relate them to modified flow structures in the near wake.