Darrieus Wind Turbine Evaluation, CFD Simulation
This project is going to simulate an airflow field close to a vertical axis Darrius wind turbine.
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Vertical Axis Wind Turbine (VAWT) is becoming ever more important in wind power generation thanks to their adaptability for domestic installations. However, it is known that VAWTs have lower efficiency, above all, if compared to HAWTs. To improve VAWTs performance, industries and researchers are trying to optimize the design of the rotors. Some numerical codes like the Vortex Method or Multiple Stream tube Model have been developed to predict VAWTs performance and maximize efficiency. Still, these codes are based on 1D simplified equations, and they need accurate experimental data for aerodynamic coefficients of the airfoils.
Furthermore, they do not provide information on the wakes, and they use semi-empirical equations to predict effects like tip vortex and dynamic stall. These codes are so mainly used to do a first attempt design, and the results have to be validated using wind tunnel experiments. However, as wind tunnel experiments are expensive in terms of both costs and time, another way to study the aerodynamic behavior of the rotors is to use CFD. As it is known, CFD resolves the fluid dynamic equations, and it is certainly more realistic. In this regard, CFD has been employed to evaluate this type of turbine evaluation in this study.
This project is going to simulate an airflow field close to a vertical axis Darrieus wind turbine. The geometry included a rotary zone for the turbine walls and a stationary zone for the rest of the domain. The inlet is considered to wind with 1 m/s, and the turbine zone is rotating with 120 RPM. The purpose of this project is to investigate the behavior of airflow and pressure distribution and study drag force.
To study a horizontal wind turbine, one must solve the flow equations in the differential form. Assuming an isothermal, incompressible condition for the air around the blades, two forces known as the Coriolis and centripetal accelerations are the important source terms exerting on the flow elements. These forces are appearing as the rotating zone starts to move in the current simulation. Briefly, the governing mass and momentum equations are written as follows:
Darrieus Geometry and Mesh
As a numerical study, the initial step towards the modeling is the production of the CAD geometry, which is depicted below. The blue face is considered as the inlet of the domain, while the red face on the other side is regarded as the outlet. The current computational domain is the representation of the wind turbine that we have evaluated the turbine. For the current problem, a mesh count of 2,289,621 elements was created to represent the geometry. Regarding the quality of the mesh, the maximum skewness of 0.70 with an average of 0.24 is a fine mesh for the current problem. In addition, for an interested reader, the quality distribution of mesh is shown as follows. Also, 5 prism layers were added adjacent to both wind tunnel walls and the turbine’s body to accurately calculate the boundary layer. Finally, the mesh is generated through ANSYS-Meshing and is as below.
As a final note, due to having a turbomachinery simulation, we separate a cylindrical zone from the whole computational geometry as the rotary zone.
Darrieus CFD Simulation Settings
By importing the mesh into the ANSYS-FLUENT software, we start the calculation procedure. As discussed before, an incompressible, isothermal condition is a valid assumption for the current simulation. However, we ignore the gravity for two main reasons. First, the gravity source would produce equivalent force for the fluid cells if we consider an isothermal condition. Thus, it won’t affect the character of the fluid flow.
Moreover, the flow field is fully turbulent. Thus, we select the k-w-SST turbulent model for the evaluation of eddies. The noted model is more accurate than any other eddy-viscosity variation due to a hybrid formulation taking care of both wall effects and the core flow strain rate. Details of the solution setup are as follows:
Table (1)- Solver Settings
|Solver settings: (Darrieus)
|Zone:||Static fluid zone: Rectangular Box: default
Rotary fluid zone: Cylindrical: Mesh-Motion
Axis point: (0,0,0)
Rotational Speed: 120 RPM
|Boundary conditions:||Turbine Walls: No-slip
Inlet: velocity inlet: 1 m/s
Outlet: pressure outlet
Wind Tunnel walls: Symmetry
|Operating Condition:||Reference Pressure Point: 101325 Pa|
|Solver Properties: (Darrieus)
|Solution methods:||Coupled Pseudo Transient|
|Pressure interpolation scheme:||Second-Order|
Time-step: 1e-3, Number of Iterations = 5000
|Initialization:||Standard > from inlet|
|Material used: (Darrieus)
|Fluid:sd||Air – constant properties
Density: 1.225 kg/(m3)
Viscosity: 1.7894×10-5 (Pa.s)
||Drag Value of Blade wall in Y-direction|
Results and Discussions
After the solution convergence, we can observe the results through post-processing. Meanwhile, as an assurance for a valid convergence, we monitor the drag value during the solution iterations. In this study, the solution converged when the drag force reached a constant rate, and the residuals were below 10-4 values. As an initial check, we evaluate the max value of y plus (Y+) to decide the consistency of the boundary layer mesh. Fortunately for this case, the maximum y plus value was less than 70.
Afterward, we present the results regarding the pressure and the velocity field below the figures. The leading edge of the turbine wall suffers from the highest-pressure gradient, which is entirely logical since the velocity has just met zero.
For the velocity field, we present both contour and streamlines to give much insight into the problem. Briefly, the velocity field adjacent to the wall of the turbine has the highest gradient. This could be, again, observed through the velocity vectors. Additionally, the streamlines vectors illustrate the quality of the flow streams resolved in the wake section, which is the core challenge of aerodynamic simulation.
Finally, the drag force is 0.1826 (N), which is accurate for a turbine with the noted specifications.
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.