Using 'A ' BC for axisymmetric formulation on elements with face ID: not 1

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Bibhas Kumar

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Dec 15, 2025, 10:32:38 PM12/15/25
to Nek5000
Hi everyone, I am generating a mesh using gmsh and gmsh2nek. I need to use 'A. ' axisymmetric boundary on one of the sides. However, it seems 'A  ' only needs to be defined on face ID 1. I am not sure how to control that face ID allocation.

Any suggestions on how to force face ID 1 on the side where I want the axisymmetric BC?

Thanks,
Bibhas

Fischer, Paul

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Dec 16, 2025, 12:58:27 AM12/16/25
to Bibhas Kumar, Nek5000
I cannot speak to the idiosyncrasies of gmsh, but I can list some of the requirements for the "A  " BC:

  1. It must be attached to Face 1, only, in the Nek5000 / SEM face ordering (illustrated below).
  2. There can be no "single vertices" touching the axis — only full faces (Face 1) can 
     touch the axis.
  3. The face attached to the axis must not be curved (e.g., no midside node definition) - 
    Even if the face is nominally straight, it must not have a midside node defined on that edge.

Hth. 

Paul

Nek .rea / preprocessor ordering:

                  ^ s       
                  | Face 3
         +---------+--------+
         |                 |               |
         |                 |               |
         |                 |               |
  Face 4 |        +--------+--> r
         |                                 |
         |                                 | Face 2
         |                                 |
         +------------------+
               Face 1

This is not the same as the user interface for genbox,
which is instead lexicographical.

From: nek...@googlegroups.com <nek...@googlegroups.com> on behalf of Bibhas Kumar <bibha...@gmail.com>
Sent: Monday, December 15, 2025 9:32 PM
To: Nek5000 <nek...@googlegroups.com>
Subject: [nek5000] Using 'A ' BC for axisymmetric formulation on elements with face ID: not 1
 
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Bibhas Kumar

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Dec 17, 2025, 12:21:00 AM12/17/25
to Nek5000
Hello Prof. Fischer,
It appears that the face located at x = 0 is being assigned the face ID as 1. Does it mean the axis of rotation needs to be x=0 instead of y=0 (genbox domains seem to have y=0 as the axis of rotation)? 


If I rotate my domain by 90 degrees and ensure the axis of rotation is x = 0, would the problem be resolved? 

Best,
Bibhas

Fischer, Paul

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Dec 17, 2025, 12:57:58 AM12/17/25
to Bibhas Kumar, Nek5000
In Nek5000, the x axis is the axis of rotation.
It is critical that vertex 1 and 2 of each element that touches the x-axis have y value of 0.

Hth. 

Best,
Paul

Sent: Tuesday, December 16, 2025 11:20 PM
To: Nek5000 <nek...@googlegroups.com>
Subject: Re: [nek5000] Using 'A ' BC for axisymmetric formulation on elements with face ID: not 1
 
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