Sign-up for free online course on ANSYS simulations!Step 7: Validate the Results
Report Force
Let's look at the forces on the airfoil. We will first investigate the Drag on the airfoil.
Main Menu > Report > Forces... 
Select Forces. Under Force Vector, enter 0.9998 next to X. Enter 0.02094 next to Y. Select airfoil under Wall Zones. Click Print.
Here's is what we see in the main menu:
Force vector: (0.99980003 0.02094 0) pressure viscous total pressure viscous total zone name force force force coefficient coefficient coefficient n n n ------------------------- -------------- -------------- -------------- -------------- -------------- -------------- airfoil 3.8125084 0 3.8125084 0.0024897052 0 0.0024897052 ------------------------- -------------- -------------- -------------- -------------- -------------- -------------- net 3.8125084 0 3.8125084 0.0024897052 0 0.0024897052
Before we can make any conclusion about the accuracy of our result, we should always make validation check. The most common validation step is grid convergence check.
A finer mesh with four times the original mesh density was created. The lift coefficient was found to be 0.649.
|
Original Mesh |
Fine Mesh |
%Dif |
|---|---|---|---|
Cl |
0.647 |
0.649 |
0.3% |
Cd |
0.00249 |
0.00137 |
45% |
We see that the difference in drag coefficient is very large. We used inviscid case for our model, so we are expecting a Cd of zero. However, since the parameter of interest is the lift coefficient, and the value lift coefficient does not deviate much from original mesh to fine mesh, we concluded that the fine mesh is good enough.
The modeling result obtained is still off from the literature result. Further validation steps are needed before we can conclude about the accuracy of our model. Other parameter that will affect the validity of our result is the choice of viscous model. We used inviscid model which basically assumed that the flow inviscid and totally ignore the effect of boundary layer near the airfoil surface. We might want to try out turbulence model for this high Reynolds number flow.
Summary
Following table shows comparison of modeling result with experimental data.
|
CL |
Cd |
|---|---|---|
FLUENT |
0.647 |
0.00249 |
Experiment |
0.6 |
0.007 |
% Dif |
7.8% |
64% |
|
CL |
Cd |
|---|---|---|
FLUENT Fine Mesh |
0.649 |
0.00137 |
Experiment |
0.6 |
0.007 |
Theory |
- |
0 |
Though further validation steps are still needed before we can come up with a model that will accurately represent the physical flow, this simple tutorial demonstrates the use of reasonable assumption and approximation in obtaining understanding of physical flow properties around an airfoil.
Reference
The experimental data is taken from Theory of Wing Sections By Ira Herbert Abbott, Albert Edward Von Doenhoff pg. 488
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