The flow mechanism around two tandem circular cylinders with different pitch ratios (P/D) was investigated. Numerical simulations were conducted on two tandem circular cylinders (P/D=1.1-5) at a low Reynolds number (Re=100). Dynamic mode decomposition (DMD) was adopted to decompose the flow around two cylinders. Reduced-order model was established based on the dominant DMD modes to reconstruct the vorticity field of two tandem circular cylinders. Results show that the typical wake flow regimes of two tandem circular cylinders were single bluff body (P/D=1.1-2)

shear layer reattachment (P/D=3)

and couple vortex shedding (P/D=4-5)

which presented distinct wake flow features. With the increase in pitch ratio

the mode structure corresponding to the vortex shedding frequency moved from the wake of the downstream cylinder to the wake of the upstream cylinder. It indicates that the dominant mode characterizes the inherent flow mechanism of the transition of the three wake flow regimes for two tandem cylinders with different pitch ratios. The wake flow pattern of couple vortex shedding presented more complicated higher-order mode characteristics than that of single bluff body and shear layer reattachment. In terms of flow reconstruction

more sub-modes were needed for couple vortex shedding to achieve similar precisions with the other two regimes. The reconstruction error was mainly concentrated near the vortex formation region.