Impact of Relative Size and Rotation Rate of an Upstream Cylinder on the Hydrodynamics of a Stationary Cylinder in Tandem

This work aims to numerically investigate the hydrodynamics of the flow past two circular cylinders in tandem using the finite volume method. The upstream cylinder rotates at a non-dimensional rotation rate, 0 ≤ α ≤ 1.6, while the main cylinder is stationary. The ratio of the diameter of the upstream cylinder, d, to the diameter of the main cylinder, D, varies as 0.25, 0.5 and 0.75. The distance of separation between the cylinders is fixed at L = 0.8D to study the flow past the cylinders in the co-shedding regime. The cylinders are exposed to an incoming fluid at Re = 500 with reference to the main cylinder. The behaviours of the root-mean-square lift ( \({C}_{l}^{\textit{RMS}}\) ) and drag coefficients ( \({C}_{d}^{\textit{RMS}})\) of the main and upstream cylinders are investigated. The drag coefficient of the upstream cylinder interestingly decreases when α = ωD/2U∞ increases, while that of the main cylinder increases when α increases. The lift coefficients of both the cylinders increase with the increase in α. The Strouhal number variations are analysed, and the vortex-shedding frequency of the upstream cylinder is found to be lesser than that of the main cylinder.