Feuchtequellen in WUFI2D

[Kai]

Liebes Support-Team,

bei den Tutorials findet man ein Beispiel für eine Eingabe einer Feuchtequelle mit 250 g/m²a,
was sich aber auf WUFI1D beziehen muss.

Rechnung dort:
250 / (1000 x 2190 x 3600) = 3,17E-08 kg/m²s

Wie geht das in WUFI2D? Ich habe ein Gefach in einem Holzflachdach mit 795 mm Breite und möchte in diesem Gefach in die unteren 5 mm einer OSB-Platte diese 250 g/m²a einbringen.

Variante1:
250 / (1000 x 2190 x 3600) x 0,795 = 2,5209E-08 kg/ms
(hier stimmt die Einheit, aber die y-Abmessung ist nicht berücksichtigt)

Variante2:
250 / (1000 x 2190 x 3600) x 0,795 x 0,005 = 1,2605E-10
(hier ist die Tiefe von 5 mm berücksichtigt, über die die Quelle sich verteilt, aber die Einheit wäre falsch)

Viele Grüße,
Kai

[Christian Bludau]

Hallo Kai,

Variante1:
250 / (1000 x 2190 x 3600) x 0,795 = 2,5209E-08 kg/ms
(hier stimmt die Einheit, aber die y-Abmessung ist nicht berücksichtigt)

Das ist die richtige Variante. Während bei 1D nur die Breite aus den Gitterelementen vorgegeben ist, ist in 2D die Breite (b) und die Höhe (d) durch die ausgewählten Gitterelemente definiert, wobei die Tiefe dann 1m beträgt (siehe Bild).
D.H. du musst die Quelle mit 5mm Höhe in die Konstruktion reinpacken.

Quelldefinition in Wufi 2D

quelle2D.png (252.26 KiB) Viewed 11200 times

Weitere Infos zur Quellstärke auch in unserem Downloadbereich:
https://wufi.de/de/service/downloads/#2d_quellstaerke


Christian

[Alberto Morales]

Hi,

What type of detils is the above one? Could you explain better how the direction of the dimension b and D work for the details? it is difficult to see it in the space with the previous explanation

Thanks

[Christian Bludau]

Hi Alberto,
please see here how the source strength is distributed:
https://wufi.de/de/service/downloads/#2d_quellstaerke
Christian

[Thomas]

Hi Alberto,

in general, you don't need to take any dimensions into account in order to use a source.

In the two-dimensional WUFI-2D, the user defines a two-dimensional cross-section of the simulated component. The full component is imagined to extend infinitely into and out of the screen, as suggested by the following diagram (which only shows a small portion of the "infinite" extent in the third dimension).

HeatSourcePerMeter_02.png (4.37 KiB) Viewed 7021 times

The same is true for all the sources which have been placed into the component. In the example shown, the red rectangle in the component is a heat source, and the full three-dimensional heat source is imagined to be a "stick" extending infinitely into and out of the screen. For example, it might be a heated cable or pipe. Accordingly, the heat or the moisture released by these infinite sticks are counted "per meter" of the stick: The source strength of heat sources has the units W/m, and the source strength of moisture sources has the units kg/(m s).

So you only have to determine how much heat or moisture you want the source to release per meter. Don't forget that the release rate is "per second", so the numbers for the source rates tend to be very small numbers. You can write those numbers to a source strength file which then controls the (possibly time-dependent) source strength during the simulation.

The size of the source cross-section in the assembly does not affect the release rate. WUFI takes the release rate specified by the user and automatically distributes it over the grid elements in the source area, as mentioned by Christian. If you change the size or shape of the source, the release rate of the source as a whole will remain the same (the release rates apportioned to the grid elements will be adjusted accordingly).

It is advisable to make the source area as large as possible, because this reduces the release rate of the individual grid elements and is easier to handle for the numerics. Also, if a moisture source is concentrated in a small area, a relatively large amount of moisture may be released locally in a short time, saturating the material at that spot and preventing further release of moisture until the previously released moisture has dispersed enough to allow the release of new moisture.

A possible application for a moisture source, and a case where the component dimensions must be taken into account, is this: You may wish to add a weak moisture source to a component in order to challenge the component's drying potential. For example, you may want to release 250 g of moisture per square meter of a roof during three winter months. The source is to be located in some wooden board within the roof.

The desired release rate refers to a square meter of the board which contains the source. However, in WUFI's 2D-assembly, that board is represented by its two-dimensional cross-section and the release rate to be specified for the source is counted per meter of the board vertical to the screen. If the cross-section of the board has a length of one meter (the width is irrelevant), then the board has an area of one square meter for each meter vertical to the screen. In this case the release rate for the WUFI-2D source would be 250 g per meter and per three months (= 3.17e-8 kg/(m s)). If the cross-section of the board in the assembly is two meters long, then the board has an area of two square meters for each meter vertical to the screen, and the release rate for WUFI-2D source would be 500 g per meter and per three months (= 6.34e-8 kg/(m s)).

