Two-stage Taguchi-based optimization on dynamically-vented blade flaps to enhance the rotor performance of a Savonius turbine

Among the vertical -axis turbine groups for small-scale wind and hydrokinetic applications, the Savonius type is considered practical due to its simple construction, easy maintenance, and good self-starting characteristics. Unfortunately, it has a poor efficiency due to the exertion of negative torques during its returning sweeps. Recently, the technique of controlled dynamic venting has been shown to reduce this negative torque on a twobladed Savonius rotor while preserving its omnidirectional capability. That investigation was extended in this numerical study by refining the controllable flaps of the same rotor to improve its performance using a two -stage optimization procedure. In the first stage, the design parameters of the controllable flaps were optimized using the Taguchi and Analysis of Variance (ANOVA) methods. In this analysis, the flap length was found to be the most significant parameter affecting the rotor performance with a contribution ratio of 79.8%, in contrast to a ratio of only 6.7% by the least effective parameter: the flap opening angle. The first optimization stage produced an improvement of 16.7% on the average power coefficient ( C P ) of the vented rotor, compared to the unvented one, at the optimal tip -speed ratio (TSR) of 1.0. In the second stage, the flap length was refined further to a shorter flap of 22.5% of the blade length, resulting in an improvement of 21% on the same metric at the same TSR. Crucially, these controllable flaps only used an energy cost of 6.1% of the total energy produced by the rotor, a significant improvement from the 12.2% energy cost obtained in the previous study.