WUFI

Hello!

I've tried to simulate a flat roof structure (picture 1). The materials are (from bottom up) hollow core slab 265 mm, EPS-concrete pouring 120...220 mm, hard mineral wool 370 mm with ventilation cuts (15 mm x 25 mm), roofing felt (bitumen), gravel. Geometry and and grid are in picture 2.

For the simulation I had to make a couple of materials from scratch. First I made a material to represent the hollow core slab as a homogenous layer (picture 3). I've done this already once before and the results seemed believable. I basically just averaged the necessary values by the area ratio (~0,55-0,45) of air pockets and concrete. The base material was [water/cement ratio = 0.5
G.Hedenblad: Moisture permeability of mature concrete, cement mortar and cement paste. PhD Thesis, Lund (Sweden)].

I also had to make a material file for the EPS-pouring (https://isoplus100.com/files/isoplus100_beps_wd_r.pdf) (Picture 4). I didn't find any values for DWW, so I left it blank.

I've already tweaked the numerics a little bit (picture 5). I raised the NSW-value from 4 to 8. I also tried using NSW 1000, but quickly found out that it's reeeally slow. During different calculations a tried URF (phi) values 0,2; 0,3 and 0,4; but it didn't seem to have a noticeable effect. I also lowered the max iteration count from 1500 to 1100.

I'm having the following problems:

- I'm hitting an iterative wall occationally (picture 6). Residual phi seems nearly constant during iterations rather than converging or oscillating.

- At some point the iteration breaks down (picture 7). Every time step is solved instantly and the residuals are both 0. After this happens, the result file size is reduced significantly and it cannot be observed with Wufi Motion (picture .

-I'm running a comparative calculation right now without the newly created materials. It seems to be going rather smoothly with materials from the Wufi database. Some of this mess might be caused by my new materials...

Edit1: I suspect that the iterative breaking down might be cause by supersaturation in some material (probably the new ones). I tried to tweak the moisture storage function by adding moisture capacity to the very end of the graph. I added values 0,97...0,999 which correspond to moisture content values 100...250 kg/m^3. This however did not help and the iteration still breaks down.

All help is greatly appreciated!

-Sami

Edit: The pictures were really disorganized when I posted them...

Here is more of the pictures:

Here are the rest of the pictures.

Hi Sami,
yes, that is proabably caused by supersaturation below the roofing felt.
Please try the following two things:
- In the enhanced solver settings switch on the "Rational interpolation of the sorption isotherm. This is new in WUFI 2D 4 an will interpolate the values from the sorption isotherm in a smoother way.
- Instead of the membrane on the top try to set an sd-value in the surface transfer coefficients in the boundary condition. The result is the same, but the calcualtion is much easier. We recommend to do that for tight membranes/foils on the outside.
Christian

Thank you Christian, I followed your instructions and the problem was solved.

Hello again!

I'm having trouble again. I made a small modification to the roof structure by adding a vapour barrier on top of the hollow core slab. All the calculation options are as before, but for some reason, after a few time steps the iteration is halted "***TERMINATED - inner iterations diverge***". There was no problem without the vapour barrier.

I turned the rational interpolation of the sorption isotherm off and the calculation seemed to carry on just fine, but this morning the calculation was finished too early (time step 3041/26280).

This is perplexing, since I get this error regardless the initial moisture content in the structure. Whether I try to simulate wet concrete strucutures (initial RH 95 %) or dry (75 %) the result is the same. When the ration interpolation setting is turned on, the calculation halts almost immediately. When it's turned on, it seems to carry on fine for a while, but it'll ultimately halt.

The last time step is also weird in Wufimotion...

Can somebody help me with this?

Dear Sami,
your last picture of WUFI Motion is related to the convergence problems. It is showing only the boundary conditions, the other parts of the constructions seems not to contain any values.
Such behavior also may be related to a special combination of materials. Please try, if you can calculate this using other materials. Start replacing the mineral wool, then try the mineral materials.
Good luck,
Christian