# Problem with natural ventilation

**URL:** <https://www.simscale.com/forum/t/problem-with-natural-ventilation/97177>\
**Category:** Post-processing\
**Created:** [June 6, 2023, 7:33am UTC](https://www.simscale.com/forum/t/problem-with-natural-ventilation/97177 "2023-06-06T07:33:40Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![s289286](https://www.simscale.com/forum/letter_avatar/s289286/32/5_5575768a8748004e209b776fc1b2916d.png) [@s289286](https://www.simscale.com/forum/u/s289286)\
**Post date:** [June 6, 2023, 7:33am UTC](https://www.simscale.com/forum/t/problem-with-natural-ventilation/97177/1 "2023-06-06T07:33:40Z")

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Hi all, I have a problem with post-processing the results with natural ventilation.  
I would be interested in understanding the flow rate of air passing through an opening.  
The problem is that if I do area integral, rightly this returns zero because everything that goes in comes out of the same opening.  
Is there any way to figure out solely the flow rate that goes in?  
I was thinking of exporting the velocities for each cell so that then through a spreadsheet I could take for example only the positive ones and thus calculate the flow rate coming in.  
Thank you all

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**Author:** ![anon77415598](https://www.simscale.com/forum/letter_avatar/anon77415598/32/5_5575768a8748004e209b776fc1b2916d.png) [@anon77415598](https://www.simscale.com/forum/u/anon77415598)\
**Post date:** [June 6, 2023, 7:42am UTC](https://www.simscale.com/forum/t/problem-with-natural-ventilation/97177/2 "2023-06-06T07:42:54Z")

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Hi @s289286,

Thanks for posting your question!  
Maybe an input from my side would be using area average instead of integral, once it will be getting you the scalar average passing through the face. Have you already tried that one?  
For more information, please take a look at that article:

> **[Surface Data | Result Control Documentation | SimScale](https://www.simscale.com/docs/simulation-setup/result-control/surface-data/)**
>
> Surface data, as a result control item, allows performing area average and integral of field variables based on geometry faces. Learn more!

I also would like to ask you for the project link, just to take a look at your project and then search for some casual issue in the setup.

best,

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**Author:** ![s289286](https://www.simscale.com/forum/letter_avatar/s289286/32/5_5575768a8748004e209b776fc1b2916d.png) [@s289286](https://www.simscale.com/forum/u/s289286)\
**Post date:** [June 6, 2023, 7:55am UTC](https://www.simscale.com/forum/t/problem-with-natural-ventilation/97177/3 "2023-06-06T07:55:20Z")

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Thank you for your reply  
I haven’t done the project yet because I wanted to understand this issue well first .  
If I do area area avarage as you suggested however I don’t think I always get zero or an unrealistic figure of the flow rate passing through the aperture since on the same surface I will have positive and negative velocities.

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**Author:** ![kaany](https://www.simscale.com/forum/user_avatar/www.simscale.com/kaany/32/57613_2.png) [@kaany](https://www.simscale.com/forum/u/kaany)\
**Post date:** [June 9, 2023, 2:07pm UTC](https://www.simscale.com/forum/t/problem-with-natural-ventilation/97177/4 "2023-06-09T14:07:24Z")

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Hello there,

In addition to what Vinicius mentioned, using a third-party software like Paraview can indeed help you analyze entering and exiting flow rates. A quick summary in Paraview would be,

- Start by exporting the results from SimScale’s post-processor by using Download,
- Open results in Paraview.
- Extract data from the surface of interest.
- Convert cell data to point data.
- Generate surface normals.
- File \> Save Data
- Filter data based on flow direction whether it’s exiting or entering the domain - this would require a conditional statement.
- Calculate volumetric flow rate using Paraview’s Integrate Variables tool or your own integration method.

All operations could be performed in Paraview environment without the need of saving data, but you would need to use, for example, Python Calculator to determine cells with entering or exiting flow.

I hope this would be helpful,  
Best.