FAQ:General:DefaultMoistureStorageFunction

(5): Default Moisture Storage Function

When I do not define a moisture storage function for a material, WUFI uses a default moisture storage function instead. What does this function look like?

WUFI needs a well-defined moisture field for each time step, so it must assign a moisture content even to materials which nominally don't have any appreciable moisture content (e.g. water-repellent mineral wool, air layers etc.).

The default moisture storage function used by WUFI is described by the function

w = a / (b - phi) + c
w	[kg/m³]:	water content
phi	[-]:	relative humidity

Since phi must be 0 for w=0, it follows immediately that
 c = -a/b
 
The constants a and b are determined as follows:
 
b is set to 1.0105.
 
The moisture content at free saturation, wf, corresponds to a relative humidity of 1 (=100%). Since WUFI also needs a unique relationship between moisture content and RH for moisture contents above free saturation, this oversaturation region is assigned RHs greater than 1, up to phimax = 1.01. This value phimax is reached when the moisture content reaches maximum saturation wmax which is determined by the porosity:
 
 wmax = porosity * 1000 kg/m³
 
Therefore we have
 
 wmax = a / (b-phimax) - a/b.
 
Solving for a yields:
 
 a = wmax * b * (b - phimax) / phimax,
 
and thus:
 
 w / wmax = phi / (b - phi) * (b - phimax) / phimax.
 
 
In particular, for phi=1 we have
 
 wf / wmax = 1 / (b - 1) * (b - phimax) / phimax = 0.047.
 
So this pseudo material has a free saturation of wf = 0.047 wmax.