Surface Flow filter gives a wrong value

Hello everyone,
I looked around the forum and found no other report of this so I create a new topic. I am currently analyzing a biphasic case (air-water) of a buried river flowing under a city. The calculation was done with Ansys CFX. I had a lot of repetitive analysis to perform on the data and CFD Post was really slow so I exported my results to CGNS and managed to analyse them with paraview which led to two conclusions:

  • Paraview is so quick to analyse it amazes me and so versatile in the same time with the python console! anyway this is obviously not my problem
  • Surface flow filter is giving me underestimations of flows whereas CFDPost does not

So I tried it the old way by calculating the projection of my velocities on the surface normal and using the integratevariable filter. The result was the same as the Surface flow, which is coherent but not better.

The error is correlated to the total area of the surface : the bigger the surface, the closer the results so for me the Paraview treatment of the velocities value on the perimeter is wrong

But I may have done something wrong myself, even if I can’t see why yet!


So you think the error is in the Integrate Variable filter ?
What makes you think the results is an underestimation ?
Do you know how CFDPost makes the computation ?

Ideally, we would need a dataset were the computation can be done by hand and which would shows that ParaView underestimate the results.

Hi Mathieu,
thank you for your reply,
I should have written that I impose 33 m3/s on one boundary and I calculated the flow in a section of the same tunnel where nothing happens to the flow yet, so the flowrate must be 33 m3/s. CFD Post gives me 32,9 m3/s which is ok and Paraview gives me 30.2m3/s, which is 8% inferior to the expected value.

I do not know how CFDPost does the calculation but I have used it a lot in the past years and I never found a noticeable issue regarding mass conservation (when correctly converged) so I am pretty confident in the result of the software


Could you share a simple dataset that shows this behavior ?

Ok, I will extract a dataset by tomorrow,

Hi again Mathieu,
here is a dataset in PVD that shows the behaviour, steps to reproduce :

-Paraview v 5.5.2 (portable version, no admin right for me :

  • create a Slice of type plane, ORIGIN : 30.9,-208.3,5.56
    NORMAL : 0.28, -0.96, -0.0137
  • apply a Surface flow filter, input variable is “eau.Velocity”
  • result is 30.4854 (m3/s), in CFDPost at the same location it is 32.9844 m3/s (2.0 MB)

Hi everyone,
Did you have a look at my dataset ?
do you have any clue about this behaviour?
Thank you

Sorry, I had no time to look into it yet.

up. Right now, the only reason I and the CFD team here in my office do not switch to paraview is because of that issue which lead to no confidence in the integrate filter. I do not think this is a big issue but the difference is there, I would really apreciate some hints.
thank you

I have almost the same results as you, 30.5249, but to check and fix the issue would require some time.

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Any news on this topic? I have a similar problem computing the mass flow rate.

A simple question. Does Surfuce flow filter use the same routing of Integrate variables?