# Velocity difference in under and overpressure

Hey,

I have a question about my results, I have two conditions I have tested my building on. A pressure inlet of 50Pa and -50Pa, so you would expect that the velocity inlet would have the same absolute value … Do you know what could cause this problem? Thanks in Advance!

Kind regards
Stijn

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Hi Stijn, thank you for using the forum!

Every simulation inside this project has multiple runs, so can you clarify which ones we should check please?

Cheers!

The two convective heat transfer simulations are the two i am talking about. the first convective heat is -50Pa at the inlet and gives an average speed of -0.11 m/s. The second convective heat transfer has a pressure inlet of 50 Pa and an average speed of 0.18 m/s. Wich is od beacuse temperature is the same so there should not be a difference in average speed …

Hi,

There is a subtle difference between having a “Pressure inlet” boundary condition work as an inlet (which is the intended configuration) and an outlet (which is not intended).

In these runs, when you set a total pressure = 50 for the total pressure inlet, it will act as a total pressure. For such cases, we have:

p_{total} = p_{static} + 0.5 (\rho U^2)

However, if you set it with a total pressure of -50, it will act as an outlet (which, again, this boundary condition should not be used for these purposes). When this happens, the boundary condition gets a fixed pressure of -50 Pa (it’s no longer a total pressure).

I can see that you have set result controls for the simulations, so you should be able to verify the above. The pressure of interest is p_rgh.

Cheers

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Hi

Okay I think I understand what you mean. Do I need to use the p_rgh to determine the true velocity?
I also tried this simulation with atmospheric pressure (101325 Pa) and than I added or subtracted 50 Pa, I got the same result. So this means that my pressure inlet also works as a pressure outlet, when my pressure inlet is lower than my environmental pressure.

For my thesis I am trying to determine certain effects on the building in over- and underpressure. Do you think it is possible to determine velocity and pressure loss through leaks with a negative pressure inlet (in airtightness measurements called underpressure)?

Kind regards

Stijn

Hi,

If I understand correctly, you are trying to use the pressure inlet boundary condition as an outlet in your simulation, and have the leaks act as inlets, correct?

In this case, I can see two main workflows:

• Change the pressure inlet BC for a velocity outlet. In this case, the leaks can be set with a pressure inlet, total pressure = 0.
• Set the leaks with a total pressure inlet > 0, and change the pressure inlet BC of the other boundary condition for a pressure outlet.

Cheers

Hi,

I will try this simulation! Let’s hope it will work !

Thank you for time and help!

Kind regards

Stijn

Hi

I tried to use the leaks as Inlet (50 Pa) and, the Fan inlet as natural convection. The graph shows the speed of the fan

The results are the same if the Fan inlet is -50 Pa and the leaks are natural convection. In overpressure the fan inlet is 50 Pa and the outlet is natural convection or pressure outlet and te results are always as the following plot.

Is there an other possible solution to get the inlet velocity (velocity of the fan) equal for +50 Pa and -50 Pa?

Kind regards

Hi

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

Stijn