FAQ:General:AbsorptionExperimentwithLimitedWaterSupply

(15): AbsorptionExperiment with Limited Water Supply

I want to evaluate the effect of different material parameters on water absorption by simulating a laboratory experiment in which different specimens are exposed to a limited water supply. I created the corresponding KLI file with a spreadsheet program to avoid typing in all those numbers by hand, but WUFI can't read this KLI file.

The reason why WUFI does not accept your spreadsheet file is probably that you did not write it in ASCII format and/or did not write the header lines in the correct format. Please consult the on-line help for details on creating *.KLI files with your own programs.

Also note that if you want to simulate a simple absorption experiment with a specified constant water supply and constant climate conditions it is sufficient to create a *.KLI file which consists of only one single line, for example the line:

[h:	rain:	rad:	t_ext:	RH_ext:	t_int:	RH_int:]
1000	5	0	20	1	20	0.8

means that for 1000 hours after the starting time of the climate file there is a constant rain load of 5 Ltr/m²h.

An alternative would be a single line like

[h:	rain:	rad:	t_ext:	RH_ext:	t_int:	RH_int:]
1 5	0	20	1	20	0.8

which states that for 1 hour after the starting time of the climate file there is a constant rain load of 5 Ltr/m²h. When WUFI reaches the end of a climate file, it starts reading the file anew from the beginning, so you can simulate an experiment which is running for 100 hours (or whatever) and the climate file will automatically be read 100 times over.

The only difference between these two files is that in the latter case WUFI does not accept a calculation time step greater than 1 hour, whereas in the former case you may also choose any convenient time step greater than one hour.

For a simple absorption experiment I usually make sure the climate file contains a rain load large enough so that it does not limit the absorption of the specimen, for example 100 Ltr/m²h, which ould not be plausible for real rain.

If you want to have a specified limited supply please don't forget that WUFI reduces the amount of rain it reads from the climate file by the rain absorption factor which allows for the fact that some rain splashes off of the wall on impact and is not available for absorption. This factor should be set to 1 during your experiments.

Furthermore, please note a small subtlety involved in using limited rain supply. Let's assume you have a specimen with a water absorption factor of 3 kg/m²h^1/2 and the climate file specifies a rain load of 3 Ltr/m²h. During each time step, WUFI performs a test step with an unlimited supply and subsequently evaluates the amount of water taken up. If this amount of water is less than the amount supplied in the climate file, then the material is the limiting factor and WUFI accepts the result of this time step and proceeds with the calculation.
However, if the amount taken up is more than the amount supplied, WUFI performs additional iteration steps in which a fictitious 'flow resistance' at the specimen surface is adjusted until the amount taken up matches the amount supplied.

If you are using 1-hour steps in your calculation, and the dry specimen absorbs 3 kg/m² of water in the first step and the climate file supplies 3 kg/m²h, then WUFI accepts the trial step done with unhindered absorption and proceeds with the calculation.
But if you are repeating the same calculation with a time step of half an hour, things are different! Since the water uptake is not linear in time, the specimen will absorb more than 1.5 kg/m⊃ in the first half hour, while WUFI compares this with 1.5 kg/m⊃ of rain in the first half hour (assuming the rain is evenly distributed over the hour) and now limits the amount absorbed to 1.5 kg/m².
This is usually of no concern with real rain data and real building materials, but it may be beneficial to be aware of these subtleties if performing test calculations with limited rain supply.