NUMERICAL INVESTIGATION OF FLOW AROUND INTERSECTING CYLINDERS

Abstract

Recent years interfering effects in the wake of bluff body are drawing more attention, especially for the intersecting cylinders. The structures with the similar shape of the intersecting cylinder are used in many industrial applications. One of the industrial applications is the aquaculture industry. Based on the assumption that the deformation of the fish-net is neglected, the flow around one isolated node of the fish-net has similarities to the flow past two intersecting circular cylinders. Moreover, intersecting cylinders can also be found in the supporting structures of the jacket platform are made of many intersecting tubular members. The vortex structure in the wake flow and the forces acting on the supports of the jackets induced by wave and current are of interests from both industrial and academic point of views. The main topics in this thesis is to use CFD tool to investigate the wake flow behind T-shaped cylinders. The interaction effects in the wake flow behind T-shaped cylinders should be documented. Additionally, validating the quality of results by comparing present numerical results with previous data is also important. The thesis are mainly divided into three parts. In the first part, the case flow past single 2D circular cylinder at Reynolds number 100 is selected. The purpose of running this simulation is to investigate the the suitable value of domain size, time step and grid topology. By comparing the numerical results from cases with different domain sizes, It can be found that when distance between flow inlet and cylinder is smaller than 10 times diameter, the effects caused by velocity inflow is obvious. The wall blockage also have a big influence on numerical simulation when the distance between lateral boundaries and cylinder is smaller than 10 times diameter. Both effects disappeared when length beyond 10 times diameter. After reviewing several previous papers, O-H mesh block is selected. By using this mesh block, a fine grid can be obtained near the cylinder. The convergence curve, the velocity and pressure contours have been plotted to improve that the convergence ability and mesh quality of O-H block is good. Finally, time-step analysis has been done by running a case under different time-step, and then checking the convergence of the numerical results. The conclusion that maximum courant number should be smaller or around 1 has been obtained. The purpose of second part is to prove that the CFD model (ANSYS FLUENT) I selected is iv able to obtain good results of flow passing T-shaped cylinders. In order to achieve this goal, the case flow pass a 3D circular cylinder at Reynolds number 300 was selected to be simulated under different mesh grids. After finishing mesh convergence test, the fundamental results, such as pressure distribution, skin friction distribution are compared with previous experimental data. And the discrepancy is very small. Based on this, it is reasonable to deduce that the correct numerical results of T-shaped cylinders can be obtained by using this CFD model. In the third part, T-shaped cylinders are simulated by ANSYS FLUENT. After doing mesh convergence test and statistical steady test, the results and discussion are presented. Durning the discussion, structure of vortex, velocity distribution along the wake center line, streamline pattern, instantaneous velocity contour and pressure distribution on the horizontal cylinder both in different cross-section and span-wise direction are shown. From these figures, some interesting phenomenons such as changes in recirculation length, local peak pressure etc. are observed and compared with previous experimental data.