Ansys Employee

There are a couple of courses on turbulence and near wall effects, look under Learning on here. 

The wall roughness models are there to work with the wall functions & turbulence models. You then have to balance the near wall cell height with the roughness: how valid is the assumption if the near wall cell is smaller than the roughness height? The manual may or may not clarify the settings, but as staff we're only able to point you at that. I think the topic is covered in more depth in the Ansys Learning Hub, but you'd need a subscription to access that - have you asked whoever set the exercise for more information?