rough wall
 

Hi experts.
I'm modeling a two phased flow water and air. I have rough wall with roughness of 4 mm. depth of water is about 10 cm and the velocity is nearly 4.5 m/s. I use k-e turbulent model. how much should be the distance of the mesh grid from the wall?

Do a mesh resolution sensitivity check and find out for yourself. There are no shortcuts - we can only guess and that is just a starting point. From there you have to refine it with a sensitivity check anyway.

thank you very much.