SebSta: hat „FAQ General:ConvergenceFailureCausedbyVapor-PermeableLayer“ nach „FAQ:General:ConvergenceFailureCausedbyVapor-PermeableLayer“ verschoben

2013-06-13T12:26:05Z

hat „FAQ General:ConvergenceFailureCausedbyVapor-PermeableLayer“ nach „FAQ:General:ConvergenceFailureCausedbyVapor-PermeableLayer“ verschoben

← Nächstältere Version	Version vom 13. Juni 2013, 14:26 Uhr
(kein Unterschied)

Len: Die Seite wurde neu angelegt: = (7): Convergence Failure Caused by Vapor-Permeable Layer = I tried to perform a WUFI simulation, but the water balance never adds up, regardless whether I make th...

2008-10-02T08:08:39Z

Die Seite wurde neu angelegt: = (7): Convergence Failure Caused by Vapor-Permeable Layer = I tried to perform a WUFI simulation, but the water balance never adds up, regardless whether I make th...

Neue Seite
= (7): Convergence Failure Caused by Vapor-Permeable Layer =

I tried to perform a WUFI simulation, but the water balance never adds up,
regardless whether I make the grid as fine as possible or whether I choose
stricter numerical parameters, as suggested in the on-line help. What can I do?


One situation where serious convergence failures tend to occur is a component
with a vapor-permeable layer (e.g. air or mineral wool) which has accumulated
a lot of moisture (RH ~ 100%) and which is now exposed to a high temperature
gradient (e.g. caused by intense solar radiation). WUFI originally wasn't
developed to treat these cases which sometimes prove too demanding for the
numerics that are mainly tuned to massive porous materials.


[[Bild:e_konvf_hivlt.gif]]


If everything else fails, you may try an alternative
[[Details:MoistureStorageFunction | moisture storage function]].
In the material database, the moisture storage functions for materials like
air or mineral wool are left undefined, so that WUFI uses an internally defined
default moisture storage function (see the preceding two questions).


This moisture storage function assumes that for RHs above ca. 50% capillary
condensation occurs which leads to increasingly higher moisture contents until
free saturation is reached at 100% RH. This is not really realistic for air
layers or hydrophobic mineral wool (it may be more appropriate for
non-hydrophobic mineral wool). 
Since it seems that the problem is mainly caused by the high water content,
reduction of the water content by choosing a different moisture storage
function often remedies the problem. 
Please note that the relative humidity in the material will remain largely
unaffected by the specific choice of the moisture storage function, as explained
above. So if you are interested in the relative humidity in the layer, your results
will be affected only slightly (but please perform a few test calculations with
different choices of the moisture storage function to be sure), and if you are
interested in the moisture content, you should not rely on the default moisture
storage function anyway, but use measured data instead which represent your
particular material.


A possible choice for the moisture storage function in these cases is a
table like this:

<TABLE>
<TR><TD><TT>phi: </TT></TD><TD><TT>w: </TT></TD><TR>
<TR><TD><TT>0 </TT></TD><TD><TT>0 </TT></TD></TR>
<TR><TD><TT>1 </TT></TD><TD><TT>wf</TT></TD></TR>
</TABLE>

Use a low value for wf (the numerics may not be able to cope
with very low values, you'll need to experiment a bit.) (*). 
This linear function is even more realistic than the default function in
that it avoids the capillary condensation for RH= 50..100%. The moisture content
remains low up to RH=100% (as it should be in air or in hydrophobic insulation
materials), and at or above 100% condensation may occur and increase the
moisture content beyond wf and up to wmax.


In particular if you are interested in moisture accumulation by condensation in
these materials, use such a linear moisture storage function with low
wf. Then you know that any moisture content exceeding
wf must have been caused by condensation. You can then analyse
this excess over wf (test calculations show that this excess is
only slightly dependent on the specific choice of wf).


(*) Note, however, that the porosity and thus wmax should remain high. If the
water content exceeds wf, WUFI reduces the vapor permeability,
in proportion to the excess, to reflect the fact that the pore volume gets
increasingly filled with water and thus vapor transport decreases. At
w=wmax the permeability reaches zero (all pores are filled).
For vapor-permeable materials like air layers or mineral wool where moisture
transport occurs mainly via vapor transport, wmax should therefore remain at a
realistic value.

FAQ:General:ConvergenceFailureCausedbyVapor-PermeableLayer - Versionsgeschichte

SebSta: hat „FAQ General:ConvergenceFailureCausedbyVapor-PermeableLayer“ nach „FAQ:General:ConvergenceFailureCausedbyVapor-PermeableLayer“ verschoben

Len: Die Seite wurde neu angelegt: = (7): Convergence Failure Caused by Vapor-Permeable Layer = I tried to perform a WUFI simulation, but the water balance never adds up, regardless whether I make th...