Three-dimensional simulations of flow around cylinders with fairing to suppress VIV

Three-dimensional simulation of stationary and moving cylinders with free-to-rotate fairings are conducted at Reynolds number 100 $\le $ Re $\le $ 10,000. Fairings are nearly-neutrally buoyant devices, which are fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIV) and reduce the drag force. The effect of gap between fairings along the cylinder axis on the hydrodynamic forces (C$_{d}$, C$_{l})$ and the translational and rotational motion of fairings (x$_{rms}$, y$_{rms,}\theta_{rms})$ are investigated. With the increase of Re, the drag coefficient C$_{d\, }$of fairing decreases. Compared to the plain cylinder case, fairings without gap can reduce C$_{d\, }$by 15{\%} while the fairing with gap can reduce C$_{d}$ by almost 50{\%}. The lift force (C$_{l})$ and angular momentum of fairing (M$_{fh})$ for different gaps are also decreased. Correspondingly, the vibration (y$_{rms})$ and rotation ($\theta_{rms})$ amplitudes of fairing are also reduced. We also investigate the change in flow structure induced by the fairing gaps. A pair of stream-wise vortices are generated in the gap region, which extracts energy from the flow in the cross-flow direction hence causing decrease of the lift force. As Re increases, pressure recovery in the wake of the fairing is observed, which is the main reason for the substantial decrease of drag force.