Effects of relative motion on the wake structure and aerodynamic characteristics of tandem cylinders at high Reynolds numbers

This study numerically investigates unsteady flow around two tandem circular cylinders at high Reynolds numbers, with the downstream cylinder approaching the upstream one at varying relative velocities. An unsteady Reynolds-Averaged Navier–Stokes approach with an overset dynamic mesh technique is employed to investigate the influence of relative motion on wake interactions, surface pressure distributions, and drag characteristics. Results reveal that the moving downstream cylinder significantly modifies the classical wake regimes, inducing enhanced vortex impingement and an increasingly widened wake as the relative velocity increases. The drag coefficient of the upstream cylinder decreases monotonically as the gap narrows, with a more rapid reduction at higher relative velocities, while that of the downstream cylinder exhibits increasing behavior with increasing velocity. Modified drag and pressure coefficients are proposed to isolate inertial effects induced by motion, yielding consistent aerodynamic metrics different dynamic conditions. These modified coefficients remain consistent across varying motion states and, at close proximity, exhibit sharp nonlinear reductions with decreasing spacing.