[Paraview] Vorticity is calculated as zero

[Burlen Loring]

> Indeed, my smaller mesh had linear triangles. And changing that to 
> quadratic triangles gives the same behavior (attachment).
I experimented a bit with the data you send and there does seem to be a 
problem with the quadratic triangles. However, I found that with linear 
triangles the result is correct.

One thing I noticed about your test data is this velocity field is a 
special case <http://en.wikipedia.org/wiki/Vortex#Vorticity_profiles> , 
where v increases proportional with distance to vortex center. For this 
case vorticity is a constant. This means in PV you'll just see a single 
color in the plot, so it looks the same for both linear(good result) and 
quadratic triangles(bad result). This is something to be aware of!





> The reason why I wanna use the vorticity function is the following: 
> eddies appear all over the domain and using lines with the 
> streamtracer filter gives regions with either no streamlines or 
> regions that are very much crowded by streamlines. Therefore, having a 
> vorticity field would be a suitable approach to have a nice and clean 
> visualization of the flow field.
In addition to visualizing vorticity the LIC visualization technique may 
be useful for you as well.
http://www.paraview.org/Wiki/ParaView/Line_Integral_Convolution

Burlen

On 05/21/2014 02:32 PM, Andreas Hessenthaler wrote:
> Indeed, my smaller mesh had linear triangles. And changing that to 
> quadratic triangles gives the same behavior (attachment).
>
> What is the best way to circumvent this?
>
> The reason why I wanna use the vorticity function is the following: 
> eddies appear all over the domain and using lines with the 
> streamtracer filter gives regions with either no streamlines or 
> regions that are very much crowded by streamlines. Therefore, having a 
> vorticity field would be a suitable approach to have a nice and clean 
> visualization of the flow field.
>
> Cheers
>
>
> Am 21-May-2014 17:00:55 +0200 schrieb andy.bauer at kitware.com:
>
>     It looks like these are quadratic triangles. I'm not sure that the
>     compute derivatives filter works with that type of cell. Did your
>     smaller grid also have quadratic cell types?
>
>
>     On Wed, May 21, 2014 at 9:51 AM, Andreas Hessenthaler wrote:
>
>         Hi Andy,
>
>         thanks for your quick reply. Please find the *.vtu file attached.
>
>         Cheers
>         Andreas
>
>
>         Am 21-May-2014 15:24:30 +0200 schrieb andy.bauer at kitware.com
>         <mailto:andy.bauer at kitware.com>:
>
>             Can you share your data set? It will be difficult to
>             diagnose without that.
>
>
>             On Wed, May 21, 2014 at 9:06 AM, Andreas Hessenthaler wrote:
>
>                 Hi there,
>
>                 I have the following problem:
>
>                 When I calculate the vorticity through Filters >
>                 Compute Derivatives > Vorticity for a simple example
>                 (driven cavity in 2D) with only few elements on an
>                 unstructured grid I get the correct values.
>
>                 Though, when I do the same for a bigger domain, i.e.
>                 more elements (~80 000 nodes), I only get zero
>                 vorticity values.
>
>                 The grids and values are all read in from *.vtu files,
>                 with the exact same format, meshes are triangluar 2D
>                 (z-components set to 0.0).
>
>                 I can reproduce the behavior on different versions of
>                 ParaView: 3.14.1 & 4.1.0 as well as different
>                 operating systems: Windows 7 & Ubuntu 14.04.
>
>                 If I calculate the gradient of the velocity field, all
>                 values are calculated as zero as well.
>
>                 Thanks in advance!
>
>                 Cheers
>                 Andreas
>
>
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