Flow over an airfoil

AIM:
To run the Flow simulations on NACA0017 airfoil at an angle of attacks 0, 2, 4, 6, 8, 10 degrees. And compare the change in lift force and drag force at these angles of attacks.

INTRODUCTION:
An airfoil or aerofoil is the cross-sectional shape of a wing, blade (of a propeller, rotor, or turbine), or sail (as seen in cross-section). An airfoil-shaped body moving through a fluid produces an aerodynamic force. The component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called drag. Subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp, often with a symmetric curvature of upper and lower surfaces. Foils of similar function designed with water as the working fluid are called hydrofoil. The lift on an airfoil is primarily the result of its angle of attack. When oriented at a suitable angle, the airfoil deflects the oncoming air (for fixed-wing aircraft, a downward force), resulting in a force on the airfoil in the direction opposite to the deflection. This force is known as aerodynamic force and can be resolved into two components: lift and drag.

THEORY:
The main function of the wing is to provide enough lift to counter the weight of the plain. Lift and weight are two of the four forces acting on the airplane, the other two are drag and thrust. The airplane generates lift using its wings. The cross-sectional shape of the wing is called an airfoil. A typical airfoil and its properties are 'chord, camber, leading-edge, airflow direction, angle of attack, trailing edge'.

CHORD: Extends from the leading edge to the trailing edge of the wing.
CAMBER LINE: Points halfway between a chord and upper wing surface.
ANGLE OF ATTACK: Angle between the direction of airflow and the chord.

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; or
-Inclined relative to the airflow direction.
There are several different kinds of airfoils, There are infinite possibilities but some of them are a)Flat bottom airfoil, b)Slightly curved Bottom, c)Symmetrical.

However, airfoils are useless without viscosity. Viscosity is essential in generating the lift. The effects of viscosity lead to the formation of the starting vortex.

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.

This phenomenon causes velocity vectors from this counter circulation to add to the free flow velocity, thus resulting in a higher velocity above the wing and lower velocity below the wing.

Using Bernoulli Equation or Euler's Equations we can show that there is lower pressure above the wing than below.
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.
EULER'S EQUATION: (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.

In this simulation, the above theory would be evident in results.

SIMULATION SETUP:

• Solidworks is used for performing this simulation. To construct the airfoil geometry, a tabular data of NACA0017 airfoil is downloaded from Airfoils website
• This tabular data is uploaded into Solidworks and is converted to sketch.
• The above sketch is extruded to a certain length.
• By default, the airfoil is at zero degrees and for increasing the angle of attack the foil can be rotated using rotate entities tool about the origin.
• Using the Flow simulation tab in Solidworks add-in the flow simulation features are activated.

Pre-Processing:

• Using the wizard tool the flow properties such as fluid type, flow type, pressure, velocity are selected.
• Fluid type: Air.
• Flow type: Time-dependent. (Total analysis time 5sec and step-time of 1sec)
• Pressure: default value.
• Velocity: 600m/s along X-direction.
• After setting the above values 2d simulation is selected and the computation domain is adjusted such that it is 10m along the positive x-axis and 5m along the negative x-axis, also 5m along both positive and negative Y-axis. This allows us to get a visualization of the simulation.
• Set the simulation goals for calculating surface force along the x and y-direction.
• Global mesh is generated with a level 5 size and Local mesh is generated at the surface of the foil and is refined to level 3 in settings.
• Run the simulation.

Solving:

• Insert the list of goals and goal plots on the solver panel.
• Observe the goal plots if it is converging to a certain value.
• Stop the calculation if the result converges and save the results.
• Load the results.

Post-Processing:

• Insert the cut plots of Velocity and Pressure.
• Observe the flow pattern.
• Insert the goal plots and observe the plots.
• Export the values of the goals to excel sheets.

Perform the steps for `2^@,4^@,6^@,8^@,10^@` angle of attack.

RESULTS AND OBSERVATION:

Velocity contours:

Angle of attack-`0^@`

Angle of attack-`2^@`

Angle of attack-`4^@`

Angle of attack-`6^@`

Angle of attack-`8^@`

Angle of attack-`10^@`

From the above images, it can be observed that as the angle of attack is increased, the velocity below the airfoil is decreasing and velocity above the foil is increasing as observed from the side color panel.

Pressure contours:

Angle of attack-`0^@`

Angle of attack-`2^@`

Angle of attack-`4^@`

Angle of attack-`6^@`

Angle of attack-`8^@`

Angle of attack-`10^@`

From the above images, it can be observed that as the angle of attack is increased, the pressure below the airfoil is increasing and pressure above the foil is descreasing as observed from the side color panel.

Lift force:

Angle of attack-`0^@`

Average value: -0.561555186627333N

Angle of attack-`2^@`

Average value: 1239.5932N

Angle of attack-`4^@`

Average value: 2466.45791744996N

Angle of attack-`6^@`

Average value: 3650.32565383483N

Angle of attack-`8^@`

Average value: 4949.02009138071N

Angle of attack-`10^@`

Average value: 6143.74480256408N

For `0^@` angle of attack the lift force plot is fluctuating since there is fluctuation in magnitude of velocity and pressure over time. But by increasing the angle of attack to plot gets refined and steadily increases and becomes constant. The average value of the lift force for each angle of attack is increasing as we increase the angle of attack.

Drag force:

Angle of attack-`0^@`

Angle of attack-`2^@`

Angle of attack-`4^@`

Angle of attack-`6^@`

Angle of attack-`8^@`

Angle of attack-`10^@`

For `0^@` angle of attack the lift force plot is fluctuating since there is fluctuation in magnitude of velocity and pressure over time. But by increasing the angle of attack to plot gets refined and steadily increases and becomes constant. The average value of the lift force for each angle of attack is increasing as we increase the angle of attack.

Lift force and Drag force:

Angle	Lift force
0	-0.56156
2	1239.593
4	2466.458
6	3650.326
8	4949.02
10	6143.745

Angle	Drag Force
0	464.715482
2	498.295587
4	620.812338
6	839.545539
8	1155.71616
10	1541.84719

CONCLUSION:

In Conclusion, The Lift force and Drag force increase as the angle of attack increases but Lift force increases significantly more than Drag force. Hence for the airplane to fly it has to overcome it's weight by increasing Lift force, this can be done by increasing the angle of attack.