On the wake of two co-rotating cylinders arranged side-by-side and subjected to a uniform crossflow

The wake behind a bluff body, such as a cylinder, exhibits unsteady patterns that directly influence the forces acting on the body and can be actively modified through rotation. When multiple rotating bluff bodies are arranged in close proximity, wake interactions critically affect flow dynamics and the resulting structural forces, necessitating detailed analysis for flow control and engineering applications. This study investigates uniform flow past two co-rotating circular cylinders in a side-by-side configuration at a Reynolds number of 200. Three-dimensional direct numerical simulations are performed for gap ratios of 0.5, 1, and 3, and rotation rates ranging from 0 to 6. The gap ratio is defined as the surface-to-surface gap between the cylinders normalized by the cylinder diameter, and the rotation rate is defined as the ratio of cylinder surface speed to the freestream velocity. A comprehensive flow regime map is constructed in the control parameter space defined by rotation rate and gap ratio. For all gap ratios, wake transitions of the top cylinder (whose gap-side surface velocity aligns with the freestream flow) occur at lower rotation rates than those of the bottom cylinder (whose gap-side surface velocity opposes the freestream flow). In several flow regimes, the wakes of the two cylinders differ not only in their patterns but also in their three-dimensionality. In three such regimes, the bottom cylinder exhibits a two-dimensional wake, while the top cylinder simultaneously develops a three-dimensional wake. Since the wake of a counterclockwise rotating cylinder is dominated by the shear layer at its upper side, the suppression of vortex shedding and the onset of three-dimensional instabilities are delayed for the bottom cylinder due to gap-induced modifications of this shear-layer dynamics. Furthermore, decreasing the gap ratio shifts the critical rotation rates for wake transitions to lower values for both cylinders. The statistics of the force coefficients are quantified across the control parameter space.