See Theory for the Single-Phase Flow Interfaces and
Theory for the Turbulent Flow Interfaces in the
CFD Module User’s Guide for a description of the theory related to laminar and turbulent single-phase flow interfaces.
where the Schmidt number at infinity is ScT∞ = 0.85 and the Schmidt number is defined as
where ν is the kinematic viscosity.
Analogous to the single-phase flow wall functions (see Wall Functions described for the Wall boundary condition), there is a theoretical gap between the solid wall and the computational domain for the fluid and relative humidity fields. This gap is often ignored when the computational geometry is drawn.
Assuming that the turbulent heat and moisture transfer in the near-wall region are analogous, the same type of wall functions used for the temperature (Ref. 30) is also applicable for the moisture transport. The moisture transfer wall function is formulated as a function of the turbulent Schmidt number, instead of the corresponding Prandtl number.
where uτ is the friction velocity,
csat is the saturation concentration at the lift-off position, and
is the dimensionless relative humidity given by (
Ref. 30):
where κ is the von Karman constant equal to 0.41,
Cμ is a turbulence modeling constant, and
k is the turbulent kinetic energy.
The distance between the computational fluid domain and the wall, δw, is always
hw/2 for automatic wall treatment where
hw is the height of the mesh cell adjacent to the wall.
hw/2 is almost always very small compared to any geometrical quantity of interest, at least if a boundary layer mesh is used. For wall function,
δw is at least
hw/2 and can be bigger if necessary to keep
δw+ higher than
11.06. The computational results should be checked so that the distance between the computational fluid domain and the wall,
δw, is everywhere small compared to any geometrical quantity of interest. The distance
δw is available for evaluation on boundaries.