Numerical study on aerodynamic characteristics of finite-length tandem rotating cylinders

Aerodynamic interference in flow around tandem rotating cylinders (e.g., Flettner rotors) is characterized by significant complexity for engineering applications. Large eddy simulation is employed to investigate aerodynamic characteristics of finite-length tandem cylinders at a subcritical Reynolds number of 3900, with focus placed on spacing ratios (L/D) of 3, 4, and 5 and speed ratios (n) ranging from 0 to 5. For stationary cylinders (n = 0), three distinct flow regimes are identified. When rotation initiates (n = 1), flow instability is intensified. The key phenomenon of “drag reversal” emerges at n ≥ 2. This transition is attributed to coupling between the blockage effect and the rotational action. At the critical speed ratio (n = 3), vortex shedding is suppressed. At high-speed ratios (n > 3), the flow transitions into a rotation-dominated regime, where a secondary instability mode is activated. A synergy between blockage effects and rotational action is revealed, enabling relative optimal operation at L/D = 3 and n = 3 with high lift-drag ratio and achieving minimal fluctuating loads. Critical theoretical and practical guidance for optimizing tandem cylinder flow configurations in Flettner rotors is provided with these findings.