Serrated Airfoil and Plain Airfoil Comparison, Darrieus VAWT, ANSYS Fluent CFD Simulation Training
The present problem compares the airflow passing over two H-type Darrieus wind turbines of plain and serrated airfoils by ANSYS Fluent software.
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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. In vertical axis turbines or rotors, the axis of rotation is perpendicular to the ground, and the blades rotate parallel to the ground.
For this reason, the surface that is moved by the wind after half a turn has to continue to move in the opposite direction of the wind flow, and this problem causes their power factor to decrease. For this reason, the blade curve is of particular importance in these rotors. The wind speed is lower since these wind turbines are installed near the ground and at lower altitudes. Therefore less energy is generated comparing to the specified size of the turbine. Also, airflow near the ground and other objects can create turbulent currents that cause vibration consequences, including noise and bearing fatigue, resulting in increased maintenance costs and reduced service life.
The performance of vertical axis wind turbines (VAWTs) is substantially affected by the dynamic stall phenomenon induced by the variations of 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, and therefore rotor torque and power output are reduced. The purpose of the present project is 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.
Plain and Serrated Airfoil Project description
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.
Plain and Serrated Airfoil Geometry and Mesh
The geometry of this project is designed in an ANSYS design modeler and contains a rotating and stationary zone. The meshing is carried out in ANSYS meshing. The mesh type used for this geometry is hybrid, and the element number is 1186185.
CFD Simulation Settings
The critical assumptions considered in this project are:
- Simulation is using a pressure-based solver.
- The present simulation and its results are transient.
- We ignore the effect of gravity.
The applied settings are summarized in the following table.
|Near wall treatment||Standard wall function|
|Cell zone condition|
|Rotational velocity||2.8285 rad/s|
|Velocity magnitude||7 m/s|
|Gauge pressure||0 Pa|
|Turbulent kinetic energy||first-order upwind|
|Turbulent dissipation rate||first-order upwind|
|gauge pressure||0 Pa|
|Velocity (x,y,z)||(0,0,7) m/s|
|Turbulent kinetic energy||0.003855735 m2/s2|
|Turbulent dissipation rate||0.008600025 m2/s3|
Results and discussion for Plain and Serrated Airfoil
After the simulation process, we obtain and present contours of pressure, velocity, streamlines, etc.
As we can see in pressure contours, due to constant variation of the position of the blades and, therefore, their angle of attack, the pressure applied on the blades changes constantly. 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 types of vertical axis turbines.
Next, if we investigate the changes of 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, and it shows 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 distribution of pressure over two surfaces of a single airfoil. In the serrated airfoils, the pressure gradient over two surfaces of each airfoil changes more smoothly comparing the plain airfoils causing the lift force to increase.
Results & Discussion
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.
You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.