I am trying to investigate the effect of surface roughness through RNG turbulence model with scalable wall treatment to capture the surface roughness effects. I have the following doubts: in spite of the turbulence model what is the correlation between the main parameters which uses in solver setting (fluent).

1. What should be the first cell height to resolve this case?

{The Reynolds number similarity hypothesis of Townsend -The Structure of Turbulent Shear Flow states that the turbulence beyond a few roughness heights from the wall is independent of the surface condition. The underlying assumption is that the boundary layer thickness is large compared to the roughness height.}

[On this basis it's good to place the first cell height around 2 times that of the roughness height Ks to resolve it?]

2.Under the solver setting there are two terms (Ks and C ) to define roughness. Perhaps in generally we define the surface roughness through CLA - Ra value. If we assume uniform roughness so roughness constant (C) value is constant so we only require the roughness height (Ks). Then how it can be selected and is there any correlation with Ra and Ks?