Roughness effect

acc

I analysing surface roughness effect on heat transfer and friction coefficient.I included roughness in the geometry as rectangular ribs.In wall boundary condition I put zero roughness height.I am getting good results for heat transfer but for fiction coefficient it gives value similar to smooth wall.If I give roughness height equals to rib height the heat ransfer anf friction coefficient both becomes very high.How I can get reasonable values of both heat transfer and friction coefficient for rib roughness in the same solution?