And finally, if you use WUFI's options to specify the heat or moisture source strength as a certain fraction of the incident solar radiation or driving rain, WUFI does all these conversions for you, and you only have to specify the desired fraction.

Kind regards,
Thomas

[Alberto Morales]

thanks a lot really good explanation so I hope to have understood it

then, in your example you will add a cte value during 3 month to one grid element to analyse the drying potential of your assembly see image

moisture source.png (63.29 KiB) Viewed 7009 times

in case of using more grids elements as you recomend you divide that source strength for each grid element that made up the whole grid area using the ratio between the total area of all grids elements together (sum area of each grid element individually) and the area of each grid element

Am I right?

another question, could you tell me how you would determine (in a real case) the moisture source of an exisiting window jamb due to the rain incident? would you measure the moisture that enters between winfdow frame and wall? which positions would you choose for your source strength? how depth from the window frame surface? would be that moisture source placed in the insulation between wall and frame? would the part of moisture source be included in the cavity closer as well? i am sure I explain my doubt really bad but I am quite new in this and I would like to know the procedure to carry out an simulation/analysis of a real case for a construction detai).

what about the air infiltration from inside? how would you take into account in the simulation?

[Thomas]

then, in your example you will add a cte value during 3 month to one grid element to analyse the drying potential of your assembly see image moisture source.png

in case of using more grids elements as you recomend you divide that source strength for each grid element that made up the whole grid area using the ratio between the total area of all grids elements together (sum area of each grid element individually) and the area of each grid element

Hi Alberto,

WUFI will automatically distribute the user-specified source strength over the grid elements which are in the source region; you don't have to do this yourself. (The procedure is as you describe, but WUFI does it for you.)

All you have to do is to specify the location, shape and size of the source region, and to specify a source strength (either as a fraction of the incident solar radiation or rain, or as a file with explicit, possibly time-dependent source strengths).

another question, could you tell me how you would determine (in a real case) the moisture source of an exisiting window jamb due to the rain incident? would you measure the moisture that enters between winfdow frame and wall? which positions would you choose for your source strength? how depth from the window frame surface? would be that moisture source placed in the insulation between wall and frame? would the part of moisture source be included in the cavity closer as well?

You don't necessarily need to include in all simulations a moisture source which simulates moisture leakages. It is an option in case you wish to "challenge" the drying potential of the construction by releasing some leakage moisture. If this is of no concern in your simulations, you can omit such a source. If it is of possible concern, you usually have some idea where the leaking moisture may appear in the construction. For example, some wall constructions inlude a weather barrier membrane; a typical spot for leaked-in moisture to appear will then be on the exterior surface of this membrane. In other cases, localized gaps or cracks may serve as entry points of moisture, the details will depend on the specific construction.

In general, the exact location of the leakage source will not be very important, because it will usually release a relatively minor amount of moisture which will spread within the construction and, in most cases, automatically end up in critical (for example, cold or vapor-trapping) locations. On the other hand, if you specifically wish to investigate a case of leakage, you will know where the potential problem occurs. You can even use WUFI to investigate the consequences of different leakage locations by running and comparing a series of relevant simulations.

what about the air infiltration from inside? how would you take into account in the simulation?

There's a "Guideline for Using the Air Infiltration Source in WUFI" in the downloads section of the WUFI website.

Kind regards,
Thomas

[Alberto Morales]

thanks a lot. last comment

"Guideline for Using the Air Infiltration Source in WUFI" is for WUFI Pro (air infiltration IBP model) but there is not that option in WUFI 2D , only heat oruce, moisture source and air exchange source.

regards

[jlg.eco]

Variante1:
250 / (1000 x 2190 x 3600) x 0,795 = 2,5209E-08 kg/ms
(hier stimmt die Einheit, aber die y-Abmessung ist nicht berücksichtigt)

Worauf beziehen sich diese Werte in den Klammern / welche Einheiten haben sie (1000 x 2190 x 3600)?
Ich würde gerne die Rechnung nachvollziehen und kann das genannte Tutorial nicht finden.

Vielen Dank
Janos

[Christian Bludau]

Hallo Janos,

das bezieht sich auf das Tutorial zur Erstellung konstanter Feuchtequelle in WUFI Pro (1D):
https://wufi.de/de/service/downloads/#k ... htequellen

Hier mit den Einheiten, die 250 g/m²a sollen gleichmäßig über 3 Monate verteilt werden. Benötigte Einheit in WUFI Pro ist kg/m²s:
250[g/m²a] / (1000[g/kg]*2190[h/a]*3600[s/h]) = 3.17e-8[kg/m²s]
Die 2190h entsprechen drei Monaten.

Für die Verwendung in WUFI 2D muss wegen der Einheit [kg/ms] noch mit der Wirkungsbreite multipliziert werden (siehe viewtopic.php?p=6251#p3166).

Christian