Abstract
A counter-rotating tidal turbine can balance the axial torque and accelerate the wake recovery, leading to a more conducive array layout. This paper aims to investigate the coaxial-rotor interaction, considering variable rotor distance and rotation speed. A numerical model based on large eddy simulation (LES) is established to study the hydrodynamic performance of this turbine, and it is validated by a flume experiment. The research results show that the upstream rotor performance is severely degraded when the two rotors are configured with narrow distance; while this distance is too wide, the downstream rotor bears a large fluctuating load induced by the fully developed wake. In terms of rotation speed, the efficiency of the upstream rotor decreases with the acceleration of the downstream rotor, but the maximum reduction is less than 10%. Moreover, a severe disturbance region will be generated behind the upstream rotor caused by blade root vortex shedding, and the downstream rotor should be positioned far away from this region. Overall, the current two-rotor system can achieve the best energy efficiency at the rotor distance of 0.4D. Compared with a single rotor, this system improves power efficiency by nearly 10%.