Serrated Airfoil and Plain Airfoil Comparison, Darrieus VAWT, ANSYS Fluent CFD Simulation Training

Description

Description

The present problem compares the airflow passing over two H-type Darrieus wind turbines of plain and serrated airfoils by ANSYS Fluent software. We perform this CFD project and investigate it by CFD analysis.

The Darrieus wind turbine is a type of vertical axis wind turbine (VAWT) used to generate electricity from wind energy. The turbine consists of several curved airfoil blades mounted on a rotating shaft or framework.

In this type of turbine, the main rotor is positioned vertically. The most important advantage of vertical wind turbines is that they do not need to be adjusted to the wind direction and can also be used at low altitudes.

The three-dimensional geometry of this project has been produced with Design Modeler software. The geometry of this project contains a rotating and stationary zone. The length and width of the calculation area are 50 cm, and the height is 300 cm.

We carry out the model’s meshing using ANSYS Meshing software. The mesh type is hybrid. The structured mesh has been generated for the stationary zone, while the unstructured mesh is generated for the rotating. The element number is 1186185.

Also, the transient solver is enabled due to the present problem in which we have used the mesh motion option.

Serrated Airfoil Methodology

The performance of vertical axis wind turbines (VAWTs) is substantially affected by the dynamic stall phenomenon induced by the variations in the angle of attack of rotating blades.

Large and sudden torque fluctuations occur when the dynamic stall vortices, formed near the blade leading-edge, are transported downstream. At a relatively low Reynolds number (Re < 105), a dynamic stall occurs periodically during the rotation of turbine blades.

This results in a sharp drop in lift coefficient, reducing rotor torque and power output.

The current project aims to investigate the concepts for improving the power performance of a conventional H-type VAWT model by implementing sinusoidal serrations on the leading edge of turbine blades to control and reduce the dynamic flow separation.

The present problem compares the airflow passing over two H-type Darrieus wind turbines of plain and serrated airfoils by ANSYS Fluent software.

In this project, the airflow enters the computational domain with a velocity of 7m/s, and we apply the RNG k-epsilon model to solve the turbulent flow equations.

Also, it should be noted that the Mesh Motion option was enabled to simulate the rotating motion of turbine blades, and the rotation velocity of the rotating domain was set to 2.8285 rad/s. Moreover, the RNG k-epsilon model is enabled to solve turbulent fluid equations.

Serrated Airfoil Conclusion

After the solution, two-dimensional contours related to the pressure, velocity, and streamlines are obtained.

For instance, as we can see in pressure contours, the pressure applied on the blades changes constantly due to constant variation of the position of the blades and, therefore, their angle of attack.

These constant changes in pressure bring up one of the significant challenges of VAWTs, which would be the dynamic stall. Also, due to changes in pressure, the blades of a VAWT are fatigue-prone due to the wide variation in applied forces during each rotation.

W can overcome these challenges by improving the design, including leading-edge serrated airfoils that can enhance the power and torque generated by these vertical axis turbines.

Next, if we investigate the changes in drag, lift, and power coefficient, we may observe a slight increase in drag force exerted on serrated airfoils compared to the plain ones, which seem to be due to the increased surface of the airfoils.

The following graph is related to the lift coefficient, showing a slight increase in the generated lift force when we use the serrated airfoils for the vertical axis turbine. The reason can be behind the more smoothed pressure distribution over two surfaces of a single airfoil.

In the serrated airfoils, the pressure gradient over two surfaces of each airfoil changes more smoothly compared to the plain airfoils causing the lift force to increase.

Finally, the generated power will slightly increase by improving the design, i.e., employing serrated airfoils. This phenomenon can be explained by the fact that the generated power is increased in each rotation cycle for each blade. This increase in power in each cycle will result in smoothed and increased power output.

The next two graphs delve more deeply into the details of power generation in each cycle and for each type of airfoils.

As was explained earlier, in the generated power coefficient graph, the smoothness of generated power for each airfoil is clearly shown.