Numerical Analysis of Forced Oscillation and Vortex-induced Motion Circular Cylinder in Cross Flow with Low Reynolds Number
|School||Shanghai Jiaotong University|
|Course||Design and manufacture of ships and marine structures|
|Keywords||Fluid-structure interaction Dynamic mesh Forced oscillation Vortex-induced motion Fluent|
With the offshore oil and gas industry striding forward to deep sea rapidly, more and more pivotal components of the circular cylinder shape representated by marine risers are widely used in the offshore platform and subsea pipeline systems. It’s of great practical significance to carry out the research of forced oscillation and vortex-induced vibration(VIV) of circular cylinder in the field of ocean engineering, since vortex-induced vibrations and the fluid-structure interaction could cause large stresses and potential fatigue damage to the marine structures, and also reduce their performance functions and affect the safety and capability of such systems. The problem of oscillation and VIV of circular cylinder is attracting increasing attention from fluid mechanics researchers as well as structural engineers, and the rapid development in numerical methods and hardware performance have resulted in advanced computer facilities which allow the extensive studies of numerical simulations of the fluid-structure interaction.This paper presents a numerical study of circular cylinder undergoing transversely forced oscillation and vortex-induced motion of an elastic-suppported circular cylinder in cross flow with low Reynolds numbers. The flow is assumed to be incompressible and viscous. The CFD software Fluent is used to solve the fluid motion which is described by the incompressbile Navier-Stokes equations and the dynamic mesh technology is applied to deal with the problem of body motion in the fluid flow. The convection term of NS equation is discretized using the second–order upwind scheme, and SIMPLE algorithm is employed to solve the pressure and velocity coupling problem.The research work and conclusions of this paper are summarized as follows:Firstly, in order to validate the reliability of the numerical method and the computional model used in this paper, numerical simulations of flows around two and three dimensional stationary circular cylinder at low Reynolds numbers as well as flow around an oscillating circular cylinder in a fluid at rest are carried out. The calculation results are compared with experimental data and numerical results in the literature, which shows rationality of the model established in this paper and it can be used in the study of transversely forced oscillation and vortex-induced motion of circular cylinder.Secondly, a numerical simulation of the transversely forced oscillation of a circular cylinder in uniform flow at low Reynolds numbers is undertaken in order to study the influence of frequency ratio and dimensionless amplitude of oscillating cylinder on the hydrodynamic forces and vortex shedding modes in the wake. A two-dimensional simulation of circular cylinder undergoing transverse oscillation in uniform flow at Re = 200 is studied. The simulation concentrates on a domain of oscillation frequencies fe near the natural vortex shedding frequency fs behind the fixed cylinder. For the cases of specified dimensionless amplitudes, the dependence of lock-in upon the ratio of the ocsillating frequency and the natural vortex shedding frequency fs are studied, and the time history of fluctuating hydrodynamic coefficients characterized by“beating”or“lock in”are presented; then the influence of oscillating amplitude and frequency on the vortex shedding mode near the lock-in region is considered. As the amplitude is increased, the Karman vortex shedding mode (2S) evolving into two different shedding modes is found. One is P+S mode in which a pair and a single vortex are shed in each motion cycle and the other is 2P shedding mode with a pair vortex in each side of the ocsillating cylinder, the transition among these shedding modes agrees very well with the concerning experimental observations. The behaviour of a three dimensional ocsillating circular cylinder at Re =200, 300 and 500 is analysed respectively and the result indicates that forcing a cylinder to oscillate transversely with certain amplitude may depress the three-dimensional effect.Finally, the vortex-induced motion of an elastically mounted circular cylinder in cross-flow is studied. The cylinder motion is modeled by a spring-mass-damper system and the equation of cylinder’s motion is solved using the Runge-Kutta method. The dynamic response of a two dimensional elastic-supported circular cylinder in flow at a range of Reynolds numbers from 90 to 150 is firstly studied by employing the experimental parameters. We successfully capture the phenomena of“lock-in”,“beating”and“phase swith”, meanwhile analyse in detail the cylinder’s displacement, the fluctuating lift and drag force on the cylinder, the vortex shedding frequency as well as vortex structure at various Reynolds numbers. Then the effect of mass ratio and mass-damping parameter on the dynamic response characteristic of vortex-induced motion of cylinder is further studied and the result shows that the mass ratio plays an important role on the response of vortex-induced motion, in which the maximun amplitude depends on the mass-damping parameter, and the range of lock-in depends on the mass ratio when the mass-damping parameter being constant. Besides, a three-dimensional numerical study to explore the three-dimensional characteristic of vortex- induced motion of circular cylinder is also presented.Therefore, the numerical analysis presented in this paper gleans some basic physics of the forced oscillation and vortex-induced motion problem of circular cylinder and develops an understanding of the interaction between the flow and the moving structures, which provides a guide leading to the further investigation of VIV of risers in deepwater.