Numerical Investigation On Vortex-Induced Vibration Of Square Cylinder And Its Control Method

The phenomenon of fluid-structure interaction has emerged as a key research areain the recent past owing to its massive engineering significance. Vortex-inducedvibration (VIV) is a frequently happen event in practical engineering, which can causestructure damage and reduce the life cycle of the object. Therefore, it is of greatimportance to understand the mechanism behind VIV and to find an effective methodto control the flow. In this thesis, a numerical study is carried out to simulate the VIVof an elastic square cylinder at Re=200and a passive control method by geometricmodification is proposed.The incompressible viscous Navier-Stokes equation is solved by the FiniteVolume Method (FVM) to capture the flow information. A mass-spring-dampersystem is used to simulate the motion of the square cylinder and the motion is solvedby applying the Newmark-β method. The self-defined code for solving the cylinder’smotion is used to get the response of the square cylinder. As a result, the interactionbetween the fluid and structure could be realized.An important parameter that influences the VIV is the ratio of the structure naturefrequency to vortex shedding frequency (fn/fs). In this work, we choose a frequencyratio range of fn/fs=0.5~3.5. The unsteady lift and drag coefficients, the cylindervibration amplitude and also the flow field structure of the vibrating square cylinderare investigated. The representative phenomena beat and phase switching aresuccessfully captured. When fn/fsis relatively small, the vibration type is gallopingand very large vibration amplitude can be observed. With the increase of thefrequency ratio, the vibration type changes to VIV and a second vibration peak can befound due to the resonance caused by vortex shedding. The stiffness of the cylinderincreases with the frequency ratio and finally the vibration of the cylinder will beignorable. For the case of high mass ratio, the above-mentioned parameters havesimilar tendency but less value when comparing with low mass ratio one. Thecomplicated fluid-structure interaction also leads to the competition of the fluidstructures between different vortex shedding modes.The passive control of the flow is realized by modifying the sharp corner shape ofthe square cylinder by changing it to round corner. Three round corner models are calculated, that is, r/D=1/8,1/5and1/4. It is found that with the increase of r/D, theamplitude of the lift coefficient and mean drag coefficient both decrease. The optimaldrag reduction model is r/D=1/4by reducing13%of the drag. Comparing withunmodified model, the VIV of the optimal model has better aerodynamic performanceand more stable flow structure. Thus we can come to the conclusion that geometricmodification is a promising technique for drag reduction and VIV control.