Vortex-induced vibration of a slender flexible riser with grooved and spanwise strips subject to uniform currents

Abstract

We report a numerical investigation of the suppression of “vortex-induced vibration” (VIV) of a cylindrical flexible riser to which are attached various grooved or strip configurations with the ensemble exposed to uniform flow. Based on the thick-strip model, the simulation is done using our in-house three-dimensional VIV solver based on the Open Field Operation and Manipulation toolbox and developed at Shanghai Jiao Tong University (referred to as “viv3D-FOAM-SJTU”). The solver is applied to calculate all the simulations; it uses the Navier–Stokes equations to calculate flow field and the Euler–Bernoulli bending-beam hypothesis to calculate the vibrational displacements of the riser. A slender flexible riser with two spanwise symmetrical strips is first used to determine the appropriate installation angle of the strips, and cylindrical or grooved risers with different strip configurations are used to improve VIV suppression. The numerical results show that the spanwise strip installation angles of 30° and 45° suppress VIV due to the secondary separation of the boundary layer, but suffer from higher crossflow vibration frequency, which brings the risk of inducing high-order mode vibration, the enhancement of the lift correlation along the spanwise direction, and the increment of total drag. The main function of spanwise strips installed at 135° and 150° is to divide the wake region, which also helps suppress VIV. The introduction of grooves in the riser combined with strips of suitable thickness reduces the correlation of lift along the span, which in turn reduces crossflow vibration frequency and the total drag, and enhances VIV suppression. Of all the configurations, the grooved riser with four staggered symmetrical strips most strongly suppresses VIV in the crossflow direction.