[petsc-users] Additiional DoF per cell

Matthew Knepley knepley at gmail.com
Wed May 6 06:39:37 CDT 2026 

On Wed, May 6, 2026 at 6:56 AM Noam T. <dontbugthedevs at proton.me> wrote:

> Hello,
>
> That is wrong. Are you sure about 22?
>
>
> That's what I get, from the argument "csize" in DMPlexGetVecClosure().
>
> Here's the output of PetscDSView:
>
> Discrete System with 2 fields
>     cell total dim 11 total comp 5
>   Field u FEM 2 components (implicit) (Nq 4 Nqc 1) 1-jet
>     PetscFE Object: u (mech_) 1 MPI process
>       type: vector
>       Vector Finite Element in 2 dimensions with 2 components
>       PetscSpace Object: Q1 (mech_) 1 MPI process
>         type: sum
>         Space in 2 variables with 2 components, size 8
>         Sum space of 2 concatenated subspaces (all identical)
>           PetscSpace Object: Q1 (mech_sumcomp_) 1 MPI process
>             type: tensor
>             Space in 2 variables with 1 components, size 4
>             Tensor space of 2 subspaces (all identical)
>               PetscSpace Object: sum component tensor component
> (mech_sumcomp_tensorcomp_) 1 MPI process
>                 type: poly
>                 Space in 1 variables with 1 components, size 2
>                 Polynomial space of degree 1
>       PetscDualSpace Object: Q1 (mech_) 1 MPI process
>         type: sum
>         Dual space with 2 components, size 8
>         Sum dual space of 2 concatenated subspaces (all identical)
>           PetscDualSpace Object: Q1 1 MPI process
>             type: lagrange
>             Dual space with 1 components, size 4
>             Continuous tensor Lagrange dual space
>         Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
>   Field p FEM 3 components (implicit) (Nq 4 Nqc 1) 1-jet
>

1. Why does pressure have 3 components in 2D?


>     PetscFE Object: p (pres_) 1 MPI process
>       type: vector
>       Vector Finite Element in 2 dimensions with 3 components
>       PetscSpace Object: Q0 (pres_) 1 MPI process
>         type: sum
>         Space in 2 variables with 3 components, size 3
>         Sum space of 3 concatenated subspaces (all identical)
>           PetscSpace Object: Q0 (pres_sumcomp_) 1 MPI process
>             type: tensor
>             Space in 2 variables with 1 components, size 1
>             Tensor space of 2 subspaces (all identical)
>               PetscSpace Object: sum component tensor component
> (pres_sumcomp_tensorcomp_) 1 MPI process
>                 type: poly
>                 Space in 1 variables with 1 components, size 1
>                 Polynomial space of degree 0
>       PetscDualSpace Object: Q0 (pres_) 1 MPI process
>         type: sum
>         Dual space with 3 components, size 3
>         Sum dual space of 3 concatenated subspaces (all identical)
>           PetscDualSpace Object: Q0 1 MPI process
>             type: lagrange
>             Dual space with 1 components, size 1
>             Discontinuous tensor Lagrange dual space
>         Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
>   Weak Form System with 2 fields
>
> using as the second field (dim = 2, nc = 3, prefix = "pres") and setting
> the FE object name to "p".
>
> The entry "cell total dim 10" agrees with the output of
> PetscFEGetTotalDimension();  "comp = 5" I assume is 2 (u) + 3 (p)
>
> However, the closure of a Vec is still 22.
>

