Flow over Airfoil - Performance characteristic simulation
INTRODUCTION:
Flow over external bodies could be broadly classified into flow over blunt bodies like a cylinder and flow over streamlined bodies. https://skill-lync.com/projects/tacoma-bridge-flow-over-cylinder, This link depicts the importance of flow over blunt bodies and concepts of wake, flow separation. These concepts hold true in case of streamlined body as well but with subtle differences in manifestation.
Airfoils are 2D projection of a 3D wing. The design of Airfoil is such that flow would be attached to the surface at zero angle of attack or in other words body can easily fit in the fluid flow without major flow disruption, known as streamlined body. Airfoils finds application not in aeroplane wing design but also in spoilers in racecars, wind turbine blades etc. Whenever we speak of objects heavier than air flying through the stream of air, we notice, four forces: Lift, Drag, Thrust and weight. Weight is the downward force which forces the body to ground under action of gravity, Lift, in order for an object to counteract weight and levitate in air, equivalent counter force needs to be generated by various means and this force is referred as the Lift.
In order for the object to propel in the air, thrust force or propelling force is supplied by means of external energy like Gas turbines, Scramjets, Motors. The object develops resistance to the movement in the fluid and this is referred as the Drag. Drag could be broadly classified into Pressure drag and Friction drag. As the name indicates, as the flow separation becomes dominant, momentum imbalance is created and is termed as the pressure drag. Friction drag is the resistance of the body through the fluid.
There are many conflicting theories to determine what really produces lift. What we know is that the Velocity on the top surface is higher than the velocity on the bottom surface which in turn creates the pressure imbalance and hence the Force. The cause of velocity or pressure difference between top and bottom surface seems more complex than widely spread incorrect theories. Bernoulli's camp where long path and equal transit theory is brought forward and newton's camp where Thrid law is applied. However, both the theories are true partially and conservation of mass, momentum and energy simulataneously applied would help realise the behaviour.
This behaviour gives raise to one important aspect. STALLING. As the airfoils angle of attack which is the angle between chord of airfoil and freestream velocity increases, flow sepration starts on the top surface, this causes rise in pressure drag and with no appreciable increase in the lift. This condition is known as Stalling. The angle of attack at which this phenomenon starts is referred as the critical angle of attack, typically critical angle of attack lies between 17-25 deg for most type of airfoil
Airfoils are typically classified into Symmetrical and cambered airfoils. In symmetric airfoil thickness from the chord to bottom and top surface is same, however, in cambered airfoil, thickness: distacnce from chord to top surface is higher than the bottom surface.
There is wide directory of airfoil designed for various coniditions, NACA, Seyligs, E Series etc. NACA is the NASA designated airfoils which follow the particular convention of naming. It consists of 4 digits.
NACA MPXX where M stands for the maximum camber as the percentage of chord, P stands for the position of maximum camber as the percentage of chord, XX stands for the thickness as the percentage of the chord.
In this work, NACA0017 is used where 00 indicates symmetrical, 17% thickness of the airfoil. The curve is generated using this website.http://airfoiltools.com/airfoil
CFD SIMULATION
Geometry - Airfoil data generated from the above site is converted into shape with length of 1 mm and extruded. The analysis is carried out in 2 dimension
Meshing - Automatic meshing feature is utilised with meshing level set to 4, local meshing is applied near the surface of airfoil with level of 3. Advance refinement is also applied.
Initial setup: Air is used as the object and velocity of air is set to 600m/s. External time dependent simulation is set for analysis. Goals are created to capture force in X,Y direction.
Computational domain: In order to prevent reflection, distance upstream to airfoil is set to 5 times chord length, downstream set to 15 times chord length, top surface and bottom surface are individually at 12 times chord length.
RESULTS AND PLOTS
Paramteric study is carried to find the critical angle of attack, however, for angle of attack from 0-10, following lift and drag values were found.
| Angle of attack | Lift(N) | Drag(N) |
| 0 | -3.5E-06 | 0.007746 |
| 2 | 0.003236 | 0.00783 |
| 4 | 0.006 | 0.008 |
| 6 | 0.009 | 0.009 |
| 8 | 0.013 | 0.009 |
| 10 | 0.016 | 0.01 |
AOA: 0Deg
AOA: 2deg
AOA: 4deg
AOA:6deg
AOA: 8deg
AOA: 10deg
DISCUSSIONS
1) As the angle of attack is increased lift increases to a point and then stays constant and starts decreasing, For NACA 0018, performance characteristic from the CFD sim is as follows
2) From, the above diagram, at 21 degrees, flow separation starts at the trailing edge and at 29degrees, stalling starts
3) For Cambered airfoil, Critical angle of attack is close to 17-20degrees and Symmetric airfoils have higher critical angle of attack than cambered but with lesser net lift force (When thickness is kept constant)
4) Drag forces increases exponentiallu with the increase in angle of attack
5) Coefficient of lift and Coefficient of drag could be calculated and plotted
6) Velocity contour during stalling
References: