Rising Damp - source

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Foo Chin Peng
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Rising Damp - source

Post by Foo Chin Peng » Wed May 09, 2018 5:11 pm -1100

hi, referring to Dr Kunzel's paper : Two-Dimensional Transient Het and Moisture Simulations of Risind Damp with WUFI 2D.

1) How is the moisture source incorporated ? I can only see moisture source by rain.
2) I am trying to simulate rising damp for masonry wall.

thank you
chin peng

Thomas
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Re: Rising Damp - source

Post by Thomas » Mon May 14, 2018 12:42 am -1100

Hi Chin Peng,

in the case of rising damp, it would not be the best choice to have the moisture released by a moisture source. Moisture sources always force a pre-determined amount of moisture into the component (possibly limited by some user-defined maximum allowed water content).

The uptake of groundwater which is in contact with the wall is instead a case of free capillary absorption: The wall is in contact with a large supply of water, and during each time step it freely absorbs an amount of water which is determined by the capillary properties and the current water content of the material .

This is the same water absorption mechanism which governs the water uptake during rain: When the weather file indicates the presence of rain, WUFI assumes that the wall surface is covered with a layer of water, and it computes the free absorption of water from this reservoir, the absorbed amount of water being determined by the capillary properties and the current water content of the material.

You can therefore simulate the uptake of (pressureless) groundwater by pretending that the surface segments of the wall which are in contact with groundwater are exposed to a large amount of rain. Create a weather file with the relevant boundary conditions: the hourly ground temperature (maybe a constant value is sufficient), relative humidity 100 % and a very large amount of "rain". The amount of "rain" need not be realistic for actual rain, it should be larger than any hourly amount that could possibly be taken up by the wall to make sure that the capillary properties and the current water content are the limiting factors for the absorbed amount of water, not the rain rate. You may safely use a rain rate of, say, 1000 mm/hour. Then assign this climate file to all the surface segments which are in contact with ground water.

Regards,
Thomas

lolisfd
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Re: Rising Damp - source

Post by lolisfd » Sun Jun 10, 2018 9:18 pm -1100

Hi Thomas,

thank you for your answer.

I am trying to simulate the same case as Chin Peng. Therefore, I have a question:

-Can I use the tab "sources, sinks" to create the rising damp in the lower part of the wall instead of adding a new climate file?

Thank you!

Lorena

Thomas
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Re: Rising Damp - source

Post by Thomas » Tue Jun 12, 2018 7:26 am -1100

Hi Lorena,

in principle, you could also use a moisture source to simulate the uptake of ground water. But I think using the capillary absorption mechanism through the surface (which is normally used to determine the absorption of rain water through the wetted surface) is more appropriate.

If you model the capillary absorption of ground water as "rain" absorption, the absorbed water enters the component through the surface, as does the ground water. If you put a moisture source into the component, you have a more or less extended region within the component which releases moisture. Of course, if the exact location where the moisture appears is not important for your simulation, both alternatives are possible.

I'm attaching a simple weather file you can use to model capillary absorption from a practically unlimited water supply (the attached file had to be zipped because for some reason the forum software does not allow the file extension *.WAC).

The file consists of one single line. When WUFI reaches the end of a weather file (in this case, after each one-hour step), it jumps back and starts reading the file again fom the beginning. The geographical coordinates and the height are not needed and have been set to a number which signals 'unspecified'.

The temperature has been set to 20 °C. You may set this to the ground temperature of your case. If you need a variable ground temperature, you must create a file like this, but with an appropriate number of lines (one for each hour. Don't forget to adjust the "Number of DataLines" parameter in the header accordingly).

The relative humidity has been set to 100 % ( = 1) and should be left at this value.

The rain has been set to a large number (1000). This number would not be realistic as a real rain rate, but the purpose here is to provide more water than the component can absorb, depending on its capillary properties and current water content. This ensures that the absorptive capacity of the component is the limiting factor for the water uptake, not the amount of water available for absorption. The parameter "adhering fraction of rain" for the relevant surface should be set to 1 to avoid limiting the amount of available water.

The rain column has been tagged as a "RM" column (= rain, measured) instead of the "RN" (= normal rain) which is usually used for rain in weather files. Usually, the rain hitting the component surface is the driving rain, while the rain data provided by meteorological stations describe the "normal" rain measured on a horizontal surface. WUFI usually feeds the RN data and the wind data to a driving rain model which estimates the resulting amount of driving rain and then offers this to the component for capillary absorption. In this case, however, no wind data exist and the driving rain model would fail. The column tag "RM" (instead of the usual "RN") tells WUFI that these data should be used as given, and should not be run through the driving rain model (they are supposed to be the measured driving rain, requiring no further modification by a driving rain model).

