Surface roughness for solid wall boundary
 

Is there any way to define roughness for a boundary designated as a solid region? I only see this option for fluid regions. If I have a solid/fluid interface where the surface of the solid has ridges, would I just change the roughness of the fluid boundary shared by that solid?

This is exactly the way to do.

Set the roughness on the boundary of the FLUID re
gion, then build an interface between the adjected boundaries of the fluid and solid. The interface ingherites the roughness of the fluid boundary.