SimScale CAE Forum

Boundary Conditions setup


For my master thesis study I am trying to simulate the airtightness measurement of a high rise building. With an airtightness test a building is put in underpressure ( a pressure difference of -50Pa) and overpressure (a pressure difference of 50Pa). I think I have the setup right for overpressure. But the underpressure setup I don’t know about.
The example of overpressure: the simulation is compressible (that is the only setup where all my speed values are positive as in reality) a pressure inlet (Blowerdoor inlet) of 101375 Pa and (leaks) pressure outlet 101 325.

I also tried a setup with convective heat transfer for overpressure with a blowerdoor inlet of 101 375 and outlet of 101 325 and one with natural convection (both plots look alike) but negative speed values so not possible

For Underpressure I have a couple of different setups i tried out.

  • With compressible simulation setup with a pressure inlet (Blowerdoor inlet) of 101 275 Pa and outlet (leaks) 101 325 Pa. This plot is very weird and should flat out at some point.

With convective heat transfer setup there are a couple more setups i tried: The setup here is a inlet of (blowerdoor inlet) 101 275 and outlet pressure of (leaks) 101 325. With this plot there is positve speed, wich is impossible in reality. The positive value represents an overpressure.

The next setup is with outlet pressure (blowerdoor inlet) 101 325 Pa and (leaks) 101 375 Pa. These value look wright but the difference between over- and underpressure is to great. I calculate these values with the next formula ( n50=V50/Vint) (n50 is value to determine infiltration, in this case we hope to get it around 1; V50 value is the volumetric flow through the blowerdoor inlets; Vint is the netto volume of the building (internal volume). A side note here is that the over and underpressure can’t have a greater difference than 20% wich in these case is greater. Underpressure is around 0.95 and overpressure is around 1.21. So this tells me the result is not wright.

The plot is the same when we change the inlet value (blowerdoorinlet) to natural convection.

I also tried to change the initail conditions (the pressure) But these wasn’t a help also. The values are pretty high
The first figure has a pressure outlet setup as (blowerdoor inlet)

The second figure has natural convection as setup for (blowerdoor inlet)

I also tried a setup with natural convetion on the leaks and a pressure outlet of 101375 Pa at the blowerdoor inlet. It couldn’t calculate a velocity.

Do you have an idea how to get a setup where these values are closer togheter between over and underpressure. I understand due to temperature difference and friction (and wind, but not yet on the model) these absolute values arent exactly the same.
I have tried a lot of setups and realy can’t seam to figure out a way to solve it.

I would like to thank you in advance for all your help

Kind regards


A post was merged into an existing topic: Velocity difference in under and overpressure