Initial Rotation Characteristic Investigation of a Hybrid Savonius-Darrieus Wind Turbine using 6 DOF Computational Fluid Dynamics

The inconsistency of the wind flow considered as one of the factors which tend to decrease the performance of the wind turbine. This paper proposes a further analysis of the initial rotation characteristic of a hybrid Savonius - Darrieus wind turbine. The addition of the Darrieus blade intends to increase the aerodynamic stability of the overlapping Savonius turbine. This study implements 2D CFD transient analysis using the 6DOF methods in 00, 300, 600, and 900 Darrieus blade position along with 2 m/s, 4 m/s, and 6 m/s wind speed variations. The results of the aerodynamic analysis show that the location of the Darrieus 300 turbine provides the greatest initial repulsion, especially when the turbine rotation is above 900, the position of the Darrieus blade can provide additional impulse force when the Savonius turbine tends to be passive. This effect occurs more significant at higher wind speeds. Savonius with 3-blade modification has a more stable level of force distribution than the 2-blade modification, although the value is smaller. This shows that the 3-blade Savonius provide a higher stability of angular velocity development.

The inconsistency of the wind flow considered as one of the factors which tend to decrease the performance of the wind turbine. This paper proposes a further analysis of the initial rotation characteristic of a hybrid Savonius -Darrieus wind turbine. The addition of the Darrieus blade intends to increase the aerodynamic stability of the overlapping Savonius turbine. This study implements 2D CFD transient analysis using the 6DOF methods in 0 0 , 30 0 , 60 0 , and 90 0 Darrieus blade position along with 2 m/s, 4 m/s, and 6 m/s wind speed variations. The results of the aerodynamic analysis show that the location of the Darrieus 30 0 turbine provides the greatest initial repulsion, especially when the turbine rotation is above 90 0 , the position of the Darrieus blade can provide additional impulse force when the Savonius turbine tends to be passive. This effect occurs more significant at higher wind speeds. Savonius with 3-blade modification has a more stable level of force distribution than the 2-blade modification, although the value is smaller. This shows that the 3-blade Savonius provide a higher stability of angular velocity development. )

Introduction
Savonius wind turbine is simple but effective to be applied in a relatively low wind speed and inconstant direction. The performance of Savonius type wind turbines decreases in relatively high wind speed. This is caused by the presence of turbulence in the airflow shortly after passing the blades on Savonius wind turbines [1]. The highest average efficiency in Savonius turbines is 16% [2]. This value is so small that certain innovations are needed to improve turbine efficiency. The presence of wind collisions in the inactive blades results in obstacles to the turbine rotation.
The effect of the number of blades on conventional Savonius turbines shows that turbines with a number of 2 blades have a slightly higher rotor rotation value than turbines with 3 blades [3]. Another study shows that using 4 Savonius turbine blades has an efficiency value of up to 10%. The efficiency value is also greatly influenced by the ratio of the gearbox used. [4] The selection of Darrieus blade types using the NACA 2412 model has a maximum generated power of 16.38 W at a wind speed of 6 m/s and the number of turbine blades of 4 units. However, the minimum 10 Volume 20 (1) 2021 power produced by the turbine is 0.45 W at wind speed is 6m/s and the number of turbine blades is 3 units [5]. There is a tendency that the increasing wind speed and the number of blades used will increase the power generated by the Darrieus turbine [6]. Natural composite materials show good strength and flexural properties along with good damping properties designed for wind turbine blades [7].
With the low efficiency of wind turbines for both Savonius and Darrieus models, further research is needed. This paper proposes the development of hybrid turbine models using a combination of Savonius and Darrieus models that are expected to take the advantage of each type of wind turbine to overcome the low efficiency of current wind turbines.
This research aims to reduce the effects of these obstacles by adding a Darrieus blade to the inactive side of the Savonius blade. So that aerodynamic effects that occur more effectively and can improve the performance of the turbine.

Materials and Method
The hybrid turbine composed of Savonius and Darrieus turbines. The Savonius turbine used in this study was the half barrel with 2 blades and 3 blades. The effective diameter of both the Savonius Turbine was 500 mm. The Darrieus turbine was built based on NACA 2412 [8] with a 100 mm chord length.   The turbine blade materials using an aluminum plate with 0.5 mm thickness. The base of Savonius blades and arms of Darrieus blades is using the composite materials to reduce energy inefficiency [9]. The static aluminum shaft used in this study. Some complex components produced using 3D printing due to its requirements to be easily assembled. In order to analyze the initial rotation characteristic, the 6 Degree of Freedom transient analysis was used in this study [10]. Figure 5 describes the setup of the 2D simulation study. The Darrieus rotor position is modified in four different locations to determine the most effective location. The flow calculation model used in this study is a segregated flow with a k-epsilon realizable turbulence model. Another parameter that needs to be considered is the use of implicit unsteady parameters with the second-order upwind discretization scheme. In addition, the time step discretization used is the second order.

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(1) The results of numerical data collected of the rotation at 0 to 120 s. The rotation value can be converted to an angular velocity value using Equation (1) [11] With is angular velocity (rad/s), n is revolution per minute (RPM). The 60 constants used to convert revolution per minute into revolution per second.

Result and Discussion
The numerical simulation used to further analysis of the initial rotation aerodynamics characteristic. The analysis divided by th e wind speed in each number of Savonius blades variation.    Variation of Savonius 3 blades at the time of 5.9 seconds in Figure 9. shows that the variation of the Darrieus turbine location at point 0 0 receives more momentum from the fluid which causes the conditions of spin initiation at this variation to run faster. The position of the turbine blade at this point has a significant impact value for the initiation of the turbine rotation. Shown in Figure 11. significant impact values occur at 2.8 s where the Darrieus 300 angle variation has the best momentum position, resulting in faster spin initiation. This is because the Darrieus turbine is able to accept fluid repulsion when the Savonius turbine is passive. Due to the balanced force in Savonius blade which always occurs in 90 0 angle [12].   Almost the same thing happened in Savonius 3 blades variation as seen in Figure 13. where the position of Darrieus 30 0 blades has the advantage in receiving turbine collisions when the rotation is above 90 0 . So the rotation value that can be generated in Darrieus 30 0 turbine blade variations has the highest rotation value compared to the others.     Figure 16 shows that the position of the Darrieus blade at 30 o has the highest angular velocity value at each time change. The effective angular velocity value for this variation is 47.2 (rad / s). While the lowest angular velocity value at each time change is obtained at a variation of the position of the Darrieus 90 o blade. The effective angular velocity value for this variation is 42.7 (rad / s).
In variation of 3 Savonius blades, Figure 17. shows that initiation of rotation occurs faster in the Darrieus 30 0 turbine variation. This is in line with experimental results which show that the variation has the highest rotation.

Conclusion
From the overall data collection, several conclusions can be obtained as follows. In both variations of the number of blades of Savonius turbine, the number of blades 2 has a higher mean rotation value than the number of blades 3. The difference in rotation is higher in the variation of wind speed 2 [m / s]. While the difference is not too large at 6 [m / s] wind speed variations. This shows that the performance of 2 blades Savonius turbine is better than 3 blades, especially in low wind speed conditions.