[petsc-users] Additiional DoF per cell

Fri May 8 06:05:45 CDT 2026

On Fri, May 8, 2026 at 5:08 AM Noam T. <dontbugthedevs at proton.me> wrote:

> On the plus side, using the flags you provided in the previous email
>
> -dm_plex_simplex 0 -mech_petscspace_degree 1 -pres_petscspace_degree 1
> -pres_petscspace_poly_tensor 0 -pres_petscdualspace_lagrange_continuity 0
>
> the output of  -ds_view is identical to yours, so setting up the fields
> should be correct.
> Thanks. I forgot about the flag "-[]_petscdualspace_lagrange_continuity 0"
> so as to have a discontinuous field.
>
> However, the closure size was still wrong.
>
> But, I figured out the reason: the manual set up of the section.
> For the argument "numDof" in DMPlexCreateSection(), I was computing the
> number of DoF with PetscSectionGetDof() (which was fine until now, since
> there was a single field), instead of PetscSectionGetFieldDof().
> So as you previously suggested
>
> [...] It sounds like somehow you have a copy of your fields.
>
>
> the number of DoF per field were indeed "doubled" with the sum of both
> fields.
>

Excellent. Let me know if you have any more issues.

  Thanks,

     Matt

> Thanks,
> Noam
>
> On Wednesday, May 6th, 2026 at 2:06 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
> Okay, this is a problem with specifying the space I believe. You want a
> discontinuous space for pressure.
>
> Here is what I get
>
> Discrete System with 2 fields
> cell total dim 11 total comp 3
> Field Q1 FEM 2 components (implicit) (Nq 4 Nqc 1) 1-jet
> PetscFE Object: Q1 (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 Q1 FEM 1 component (implicit) (Nq 4 Nqc 1) 1-jet
> PetscFE Object: Q1 (pres_) 1 MPI process
> type: basic
> Basic Finite Element in 2 dimensions with 1 components
> PetscSpace Object: Q1 (pres_) 1 MPI process
> type: poly
> Space in 2 variables with 1 components, size 3
> Polynomial space of degree 1
> PetscDualSpace Object: Q1 (pres_) 1 MPI process
> type: lagrange
> Dual space with 1 components, size 3
> Discontinuous Lagrange dual space
> Quadrature on a quadrilateral of order 3 on 4 points (dim 2)
> Weak Form System with 2 fields
>
> I have attached the code I used.
>
> Thanks,
>
> Matt
>
> On Wed, May 6, 2026 at 9:32 AM Noam T. <dontbugthedevs at proton.me> wrote:
>
>>
>> Field p FEM 3 components (implicit) (Nq 4 Nqc 1) 1-jet
>>
>>
>> 1. Why does pressure have 3 components in 2D?
>>
>>
>> I used nc = 3 when calling PetscFECreateDefault() for the pressure field.
>> Confusion between wanting 3 addditional DoF and components.
>>
>> With nc = 1 the closure size is 18 instead.
>>
>> 2. Did you look at the incompressible example SNES ex69? It sets up this
>> exact element.
>>
>>
>> I changed some of my code following the example and got rid of some
>> errors. But I'll have a more thorough look again.
>>
>> 3. Please send me that code that sets up your DM. It sounds like somehow
>> you have a copy of your fields.
>>
>> Thanks,
>>
>>
>> For reference, -ds_view with only one (displacements) fields shows:
>>
>> ---
>> Discrete System with 1 fields
>> cell total dim 8 total comp 2
>> Field u FEM 2 components (implicit) (Nq 4 Nqc 1) 1-jet
>> PetscFE Object: u (disp_) 1 MPI process
>> type: vector
>> Vector Finite Element in 2 dimensions with 2 components
>> PetscSpace Object: Q1 (disp_) 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 (disp_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
>> (disp_sumcomp_tensorcomp_) 1 MPI process
>> type: poly
>> Space in 1 variables with 1 components, size 2
>> Polynomial space of degree 1
>> PetscDualSpace Object: Q1 (disp_) 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)
>> Weak Form System with 1 fields
>> ---
>>
>> and -dm_view
>>
>> ---
>> DM Object: 1 MPI process
>> type: plex
>> DM_0x55c117db1b00_2 in 2 dimensions:
>> Number of 0-cells per rank: 4
>> Number of 1-cells per rank: 4
>> Number of 2-cells per rank: 1
>> Labels:
>> celltype: 3 strata with value/size (0 (4), 1 (4), 4 (1))
>> depth: 3 strata with value/size (0 (4), 1 (4), 2 (1))
>> Cell Sets: 1 strata with value/size (1 (1))
>> surf: 1 strata with value/size (1 (1))
>> Face Sets: 2 strata with value/size (2 (1), 3 (1))
>> edge_right: 1 strata with value/size (3 (1))
>> edge_left: 1 strata with value/size (2 (1))
>> Vertex Sets: 4 strata with value/size (4 (1), 5 (1), 6 (1), 7 (1))
>> P1: 1 strata with value/size (4 (1))
>> P2: 1 strata with value/size (5 (1))
>> P3: 1 strata with value/size (6 (1))
