Compared with a drag-type vertical axis wind turbines, one of the greatest advantages for a lift-type vertical axis wind turbines is its higher power coefficient ( C_p). However, the lift-type vertical axis wind turbines is not a self-starting turbine as its starting torque is very low. In order to combine the advantage of both the drag-type and the lift-type vertical axis wind turbines, a lift drag hybrid vertical axis wind turbines was designed in this article and its aerodynamics and starting performance was studied in detail with the aid of computational fluid dynamics simulations. Numerical results indicate that the power coefficient of this lift drag hybrid vertical axis wind turbines declines when the distance between its drag-type blades and the center of rotation of the turbine rotor increases, whereas its starting torque can be significantly improved. Studies also show that unlike the lift-type vertical axis wind turbines, this lift drag hybrid-type vertical axis wind turbines could be able to solve the problem of low start-up torque. However, the installation position of the drag blade is very important. If the drag blade is mounted very close to the spindle, the starting torque of the lift drag hybrid-type vertical axis wind turbines may not be improved at all. In addition, it has been found that the power coefficient of the studied vertical axis wind turbines is not as good as expected and possible reasons have been provided in this article after the pressure distribution along the surfaces of the airfoil-shaped blades of the hybrid turbine was analyzed.

The co-flow jet (CFJ) based active flow control method is a new approach for suppressing the flow separation on the blade, delaying the stall, and enhancing the lift. In this study, a detailed numerical analysis of the flow field developing around a Darrieus wind rotor equipped with CFJ is presented in order to explore the effectiveness of CFJ in the efficiency improvement of vertical axis wind turbines (VAWTs). Unsteady Computational Fluid Dynamics simulations of the straight one-bladed and three-bladed rotors with the NACA0015 profile, using the shear stress transport (SST) k-ω turbulence model, were conducted. As a result, the effects of jet operational parameters such as the injection site and the momentum coefficient on the aerodynamic performance of the straight-bladed Darrieus vertical axis wind turbine (VAWT) have been comprehensively studied. The numerical results suggest that CFJ would be capable of recovering some of the power loss due to blade stall and its control effectiveness is more pronounced with the increase in the jet momentum coefficient. In addition, a power saving method for intermittent injection of a CFJ in accordance with the cyclic variation of the angle of attack is proposed and our results show that the stalling characteristics around a VAWT blade especially at low tip speed ratios can be considerably improved using intermittent CFJ which requires much less additional energy input.