Roughness function  small diameter tube
Hi all
Does anybody have any experience using the roughness function in StarCCM+? I have a simulation where I have flow through a tube with a very small diameter. I suspect that the relative surface roughness plays an important role. I have tried changing the "Wall surface specification" of the tube to "rough", and setting a roughness height, but I haven't seen any change in my results. I'm making use of the komega turbulence model, and a low yplus grid to resolve the boundary layer completely. Does the roughness function only affect the wall function the turbulence model uses, and therefor have no effect if you are resolving the boundary layer? I would ideally not like to explicitly add the roughness (ie draw it into the 3d model). Any suggestions are appreciated. Thanks 
You are right  roughness function affects only the wall function for high Reynolds turbulence models

Thanks for the reply.
Do you have any idea how I can add roughness while using a low Reynolds number turbulence model? 
For LoReynolds you'll have to model it directly, sorry :(
I've seen a paper about accounting for roughness for twolayer models, but there are no twolayer models in STARCCM+, only in STARCD It's just coming from the nature of different types of turbulence models  for HiReynolds and for twolayer you're not solving the equations all the way down to the wall, you use some simple relations instead which you can modify to account for roughness. For LoReynolds you're solving the equations in the whole nearwall region. Well, may be there is some way to modify damping terms, but I haven't seen any papers about it. 
Thanks, I suspected that that might be the case. I will have a look at the difference between explicitly modeling it, using a low yplus grid, compared to making use of the roughness function with a high Reynolds number turbulence model. If I find something interesting I'll post it here.

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