Flow over an airfoil
Project Title: Flow over an airfoil
Objective:
1. Study the flow around the airfoil.
2. Study the pressure distribution over an airfoil.
3. Study the different force acting on to the airfoil.
4. Study the drag force and lift force acting on the airfoil.
Theory:
AirFoil
MAIN FUNCTIONAL REQUIREMENT:
Lift and weight are two of the four forces acting on an aeroplane, the other two are drag and thrust (see Figure 1).
GEOMETRY/STRUCTURE:
The aeroplane generates lift using its wings. The cross-sectional shape of the wing is called an airfoil. A typical airfoil and its properties are shown in Figure 2, and are described below.
EXPLANATION OF HOW IT WORKS/ IS USED:
The wings provide lift by creating a situation where the pressure above the wing is lower than the pressure below the wing. Since the pressure below the wing is higher than the pressure above the wing, there is a net force upwards.
To create this pressure difference, the surface of the wing must satisfy one or both of the following conditions. The wing surface must be:
- Cambered (curved); and/or
- Inclined relative to the airflow direction.
Several airfoils are shown in Figure 3. However, the air foils shown in Figure 3 are useless without viscosity.
Viscosity is essential in generating lift. The effects of viscosity lead to the formation of the starting vortex (see Figure 4), which, in turn is responsible for producing the proper conditions for lift.
As shown in Figure 4, the starting vortex rotates in a counter-clockwise direction. To satisfy the conservation of angular momentum, there must be an equivalent motion to oppose the vortex movement. This takes the form of circulation around the wing, as shown in Figure 5. The velocity vectors from this counter circulation add to the free flow velocity vectors, thus resulting in a higher velocity above the wing and a lower velocity below the wing (see Figure 6).
The following presents two of several ways to show that there is a lower pressure above the wing than below.
One method is with the Bernoulli Equation, which shows that because the velocity of the fluid below the wing is lower than the velocity of the fluid above the wing, the pressure below the wing is higher than the pressure above the wing.
A second approach uses Euler's Equations (which the Bernoulli equation is derived from) across the streamlines. Due to the curvature of the wing, the higher velocities and acceleration over the top of the wing requires a pressure above the wing lower than the ambient pressure.
Thus, using either of the two methods, it is shown that the pressure below the wing is higher than the pressure above the wing. This pressure difference results in an upward lifting force on the wing, allowing the aeroplane to fly in the air.
Forces acting on airfoil and pressure distribution around the aerofoil.
Variation of the lift coefficient with the angle of attack for a symmetrical and non-symmetrical airfoil
Problem Statement:
- Model the flow over a NACA0017 airfoil
- For the angle of attacks 0,2,4,6,8,10 compare the lift and drag forces
You can use any Airfoil from the AirFoil Data Base. While copying the Airfoil data to a text file, make sure that you format it in a way that SolidWorks can read it. A sample file is attached in the link below,
Solution
Geometry
NACA0017 airfoil
Computational domain
At the origin, the airfoil is placed.
COmputational domain dimensions
Flow is supersonic so the computational domain is kept big at top, bottom and behind the airfoil.
Meshing
Hex mesh is used in this simulation.
Accuracy of hex mesh is more compared to triangular mesh.
Global mesh setting
Global mesh refinement settings.
Advanced channel refinement is done around the airfoil.
Local mesh setting
Local mesh refinement settings.
Flow setup
Steady-state external flow simulation is performed
Working fluid
Working fluid in this simulation is considered is air
Initial boundary condition
The initial pressure is 101325 Pa
The initial temperature is 293.2 K
Velocity in x-direction 600 m/sec.
Result
Case 1
The 0-degree inclination of the airfoil
Convergence
Solution is converged around 300 number of iterations.
Drag force is considered for convergence criteria due to uncertainty with software convergence criteria.
Pressure distribution
The pressure is maximum at stagnation point due to all kinetic energy is converted into the potential energy.
As air move around the airfoil, pressure starts reducing
Maximum pressure 455150 Pa
Minimum pressure 56471.12 Pa
velocity distribution
Velocity and pressure have a reverse correlation with each other.
