| dc.description.abstract | Flow around a circular cylinder has been extensively studied, both numerically and experimentally, for a number of years. With the increase in flow-structure interactions around marine structures such as platform legs/columns, pipelines and risers, the study of the complex flow mechanisms that is caused around cylinders at high Reynolds numbers has become increasingly important. The use of computational fluid dynamics (CFD) have proved to be an important tool in order to understand these mechanisms. In the present thesis flow around a circular cylinder in steady stream at Reynolds number 3900, is investigated, as well as the flow around two cylinders in tandem subjected to uniform current. Both of these cases where then investigated in the proximity of a wall at a gap ratio (G/D) equal to 0.2D. The open-source code OpenFOAM has been utilized in the simulations. A URANS equation model with a standard high Reynolds number $k - \varepsilon$ turbulence model is applied. This numerical method is shortly described, and an overview of the functions and utilities of OpenFOAM is given. In 2009, Muk Chen Ong studied the applicability of the standard high Reynolds number $k - \varepsilon$ model in flows around a circular cylinder at high Reynolds numbers. He also performed a simulation of the flow around a cylinder near a wall for Reynolds number 13100. He concluded that although the utilized model give less accurate predictions of flow with strong anisotropic turbulence, the model gave satisfactory qualitative agreements with the experimental data and numerical results for high Reynolds number flows. [Ong, 2009]The grid generation for all simulations were based on experiences from the convergence study performed by [Prsic et al., 2012] and [Abrahamsen-Prsic et al., 2013], and kept as equal to this as possible. Hence, no convergence study was performed in this thesis. The mesh domain for Case 3 and 4 was extended in the streamwise direction, in order to account for the additional cylinder. However, for these cases a convergence study should have been performed, in order to further investigate the accuracy of the obtained results.The first case to be simulated, was the benchmark case, one cylinder in free steady current at Reynolds number 3900. The results from Case 1, were compared with the available numerical and experimental results from LES, DNS and PIV simulations. Due to the nature of the applied numerical method, some discrepancies were expected when comparing the results. However, they showed sufficient qualitative agreement with the numerical and experimental results. For Case 2, the cylinder was placed 0.2D over a wall and exerted to a uniform current. Results were compared to results obtained from large eddy simulations performed by [Thingbø, 2013].In Case 3, the flow over two cylinders in tandem with a center to center distance at 5D was simulated. The results showed that the introduction of the second cylinder did not affect the upstream cylinder, however the upstream cylinder had a major affect on the forces on and the pressure distribution around the downstream cylinder. The final case to be investigated was two cylinders in tandem near a wall. The distance between the cylinders were kept equal to Case 3, and the distance from the wall, was equal to that of Case 2. The results showed that the wall affected the downstream cylinder further, and seemed to enhance the effects observed in Case 2 and Case 3. | nb_NO |
