Combined effect of diameter ratio and spacing ratio on flow around two tandem circular cylinders at a moderate Reynolds number (<i>Re</i>)

In order to ascertain the combined effect of diameter ratio (d/D) and spacing ratio (L/D) on the flow around two tandem circular cylinders, a total of 43 research cases are numerically simulated using a large eddy simulation model in this study, including d/D = 0.2 (L/D = 0.80–6.00), d/D = 0.4 (L/D = 0.80–6.00), and d/D = 0.6 (L/D = 0.90–6.00) at Re = 3900. By systematically analyzing the instantaneous/time-averaged spanwise vorticity contours, the three-dimensional time-averaged Q-criterion iso-surfaces, the spanwise periodicity within the gap, and the Strouhal number (St), eight sub-flow regimes can be identified, namely, non-periodic reattachment-(1), small-scale periodic reattachment, large-scale periodic reattachment, non-periodic reattachment-(2), bi-stable flow, intermittent lock-in co-shedding, subharmonic lock-in co-shedding and no lock-in co-shedding. The occurrence of these sub-flow regimes depends on the combination of d/D and L/D. Besides, with regard to the larger downstream cylinder (DC), there are three possible dominant St values for the DC instantaneous drag coefficient (i.e. StCD-D1, StCD-D2, and StCD-D3). It is quite clear that StCD-D1 has almost the same value as StCL-D1, StCD-D2 is roughly equal to 2StCL-D1, and StCD-D3 is approximately the summation of StCL-D1 and StCL-D2, where StCL-D1 and StCL-D2 denote the primary and secondary dominant St values of the DC instantaneous lift coefficient, respectively. Additionally, various flow statistical parameters are obtained and compared for different combinations of d/D and L/D, such as the velocity, vorticity, St value, spanwise periodicity, force coefficient, base pressure coefficient, reattachment angle, and separation angle.