# Q criterion visualizations

**URL:** <https://pyfr.discourse.group/t/q-criterion-visualizations/818>\
**Category:** Cases\
**Tags:** visualisation\
**Created:** [25 May 2023 05:31 UTC](https://pyfr.discourse.group/t/q-criterion-visualizations/818 "2023-05-25T05:31:12Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Frankx9](https://avatars.discourse-cdn.com/v4/letter/f/3e96dc/32.png) [@Frankx9](https://pyfr.discourse.group/u/Frankx9)\
**Post date:** [25 May 2023 05:31 UTC](https://pyfr.discourse.group/t/q-criterion-visualizations/818/1 "2023-05-25T05:31:12Z")

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Hi,

I have a question regarding Q criterion visualizations: given that the grids employed are pretty coarse, when I use ParaView to compute the second invariant of the velocity gradient, I get isosurfaces that look “blocky”. In one of your seminars I saw a rather smooth view of Q, was it just a matter of a much finer grid ?

Regards

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**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\
**Post date:** [25 May 2023 13:44 UTC](https://pyfr.discourse.group/t/q-criterion-visualizations/818/2 "2023-05-25T13:44:52Z")

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When you export your solution use the `-d` flag in PyFR. For a p = 3 simulation you probably want something on the order of `-d5`. Then, when you open your solution in ParaView do not forget to run the clean to grid filter _before_ taking gradients/Q.

Regards, Freddie.