The objective of the present study is estimating hydrodynamic forces acting on cylinders undergoing vortex-induced-vibration (VIV) using Dynamic Mode Decomposition (DMD). The cylinders are subjected to a uniform incoming flow at a laminar Reynolds number (Re=250) and an upper transition Reynolds number (Re = 3.6 × 106) (Re = UD/v defined based on the incoming flow U, the diameter of the cylinder D and the viscosity of the fluid v). Both a single cylinder and a configuration of piggyback cylinders are considered. Numerical simulations based on two-dimensional URANS (Unsteady Reynolds Averaged Navier-Stokes) equations combined with the k-ω SST turbulence model are carried out to obtain the snapshots of the surrounding flow fields for DMD analysis. The DMD method is a powerful tool to obtain the spatial-temporal evolution characteristics of the coherent structures in the wake flow behind the cylinders. In the present study, this modal decomposition method is combined with a moving reference frame around the cylinders. The dominant DMD modes with their corresponding frequencies of the wake flows are identified and are used to reconstruct the flow fields. The large-scale shedding vortices are captured by the dominant modes. The reconstructed wake flow behind the cylinders is used to estimate the drag and lift forces on the cylinders combined with a force partitioning analysis.

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