Large Eddy Simulations of flow around two circular cylinders in tandem in the vicinity of a plane wall at small gap ratios

Large Eddy Simulations (LES) with Smagorinsky subgrid scale model have been performed for the flow past two circular cylinders in tandem placed in the vicinity of a horizontal plane wall at very small gap ratios, namely G∕D=0.1,0.3 and 0.5, in three-dimension (3D). The ratio of cylinder center-to-center distance to cylinder diameter, or the pitch ratio, L∕D, considered in the simulations is L∕D=2 and 5. This work serves as an extension of Abrahamsen Prsic et al. (2015). In essence, six sets of simulations have been performed in the subcritical Reynolds number regime at Re=1.31×104. Our major findings can be summarized as follows. (1) At both pitch ratios, the wall proximity has a decreasing effect on the mean drag coefficient of the upstream cylinder. At L∕D=2, the mean drag coefficient of the downstream cylinder is negative since it is located within the drag inversion separation distance. (2) At L∕D=2, a squarish cavity-like flow exists between the cylinders and the flow circulates within the cavity. A long lee-wake recirculation zone is found behind the downstream cylinder at G∕D=0.1. However, a much smaller lee-wake recirculation zone is noticed at L∕D=5 with G∕D=0.1. (3) At L∕D=2, the reattachment is biased to the bottom shear layer due to the deflection from the plane wall, which leads to the formation of the slanted squarish cavity-like flow. At both pitch ratios, as G∕D becomes smaller, stronger vortices are found between the two cylinders. Less intense vortices are observed in the near wake of the downstream cylinder due to the vortex shedding suppression of the neighboring wall.