Numerical Simulations of Vortex-Induced Vibration of Flexible Cylinders

Abstract

The main purpose of this paper is to acquire a better understanding of the hydroelastic interactions, which take place between oscillating flexible cylinders and fluid forces. The cylinders are subjected to currents and shear flow, and the hydrodynamic forces are estimated by CFD tools. This article presents the results of an investigation being carried out at the University of Sa˜o Paulo, in which a discrete vortex method is used to simulate the flow around a flexible cylinder. The calculations are compared with results obtained employing the quasi-steady theory, as proposed by Ferrari [2]. Also, the calculations are compared with experiments of a cantilever flexible cylinder immersed in a current, see Fujarra [6]. The reduced velocity vs. non-dimensional amplitude curve obtained in our calculations is compared with the experimental results. Visualizations of the wake indicate a hybrid mode of vortex shedding along the span. A 2S mode is found in regions of low amplitudes, changing to a 2P mode in the regions of larger amplitudes. The position of the transition of the modes varies with the reduced velocity. Our intention is to apply this model to problems occurring in the offshore industry. In this industry fluids are conveyed from the seabed to the platform through slender structures called risers. These risers are subject to shear and oscillatory flows due to currents and waves, respectively, flows with a very high degree of complexity, with changes of intensity and direction the deeper the water depth. A finite element structural model based on the Euler-Bernoulli beam theory was developed. In order to evaluate the dynamic response, a general equation of motion is solved through a numerical integration scheme in the time domain. The hydrodynamic forces are evaluated in two-dimensional strips. The technique used is the Discrete Vortex Method, which is a Lagrangian numerical scheme to simulate an incompressible and viscous fluid flow. A practical case of marine risers is also presented. In this case the results for various uniform currents acting on a single, flexible cylinder, representing a riser of 120m with 100m under water, are shown. Envelopes of maximum and minimum in-line and transverse displacements are presented. There is also a comparison of a shear flow case between the CFD numerical code with the quasi-steady theory code developed by Ferrari [2].