|dc.description.abstract||Marine pipelines placed at or near the seabed are exposed to currents and waves. In the presence of a sedimentary seabed, the interaction between the surrounding water motions and the pipeline may cause erosion of sediments beneath the pipeline. Due to this erosion or due to the installation procedure, free spans may occur in sections along the span of the pipeline. A simplified description of the flow is obtained by approximating the seabed as a plane and impermeable wall. Most of the previous experimental and numerical studies on this flow are performed at Reynolds numbers, Re, in the range 103-105 as this range is of most relevance for offshore engineering purposes. Numerical modeling at such Reynolds numbers generally involves different kinds of artificial flow modeling.
In this study, the software OpenFOAM is applied for numerical simulations of the viscous flow around a circular cylinder at Re= 100. At this Reynolds number the flow around an unconfined circular cylinder is characterized by vortex shedding and yet turbulence is avoided, which provides the possibility of accurate calculations of the flow. The flow is investigated with reference to visualizations of pressure, velocity and vorticity; and the flow characteristics are quantified in terms of drag and lift acting on the cylinder and vortex shedding frequency. Numerical simulations of an unconfined cylinder in uniform cross-flow constitutes the basis of the present study, and these simulations are included in an attempt to validate the results and the applied computational method. The results for this flow are within the scatter of the reported values in the literature.
In this study, the flow around a cylinder near a plane wall is of main interest. Three different gap ratios G/D=0:2;0:5;1:0 are applied. The results are generally in accordance with published data from numerical simulations; best agreement is found at G/D=0:2 and G/D= 0:5. The results are supportive to the suggestion of vortex shedding suppression to be caused by the interaction between the lee-side recirculating flow and the gap flow, which inhibits large-scale vortex roll-up. Further, the results at G/D= 1:0 indicates cancellation of opposite signed vorticity in the near-wall region, in accordance with suggestions in the literature.
Additionally, the geometry of the wall is altered, introducing a hollow below the cylinder. This shape imitates a fully developed scour profile. These simulations are expected to bring new results to this topic of research. The flow is characterized by evident vortex shedding. Further, at this gap a distinct mean lift in the direction towards the wall is observed and both the drag coefficient and the frequency of vortex shedding is reduced as compared to the flow around a cylinder in uniform cross-flow. The obtained results exhibits similarity to published experimental data for the flow at Re= 1:104.
Two-dimensional simulations are performed for all of the flow configurations, and a few three-dimensional simulations are performed for a cylinder in uniform cross-flow and a cylinder located a distance G/D= 0:5 from a plane wall. Due to the two-dimensional flow patterns, insignificant differences are found between the two-dimensional and three- dimensional simulations.
In this study, emphasize is given to the influences on the solution of the following numerical parameters: time step, domain size, grid geometry, element size and element spacing. These parameters are thoroughly investigated in terms of convergence studies.
Also included in this thesis is a review of some features of these flows and an overview of the governing equations, OpenFOAM and the applied solver icoFoam.