Flow over an airfoil

Aim

To perform an external flow simulation on a NACA0017 airfoil and compare the drag and lift force for every angle of attack at 0,2,4,6,8,10 deg.

Theory:

Airfoil : An airfoil is term used to define the waterdrop shape of cross-section of an object like wings,blades,sails that when in flow created an aerodynamic force i.e lift or thrust.An airfoil is the cross-sectional shape of a wing, blade (of a propeller, rotor, or turbine), or sail. 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 trailing edge, often with a symmetric curvature of upper and lower surfaces. Foils of similar function designed with water as the working fluid are called hydrofoils.

NACA Airfoil : NACA stands for National Advisory Committee for Aeronautics. The NACA airfoils are used to design aircraft wings, which consists of a series of numbers. The first digit describes maximum camber as a percentage of chord. The second digit describes the distance of maximum camber from airfoil leading edge intents of chord. The last two digits gives the maximum thickness of the airfoil as percent of chord.

Drag force - As with lift, the drag of an airfoil depends on the density of the air, the velocity of the airflow, the viscosity and compressibility of the air, the surface area of the airfoil, the shape of the airfoil, and the angle of attack.The complexities associated with drag and the airfoil's shape, angle of attack, the air's viscosity, and air's compressibility are simplified in the drag equation by use of the drag coefficient. The drag coefficient is generally found through testing in a wind tunnel, where the drag can be measured, and the drag coefficient is calculated by rearranging the drag equation,

where

is the drag force,

is the density of the fluid,

is the speed of the object relative to the fluid,

is the cross-section area, and

is the drag coefficient

(The drag coefficient depends on the shape of the object and on the Reynolds number)

Procedure for setup :

• Modelling - At first, the coordinates of airfoil is imported into Solidworks through insert curve options and in a suitable plane its converted into an entity for further sketching. The tail section of the airfoil is not closed so a line is drawn between the two point of tail to make it a closed sketch. For simulation for different angle of attacks, it can be rotated about a reference point by providing angle of rotation. Then it is extruded to a certain width (5mm) .

Flow simulation setup:

Parameters:

• Flow Simulation type = External
• Pressure = 101325 Pa
• Temperature = 293.2 K
• Velocity along x-axis = 600 m/s
• Medium = Air
• Type of flow: Laminar & Turbulent,
• Wall Condition: Adiabatic Wall with 0 micro-meter roughness,

Computational domain :

At inletside 5 times the chord length and at outlets side usually 10-15 times the chord length.

Global and Local mesh :

Global goals and surface goals were given for forces in x and y directions. A local mesh was inserted for the top surface of airfoil for a much refinement around it.

results :

case 1 : Angle of attack 0

pressure cut plot contour -

Velocity cut plot contour -

Case 2 : Angle of attack 2 deg

Pressure contour cut plot-

Velocity contour cut plot-

Case 3 : Angle of attack 6 deg

Pressure contour cut plot-

Velocity contour cut plot-

Case 4 : Angle of attack 8 deg

Pressure contour cut plot-

Velocity contour cut plot-

Case 5 : Angle of attack 10 deg

Pressure contour cut plot-

Velocity contour cut plot-

Conclusion:

• As the angle of attack increases the lift force increases over drag force
• As the angle of attack increases the velocity profile and pressure distribution above the airfoil increases as compared to the lower face of an airfoil.
• The pressure force is more than the pressure above the airfoil. it affects to push airfoil upward. As per the relation between the pressure and velocity higher the pressure lower the velocity, low pressure above the airfoil feels free to lift upward.
• As angle of attack changes airflow direction changes by the boundary layer separation pressure and velocity values changes and tends to changes in lift and drag force.