Result Error for Passive Scalar Simulation

hi, I wanna ask for help about the result in SimScale incompressible simulation.

I want to try simulate SO2 in a house. This is the link to my project.

I want to get similar result with a particular project I’ve seen before because it is similar to my project. The difference is only the gas emission; I choose SO2 as the gas variable (the diffusion coefficient and the flux are the differences).

The similar project;

The problem is my passive scalar result is different with the project example; my passive scalar result is too high from what it has to be. Is it because the variable or something else? I am still learning about CFD software as well, so I don’t really understand about this. And also, my area average isn’t showing any result. Do you know why?

I really appreciate for the help @tsite

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Hi there, thank you for using the forum!

The first thing I noticed, that is extremely crucial, is the direction of the flow that enters the house:

Fixing this should solve a lot of issues :wink:

Also, keep in mind that your mesh is very coarse, I would recommend that you increase the global fineness level a bit.

You can always check the mesh quality using this guide: How to Check and Improve Mesh Quality | SimScale

Best regards,

hi, me again. I want to ask for the result again.
I want to search for ppm result for this simulation, but why my result still exceeds the project example? The result is supposed to be less than 1000 but mine is exceeding the number. Do you know why? And I’ve already done what you’ve told me to do, but the result is still the same.

This is my passive scalar result:
rumah minimum

And this is the example result:

Thank you so much for any helps!

Hi again, which run are you referring to?

every run is exceeding the number :sweat_smile: but you can look at run “velocity 3.3” because it has final variable. I am confused about the passive scalar unit, I followed the project example to get similar ppm, but I don’t understand why the passive scalar result is so high; is my ppm concentration really that high or there is still something wrong with my model’s variable?

Thank you once again for your help.


I noticed that you are going through my old tutorial :smiley:

The issue here is the scale of the model. As the name suggests, the volumetric scalar source is based on the assigned volume. In your case, the model seems to be oversized by a factor of 1000 (I’d guess):

So the concentration of scalar goes up pretty quick. It would be best to resize the model back to the appropriate dimensions.



Hi, thank you for the fast response. I have repeated the process and scaled the model and it helps with the number of the passive scalar! :ok_hand:


Hi, it’s me again!
I wanna ask, I want to run 2nd simulation with same model but it turns out the result of the scalar has bigger number again. How can I solve this?

First simulation

Second simulation

I appreciate very much for the help.

Hi @dprimalaila ,

You can think of the T1 values as a concentration of passive scalar. For your first simulation, the velocity inlet of fresh air was 10 meters per second, and for the second simulation you have 0.15 meters per second.

Since we have less fresh air going in and the same quantity of scalar being generated, the concentration of the passive scalar goes up. In short, this is expected.


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Thank you for the explanation, this helps a lot with my understanding :ok_hand:

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Hi, I want to ask about a similar issue from the above.
Now I want to simulate NO2 within the same model of the house.
Using the same model, I am not able to simulate as the run time always exceeds the maximum runtime.
Is there any error within my model? should I erase some simulation results?

Thank you.