How to Model Wall Roughness in CFD?

Question

In CFD simulations, can users model wall roughness? If so, how is it done?

Solution

The surfaces of all materials used in engineering applications are rough. Levels of roughness can vary based on material, mechanical finishing, and polishing, amongst others.

By default, SimScale considers walls to be smooth. In applications where the wall roughness is considered important, it can be accounted for via custom boundary conditions in the OpenFOAM®-based solvers and directly via the wall boundary condition in the Subsonic solver.

After enabling wall roughness, the user has to input two parameters: roughness height and/or roughness constant.

Roughness height (Ks)

This parameter is a representative distance to the wall. Therefore, a value of 0 results in smooth surfaces. A sand-grain type of roughness is assumed.

As shown above, when the sand-grain roughness is uniform, k_s is easy to obtain. For irregular grain sizes, the user can specify k_s based on average values.

In some cases, the wall roughness will be very irregular and won’t be similar to a sand-grain type. In this situation, users should estimate the equivalent sand-grain roughness.

Some authors explore correlations between measured surface roughness and equivalent sand grain roughness (Adams and Watson, 2012). It’s also possible to find estimates based on the material of the wall.

Roughness constant

Roughness constant is a parameter that measures how uniform the roughness is. Inputs may range from 0.5 to 1:

• 0.5 indicates a uniform wall roughness;
• 1 means that the roughness is strongly non-uniform, both in terms of spacing and height.

References