Rotation–wake coupling mechanisms and aerodynamic responses of tandem co-rotating dual cylinders at a critical Reynolds number

To elucidate aerodynamic interference mechanisms in multi-Flettner-rotor systems, this study investigates tandem, co-rotating dual circular cylinders at a critical Reynolds number of Re = 8.901 62 × 105 using two-dimensional unsteady numerical simulations. With a fixed spacing ratio of L/D = 8, the evolution of aerodynamic performance and wake structures is examined over a wide range of rotation ratios (α = 0–14). The results show that the tandem flow exhibits three distinct regimes as α increases: an unsteady regime dominated by alternating vortex shedding, a quasi-steady regime characterized by a stable deflected wake, and a high-α regime with re-emergent unsteadiness. The upstream cylinder reaches its maximum lift at α = 5; at higher α, enhanced shear-layer re-instability leads to aerodynamic efficiency degradation. In contrast, the downstream cylinder attains its lift peak earlier at α = 4. Within the intermediate-α range, a cooperative amplification between rotation and the upstream wake markedly enhances pressure asymmetry and suction on the co-rotating side, whereas this synergistic effect weakens at high α, resulting in performance deterioration. Joint analyses of pressure coefficients and wake topology reveal that rotation reshapes the mechanisms of aerodynamic force generation by modulating near-wall shear layers and wake deflection, while the upstream and downstream cylinders exhibit distinctly different control authority and response characteristics during the coupling process. These findings provide a fluid-dynamic basis for modeling assumptions and parameterization of rotation–wake coupling in multi-rotor rotating systems.