ineffective wall roughness
 

Hi experts.
I'm simulating a weir. I want to know the effect of roughness of weir on velocity. I've done mesh sensitivity analysis and I checked various values of roughness but nothing has been changed. does anyone know where the problem is?
any help would be appreciated.

Maybe roughness does not significantly influence your flow?

thank you for your reply.
the effect of various roughnesses are the same. roughness of 4 mm, 8 mm and 20 mm!
could it be from fault or inappropriate setting or something else?

How do you specify the roughness parameter in your simulation?
Is it part of the wall boundary condition or do you have a geometrical roughness? What about the rest of your setup?
Be descriptive!

Let me state my point a different way. If a hurricane was heading your way, do you think putting a piece of sand paper on the ground would make a difference? What about if you put a piece of sand paper on the ground which had double the surface roughness?

Obviously a hurricane is so big it does not matter what you do, the hurricane is going to progress unaffected. So my point is that if the surface roughness does not significantly affect the flow then it does not matter what surface roughness you use you will still get the same result.

thank you all for your replays.

it's wall boundary condition. I'm trying geometrical roughness now.