Wake interactions and aerodynamic performance of phase-locked tandem rotors at low Reynolds numbers

Abstract

Rotor–wake interactions strongly influence the aerodynamic performance of compact multirotor unmanned aerial vehicles operating at low Reynolds numbers. Overlapping rotor configurations such as tandem arrangements enable compact designs but introduce strong aerodynamic coupling between rotors. Although prior studies have shown that these interactions affect rotor performance, the influence of specific wake structures remains unclear, as most investigations rely on asynchronous rotor operation, where the relative azimuthal phase between rotors continuously varies. The present study investigates the controlled interaction between upstream rotor wake structures and downstream rotor blades in tandem rotors and their effect on rotor performance. This is achieved by operating the rotors at precisely the same speed under phase-locked conditions (maintaining a fixed relative phase angle between rotor blades). Experiments are conducted over a range of rotor separations and relative phase angles. Ensemble-averaged flow fields for the phase-locked rotors are obtained using two-dimensional two-component particle image velocimetry (2D-2C PIV), together with simultaneous thrust and torque measurements. The results show that the relative phase angle acts as an additional governing parameter, alongside rotor spacing, in determining rotor–wake interaction behavior and the resultant rotor performance. Certain phase configurations produce strong blade–vortex interactions and notable thrust variations, while others reduce wake interference. These findings provide new insight into rotor–rotor interaction mechanisms and demonstrate the potential of rotor phase control for improving the performance of compact multirotor systems.