Numerical Investigation of Viscous Flow Around Two Tandem Circular Cylinders Ending on a Flat Plate

Flow around cylindrical shapes are frequently seen both within the academics and for applications in ocean and marine engineering. Viscous flow around tandem cylinders fixed to a vertical wall is numerically simulated for a Reynolds number of 500. Direct Numerical Simulations with body-fitted structured hexahedron mesh using OpenFOAM have been performed. Important parameters in the simulations have been chosen based on technical specifications for the Submerged Floating Tube Bridge (SFTB) \citep{Team2016} in the E39 Coastal Highway Route-project. This gives a diameter of 1 m, free stream velocity of 1 m/s and a center-to-center spacing between the cylinders of $S/D = 4$. The physics of the flow around the cylinders and in the plate area have been investigated by looking at velocity- and pressure fields. These have been visualized in several ways using different figures, and interesting features have been noted and compared to similar cases wherever feasible. The flow pattern revealed many interesting features along the span of the two cylinders. The gap flow was quite affected by the horseshoe vortex forming in front of the upstream cylinder, and show clear three-dimensional effects. The horseshoe vortex was rolling up behind the upstream cylinder, disturbing the flow onto the downstream cylinder. A horseshoe-like vortex also developed in front of the downstream cylinder, suggesting some of the same characteristics. Despite having some similarities, the difference in the flow onto the two cylinders indicate that this vortex is not a horseshoe vortex. The effect of the wall is observed to be quite prominent by looking in the wake for different locations along the cylinder z-axis. A cylinder length of 16D is sufficient to capture signs of this, as the wake further away from the wall has similar results as the experiments and simulations for tandem cylinders in free flow. Viewing the pressure distributions, the effect of the wall is apparent as they are vastly different when comparing the time-averaged pressure at the junction and further out. The Strouhal number was found to be similar for both cylinder, indicating synchronized vortex shedding, which was visualized through streamlines along the span of the cylinders. The flow picture for the upstream and downstream cylinders are entirely different, and the effect of the wall is evident. Vorticity contours show some similarity in the flow picture around each cylinder, but the streamlines reveal a different flow picture for the upstream and downstream cylinders. Some of the effects of the wall seem to disappear as one moves away from the wall. The study presented in this thesis helps to improve the understanding of flows with wake interference from having two cylinders and a wall present in the simulation. This can be very useful for future investigations of other aspects of such flows