Numerical simulations of the flow around single and tandem circular cylinders close to a plane wall
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- Institutt for marin teknikk 
In order to model the turbulent flow around cylindrical structures in various configurations, Large Eddy Simulations (LES) are used in the present work. LES utilize a highly accurate model at reasonable time-consumption, offering an insight into the details of a complex flow. Standard Smagorinsky subgrid scale model is used to include the effects of the subgrid scale motions. Three configurations of cylindrical structures are chosen in this study, inspired by the arrangements of the subsea pipelines in the marine environment. The simplest and most extensive studied arrangement is the flow around a single, smooth circular cylinder in a uniform current. Hence, this configuration is used for the assessment of the numerical tools’ performance. The second configuration, a circular cylinder in the vicinity of a plane wall is chosen to represent the flow around free-spanning subsea pipelines. Performed for various gap to diameter ratios (G/D), the simulations successfully capture the details of the flow in the cylinder wake and the interaction with the seabed boundary layer. The influence of the less explored parameters, the thickness and the shape of the inflow profile, is explored. The three-dimensional (3D) LES results show clear improvements over the previously published two-dimensional (2D) simulations. Two circular cylinders, placed one behind the other, relative to the incoming current, are called the tandem arrangement. The third configuration, tandem cylinders in the vicinity of a plane wall correspond to double pipelines or flowlines in the subsea systems. This configuration has received little research attention. The flow around tandem cylinders close to the wall is therefore compared to the simpler and more thoroughly understood configurations. It is concluded that the existing classification of the flow around one cylinder close to the wall describes well the flow around the upstream cylinder at large horizontal span ratios (L/D). Behaviour of the flow around tandem cylinders at large G/D can be described through the flow classification for the tandem cylinders in the infinite fluid. At intermediate G/D (and various L/D), the presence of the wall causes stronger influence than recorded for the flow around one cylinder at the same G/D.