Investigating vortex shedding flow in the wake of circular cylinders is of importance in order to understand the hydrodynamics around offshore cylindrical structures. In this thesis, three-dimensional numerical simulations of steady flow past inclined circular cylinders have been performed for various Reynolds numbers. The incompressible Navier-Stokes (NS) equations are solved to simulate the vortex shedding flows using the Petrov-Galerkin finite element method (PG-FEM) developed by Zhao et al. (2009). The so-called periodic boundary condition was applied at the two end boundaries of the cylinder(s) to eliminate any end effects. The effects of the cylinder inclination angle on the flow and the forces of single and double cylinders are the focus of this study. In the present thesis, the cylinder inclination angle, based on which, 0° corresponds to the case where cylinder is perpendicular to the flow direction and 90° corresponds to the case where cylinder is parallel to the flow direction and the applicability of the independence principle is examined. At first, vortex shedding flows around a circular cylinder at two different inclination angles, namely 0° and 45°, and Reynolds numbers between 100 and 1000 were simulated to examine the effects of the inclination angle on the transition from two-dimensional (2D) to three-dimensional (3D) wake flows. It was found that the transition from 2D to 3D at a = 45° was distinctively different from that at a = 0°. The vortex dislocation in Mode A, which is characterised by the inception of vortex loops and the formation of vortex pairs due to the deformation of primary vortices as they shed from the cylinder, transition for a = 0°was not observed at a = 45°. Then flow past an inclined circular cylinder close to a plane boundary, which is relevant to the pipeline industry, was studied at Re = 500 for gap-to-diameter ratios (e/D) of 0.4 and 0.8, where e and D are the gap between the cylinder surface and the solid boundary and the diameter of the cylinder, respectively. It was found that the independence principle is better satisfied at e/D = 0.8 than that at e/D = 0.4 due to the more apparent influence of the plane boundary in the latter case. To broaden our knowledge of flow past multiple inclined cylindrical structures, three dimensional flow past two identical cylinders in side-by-side and tandem arrangements at oblique attacks were simulated. In the first case, it was found that the normal Reynolds number, which is based on the velocity component perpendicular to the cylinder, should be used in the implementation of the independence principle. The well-defined biased flow observed at 0° is also identified at 45°. When two inclined cylinders are arranged in tandem at Re = 500, it was found that the influence of the gap between the two cylinders is similar to that when the flow passes the cylinders at a right attack angle. Finally, flow past two cylinders of different diameters, which resembles the piggyback pipelines in offshore oil and gas engineering, was studied using 3D model at a constant diameter ratio of 0.45 and gap ratio of 0.0625. The main focus is to identify the effects of the position angle of the small cylinder relative to the large cylinder on the forces of the cylinders. The reduction of the oscillatory forces on the large cylinder by placing a small cylinder close to it was successfully predicted by the 3D numerical model, which cannot be predicted by two-dimensional models.
Date of Award | 2015 |
---|
Original language | English |
---|
- offshore structures
- cylinders
- vibration
- vortex-motion
- fluid dynamics
Numerical study of vortex and force characteristics of inclined offshore cylindrical structures
Thapa, J. (Author). 2015
Western Sydney University thesis: Doctoral thesis