Water which is not absorbed simply "runs off" and is not considered further in the simulation, so for the present purpose we may choose an arbitrarily large number without compromising the simulation result.

Code: Select all


WUFI®_WAC_02
10	Line Offset to 'Number of Data Columns'
-
Climate file for free capillary water absorption
-1e11	Longitude [°]; East is positive
-1e11	Latitude [°]; North is positive
-1e11	HeightAMSL [m]
1.0	Time Zone [h from UTC]; East is positive
1	Time Step [h]
1	Number of DataLines
3	Number of DataColumns
TA	HREL	RM
20	1.0	1000
Additional note:
In addition to the uptake of ground water, this file could also be used to simulate a water absorption experiment. In such an experiment, one face of a material specimen is submerged in water so that its capillary forces can absorb water from this unlimited supply, the absorbed amount depending on the capillary properties and the current water content. The amount of water which an initially dry specimen absorbs per square meter of wetted area and per square-root of absorbing time is the water absorption coefficient or A-value [kg/(m^2h^0.5)]. By simulating such an experiment you can determine the A-value corresponding to the set of liquid transport coefficients of the material. (I'm adding this note so that I can simply refer to this posting if a question about simulating water absorption experiments comes up.)

Regards,
Thomas
Attachments
WaterAbsorption.WAC.zip
(401 Bytes) Downloaded 40 times

lolisfd
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Re: Rising Damp - source

Post by lolisfd » Fri Jun 15, 2018 2:21 am -1100

Hello Thomas !

Thank you a lot ! Indeed, I am also performing the water absorption experiment as you guessed ! Your information will be very useful to compare my experimental data to WUFI results. In this case, I suppose I have to set all the surfaces to adiabatic (except the one in contact with water of course) and see water content results during time? Then, I can calculate A-value? Or maybe WUFI delivers the A-value directly?

Another question: I am simulating a wall with rising damp but now I have to introduce a ventilation system between the wall and the insulation. Should I create an air layer and then put an 'Air exchange source' in the 'sources/sinks' tab? In this case, which boundary conditions should I use for the air layer ?

Best regards,

Lorena

Thomas
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Re: Rising Damp - source

Post by Thomas » Fri Jun 15, 2018 3:04 am -1100

lolisfd wrote:In this case, I suppose I have to set all the surfaces to adiabatic (except the one in contact with water of course) and see water content results during time? Then, I can calculate A-value? Or maybe WUFI delivers the A-value directly?
Hi Lorena,
as to the water absorption experiment: Yes, set the other surfaces to adiabatic and set the initial water content to zero. If you run the calculation for 100 hours, the square root of the elapsed time is simply ten (but any other time will work as well). Look at the water content at the end of the calculation, multiply the kg/m3 with the sample thickness to determine the amount of absorbed water in kg/m2 and divide by the square root of the elapsed time to determine the A-value in kg/m^2h^0.5. Divide by another factor 60 if you prefer the A-value in kg/m 2s^ 0.5.

For example, the Baumberger sandstone from the material database should have an A-value of about 2.6 kg/m^2h^0.5.

Regards,
Thomas

[keywords: water absorption experiment, water absorption coefficient, A-value, Saugversuch, w-Wert, Wasseraufnahmekoeffizient]

Christian Bludau
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Re: Rising Damp - source

Post by Christian Bludau » Sun Jun 17, 2018 9:27 pm -1100

Hi Lorena,
Another question: I am simulating a wall with rising damp but now I have to introduce a ventilation system between the wall and the insulation. Should I create an air layer and then put an 'Air exchange source' in the 'sources/sinks' tab? In this case, which boundary conditions should I use for the air layer ?
You should create an air layer in the between the wall and the insulation. Adapt an air exchange source to the whole layer. The boundary condition is that one, where the ventilation comes from.

Simply spoken what the air exchange source is doing: I is mixing the conditions (T/RH) in the air layer with the conditions in the related boundary condition the often you set in the air exchange rate.

Regards,
Christian

sarinpinich
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Re: Rising Damp - source

Post by sarinpinich » Mon Jan 14, 2019 5:03 am -1100

Hello Thomas,

Thank you so much for your information about how to simulate rising damp for masonry wall.

I have been studying Phd with the topics of hygrothremal analysis of historical wall in Tropical climate. Now I am trying to simulate rising damp with my case study which has a solid brick (interior) wall connecting to a solid brick foundation (which is sink into a ground). So I followed your suggestions above and applied your climate file with the surface of my case study's foundation. And then I got the error results when I use "1" as a adhering fraction of rain.

As a result, I would like to ask if there are any possibilities that I did somethings wrong with the input values. Because I also tried to use 0.1 or 0.7, and then still got an error. It worked if I apply just "0" number. But in my opinion, that results are not correct aswell.

Thank you,
Sarin Pinich

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