Flow over an airfoil
AIM:
To perform flow over an airfoil at different angle of attacks and study the velocity plots and ovserve the trends in the change of drag and lift produced in each cases with angle of attack of 0, 2, 4, 6 , 8, 10.
INTRODUCTION:
In this simulation, we will perform flow simulation over a NACA airfoil. It can be done by importing the cordinates to the solidworks curve, and then simply extruding it with the help of convert entities. Then, we campare the lift and drag forces caused due to the air flow and compare them with diifferent angle of attacks. The lift and drag forces are simply the forces along X-axis and Y-axis of the airfoil and can be calculated with the ongoing simulation.
After getting the results, we can change the angle of attack by rotating the airfoil with the help of the feature 'rotate entities' on the ribbon menu of sketch by our desired angle and then perform the same steps to get the results for that case of angle of attack.
THEORY:
An airfoil is the cross-sectional figure of a wing/blade. It represents the wings that are supposes to provide flight. 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.In oor case, the fluid is air, moving at 600m/s at X-direction and the drag is the force exerted on the airfoil on X-axis while lift is the force on the airfoil on Y-axis. Lift is the force responsible for elevating the body while drag is the resisting force from the fluid on the body. Theese forces are directly related to the angle of attack. The higher the angle of attack, the higher the drag and lift forces. But its still favourable, since we get better lift even at the cost of increasing drag. So, we need to find a suitable angle of attack for maximum efficiency with higher lift force and minimal drag force, since drag is not good for the flight and a lot of fuel energy is wasted to overcome it.
Angle of attack is the angle between the oncoming air or relative wind and a reference line on the airplane or wing. It is responsible for both the forces on the airfoil. Its very important to chose the right AOA since it crucially influences the parameters for the flight.
The shape of the airfoil is constructed in such a way, to maximize the lift produced with as low of drag force as possible. Even the shape is made spherical, since we know that, the inncer face of the sphere always experiences a higher pressure, so this geometry also helps in further increasing the lift of the airfoil.
SIMULATION:
The following procedure is done with an angle of attack of 0 degrees.
To carry out the simulation, we first have to import the cordinates of the airfoil we need(AG13(agi1-il). It was done by copying the cordinates for that airfoil into a textfile and then importing those cordinates in solidworks with the insert curve feature. It should be noted that the data should be in a format in which solidworks can read it ie- it shoud only contain X-Y-Z ccordinates only, as columns. It should also be taken into account that the unit system should be MKS, or else the airfoil becomes too small for our simulation.
After inserting the curve, it is to be converted into a sketch in the front place and extruded. This completes the modelling of the airfoil.
To begin the simulation, we create a new project wizard with the following parameters:
- Unit system as Si
- Flow type as external and time dependent
- Total analysis time at 5s, and timestep at 1s
- Add air as our flowing fluid
- Add a velocity of 600m/s along the X- direction
After completing the above steps, we are presented with a computational fluid domain. Since we are performing fluid simulation over an airfoil, we need a larger domain than the default one. So, we change the domain size by editing its definition, since the normal length of the domain should be 5 times the length of the airfoil at the inlet of the airfoil and 10-15 times after the outlet and our chord length of the airfoil is 1m.
After sitting the length of the domain, we check the basic mesh of the domain. The basic will get the job done for our simulation, but we add a local mesh on the surface of the airfoil for better results.
Finally, we add goals for our simulation, as force along X-axis and force along Y-axis as we have to compare the drag and lift forces for each cases and finally, we hit the run button.
It is not necessary to wait for the whole simulation to be over. We can visualize the simulation in real time and analyze the results. It can be done with the preview and goal plot options in the 'insert menu'. After around 200 iterations, we see that, the lift force doestnt change anymore ie, it converges. Which mean, the value wont change much even after successive iterations so it would be wise choice to stop the simulation at that point and save our results.
After saving the results from the calculations, we can access them from the results pannel.
velocity contour for AOA at 0 degrees:
drag force for airfoil at AOA at 0 degrees:
lift force for airfoil at AOA at 0 degrees:
Average value of drag force = 4847 N
Average value of lift force = 2480 N
From the obtained results, we can see that the combination of the drag and lift force is very bad. The drag force exceeds the lift. Hence, this AOA is not very favourable.
The simulation for the other case of AOA can be done by rotating our base sketch by -2 degrees after each run.
CASE-2(AOA=2 degrees):
velocity contour for AOA at 2 degrees:
drag force for airfoil at AOA at 2 degrees:
lift force for airfoil at AOA at 2 degrees:
Average value of drag force = 3389.69 N
Average value of lift force = 7710.95 N
By increasing the AOA even slightly, we see a lot of improvement in our result. The lift force drastically increased and the drag florce also drastically decreased, making this AOA much better than the previous.
CASE-3(AOA=4 degrees):
velocity contour for AOA at 4 degrees:
drag force for airfoil at AOA at 4 degrees:
lift force for airfoil at AOA at 4 degrees:
Average value of drag force = 7295.25 N
Average value of lift force = 11993.3 N
In this case, increasing the AOA improved the produced lift, but it drastically increased the drag force produced as well. If the increase in the lift is not worth the extra drag, then we should not consider this AOA over the previous one.
CASE-4(AOA=6 degrees):
velocity contour for AOA at 6 degrees:
drag force for airfoil at AOA at 6 degrees:
lift force for airfoil at AOA at 6 degrees:
Average value of drag force = 7603.94 N
Average value of lift force = 17282.2 N
After further increasing the AOA, we see a much better improvement again. The lift foce drastically increases without any significant change in the drag force, making it a much better choice than the previous one.
CASE-5(AOA=8 degrees):
velocity contour for AOA at 8 degrees:
drag force for airfoil at AOA at 8 degrees:
lift force for airfoil at AOA at 8 degrees:
Average value of drag force = 8216.49 N
Average value of lift force = 26634.4 N
Yet again, we see an improvement in our results after increasing the AOA. The results follow the trend and we get a better drift at the cost just a mere increase in the drag force.
CASE-6(AOA=10 degrees):
velocity contour for AOA at 10 degrees:
drag force for airfoil at AOA at 10 degrees:
lift force for airfoil at AOA at 10 degrees:
Average value of drag force = 9208.63 N
Average value of lift force = 31675.8 N
The final case of the AOA brings the highest lift force without any significant increase in the drag force. Hence, this is the best AOA to chose among the above cases.
OBSERVATIONS AND CONCLUSION:
From the above simulations and obtained results, we observe that, no matter how good the aerodynamic design of the airfoil is, eventually it all comes down to the angle of attack. At case-1, the angle of attack is 0 degrees hence,we get a very poor combination of drag and lift force. The average drag force value is evn highter than the lift produced. So, its a poor angle of attack.
This result can be justified because of the bad selection of the angle of attack. Since its 0 degrees, the incoming air hits head on with the airfoil. Compared to other cases, they hit the airfoil at an angle. So in a way, they hit the lower face of the airfoil and naturally create a force on the airfoil to lift it.
And incase of 0 degrees head on collision, the poor angle infact produces even more drag. By increasing the angle upto a certain threshold, we keep on getting better combination of the two forces ie, better lift force and minimal drag. Hence its very important to decide the best angle of attack according to our needs.
The same procedure can be repeated to get results for even higher AOA but the results will improve only till a certain AOA as after a certain AOA, the drag force would be just too much.
DATABESE OF THE AIRFOIL CORDINATES: