1. Introduction:
Blades are an important part of the horizontal axis wind turbine (HAWT) as they are used to generate aerodynamic forces such as lift and drag. The main objective is to carry out a two-dimensional CFD analysis on a HAWT blade aerofoil using the ANSYS Fluent Solver and to compare the numerical results against the experimental data. In this study, NACA 63(4)-221 aerofoil profile is chosen for the 2D simulation. In the CFD analysis, the lift, drag, and moment forces should be calculated for the blade profile at a wide range of angles of attack.
2. Blade Profile:
NACA 63(4)-221 aerofoil profile is shown in the below figure. The maximum thickness of the aerofoil is 21%, which is located at 35% of the chord length.
3. Flow Conditions:
In this simulation, the flow Reynolds number is 𝑅𝑒=3×106, based on the free stream velocity and blade chord length (𝑐=1 𝑚). The air kinematic viscosity is 𝜈=1.5×10−5𝑚2𝑠⁄ and the turbulence intensity is set at 0.07%. The aerodynamic forces and flow field characteristics are obtained at angles of attack ranging from α=0.0 to 8.0 degrees, in the step of 2.0 degrees.
4. CFD steps:
4.1: Introduction
- Our main objective is to get the results of the drag, lift, and moment coefficient for NACA 63(4)-221 at different Angles of Attack (0-2-4-6-8) Also to make comparisons between the contours of pressure and velocity and to plot the Pressure coefficient and shear wall stress around the airfoil.
- Carrying out CFD analysis of a rotor-blade design allows an engineer to virtually assess how the turbine will function in real-life conditions once constructed. For example, designs can be tested under varied conditions by simply changing the boundary conditions to mimic fluctuating temperatures and average wind speeds.
- For blade design, engineers can use simulation to analyze how blade length, chord length of the blade airfoil, the twist angle in the blade, or pitching to adjust the angle of attack, affect the overall performance of a turbine.
4.2: Pre-proccessing stage
- Only one mesh is created for the five results of the five (0:8) angles of attack. And five setups are performed, and for each setup, the directions of flow are changed:
Table.1
- From Re= 3×10^6 & Kinematic viscosity= 1.5x 10^-5 m2/s. So velocity= 45 m/s which is not a high velocity and we are subsonic.
- As we are at a low Mach number (0.1312), so no need to use energy equations or ideal-gas properties
- a2D Geometry is chosen for better convergence and simpler simulation. NACA 63(4)-221 is modeled as fifty-one points for design modeler ANSYS. Then a fluid domain was created.
- A concentrated meshing around the airfoil is required for getting high-realistic results. Much fewer elements are used in mesh size around the airfoil.
Names are selected as shown:
- sizing and quality of mesh
- Nodes: 297752
- Elements: 296836
- Skewness average: 8.4203e-002
- Orthogonal Quality: 0.9853
- As introduced we have to make more inflation cells and more concentrated mesh around the airfoil. So Y+ checking is needed, more concentrated mesh at the tip of the airfoil as this area is so much interesting in the simulation.
4.3. Solver setting and solution algorithm
- As we are at a low Mach number, a pressure-based solver is used. A gravity force can be added for a more realistic result.
- At inlet boundary condition; we define velocity as a (Magnitude and Direction)
- for magnitude, we use 45 m/s for the whole simulations. However, the X and Y-components of Flow Direction will be set as we it is shown in Table 1.
- For Viscous Mode: K- omega SST is suitable for our simulation as we are at not high Mach and the flow is not inner.
- About solution methods; A “coupled” scheme Pressure-Velocity coupling is used. In addition to, “second-order” for pressure, Momentum, and Turbulent kinetic Energy.
- For the results of coefficients of drag, lift, and moment;
- Force vector at Cd and Cl are needed to change as changing of the angle of attack, according to the values of Table.1. for example, at the angle of attack= 4;
- Drag force vectors (0.99756, 0.0698)
- Lift Force vectors (-0.0698, 0.99756)
- The moment center is 0.25*C as “C” is the cord of the airfoil. And the Moment Axis is always in the Z direction.
- We set convergence at 1e-6 instead of 1e-4 for getting more accurate results at the number of 1000 iterations. Fortunately, the solution converged at the 326th iteration at Angle of Attack=0.
- CFD post is used to get the results of pressure coefficient (Cp), Wall shear stress, and contours of pressure, Velocity, Mach number.
- As ANSYS supports defining users’ variables, so “Pressure Coefficient” and “Mach Number” are added.
- As Cp= P/0.5*Density*Gravit*Velocity^2
- Mach number= Velocity/ Sound speed.
5. Results:
5.1. Coefficients of lift, drag, and moment:
| Angle of attack | 0 | 2 | 4 | 6 | 8 |
| CL | 0.1624 | 0.26975 | 0.3769 | 0.4825 | 0.585 |
| CD | 0.0107 | 0.01556 | 0.024 | 0.036 | 0.0515 |
| CM | 0.0359 | 0.0363 | 0.03662 | 0.0368 | 0.037 |
5.2. Pressure coefficient (Cp) on the airfoil at each angle of attack:
5.2.1. at an angle of attack= 0
5.2.5 At an angle of attack =8
5.3. Shear wall stress on the airfoil at each angle of attack:
5.3.1 At an angle of attack= 0
5.3.5 At an angle of attack= 8
5.4. Result contours of Pressure around the airfoil
5.4.1 At an angle of attack= 0
5.4.5 At an angle of attack= 8
5.4. Resulting contours of Mach number around the airfoil:
5.4.1. At angle of Attack =0
5.4.5. At angle of Attack = 8
6. Conclusion
The results show that increasing the angle of attack increases the lift and drag values, but the increase in lift is much more significant than the slight increase in drag. The coefficient of pressure charts indicate that increasing the angle of attack results in a greater difference between the pressure above and below the airfoil, which increases the lift force. The pressure contours show that the pressure below the airfoil increases with the angle of attack, while the pressure above the airfoil decreases. The velocity and Mach number contours show that the velocity below the airfoil decreases with the angle of attack, while the velocity above the airfoil increases. The shear wall stress also shows an increase in stress on the airfoil and a decrease in stress above the airfoil with an increase in the angle of attack.