SimScale CAE Forum

Can you measure fluid flow rate through a specific area within an enclosure?

Hello! Brand new SimScale user here and I have a question about what data you can extract from an enclosure operation. Here is a version of my project that demonstrates what I’m looking to find:

This is basically a primitive transit bus model with open passenger windows on both sides in a “wind tunnel.” The finished simulation shows that air flows into certain window openings and out of certain other window openings due to pressure differences as the “bus” moves through the air. What I am wanting to do is find a way to measure how much air flows through a specific window opening over a given unit of time (flow rate). Is there a way to find that using SimScale or possibly ParaView?

Thank you so much to anyone able and willing to offer guidance!

Hey there, welcome!

It is possible to achieve that within SimScale. I’ll post the steps below:

  1. Create a cutting plane and place it exactly over the windows:

  2. Go to Result Statistics and enable the second option:

  3. Select the cutting plane and choose the velocity that you’re interested in for the cutting plane plot (the volumetric flow rate in this case is given by y velocity). You will see a bunch of boxes with information about each one of the sections (minimum, maximum, average, integrals):

And that’s it! It’s not necessary to take it to ParaView.


@Ricardopg that was perfect! I had done step 1, but was not aware of the feature in step 3. Thank you so very much for your incredibly helpful response!!! :beer: Wish I could buy you a beer :smiley:

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@Ricardopg, I have another question I’m hoping you know the answer to. As you can see in the screenshots you so generously provided to help me with my question, there are two values given at the bottom: Integral and Volumetric Flow Rate. Over a surface like this, I would think that they should be equal, but they are not. My thought is that the integral of velocity over one of these openings (which is the “Integral” value given) IS the volumetric flow rate, is it not? Do you know what the difference is in this case? Do you think you could explain why the given integral and volumetric flow rates are different from each other?

Also, there are only 11 openings on this model through which air can flow (6 road-side windows and 5 curb-side windows). In theory, the sum of all outward flow rates should be the same as the sum of all inward flow rates through these openings (since air flowing in should equal air flowing out). But they’re not equal. The sum of outward flow rates is 241.92 m^3/s and the sum of inward flow rates is 246.06 m^3/s. Do you know why that is?

Thank you so much!


So, in that picture, the parameter being considered is “All velocity”. The integral values will take into account the magnitude of the velocity.

Naturally, the velocity vector won’t be necessarily normal to the windows, and that’s where the difference occurs. In your case, since the windows are in the XZ plane, we can check the integral values for Vy, which should be the volumetric flow rate. The results do match, as you can see below:


All velocity:

In a fully converged solution, I do agree! The inlet and outlet rates should be equal. Have you done any detailed analysis with regards to convergence, sensitivity studies (domain size, etc)?



@Ricardopg, Thank you for your detailed response once again. That definitely makes sense now that you explained it that way. Thanks for taking the time to show how the Vy intergral and volumetric flow rate are the same!

I have not done any detailed analysis regarding convergence/sensitivity studies. Am I to understand that if the simulation were to run longer to a tighter convergence, the inlet and outlet rates would get closer to equal? If so, is there a way to adjust convergence parameters to get a more accurate solution?

The reason I ask is because I have since been doing other sim runs looking for this same volumetric flow rate, but with more detailed models. Some of the runs end up with solutions in which the inlet and outlet rates are FAR from the same and I can’t figure out why that would be. Unfortunately, I’m not familiar with sensitivity studies and domain size that you mentioned.

I’m not asking for a detailed explanation or anything, but perhaps you could point me in the right direction?

Thanks again for all you help!