Simulation Of Three-dimensional Flow With Moving Bodies And Analysis Of Vortical Structures In The Wake

In the first part of this thesis, we introduce the computational methods, which will be used in our studies. First, the high-order domain decomposition method is exhibited. This method is designed for incompressible viscous fluid flow, and based on the general curvilinear coordinates system. For moving boundary problems, the arbitrary lagrangian-Eulerian description is involved. Furthermore, the N-S equation's convection term and dissipation term are discretized using the third-order upwind compact scheme and the fourth-order central compact scheme, respectively. The author's unique work is to develop a parallel procedure based on the natural parallel feature of the domain decomposition method. Two different parallel strategies are employed, one is based on the domain decomposition, the other is according to the considering of load balancing among processes. We found out that the later strategy works better.Second, an improved Immersed Boundary Method (IMB) is presented, which is based on the Cartesian grid system, therefore, much time consuming can be saved from grid generating. This method is especially suitable for moving problems, because it is no need to regenerate the grid when the immersed body moves. We first review the previous studies on immersed boundary method, then develop an improved version of the this method, on both staggered and non-staggered grid systems. Moreover, the Large Eddy Simulation (LES) method is also combined in this solver, and we make an attempt to perform the simulations on high Reynolds number problems for turbulence.The galloping and vortex-induced vibrations of a square cylinder are investigated. The square cylinder is modeled by a spring-damper-mass system, the motion equation of which is solved by using the Runge-Kutta method. We successfully captured the "locked-in", "beat" and "phase switch" phenomena, and successfully captured the transition from vortex-induced vibrations to galloping.Three-dimensional flow around a single circular cylinder is investigated. A critical Reynolds number of 195 is found, and the wake flow below this Reynolds number is purely two-dimensional. Whereas, when the Reynolds number go beyond this critical point, the wake flow becomes instable to three-dimensional small disturb. The transition regime involves two modes of small-scale three-dimensional instability (modes A an B), depending on the regime of Reynolds number (Re). It is found that the two different modes A and B scale on different physical features of the flow, and many other important questions are addressed in corresponding sections.The spatial evolutions of vortices and transition to three-dimensionality in the wake of two tandem circular cylinders are studied. Two different aspects of this problem are considered. First, the spacing ratio L/D is varied from 1.5 to 8 and the Reynolds number is set unchanged at Re = 220. It is shown that three-dimensionality appears in the wake for L/D≥4, whereas the flow wake keeps a two-dimensional state for L/D≤3.5. The critical spacing for the appearance of three-dimensional instability is then deduced at the range of 3.5 < L/D < 4, which is similar to the critical spacing found in two-dimensional case. Second, we set L/D = 3.5 and make the Reynolds number varied. A mode of small-scale three-dimensional instability, named mode A, is observed to appear at Re = 250 and persists over the Reynolds number range of 250～270. The conclusion is therefore drawn that the cylinder in the wake of another one can sometimes suppress the transition to three-dimensionality, then put off the wake from transition to turbulence.Two circular cylinders in cruciform arrangement confined in a square channel are studied. The distance between the axis of this two cylinders is five diameters, the Reynolds number is Re = 200. A complex three-dimensional flow regime is found between these two cylinders. The regular vortex shedding from upstream cylinder is obstructed into complicated twist structures by downstream cylinder. Furthermore, the influence from the channel wall is also considered.Two long circular cylinders in cruciform arrangement are investigated, besides when the downstream cylinders is forced to vibrate, or endures the vortex-induced vibration. A critical spacing is found at about three diameters (L/D = 3). When beyond this critical spacing, the influence of the downstream cylinder on the wake of the upstream cylinder is restricted in the mixed region, whereas below this critical spacing, the influence of the downstream cylinder on the wake of the upstream cylinder is enlarged. If letting the downstream cylinder freely vibrate or forced vibrate, since suffering from the upstream wake, the vortical modes in the wake of the downstream cylinder will vary along the spanwise direction.In the last section, we apply the computational fluid dynamics to fish-like swimming, and focus on the propulsion mechanism of this motion. For the investigations that experiments is difficult to cover, we make an attempt. For instance, in cruising state, to study a certain shape of fish schooling, which will be helpful for drag reduction and efficiency enhancement. We pick out three fishes from a diamond-shaped fish schooling, and found that a fish situated laterally midway between two fish of the preceding column can benefit from the reversed Karman vortical street shedding from the upstream fishes, therefore the propulsion efficiency is increased, whereas the power consumed is reduced. It is the first time to confirm the hypothesis of the drag reduction and efficiency enhancement mechanisms of fish schooling.