Two-dimensional Numerical Simulation Of Flow Around Oscillating Cylinder Using A CIP-based Model

Flow past cylinders is a complex engineering problem, which involves a variety of scientific problems, such as the turbulence, flow separation, vortex mode and so on. Precise prediction of the load on the structure, the frequency of vortex shedding and the lock-in mode is of grate scientific value and engineering significance. The main issue of present work is to discuss the hydrodynamic characteristics of flow past in-line imposed oscillating cylinders. Active flow control problem and complex fluid-structure interaction are involved in this subject. We also study the phenomenon of flow around stationary cylinders in this thesis.In this paper, a CIP-ZJU (Constrained Interpolation Profile method in Zhejiang University) model is developed to study this problem. The model is established in the Cartesian coordinate system, using the CIP method as the base flow solver to discretise the Navier-Stokes equations. The fluid-structure interaction is treated as a multiphase flow with liquid and solid phases solved simultaneously. An immersed boundary method is used to deal with the solid body boundary.The CFD model is firstly applied to compute flow past a circular/square cylinder for low Reynolds number Re. Computations are compared with available results and good agreements are obtained. By introducing the pressure drag and viscous drag, the variations of Strouhal S,drag coefficient Cd and lift coefficient Cl with Reynolds are analyzed. The differences of dynamic characteristics between flow past a circular cylinder and a square cylinder are also discussed. Results show that at Re≈140, the separation point of flow past a square cylinder moves from rear corners to upstream corners, resulting in viscous drag changing its direction. This phenomenon does not occur for flow past a circular cylinder. The dominance of pressure drag for a square cylinder is more pronounced than that of a circular cylinder. The primary wavelength of flow past a square cylinder is longer than that of a circular cylinder. The amplitudes of lift coefficient are about one order of magnitude larger than the corresponding drag coefficient values. With the increase of Re, the amplitudes of a square cylinder increase much faster than that of a circular cylinder.The model of flow around oscillating cylinder is established to study the phenomenon of flow past an in-line forced oscillating circular/square cylinder. The disciplines of hydrodynamic coefficients, vortex modes and the lock-in region are analyzed at different oscillation amplitudes and oscillation frequencies. It shows that the 2T lock-in modes can be both observed from flow past an oscillating circular or square cylinder. The lift portrait plots show a good phase lock phenomenon when the oscillation frequencies are in lock-in region, and the maximum of Cd and Cl are got.The flow field of flow past two tandem stationary cylinders is much more complex than that of flow past a stationary cylinder. Flow past two stationary cylinders in tandem at Re=100 is computed with different spacing ratios s=L/D=1.5-7.0 (L is the center-to-center distances), to discuss the influence of s on the flow field. It shows that St is the same between flow past circular cylinders and square cylinders. The critical spacing of square is larger than that of circular. At L≈Lc a sudden jump of hydrodynamic coefficients can be observed.Computations are then performed with flow past two tandem cylinders with an upstream in-line oscillating cylinder with a small spacing L=2D at Re=100. Considerable attention is paid to the spectrum characteristics and vortex modes. The spectrum exhibits different peaks:the natural shedding frequency from two tandem fixed cylinders, the frequency of oscillating cylinder and its harmonics and sum of different components of the aforementioned. Vortex modes of symmetric S mode similar with flow past an oscillating cylinder and 2P mode are observed in the "lock-in" region.