Vertical Axis Wind Turbine (VAWT), Numerical Study (Validation)
The simulation is based on a reference paper, “Wind tunnel and numerical study of a small vertical axis wind turbine.” Its results are compared and validated with the results in the article.
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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 crucial 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 rotation axis 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. Since these wind turbines are installed near the ground and at lower altitudes, the wind speed is lower. Therefore, less energy is generated than the turbine’s specified size. 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 present problem simulates the airflow passing over an H-type darriues wind turbine. The simulation is based on a reference paper, “Wind tunnel and numerical study of a small vertical axis wind turbine.” Its results are compared and validated with the results in the article. In this project, the airflow enters the computational domain with a velocity of 5.07m/s, and the RNG k-epsilon model is exploited to solve the turbulent flow equations, as the paper stated. Also, it should be noted that the Mesh Motion option was enabled to simulate the rotating motion of turbine blades.
VAWT Geometry & Mesh
The geometry of this project is designed in ANSYS design modeler and meshed in ANSYS meshing. The mesh type used for this geometry is structured, and the element number is 1650940.
VAWT CFD Simulation Settings
The critical assumptions considered in this project are:
- Simulation is done using a pressure-based solver.
- The present simulation and its results are transient
- The effect of gravity has not been taken into account.
The following table presents a summary of the CFD simulation settings:
|Near wall treatment||Standard wall function|
|Cell zone condition|
|Rotational velocity||42.25 rad/s|
|Velocity magnitude||5.07 m/s|
|Gauge pressure||0 Pa|
|Spatial discretization||Pressure||Second order|
|Momentum||second order upwind|
|Turbulent kinetic energy||second order upwind|
|Turbulent dissipation rate||second order upwind|
|gauge pressure||0 Pa|
|Velocity (x,y,z)||(5.07,0,0) m/s|
|Turbulent kinetic energy||0.003855735 m2/s2|
|Turbulent dissipation rate||0.008600025 m2/s3|
At the end of this simulation, we compared the present work results and validated with results obtained by the paper. For this purpose, we use the diagram in figure 14, which shows the Torque coefficient changes over time for each rotor blade.
The present work is done considering a three-blade turbine model and the TSR value is considered equal to 2.5; The open air speed is equal to 5.07 m.s-1 and the length of the blades or the radius of the turbine is equal to 0.3 m and also the value of the rotational speed of the blades is equal to 42.25 rad.s-1.
Results & Discussion
After the simulation process, we obtain and present the contours of pressure, velocity, streamlines, etc.
In pressure contours, due to constant variation of the blades’ position and their angle of attack, the blades’ pressure continually changes. These constant pressure changes bring up one of the significant challenges of VAWTs, which would be the dynamic stall. Also, due to pressure changes, the blades of a VAWT are fatigue-prone due to the wide variation in applied forces during each rotation. We can overcome these challenges by using modern composite materials and improvements in design, including aerodynamic wingtips that cause the spreader wing connections to have a static load since the vertically oriented blades can twist and bend during each turn, causing them to break apart.
Results & Discussion
Another prominent factor regarding the rotation of VAWTs is the generation of tip vortex on the blades’ top tip. Based on the phase angle of each blade, the strength of the tip vortex is different. The power of the tip vortex is a sign of lift force on each blade. The stronger the tip vortex core, the stronger the lift force applied on the blade. This change in strength is due to the turbine rotor blade’s changing lift as it rotates through different phase angles. We should note that there is likely to be a small delay between the maximum lift and the maximum strength of the tip vortex occurring. This will be due to the flow’s time to respond to the changing lift around the rotor blade.
Results & Discussion
As previously discussed, the generated tip vortex on the blades shows that turbulent viscosity values have changed after the blades and in the flow direction. Specifically, we can observe the wake area in the middle section between the blades. As the wake segment continues to flow downstream, it will interact with the blade that created it and cause changes in the surface pressure distribution of that blade, causing vibrations and other detrimental effects on the rotor blades.
There is a mesh file in this product. By the way, the Training File presents how to solve the problem and extract all desired results.