Dear all, For my LES study on an airfoil I would like to trip the boundary layer artificially by blocking a few cells in CFX (treating them as walls). Is that possible without the necessity of changing the grid prior to CFX?

Thanks, maxflow

Hi,

Do you want to trip turbulence by putting a little wall on the airfoil surface? This can only be done by going back to the mesh and editting it. If you have ICEM you will not need to remesh, you can just edit the mesh you already have (easy if it is a 2D or extruded mesh, tricky if a 3D mesh). Otherwise you will have to put a little wall in and remesh.

Also, other options: have you considered using a patch of high surface roughness on the foil surface? Also, you can explicitly set the location of the boundary layer trip in the SST transition turbulence model so you don't need to artificially trip it if you already know where it trips.

Glenn Horrocks

Hi Glenn,

thanks for your quick response. My idea is to artificially trip the boundary layer for reproducing experimental conditions. I have relatively low Reynolds number flow over the airfoil (2x10^5) so that no natural transition should occur. The problem is that I want to run a LES with this tripping. The transition model as far as I know works only work with SST model. Is that true?

I thought it could be possible to inactivate single cells in the mesh. But it seems like I have to go back and modify the grid.

If you have any more comments, please let me know.

Thanks

Hi,

The transition model only works with SST - Correct.

Deactivate single cells - not easily. Does not sound like a good idea anyway.

I recommend using the SST transition model and specifying where transition occurs. Alternately you could let it find the transition point and see if it agrees with experiment (or was the experiment artifically tripped too?)

For an airfoil at Re 2e5 I would expect a turbulent transition point on top and bottom surfaces (unless your airfoil is very thin and low AOA).

Glenn Horrocks