Experimental and numerical studies on the three-dimensional flow around single and two tandem circular cylinders in a duct

Experimental and numerical investigations are conducted in order to understand the flow around identical tandem circular cylinders confined in a duct (blockage ratio b = 0.1 and aspect ratio a = 5). In this work, the Reynolds number Re ranges from 40 to 200 and the spacing ratio (distance between two centers of tandem circular cylinders to diameter, L/d) from 0 to 8. When fluid flows around a single cylinder placed symmetrically in the duct (L/d=0), it is found that the Strouhal numbers St and mean drag coefficients Cd¯ increase with the increase in Re, although the separation angle θs decreases. For the flows around two tandem circular cylinders, the research is focused on examining the coupling effect of Re and L/d on St, flow structures, θs, Cd¯ and reverse region length Lr. Based on numerical results, the mathematical descriptions are established, Cd¯∼Re−1, θs∼Re−1/2, and St∼Re−1. The dependence of flow structures, drag forces, θs and Lr, on L/d is described in detail with the help of approaching velocity profiles, pressure coefficient Cp, and vorticity ωz* distribution on a cylinder's surface. Through numerical simulations, the detailed mechanisms about influences of these factors on the flow properties are revealed. The experimental results evidence the occurrence of three flow modes, i.e., no vortex shedding mode, single body mode, and reattachment mode, which are determined practically by the Reynolds number and space ratio. Both the measured St based on the dominate frequencies of vortex shedding and the visual flow field match well with numerical simulations.