U-shaped fairings suppress vortex-induced vibrations for cylinders in cross-flow

Abstract

We employ three-dimensional direct and large-eddy numerical simulations of the vibrations and flow past cylinders fitted with free-to-rotate U-shaped fairings placed in a cross-flow at Reynolds number 100Re 10, 000. Such fairings are nearly-neutrally buoyant devices fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIV). We consider three different geometric configurations, a homogeneous fairing, and two configurations (denoted A and AB) involving a gap between adjacent segments. For the latter two cases, we investigate the effect of the gap on the hydrodynamic force coefficients and the translational and rotational motions of the system. For all configurations, as the Reynolds number increases beyond 500, both the lift and drag coefficients decrease. Compared to a plain cylinder, a homogeneous fairing system (no gaps) can help reduce the drag force coefficient by 15% for reduced velocity U * = 4.65, while a type A gap system can reduce the drag force coefficient by almost 50% for reduced velocity U * = 3.5, 4.65, 6, and, correspondingly, the vibration response of the combined system, as well as the fairing rotation amplitude are substantially reduced. For a homogeneous fairing, the cross-flow amplitude is reduced by about 80%, whereas for fairings with a gap longer than half a cylinder diameter, VIV are completely eliminated, resulting in additional reduction in the drag coefficient. We have related such VIV suppression or elimination to the features of the wake flow structure. We find that a gap causes the generation of strong streamwise vorticity in the gap region that interferes destructively with the vorticity generated by the fairings, hence disorganizing the formation of coherent spanwise cortical patterns. We provide visualization of the incoherent wake flow that leads to total elimination of the vibration and rotation of the fairing-cylinder system. Finally, we investigate the effect of the frictional coefficient between cylinder and fairing; the effect overall is small, even when the frictional coefficients of adjacent segments are different. In some cases the equilibrium positions of the fairings are rotated by a small angle on either side of the centerline, in a symmetry-breaking bifurcation, which depends strongly on Reynolds number.

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Experimental Study on Wake Characteristics of Secondary Grooved Cylinders with Different Depths

“…In an effort to mitigate these negative consequences, many academics have modified the wake flows behind the cylinder using passive devices on its surface. In the past, the rigid splitter plate (Hwang and Yang 2007;Xie et al 2015;Liu et al 2016) and fairing structures (Gu et al 2012;Assi et al 2014) were the most successful and widely used vortex suppression devices. Fairings and a stiff splitter plate can reduce drag the most, but they are only useful in certain conditions.…”