2. Did you look at the incompressible example SNES ex69? It sets up this
exact element.

3. Please send me that code that sets up your DM. It sounds like somehow
you have a copy of your fields.

  Thanks,

     Matt


> On Tuesday, May 5th, 2026 at 1:58 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
> On Tue, May 5, 2026 at 9:06 AM Noam T. via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
>> Hello,
>>
>> I am trying to work with a "mixed" FE discretization, where besides the
>> usual displacements DoF in nodes, there is an additional field (e.g.
>> pressure) that is also part of the system. This additional field has a
>> certain number of additional dof : p0, p1, p2...
>>
>> Looking at example 77 (
>> https://urldefense.us/v3/__https://petsc.org/release/src/snes/tutorials/ex77.c.html__;!!G_uCfscf7eWS!dgCGMDKX0lQYHlCGpF8h1ZTP6zOmpezZRQiG5zqOHKm8pCFYv_yUAKaXAsHTEuxUZNlUWgy026NskhiSy0Mt$ 
>> <https://urldefense.us/v3/__https://petsc.org/release/src/snes/tutorials/ex77.c.html__;!!G_uCfscf7eWS!aZNOxZZeoHYqe6lRZV0wHMVVTu3YIEfc1Sr-dca7xiGVfG3enULFD4_g6_fxWIa7A_Cu9LcXU-MuxRj4sOwDRyWqvsdGdrEd$>)
>> this seems to be handled with an additional field, added to the DM. I've
>> tried so, but then I am getting from the DM arrays whose size/contents are
>> not what I expected.
>>
>> For example, a four-noded Q1 element (PetscFE created with a 2x2 Gauss
>> rule for quadrature), with just one field, a call to DMPlexGetVecClosure()
>> gives me an array with 8 entries (say, the coordinates of the initial
>> mesh): x = x0, y0, .... x3, y3
>>
>> Then add the new field:
>>
>> PetscFECreateDefault(..., nc = 3, ..., p_FE) /* not sure about the value
>> of nc here */
>> DMAddField(dm, ..., p_FE)
>>
>> The total dimension, from PetscDSGetTotalDimension(), is now 11 (8 + 3).
>> This results in a closure of size 22.
>>
>
> That is wrong. Are you sure about 22?
>
>> However, what I am looking for is a closure of size 8 + 3 i.e. the
>> original 8 DoF at the nodes, plus exactly 3 DoF (for the cell, so to speak)
>> p0, p1, p2, so that in a system with "block" matrices of the form
>>
>> [K_uu, K_up | K_pu, K_pp] { u | p } = RHS
>>
>> the unknowns {p} has size 3 per cell. Is this possible? I tried some
>> combinations of dim / nc for the new PetscFE, but when creating the DM
>> section I get errors e.g.
>>
>
> This is what you should get. I definitely have examples that do this. For
> example, here is Q1-P0 (I think that is what you are suggesting) for
> incompressible Stokes
>
>
> https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/blob/main/src/snes/tutorials/ex69.c?ref_type=heads*L3454__;Iw!!G_uCfscf7eWS!dgCGMDKX0lQYHlCGpF8h1ZTP6zOmpezZRQiG5zqOHKm8pCFYv_yUAKaXAsHTEuxUZNlUWgy026NskqJzzOX5$ 
>
> Thanks,
>
> Matt
>
>> "point X has a number of DoF not divisible by 2 field components"
>>
>> Is this a hint that I should have 3 x dim new DoF, and simply not deal
>> with entries that I don't need? Or I am not setting up the section properly
>> (works with just one field)?
>>
>> Thank you,
>> Noam.
>>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
> https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!dgCGMDKX0lQYHlCGpF8h1ZTP6zOmpezZRQiG5zqOHKm8pCFYv_yUAKaXAsHTEuxUZNlUWgy026NskuDOEF3q$ 
> <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!dgCGMDKX0lQYHlCGpF8h1ZTP6zOmpezZRQiG5zqOHKm8pCFYv_yUAKaXAsHTEuxUZNlUWgy026Nskt9MckXB$ >
>
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!dgCGMDKX0lQYHlCGpF8h1ZTP6zOmpezZRQiG5zqOHKm8pCFYv_yUAKaXAsHTEuxUZNlUWgy026NskuDOEF3q$  <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!dgCGMDKX0lQYHlCGpF8h1ZTP6zOmpezZRQiG5zqOHKm8pCFYv_yUAKaXAsHTEuxUZNlUWgy026Nskt9MckXB$ >
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