The Darrieus Vertical Axis Wind Turbine (VAWT) with straight blades, also known as the H-rotor or Giromill, is a unique wind energy conversion device. Unlike traditional horizontal-axis wind turbines, this design features vertical blades that rotate around a central shaft. The straight-bladed Darrieus VAWT offers several advantages, including omnidirectional operation, eliminating the need for complex yaw mechanisms to track wind direction. Considering the Transient nature of the simulation, validating a numerical paper is very tough. Nevertheless, in the current study, an invaluable paper entitled “ The Darrieus wind turbine: Proposal for a new performance prediction model based on CFD” published in Energy Journal is validated.

Figure 1: 3D experimental setup (reference paper)

Simulation Process

As represented in Table 3 in the reference paper, NACA 0021 profiles are installed. The 2D geometrical domain is created using Design Modeler. It requires the use of a specific template to order design points and import them to the software. Given the extremely high computational cost of the simulation, sub-grids are carried out, particularly in critical regions such as around the airfoils and near the interface (Fig. 2). Besides, the formation of separation on the blades is very crucial. Thus a suitable generation of the boundary layer is necessary. Clearly, a sliding mesh (mesh motion) module is included to predict the rotational movement of the rotor.

Figure 2: grid generation with a focus on critical boundaries

Post-processing

The graph presents a comparison of power coefficient (Cp) values for a Straight-Bladed Vertical Axis Wind Turbine (SB-VAWT) across various Tip Speed Ratios (TSRs), ranging from 1.5 to 3.5. It compares results from Castelli’s 2D simulation with the current 2D simulation. Both simulations show remarkably similar trends, validating the current study’s approach.

The Cp, which is calculated as the product of the moment coefficient (Cm) and TSR (Cp = Cm * TSR), increases steadily with TSR up to a peak value of approximately 0.57 at a TSR of about 2.5. After this optimal point, the Cp gradually decreases as TSR further increases. This behavior is typical for wind turbines, demonstrating the importance of operating at the optimal TSR to maximize power extraction efficiency.

The close agreement between the two simulations across the entire TSR range suggests that the current 2D simulation accurately replicates the performance characteristics of the SB-VAWT model studied in Castelli’s work.