# Visualising structures using Q-criterion in Paraview
**URL:** https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528
**Category:** ParaView Support
**Tags:** vtk
**Created:** [December 16, 2023, 9:51pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528 "2023-12-16T21:51:45Z")
**Posts on this page:** 17
**Page:** 1
### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 16, 2023, 9:51pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/1 "2023-12-16T21:51:46Z")
I have loaded a state file (.pvsm) of a turbulent channel flow solved using DNS and I want to visualise vortical structures using the Q-criterion but it’s not working out.
I’m not sure what I’m missing because the tutorials seem straightforward.
The velocity components are all stored as points in 3 different arrays (u,v,w) but I can’t seem to be able to calculate the velocity magnitude with the Calculator filter. I use the equation Umag= sqrt(u^2+v^2+w^2) to find the velocity magnitude.
Any help will be appreciated and I am happy to give more information about the setup
Thanks!
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 18, 2023, 9:03am UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/2 "2023-12-18T09:03:25Z")
Please share your data and pvsm file
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 18, 2023, 9:38am UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/3 "2023-12-18T09:38:14Z")
[flow\_field.zip](https://discourse.paraview.org/uploads/short-url/6bPS7e6oR4bd5Gz9NnydqgTCXmQ.zip) (58.3 KB)
I don’t know if you can or would like to open zip files.
the u, v and w velocity data are stored within .xmf files separate from the .pvsm file so I had to compress and zip them all together
thanls.
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 18, 2023, 11:16am UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/4 "2023-12-18T11:16:46Z")
HI @akintunde_akinneye
Please share the .h5 files too.
Best,
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 18, 2023, 11:25am UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/5 "2023-12-18T11:25:19Z")
[grid3dc.h5sp](https://discourse.paraview.org/uploads/short-url/kEgN38UWTPTmEokSZnEO5ogNZsr.h5sp) (5.3 KB)
[grid3du.h5sp](https://discourse.paraview.org/uploads/short-url/yv7EJexxi1EPhYliDEzrDqFmOb2.h5sp) (5.3 KB)
[grid3dv.h5sp](https://discourse.paraview.org/uploads/short-url/iMQoF3j1PYuVCNEVtSLhH1o3G1R.h5sp) (5.3 KB)
[grid3dw.h5sp](https://discourse.paraview.org/uploads/short-url/4D7uO6S65rDaT5pAiYv5BmpJ4wn.h5sp) (5.3 KB)
these are .h5sp files, I hope they are the same?
Many thanks
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 18, 2023, 12:14pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/6 "2023-12-18T12:14:34Z")
the files should be named `u.res.hdf5` or similar
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 18, 2023, 12:21pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/7 "2023-12-18T12:21:53Z")
The files are quite heavy and for some reason I cannot upload them in this chatbox.
Is there any other way to send them to you or can something else be done?
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 18, 2023, 12:24pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/8 "2023-12-18T12:24:21Z")
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 18, 2023, 12:31pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/9 "2023-12-18T12:31:07Z")
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Many thanks!
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 18, 2023, 3:57pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/10 "2023-12-18T15:57:29Z")
> [@akintunde\_akinneye](#):
>
> sqrt(u^2+v^2+w^2)
Works fine here:
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 18, 2023, 4:43pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/11 "2023-12-18T16:43:51Z")
Yes I can visualise the results but I seem to be having trouble calculating the Q-criterion
if you were able to, could you provide a brief walkthrough of how you did it?
thanks
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 18, 2023, 4:45pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/12 "2023-12-18T16:45:49Z")
> [@akintunde\_akinneye](#):
>
> I seem to be having trouble calculating the Q-criterion
How do you compute it ?
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 18, 2023, 4:51pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/13 "2023-12-18T16:51:59Z")
Well, assuming the research I have made is correct, the first step is to create an array (Umag) with the three velocity components (u,v,w) using the calculator filter.
I have been struggling to do this, then you use the Gradient of Unstructured Dataset 1 filter to calculate vorticity and Q criterion.
So, while my main goal is to calculate the Q criterion I haven’t been lucky to find the Umag. I get a blank box when I apply the calculator filter as shown in the image below:
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 18, 2023, 4:58pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/14 "2023-12-18T16:58:35Z")
It is actually pretty simple.
> I get a blank box when I apply the calculator filter as shown in the image below:
This is an outline, switch representation to surface to see the surface instead of the outline
> the first step is to create an array (Umag) with the three velocity components (u,v,w) using the calculator filter.
The first step is to create a **Vector** with the three velocity components (u,v,w) using the calculator filter or the dedicated filter, **MergeVectorComponents**
Here is a state file: [3d\_visualisation\_kw.pvsm](https://discourse.paraview.org/uploads/short-url/6rtcKdHjU9TRmHo74SsY00Rv9rl.pvsm) (971.6 KB)
Here are the manual steps: -
- run ParaView
- open u.res.xmf, Apply
- open v.res.xmf, Apply
- open w.res.xmf, Apply
- select all three readers in the pipeline browser by keeping Ctrl pressed,
- Filters → AppendAttributes, Apply
====== (new steps below)
- Filters → MergeVectorComponents, select u,v,w, Apply
- Filters → Gradient, Select “Vector”, show advanced options (cogwheel), check QCriterion, Apply
- Switch representation to surface, show QCriterion
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 18, 2023, 5:56pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/15 "2023-12-18T17:56:29Z")
Thanks, I’ve been able to visualise the velocity magnitude
However, I don’t seem to have the Mergevectorcomponents filter. Perhaps it’s in a later release of Paraview?
I’m currently using v5.4.1
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### Author:  [@mwestphal](https://discourse.paraview.org/u/mwestphal)
#### Post date: [December 19, 2023, 8:52am UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/16 "2023-12-19T08:52:16Z")
Yes, this is a recent filter. Please update your ParaView.
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### Author:  [@akintunde\_akinneye](https://discourse.paraview.org/u/akintunde_akinneye)
#### Post date: [December 19, 2023, 2:50pm UTC](https://discourse.paraview.org/t/visualising-structures-using-q-criterion-in-paraview/13528/17 "2023-12-19T14:50:36Z")
Many thanks!
It’s worked fine