"Unefficient" wall boundary
 

Hi,

I'm working on an NACA 0012 airfoil equipped with an airbrake.
But when running the calculations, I have some pretty high residus (10^-5 for the velocities and 10^-2 for the continuity) though Cl and Cd are more or less converging.

I've done tons of tests and I noticed that each times, I had some strange behaviours around my airfoil :

Here's my mesh, Quad for the entire area except around the airfoil where I couldn't use Quad Elements, so it's "Tri Elements" (I got the boundary layer refinement by meshing the edges first).

http://img28.imageshack.us/img28/6379/mesho.jpg



And there is a close look of my transparent wall in Fluent :

http://img851.imageshack.us/img851/1020/wall1k.jpg




The vectors looks very strange close to the Wall. It's easier to spot with high angles of attack (20° in that case).
My Wall boundary is attached to the airfoil edges, and I double checked for double edges, faces etc....
My idea was to attach the Boundary condition to the foil Face but when I create one Gambit can't export it to Fluent because it's not meshed, a problem I couldn't solve...


Someone got an idea of my problem ?

Thanks,
Charles

hi,

I didnt quite understand ur problem. But some initial comments on ur grid,

1. The transiiton between the tri to quad cells at the trailing edge is not quite good. try clustering near the trailing edge.
2. what is the y+ of the grid?

If you dont specify the BC to the aerofoil in Gambit, you can specify it as noslip wall in Fluent.

Cheers

Hi,

Thanks for your advice, I've add some extra mesh just behind the trailing edge.
I change my strategy and made the entire mesh with Quad Elements, hoping that it could solve my problem. Murphy's law, it didn't solve anything.
I get some amazing results with low pressure under the foil and high pressure over the foil. I can understand the high pressure zone above the foil due of the airbrake, but low pression under, it's impossible....

I also noticed that the air is hitting the foil quite higher that excepted. I rotated the grid by 5° and set up 30m/s on the X axis, so I should have a 5° angle of attack but if you take a close look, I get a negative one instead. The vectors are straight when exiting the inlet but their direction is changing and they are progressively heading down when approching the foil.

k_k, you advised to check the Yplus but I'm using a laminar model, so I can't check Yplus right ?

After a week of work, I'm iteratively losing all hope in life... :o I really need some help. If someone is okay with that, I can send him or her my .msh file.

There are my screens :

Mesh
http://img685.imageshack.us/img685/2746/meshy.jpg




My unrealistic pressure

http://img121.imageshack.us/img121/2186/pressure.jpg


And the velocity, note the strange direction of the vectors before hitting the foil

http://img823.imageshack.us/img823/5258/velocityc.jpg

hi,

Are you running a incompressible, laminar case?

How far are the boundaries from the aerofoil?

What is the shape or your inlet boundary? straight or curved? if it is curved, in the inlet BC check if you have specified velocity components rather than normal to boundary. If possible, try to create a domain with straight inlet boundary.

For an incompressible case, it is better to rotate the aerofoil alone to the required angle of attack (not the complete domain).

cheers

your new grid, the lower surface of the aerofoil is very coarse to resolve the boundary layer. Make it more dense.