Numerical simulation of flow around cylinder with an upstream rod in tandem at low Reynolds numbers

This study is concerned with the mechanism of the formation and the convection of vortices shedding from the cylinder with an upstream rod. The aerodynamic characteristics of the rod and cylinder are modelled in two dimensions by using Fluent® with laminar flow. It is found that the mean drag and the lift fluctuation of the cylinder can be reduced by the upstream rod, especially in the cavity flow mode with the rod diameters d/D=0.3d/D=0.3 and 0.5 (here dd and DD are the diameters of the rod and the cylinder, respectively). With rod diameters d/D=0.3d/D=0.3 and 0.5, the simulation indicates that there are two flow modes of the flow field around the rod and cylinder, which appear with the variation of the center-to-center spacing ratio L/DL/D: the cavity flow mode and the wake impinge mode. There is a pair of standing vortices between the rod and the cylinder in the cavity mode, which results in the pressure decrease on the windward side of the cylinder. It is found that the characteristics of the flow significantly depend on the spacing ratio L/DL/D. Instantaneous flow fields, force fluctuation, force spectrum and so on are presented to identify the flow modes and to understand the underlying mechanism of the cylinder drag reduction.