Correction of sidewall blockage effect for twin-rotor vertical axis tidal stream turbine subjected to different solidities

This study investigates the impact of sidewall blockage effect on the hydrodynamic performance of twin-rotor vertical axis tidal stream turbine (TR-VATST) with varying solidities. A hydrodynamic analysis model for TR-VATST under sidewall blockage effect is developed using computational fluid dynamics. The hydrodynamic characteristics of TR-VATST in an unsteady flow field are numerically simulated, focusing on the influence of sidewall blockage effect on the energy extraction efficiency across different tip speed ratios and solidities. Base on these analysis, a modified model for sidewall blockage effect applicable to TR-VATST is proposed. The results indicate that: (1) the mean power coefficient of TR-VATST increases with the blockage ratio across different solidities, and the improvement is faster at higher solidity; (2) as the blockage ratio increases, the influence of sidewall blockage effect on TR-VATST intensifies, leading to an increase in the flow velocity near the blades, thereby enhancing energy from the incoming flow and improving the mean power coefficient; (3) The modified model for sidewall blockage effect accurately predict the mean power coefficient of TR-VATST within a specific range of tip speed ratios, solidities, and blockage ratios. Moreover, the model effectively eliminates the influence of laboratory flume sidewalls, enabling correction of experimental results to reflect offshore prototype performance in an unbounded flow environment. These findings provide valuable engineering insights for the performance prediction and optimization of TR-VATST systems.