### 4.2.5. Treatment of Rough Walls

The near wall treatment that is discussed above (Scalable Wall Functions, Automatic Wall Treatment) is appropriate when walls can be considered as hydraulically smooth. For rough walls, the logarithmic profile exists, but moves closer to the wall. The near wall treatment becomes more complex, because it depends now on two variables: the dimensionless wall distance y + and the roughness height.

The roughness height that must be specified in the wall boundary section (see also Wall Roughness), is the equivalent sand grain roughness height. Note that the sand-grain roughness is not equal to the geometric roughness height of the surface under consideration. Wall friction depends not only on roughness height but also on the type of roughness (shape, distribution, and so on). Therefore, one must determine the appropriate equivalent sand-grain roughness height. Guidance can be obtained from White [14], Schlichting [77] and Coleman et al. [193]. Details on the formulation of the rough wall treatment for turbulence models based on the and -equation can be found in Treatment of Rough Walls in the CFX-Solver Theory Guide.

If the ANSYS-CFX two-equation transition model is used together with rough walls, then the roughness correlation must be turned on in the user interface. This correlation requires the geometric roughness height as input parameter. More details can be found in ANSYS CFX Transition Models in the CFX-Solver Theory Guide.

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