Velocity is maximum where pressure is minimum and vice versa.
Drag force & lift force
Lift force is in the y-direction. the value of the lift force is 4.47 N.
Drag force is the flow x-direction. the value of drag force is 1561.37 N.
Case 2
2 degrees inclination of the airfoil
Convergence
The solution is converged around 270 number of iterations.
Drag force is considered for convergence criteria due to uncertainty with software convergence criteria.
Pressure distribution
Maximum pressure 450652.70 Pa
Minimum pressure 51607 Pa
The pressure is maximum at stagnation point due to all kinetic energy is converted into the potential energy.
As air move around the airfoil, pressure starts reducing
velocity distribution
Velocity and pressure have a reverse correlation with each other.
Velocity is maximum where pressure is minimum and vice versa.
Drag force & lift force
Lift force is in the y-direction. the value of the lift force is 1599.261 N.
Drag force is the flow x-direction. the value of the drag force is 747.932 N.
case 3
4 degrees inclination of the airfoil
Convergence
The solution is converged around 270 number of iterations.
Drag force is considered for convergence criteria due to uncertainty with software convergence criteria.
Pressure distribution
Maximum pressure 450279 Pa
Minimum pressure 46496 Pa
The pressure is maximum at stagnation point due to all kinetic energy is converted into the potential energy.
As air move around the airfoil, pressure starts reducing
velocity distribution
Velocity and pressure have a reverse correlation with each other.
Velocity is maximum where pressure is minimum and vice versa.
Drag force & lift force
Lift force is in the y-direction. the value of the lift force is 1671597 N.
Drag force is the flow x-direction. the value of the drag force is 1425.978 N.
case 4
6 degrees inclination of the airfoil
Convergence
The solution is converged around 270 number of iterations.
Drag force is considered for convergence criteria due to uncertainty with software convergence criteria.
Pressure distribution
Maximum pressure 450831 Pa
Minimum pressure 40709 Pa
The pressure is maximum at stagnation point due to all kinetic energy is converted into the potential energy.
As air move around the airfoil, pressure starts reducing
velocity distribution
Velocity and pressure have a reverse correlation with each other.
Velocity is maximum where pressure is minimum and vice versa.
Drag force & lift force
Lift force is in the y-direction. the value of the lift force is 1780.759 N.
Drag force is the flow x-direction. the value of the drag force is 227.578 N.
case 5
8 degrees inclination of the airfoil
Convergence
The solution is converged around 270 number of iterations.
Drag force is considered for convergence criteria due to uncertainty with software convergence criteria.
Pressure distribution
Maximum pressure 447872 Pa
Minimum pressure 37848 Pa
The pressure is maximum at stagnation point due to all kinetic energy is converted into the potential energy.
As air move around the airfoil, pressure starts reducing
velocity distribution
Velocity and pressure have a reverse correlation with each other.
Velocity is maximum where pressure is minimum and vice versa.
Drag force & lift force
Lift force is in the y-direction. the value of the lift force is 1933.093 N.
Drag force is the flow x-direction. the value of the drag force is 2967.179 N.
case 6
10 degrees inclination of the airfoil
Convergence
The solution is converged around 270 number of iterations.
Drag force is considered for convergence criteria due to uncertainty with software convergence criteria.
Pressure distribution
Maximum pressure 465947 Pa
Minimum pressure 32649 Pa
The pressure is maximum at stagnation point due to all kinetic energy is converted into the potential energy.
As air move around the airfoil, pressure starts reducing
velocity distribution
Velocity and pressure have a reverse correlation with each other.
Velocity is maximum where pressure is minimum and vice versa.
Drag force & lift force
Lift force is in the y-direction. the value of the lift force is 2146.824 N.
Drag force is the flow x-direction. the value of the drag force is 3776.794 N.
Conclusion:
1. As the angle of attack increase lift force and drag force increases.
2. The angle of a stall is the angle at which airfoil get maximum lift.
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number.