Hydrodynamics and vortex shedding characteristics of two tandem cylinders of different diameters in steady flow

Viscous flow past two tandem circular cylinders of different diameters is numerically investigated by using finite volume method. The diameter ratio between the two cylinders ranges from 0.1 to 1.6. The Reynolds number based on the diameter of cylinders is 800 for the large cylinder and 100 for the smallest cylinder. The spacing between two tandem cylinders ranges from 0.1 to 1.6times the diameter of the large cylinder. Identified from the numerical results for all the cases of tandem configurations, spacing ratio* (S/d) = 1.5 is the critical point. The root mean square lift coefficient on both of the two cylinders abruptly increases at the critical point. The values of the fluctuating forces acting on the large cylinder are generally lower than those on a single cylinder. Three different wake modes are identified based on the vortex shedding characteristics behind two tandem cylinders, the one wake mode, the transitional mode and the reappearance mode. Attachment has been found in the transitional mode for the diameter ratio of d/D = 0.75 at the critical point. The numerical results are powerful basis for design of piggyback pipeline.