In CFD simulation of building flows, two different boundary layer types must be considered.
- The atmospheric boundary layer as an inflow condition.
- Local boundary layers on solid surfaces, often modeled through wall functions.
If the flow is not directly resolved at the structural surface, wall laws are applied, which place specific requirements on surface discretization. The application of wall laws in separation areas should be critically examined.
Although both boundary layer types are based on the universal logarithmic wall law, they use different parameters to describe surface roughness:
- The atmospheric boundary layer uses the implicit roughness length z0.
- Wall functions typically use the explicit equivalent sand grain roughness ks.
These roughness parameters originate from different perspectives and have varying magnitudes ( ks : 0.9 to 60 m; z0:0.01 to 1 m). Direct equivalence is therefore not permissible.
For applying terrain roughness in wall functions, converting z0 to ks is necessary. Depending on the implementation, an approximation applies: ks≈20⋅z0 to 30⋅z0 (according to Schlichting and Blocken).
This distinction is crucial for the correct modeling of boundary layers in CFD simulations of building flows.