Over the past decade, research on Vertical Axis Wind Turbines (VAWTs) has flourished as they are being considered as possible alternatives for the traditional horizontal axis wind turbines (HAWTs) for offshore and small scale applications. The work of Dabiri [1] showed an improvement in the efficiency of the VAWTs when two rotors were placed in close proximity. An experimental validation of a pair of counter-rotating 2 bladed VAWTs separated by small shaft distances by Vergaerde et al [2] shows a 13-16% improvement in the performance of the rotors. Aside from this power enhancement, a peculiar spontaneous synchronisation behaviour between the rotors in certain regimes was observed during these experiments. The angular velocities of the rotors tend to equalise. This implies that if one of the rotors is sped up, the other rotor also speeds up. The significance and effect of this unique behaviour are currently unexplored. The aim of this work is to understand the dynamics of the paired VAWTs and the nature and physics of this synchronisation behaviour. Along with this, the effect of the synchronisation and the oscillations on the power production will be studied.

The preliminary analysis of the instantaneous rotational velocity of the rotors obtained during the experiments has provided information on the weak coupling between the turbines. During synchronisation, the rotors oscillate slowly around a common average rotational velocity with a phase difference less than π/2. Further data analysis to study the robustness of the lock-in behaviour of the rotors where the rotation of one rotor influences the rotation of the other rotor will be done. Computational fluid dynamics approach has been used to investigate the aerodynamic aspect of this behaviour. 2D RANS simulations are carried out using OpenFOAM under the same conditions as the experiments with a Spalart-Almaras model to account for the turbulence in the flow. The outcomes of these simulations provide an insight into the power enhancement due to the close proximity of the rotors, the optimum operational tip speed ratios, torque variations, and the forces on the blades of each rotor. The synchronisation behaviour is analysed to identify the restoring forces responsible for the oscillations of the rotors.

The results from the simulations are compared with the results from the experiments to ensure that the synchronisations are the result of aerodynamic processes.