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Why is vorticity ALWAYS ZERO on surfaces in Tecplot

Posted: Sat Feb 19, 2011 7:32 pm
by jon98064
Can someone explain why vorticity is ALWAYS ZERO on surfaces in Tecplot.

In particular, I am looking at Vorticity Magnitude on the wall beneath a boundary layer.

Is this some gigantic bug that has managed to survive through the decades?
or
Am I missing something?

Whatever, this is a REALLY irritating problem because the vorticity distribution on surfaces is one of the main reasons for purchasing Tecplot in the first place (please don't tell me that vorticity should be zero on surfaces)

Re: Why is vorticity ALWAYS ZERO on surfaces in Tecplot

Posted: Sat Feb 19, 2011 11:31 pm
by jon98064
I see the following variables appear in the list of variables for contours
Wall shear-1, Wall shear-2 and Wall shear-3
They look useful since the different contour levels do appear on the surfaces

However, these terms cannot be found in any of the Tecplot manuals?
My guess is that these are wall shear stress in the x, y and z directions.
Is this correct?

I also tried looking at contours of dU/dY (x wall shear) but Tecplot says this quantity is zero on the wall beneath my boundary layer too?

Re: Why is vorticity ALWAYS ZERO on surfaces in Tecplot

Posted: Tue Feb 22, 2011 8:00 am
by salter
First off, the vorticity is defined as the curl of velocity, or the gradient cross-product with the velocity field. In viscous flow, velocity should be zero at the wall, a boundary condition that causes the build-up of the boundary layer. So, unless you have a slip condition in the computation of the flow, your vorticity should be zero at the wall.

Second, if you see variables in the contour dialog for contours on surfaces, those variables are typically read in from your dataset. I'm not seeing where they are computed from the analyzer.

Re: Why is vorticity ALWAYS ZERO on surfaces in Tecplot

Posted: Tue Feb 22, 2011 8:18 am
by davetaflin
You can interpolate vorticity from the volume zones to the surface zones using Tecplot's Data/Interpolate... options. If the nodes of your surface zones are coincident with nodes in your volume zones, linear interpolation should be fine (and quick). Otherwise, you may want to consider Kriging, a much slower option.

I'll enter this issue for consideration in future Tecplot versions.

Re: Why is vorticity ALWAYS ZERO on surfaces in Tecplot

Posted: Tue Feb 22, 2011 10:38 am
by salter
I stand corrected. The vorticity is based on the gradient of velocity through the boundary layer to the wall. So as long as the flow is moving, even if the velocity at the wall is zero, the vorticity can be non-zero for a viscous flow field. As to the issue of the vorticity being zero on a surface, if the zone is only a surface, there is no information to volume data that would give some analysis the ability to compute the gradients.

Re: Why is vorticity ALWAYS ZERO on surfaces in Tecplot

Posted: Sat Mar 16, 2013 5:26 am
by jon98064
It is basic undergraduate fluid mechanics that vorticity exists on surfaces, even though the velocity may be zero, such as on a non-slip wall.

Yet Tecplot fails to display vorticity contours on such surfaces.

It is almost unbelievable - but true,
that this very significant and basic flaw has been known about for years.

The Tecplot developers need to explain why such a basic flaw continues to exist even though it is a flaw that has been acknowledged and there have been promises to get it fixed.