Step 7: Validate the Results
Report Force
Info | ||||||||
---|---|---|---|---|---|---|---|---|
| ||||||||
FLUENT report forces in term of pressure force and viscous force. For instance, we are interested in the drag on the airfoil, (Drag)total = (Drag)pressure + (Drag)viscous Drag due to pressure:
Drag due to viscous effect:
where ed is the unit vector parallel to the flow direction. n is unit vector perpendicular to the surface of airfoil. t is unit vector parallel to the surface of airfoil. Similarly, if we are interested in the lift on the airfoil, (Lift) = (Lift)pressure + (Lift)viscous Lift due to pressure:
Lift due to viscous effect:
where el is the unit vector perpendicular to the flow direction. n is unit vector perpendicular to the surface of airfoil. t is unit vector parallel to the surface of airfoil. |
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:
...
No Format |
---|
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 |
...
Cd = (Cd)pressure + (Cd)skin friction
where
(Cd)pressure is due to pressure force.
(Cd)skin friction is due to viscous force.
Indeed, we see that the (Cd)skin friction is zero because of the inviscid model.
Info | ||
---|---|---|
| ||
In reality, (Cd)skin friction has biggest contribution to drag but ignored because of the inviscid model that we specify. (Cd)pressure should be zero, but it is not zero because of inaccuracies and numerical dissipation during the computation. |
Now, let's look at the lift coefficient.
Main Menu > Report > Forces...
Select Forces. Under Force Vector, enter -0.02094
next to X. Enter 0.9998
next to Y. Select airfoil under Wall Zones. Click Print.
Here's is what we see in the main menu:
No Format |
---|
Force vector: (-0.02094 0.99980003 0)
pressure viscous total pressure viscous total
zone name force force force coefficient coefficient coefficient
n n n
------------------------- -------------- -------------- -------------- -------------- -------------- --------------
airfoil 1008.3759 0 1008.3759 0.6585058 0 0.6585058
------------------------- -------------- -------------- -------------- -------------- -------------- --------------
net 1008.3759 0 1008.3759 0.6585058 0 0.6585058
|
Similarly, lift force is due to the contribution of pressure force and viscous force.
Cl = (Cl)pressure + (Cl)skin friction
where
(Cl)pressure is due to pressure force.
(Cl)skin friction is due to viscous force.
Since our model is inviscid, (Cl)skin friction is zero. We see that the lift coefficient compare well with the experimental value of 0.6.
Info | ||
---|---|---|
| ||
Do note that the lift coefficient for inviscid model is higher than the experimental value. In reality, if we take into account the effect of viscosity, we will have (Cl)skin friction of negative value. The viscous effect will lower the overall lift coefficient. Since our inviscid model neglect the effect of viscosity, we have a slightly higher lift coefficient compared to the experimental data. |
Grid Convergence
A finer mesh with four times the original mesh density was created. The lift coefficient was found to be 0.649.
...
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 l | Cd | |
FLUENT Fine Mesh | 0.649 | 0.00137 |
Experiment | 0.6 | 0.007 |
Theory | - | 0 |
...