Fluids

2D Simulation of NACA 6409. Lift is right, but Drag off way off. Why?

• oll5hbdfk
Subscriber

Hi,

I did a 2D Simulation of the NACA 6409 in Fluent similar to this tutorial https://confluence.cornell.edu/pages/viewpage.action?pageId=144976434. The lift I get is the same as I found in literature, but my drag is completly wrong.

• Lift: Airfoiltools: 1.25 -- Windtunnel: 1.17 -- My Simulation: 1.14
• Drag: Airfoiltools: 0.015 -- Windtunnel: 0.016 -- My Simulation: 0.04

My Mesh:

• Nikhil Narale
Ansys Employee
Hello,

Please check if the mesh downstream of the airfoil is sufficient to capture the wake accurately. For correct drag estimation, wake needs to be resolved accurately.
Also, can you please provide velocity and pressure contours?

Moreover, the airfoil looks inverted. And based on your velocity components, it looks like the AOA will be negative. Let me know your thoughts on this.
Just a thought, can you try using x=1 and y=0 force vectors for drag coefficient calculation?

Nikhil
• oll5hbdfk
Subscriber
Please check if the mesh downstream of the airfoil is sufficient to capture the wake accurately. For correct drag estimation, wake needs to be resolved accurately.
Do you have any documentation on how to do that exactly?
Also, can you please provide velocity and pressure contours?
Moreover, the airfoil looks inverted. And based on your velocity components, it looks like the AOA will be negative. Let me know your thoughts on this.
Yes, it's inverted, because my goal is to simulate a racecar wing. But because the airflow is slightly downwards the AOA should be positive aver all.
Just a thought, can you try using x=1 and y=0 force vectors for drag coefficient calculation?
Cd -------------------------------- -------------------
wing -0.079260993

Cl -------------------------------- -------------------
wing -1.1405325
´╗┐Another idea I had, since the angle at which Cl and Cd are calculated affect Cd way more than Cl, is that maybe I got the angle wrong. Or in other words, the images of the airfoil I posted do not show the airfoil at an AOA of 0┬░
• Nikhil Narale
Ansys Employee
Hello,

From the contours, it looks like the wake region in pretty wide (y direction) and the existing mesh might not resolve it accurately, which might give incorrect cd value. Having said that, you can refer to the following course for some reference.
However, if you are comparing your results with the experiment, make sure the AOA value is matching with that of the experiment. Please check!

Nikhil

• oll5hbdfk
Subscriber
I've check the angle with the experiment and everything matches.
I also tried a unstructured mesh similar to the one in the course you linked. I felt like I didn't have enough control over the structured mesh to get it the way I wanted. But the results of the unstructured mesh are basically the same.
cl = -0.92268898; cd = 0.041851928
Would you still say that whe wake is not properly resolved? I'd say the mesh is quite a bit finer than the one from the ansys course

• oll5hbdfk
Subscriber
I also conducted a small mesh study. "size factor" 1 means it's about the number of elements as in the course. 2 is twice as big and 0.5 is half as big
for me it doesn't look like a finer mesh would improve the result

• Rob
Ansys Employee
Looking at that I wonder if you need more resolution along the wing and into the wake. y+ gives us the boundary layer, but here the wake only has a few cells across it. Use adaption around the wing to avoid the need to remesh the whole domain.
• oll5hbdfk
Subscriber
Thanks for your suggestion. Though, I don't know how to set up mash adaption as I never used it before.
I tried to solve the issue by just throwing more cells on it. The hight of the "green" wake is now resolved into ~100 cells. The result is still the same: cl=-1.0844292; cd=0.042461194

• Rob
Ansys Employee
What temperature & pressure were the wind tunnel at? How does k-w SST compare?
• oll5hbdfk
Subscriber
The pressure at the boundaries is 0. I set the density of the air as constant, so the absolute pressure shouldn't matter. The operating pressure is 101325 pascal
Regarding the temperature, I'm not sure. In the Models Energy and Heat Exchanger (and everything else exept Viscous) are off. Also the Thermal tab in the boundary condition window is greyed out

I'll try k-w SST and will report back
• oll5hbdfk
Subscriber
k-w looks quite a bit better. cl=-1.1385 and cd=0.02395. That's 98% and 158% respectivly of the value I found in the literature