Vortex-induced vibrations of circular cylinders in steady and oscillatory flow

Abstract

Vortex-induced vibration (VIV) of cylindrical structures in fluid flow is of interest to many fields of engineering. For example, it influences the dynamics of offshore riser tubes bringing oil from the seabed to the surface and is a key issue in deep water riser design. Water depths up to 3000 m are found in typical oil extraction areas. Understanding VIV is also important in many other offshore engineering applications, such as mooring lines of floating offshore wind turbines, undersea pipelines and flexible slender pipes. These slender structures can experience VIV when exposed to marine current, because the dynamic vortex shedding flow leads to oscillatory forces. This study focuses on the Vortex-induced vibration (VIV) of circular cylinders in steady and oscillatory flows. As the first step, a two-dimensional (2D) numerical model is used to study VIV of a single circular cylinder in combined steady and oscillatory flow. The numerical model is based on the Reynolds-Averaged Navier-Stokes equations. Special focus is to investigate the effects of flow ratio (the percentage of the steady current velocity in the total fluid velocity) on the response of the cylinder. The simulations are carried out for a constant Keulegan""Carpenter (KC) number. The second step is to investigate the vortex-induced vibration (VIV) of multiple circular cylinders elastically connected together in a side-by-side arrangement subject to steady flow at a low Reynolds number of 150 and a low mass ratio of 2. Simulations are conducted for two-, five- and ten-cylinder systems over a wide range of reduced velocity that covers the whole lock-in regime for each of the cases. The differences between the responses of a multiple-cylinder system and a single cylinder are discussed. The shedding of vortices and flow interference between multiple circular cylinders in side-by-side arrangement in steady flow are examined. Then, a numerical study of vortex-induced vibration of four rigidly connected and four separately mounted circular cylinders in an inline square configuration at a Reynolds number of 150, a low mass ratio of 2 and a range of spacing ratio L from 1.5 to 4 is carried out. For a rigidly connected four-cylinder array, the maximum and minimum response amplitudes occur at L=1.5 and L=2.0, respectively, for the range of spacing ratio covered in this study and the maximum respond amplitude at L=1.5 is accompanied by a wider lock-in range than a single isolated cylinder case. For spacing ratios L ≥ 2.5, the lock-in regime of four rigidly connected cylinders is similar to that of a single cylinder and the response amplitudes in the lock-in regime are slightly higher than that of a single cylinder. The biased vortex street leads to a shift of the mean position of the cylinder array with the largest mean position shift being observed at L=3. Four response modes are identified for four separately mounted cylinders. These are the in-phase mode, the anti-phase mode, the correlated out-of-phase mode and the uncorrelated mode. The response mode for a cylinder in the four cylinder system is dependent not only on the spacing ratio, but also on the initial condition of the flow. The response amplitude under the in-phase mode is generally higher than that under the anti-phase mode at identical spacing ratios. This is attributed to the interaction of vortices in the wake of the cylinders. Two-dimensional studies have been popularly used to investigate the fundamental mechanisms of VIV due to its efficiency. Because the flow in the wake of a circular cylinder is in a three-dimensional fashion when the Reynolds number is in the turbulent regime, even when the cylinder is rigid and the free-stream flow is uniform, three-dimensional simulation of VIV is necessary and has been carried out in this study. Vortex-induced vibration (VIV) of tapered and uniform cylinders is investigated numerically at a constant Reynolds number of 500 using three-dimensional numerical simulations. The objectives of the study are to identify the difference between the responses of tapered and uniform cylinders. Simulations are conducted using parameters as close to the experimental condition as possible. Two cylinders are considered: one with a length to diameter ratio of 4.3 and a mass ratio of 2.27 and another one with a length to diameter ratio of 12.3 and a mass ratio of 6.1. While the simulation of the shorter cylinder is mainly for validating the numerical model, detailed analysis of the vibration amplitude and frequency, the vortex shedding flow mode and the lift coefficient are performed for the longer cylinder. For some reduced velocities, it is found that vortex shedding is in 2P mode at the small-diameter part of the cylinder and 2S mode at the larger-diameter part, forming a hybrid flow mode.

Date of Award	2015
Original language	English