Flow structure around two tandem square cylinders close to a free surface

Abstract

In this study, two dimensional unsteady flow between and in the downstream region of two tandem cylinders close to a free surface was numerically investigated. The governing momentum equations were solved by using the Semi Implicit Method for Pressure Linked Equations (SIMPLE). To simulate a free surface, the Volume of Fluid (VOF) method was used. Central differences and Quadratic Upwind Interpolation (QUICK) methods were applied to discretize the viscous and convective terms respectively. Non-uniform grid structures were used in the simulation with denser grids near the free surface, bottom plate, and in the vicinity of the cylinders. The working fluid was water (Fr = 0.3). The degree of submergence of the cylinders (G) ranged from 0.25D to 5.5D, the distance between the cylinders (L) ranged from 1.5D to 4.0D, and the channel depths (H) were 16D, 8D and 4D. The simulations were performed for the Reynolds numbers of 180, 250 and 400. It was shown that the flow characteristics were influenced by the submergence depth, the distance between the cylinders and the depth of the channel as well as by the Reynolds numbers. As the distance to the free surface increased, the vortices in the downstream region were formed at longer distances. As the flow depth decreased, the flow parameters on the cylinders (CDmean, CLmax, and St) increased due to the influence of the boundary layer that was formed on the bottom wall. When the distance between the cylinders was increased (L/D = 4), the cylinders had higher CDmean and CLmax values as the vortex was formed between the cylinders. Increases were observed in all flow parameters as the Re number increased.