The flow-induced rotation of the modified Savonius rotor, for which the blade consists of a semicircular profile and an elliptical shape, is studied using a series of unsteady computational fluid dynamics (CFD) simulations. This study first concentrates on the validation of the numerical scheme against Blackwell's experimental data of the conventional rotor. The computed flow physics around the modified rotor with the same diameter is then analyzed and compared with that of the conventional rotor during one rotation cycle. As the result, the modified rotor is outperforming the conventional one but keeping its unique features. The modified rotor offers exceeding performance at a tip speed ratio (TSR) greater than 0.8. The new peak of the power coefficient Cp is reached at TSR = 1.4 which is a typical operating condition of the wind turbine in urban areas. The remarkable finding is that the suppression of the flow separation on the blade is an effective way to improve the rotor's aerodynamic performance. As expected, the additional elliptical profile plays a key role in increasing the positive torque and in preventing the flow separation on the blade, especially at high TSR > 0.8. Finally, this study points to not only advances the fundamental understanding of flow mechanism around the rotor but also proposes a good practical energy harvesting application in urban environments.

In this study, skin friction around a ½-scale Ahmed body was measured experimentally at a Reynolds number of Re = 2 × 10⁵. The slant angle of the Ahmed body was 25 deg, and the yaw angles ranged from 0 deg to 8 deg. This study focused on the flow structure on the slant surface under different cross-wind conditions. A force balance system was applied to measure the aerodynamic drag of the model. The global skin-friction topology was measured by applying a luminescent oil layer with a subgrid data processing algorithm. The method used to measure the skin friction was conducted for the first time on the Ahmed body. The results indicated that the technique is highly capable of extracting the skin-friction topology. For a yaw angle below 3 deg, the flow on the slant surface was not significantly affected by the cross-wind condition, and the drag of the model was nearly constant. However, at yaw angles above 3 deg, the flow on the slant surface was highly affected by the roof longitudinal vortexes on the windward side, leading to a dramatic increase in the drag of the model. High consistency in the drag and skin-friction fields was observed. The detailed skin-friction structure at different yaw angles will be discussed in this study.

This study proposes a novel blade configuration, made with a multicurve profile, to further maximize the advanced benefits and the performance of a Savonius turbine for energy harvesting applications. The advantages of the presently designed blade are proved by a detailed analysis of the flow mechanism and the performance through a sequence of two-dimensional (2D) unsteady computational fluid dynamics simulation. The remarkable finding states that the main blade section with a length ratio of 0.9 combined with an elliptical auxiliary section effectively enhances the positive wind flow phenomena around the blade, especially for the returning position, which either smooths the flow or reduces the pressure on the blade’s convex side. As a result, greater torque is obtained by the present configuration, responding to the improvement of the power generation to 0.24 – about 5.5% at the low TSR = 0.8 and up to 0.28 - about 185% at the high TSR = 1.5 over the conventional configurations. The effectiveness of the presently designed blade is further proved with more than 20% improvement in self-staring speed through six degrees of freedom analysis. The results above state a high potential application of the turbine with the presently designed blade for renewable energy applications and for supporting vulnerable communities without access to electricity.

A new blade configuration is proposed to further increase the performance of a Savonius rotor through a sequence of unsteady two-dimensional (2D) computational fluid dynamics (CFD) simulations. The blade is made by a multicurve and auxiliary profiles for a reduction of the negative drag on the rotor. The flow aspects around the new blade are analyzed and quantitatively compared with that of the conventional and other blade configurations. The results imply a dependency of the rotor performance on the blade shape, demonstrating an appropriate configuration that produces the highest coefficient of torque and power. The newly optimized configuration is recognized with the peak of power coefficient at a tip speed ratio (TSR) of 1.5, which is more than two times higher than the conventional one. This makes the Savonius rotor better applicable to the urban environment. Importantly, this blade significantly increases the power coefficient by 6.3% at TSR < 1.0, known as a typical working condition in rural areas. The present results thus point out a feasible solution for powering the poor-households with no access to the grid and reducing the harmfulness to the environment, with high efficiency in wide operating conditions over the previous designs.

This study focuses on the unsteady behavior of the flow around axisymmetric conical boattail models under low-speed conditions. Particle image velocimetry was conducted on the symmetric plane for four boattail models with angles of 0°, 10°, 16°, and 22°. Different data processing techniques, including variational mode decomposition (VMD), fast Fourier transform, proper orthogonal decomposition (POD), and multiscale proper orthogonal decomposition (mPOD), were applied to understand the effects of boattail angles on the characteristic frequency of the wake flow. Our results indicated that vortex shedding, bubble pumping, and the rotation of vortex shedding are the three dominant modes for four boattail configurations. However, the energy of vortex shedding mode becomes comparable to that of bubble pumping for the model with the boattail angle of 22°. The orientation of the shear layer remarkably changes near the boattail surfaces for different angles, which is connected to the flow behavior on the surface. This study also suggests that VMD and mPOD are advantageous data-driven methods for analyzing turbulent flows.