Numerical study of wake flow across two circular cylinders in tandem arrangement with high rotation speed

Flow behind a bluff body is marked with unstable wake pattern eventually impacting the forces acting on the body, which can be actively controlled by rotation. When multiple rotating bluff bodies are placed in close proximity, the wake flow and resulting forces are significantly affected by the mutual flow interaction, necessitating detailed analysis for practical applications. In this vein, this study investigates the variation in wake flow across two circular cylinders, placed in a tandem arrangement, for both co-rotation and counter-rotation configurations. Two-dimensional numerical simulations are performed at a low Reynolds number of 100 for three gap ratios of L/D = 1.5, 2, and 4, where L and D are the center-to-center distance between the cylinders and the cylinder diameter, respectively. The non-dimensional rotation rate is varied from 0 to 6 for the co-rotation configuration and the range is extended till 15 for counter-rotation configuration to capture the secondary instability regime of the system. Flow regimes and force coefficients are analyzed to qualitatively and quantitatively map the overall system behavior, respectively. The flow regimes observed for different rotation rates and L/D are noticed to be different combinations of the flow regimes observed for a single rotating cylinder. Irrespective of the co-rotation or counter-rotation configuration, the dominant frequency of secondary vortex shedding for both upstream and downstream cylinder is noticed to be same, indicating that either a single vortex is shed from the system or a synchronized vortex pair is shed with one vortex from each cylinder. Compared to the co-rotating cylinders, counter-rotating cylinders demonstrate predominant inclination toward steady flow behavior over majority of the studied rotation rates.