>> P4: 1 strata with value/size (7 (1))
>> Field u:
>> adjacency FEM
>> /// With the second field, the two lines below are added
>> Field p:
>> adjacency FEM
>> ---
>>
>> DM set up
>>
>> DM :: dm_mesh
>> PetscFE :: u_FE, p_FE
>> PetscInt :: dim, dim_u_FE, tdim
>> PetscDS :: dm_ds
>>
>>
>> DMCreate(PETSC_COMM_WORLD, dm_mesh)
>> DMSetType(dm_mesh, DMPLEX)
>> DMGetDimension(dm_mesh, dim) // dim = 2
>> DMSetFromOptions(dm_mesh) // this was called twice by mistake, now
>> removed; no changes that I can see
>>
>> -- using flags
>>
>> -dm_plex_filename QUAD1.msh
>> -dm_plex_interpolate 1
>> -dm_plex_gmsh_use_generic
>> -dm_plex_gmsh_use_regions
>> -dm_plex_gmsh_multiple_tags
>> -dm_plex_gmsh_mark_vertices
>>
>> -- some operations with labels
>>
>> // u field
>> PetscFECreateDefault(PETSC_COMM_SELF, dim, dim, PETSC_FALSE, "mech_",
>> PETSC_DETERMINE, u_FE)
>> PetscFEGetDimension(u_FE, dim_u_FE) // dim_u_FE = 8
>> DMAddField(dm_mesh, NULL, (PetscObject)u_FE)
>>
>> // p field
>> PetscFECreateDefault(PETSC_COMM_SELF, dim , 1, PETSC_FALSE, "pres_",
>> PETSC_DETERMINE, p_FE)
>> PetscFEGetDimension(p_FE, dim_p_FE) // dim_p_FE = 1
>> DMAddField(dm_mesh, NULL, (PetscObject)p_FE)
>>
>> DMCreateDS(dm_mesh)
>> DMGetDS(dm_mesh, dm_DS)
>> PetscDSGetTotalDimension(dm_DS, tdim) // tdim
>>
>> with flags
>>
>> -disp_petscdualspace_lagrange_node_type equispaced
>> -disp_petscdualspace_lagrange_node_endpoints 1
>>
>>
>> I believe these are all the DM related calls.
>>
>> Thanks,
>> Noam
>>
>> On Wednesday, May 6th, 2026 at 11:39 AM, Matthew Knepley <
>> knepley at gmail.com> wrote:
>>
>> 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!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJDWQyy5f$ 
>>>> <https://urldefense.us/v3/__https://petsc.org/release/src/snes/tutorials/ex77.c.html__;!!G_uCfscf7eWS!aZNOxZZeoHYqe6lRZV0wHMVVTu3YIEfc1Sr-dca7xiGVfG3enULFD4_g6_fxWIa7A_Cu9LcXU-MuxRj4sOwDRyWqvsdGdrEd$>)
>>>> this seems to be handled with an additional field, added to the DM. I've
>>>> tried so, but then I am getting from the DM arrays whose size/contents are
>>>> not what I expected.
>>>>
>>>> For example, a four-noded Q1 element (PetscFE created with a 2x2 Gauss
>>>> rule for quadrature), with just one field, a call to DMPlexGetVecClosure()
>>>> gives me an array with 8 entries (say, the coordinates of the initial
>>>> mesh): x = x0, y0, .... x3, y3
>>>>
>>>> Then add the new field:
>>>>
>>>> PetscFECreateDefault(..., nc = 3, ..., p_FE) /* not sure about the
>>>> value of nc here */
>>>> DMAddField(dm, ..., p_FE)
>>>>
>>>> The total dimension, from PetscDSGetTotalDimension(), is now 11 (8 +
>>>> 3). This results in a closure of size 22.
>>>>
>>>
>>> That is wrong. Are you sure about 22?
>>>
>>>> However, what I am looking for is a closure of size 8 + 3 i.e. the
>>>> original 8 DoF at the nodes, plus exactly 3 DoF (for the cell, so to speak)
>>>> p0, p1, p2, so that in a system with "block" matrices of the form
>>>>
>>>> [K_uu, K_up | K_pu, K_pp] { u | p } = RHS
>>>>
>>>> the unknowns {p} has size 3 per cell. Is this possible? I tried some
>>>> combinations of dim / nc for the new PetscFE, but when creating the DM
>>>> section I get errors e.g.
>>>>
>>>
>>> This is what you should get. I definitely have examples that do this.
>>> For example, here is Q1-P0 (I think that is what you are suggesting) for
>>> incompressible Stokes
>>>
>>>
>>> https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/blob/main/src/snes/tutorials/ex69.c?ref_type=heads*L3454__;Iw!!G_uCfscf7eWS!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJOYF2hni$ 
>>>
>>> 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!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJISojy8A$ 
>>> <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJHvAUc1W$ >
>>>
>>>
>>>
>>
>> --
>> 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!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJISojy8A$ 
>> <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJHvAUc1W$ >
>>
>>
>>
>
> --
> 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!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJISojy8A$ 
> <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJHvAUc1W$ >
>
>
>

-- 
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!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJISojy8A$  <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!d7n2bBRse7a9tBVL6RV7XqGopPgMbm6YlXOKdBzOEHT8exs7pN-id9yJDURq8w5KZpeKwclBsFSWJHvAUc1W$ >
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