Constant Monthly moisture source

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Axelarris
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Constant Monthly moisture source

Post by Axelarris » Thu May 14, 2015 8:53 am -1100

I am not sure how to correctly enter a "constant monthly moisture load" in the moisture source dialogue. The help function does not appear to have this option described yet. The problem I am having is understanding the units (lb/a ft2).
In which plane is the ft2 measured? I am assuming it is in the plane normal to the screen - in other words, this would be "per square foot of wall area". Is that correct?
I am attempting to convert precipitation data (2" rain per month) into a constant monthly moisture load applied to a soil layer so I can analyze a sub-grade wall assembly. Currently, I am converting the 2" of monthly rain into a "constant monthly moisture load" applied to the whole soil layer (which I modeled as a 24" thick layer). If the moisture source is supposed to be "per square foot of wall area", then I get 0.33 ft3 water per square foot of wall (which is 20.6 lb/SF). this was calculated as 2" (rain depth) X 24" (layer length) X 12" (assumed layer width to match a square foot of wall area).
Where did I go wrong?

Thomas
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Re: Constant Monthly moisture source

Post by Thomas » Thu May 21, 2015 4:58 am -1100

Axelarris wrote:In which plane is the ft2 measured? I am assuming it is in the plane normal to the screen - in other words, this would be "per square foot of wall area". Is that correct?
Hi Axelarris,
yes, that is correct. In the one-dimensional WUFI versions you specify the layer sequence in one direction (from left-side-of-the-screen towards right-side-of-the-screen). In the other two directions (from bottom-of-the-screen towards top-of-the-screen, and from behind-the-screen towards in-front-of-the-screen) the layers are supposed to extend infinitely. The resulting water content in each layer, for example, must thus be given in pounds per square foot of the infinitely extended wall. Similarly, a moisture source is active everywhere along the infinitely extending wall, releasing an infinite amount of moisture altogether, so the amount of released moisture must be specified as so-and-so many pounds per second and per square foot of the infinitely extended wall.
I am attempting to convert precipitation data (2" rain per month) into a constant monthly moisture load applied to a soil layer so I can analyze a sub-grade wall assembly. Currently, I am converting the 2" of monthly rain into a "constant monthly moisture load" applied to the whole soil layer (which I modeled as a 24" thick layer). If the moisture source is supposed to be "per square foot of wall area", then I get 0.33 ft3 water per square foot of wall (which is 20.6 lb/SF). this was calculated as 2" (rain depth) X 24" (layer length) X 12" (assumed layer width to match a square foot of wall area).
Where did I go wrong?
Mathematically, your calculation is correct (but see below). Each month, the rain deposits an imaginary layer of water with dimensions 2" x 24" x 12" = 576 in^3 = 0.333 ft^3 on the ground in front of the considered square foot (12" x 12") of the wall. You redistribute this volume of water across the considered square foot of the construction and tell the moisture source located in the soil to release 0.333 ft^3 per ft^2 per month = 20.8 lb per ft^2 per month.

If those 2" of monthly rain are falling every month of the year, multiply this by 12 to get the yearly amount of water to be released, enter this number as the "Total moisture load to be distributed [lb/a ft^2]" in WUFI's moisture source dialog and tick all the 12 months.

If you simulate the soil by a thicker layer than 24", the accumulating amount of rain and the source strength will be proportionately larger, resulting in the same amount of moisture being released per unit volume of soil.

HOWEVER...
this will probably not give you the results you intended. The calculation discussed above is mathematically correct but not applicable to this situation.

As I understand, you are simulating a wall in contact with humid soil. The moisture content of the soil is due to rain, the rain water seeping down and keeping the soil moist. Please note that although new rain water is continuously being added, the moisture content of the soil does not increase continuously. A certain rate of rain water reaches a given depth in the ground, but it will seep further down into the ground at the same rate, so it will keep the soil moist but it will not accumulate.

Simulating the rain by a moisture source in the ground layer (which is infinitely extending in two directions) will create new moisture everywhere in this infinitely extending ground layer, with nowhere for the moisture to go. The moisture in this moisture-generating soil will accumulate instead of just passing through and keeping the soil moist. The moisture will accumulate until WUFI's cut-off limit for this source prevents further accumulation. This is probably not what you intend.

I assume your intention is to simply keep the soil at some given moisture level throughout the simulation. WUFI can not tell you what moisture level will result in the ground for a given rain rate (maybe a dedicated WUFI simulation of a vertical cross-section of the ground could do that if you know the structure and the hydraulic properties of the soil - I have never tried that).

What you could do is this: you guess a typical soil moisture content (or take a typical value from literature) which shall be kept constant throughout the simulation. You make the "outer" end of your soil layer vapor tight by applying a permeance = 0 there and start the simulation after setting the initial moisture content of the soil to the desired value. If the wall takes up only an insignificant amount of soil moisture, the moisture content of the soil will not change appreciably, and you get the desired constant moisture content in the soil without having to do anything else (using a soil with high moisture storage function and a thick soil layer will help).

If the soil loses more moisture during the simulation than you are willing to tolerate, put a moisture source in the soil and give it a small source strength which is just slightly larger than needed to compensate for the moisture gone into the wall. Setting the "Source Term Cut-Off" of the source to the desired moisture content ("User-Defined") will ensure that the source just releases enough moisture to keep the moisture content at the desired level.

Kind regards,
Thomas

Jesse Clarke
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Re: Constant Monthly moisture source

Post by Jesse Clarke » Thu Oct 08, 2015 5:38 pm -1100

Hi,

The units for the Monthy Moisture Load is stated as [g/a m²] and you select the months that you would like to distribute the moisture over.

Is the moisture load [g/a m²] = grams/annum.m² ?

So if I distribute 120 g/a m² over Nov/Dec/Jan/Feb (120 days) = 1 g/day.m² or 0.0416 g/hr.m²

Is this how WUFI applies the load?

Thanks

Christian Bludau
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Re: Constant Monthly moisture source

Post by Christian Bludau » Sun Oct 11, 2015 7:53 pm -1100

Hi Jesse,

thats right, but you have to use

kg / m² second

In WUFI Pro we have a automatic source which can do that, see "constant monthly moisture load". For 2D you have to enter as file.

Christian

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