In fluid dynamics calculations, it is common practice to assume that boundary surfaces are perfectly smooth. In laminar flow, this assumption is valid, because the length scales of typical rough surfaces are much smaller than the length scales of the flow. In turbulent flow, however, the length scales of the flow eddies are much smaller than laminar length scales; therefore, it is sometimes necessary to account for surface roughness. Surface roughness acts to increase resistance to flow, leading to higher rates of heat transfer.
Ansys Icepak assumes, by default, that all surfaces of a wall in contact with a fluid are hydrodynamically smooth, and applies standard no-slip boundary conditions. For turbulent-flow simulations in which roughness is significant, however, you can specify a roughness factor for the entire wall. This roughness factor is defined as part of the properties of the surface material specified for the wall. The purpose of the roughness factor is to approximate the average height of the surface texture on the wall.
Note: Fluid flows over rough surfaces are encountered in diverse situations. See Wall Roughness Effects in Turbulent Wall-Bounded Flows in the Fluent User Guide for more information.
Surface roughness is only applicable to turbulent models that use wall function. That is, Two equation, Enhanced two equation, Enhanced RNG, Enhanced realizable two equation used with the mesh size with Wall Y Plus > 35. The turbulent models other than that ignore wall roughness.
With the applicable turbulent model, the first mesh size must be smaller than half of the specified surface roughness.
Small roughness size requires a smaller mesh size, but this may generate an inappropriate mesh size (approximately Wall Y Plus > 35) for turbulent model with wall function. This leads to the incorrect results.