rough wall
 

If I have a fluid/solid interface between a heat generating object and air, if I wanted to represent the object as having a roughness (for example, a transformer with bundles of windings), where would I apply this? Would I go under the fluid region and find the interface corresponding to the same solid surface and change the wall from smooth to rough? Or would I change the boundary and not the interface to rough? Or both? There is no smooth/rough option with solid region boundaries.

I'm looking at how the roughness affects the heat transfer from the object, but so far after changing the fluid interface to rough and rerunning the simulations everything looks the same. Maybe this is the correct result, or maybe I'm applying the roughness incorrectly I do not know.