Numerical Investigation of Flow around Two Tandem Cylinders in the Upper Transition Reynolds Number Regime Using Modal Analysis

Flow around two tandem cylinders at Re = 3.6 × 10⁶ for different center-to-center spacing ratio ($L/D$) is investigated numerically using two-dimensional (2D) Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations combined with a standard $k-\omega \mathrm{SST}$ turbulence model. The instantaneous flow structures around the cylinders, hydrodynamic forces on the cylinders and Strouhal number $\left(St\right)$ are analyzed and discussed. Dynamic Mode Decomposition (DMD) is used to extract the spatiotemporal information of the coherent flow structures in the wake regions behind the upstream (UC) and downstream (DC) cylinders. A sparsity-promoted algorithm is implemented to select the dominant modes which contribute the most to the dynamics of the system. Based on the dominant modes, a reduced-order representation of the flows is built. A comparison of the lift and drag force–time histories, obtained by simulation results and the reduced-order representations, shows a high capability of the latter to reproduce the surrounding flow and hydrodynamic properties of the tandem cylinders at the high Reynolds number.