[petsc-users] Additiional DoF per cell

Wed May 6 08:32:05 CDT 2026

>> 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*5D(https:/*urldefense.us/v3/__https:/*petsc.org/release/src/snes/tutorials/ex77.c.html__;!!G_uCfscf7eWS!aZNOxZZeoHYqe6lRZV0wHMVVTu3YIEfc1Sr-dca7xiGVfG3enULFD4_g6_fxWIa7A_Cu9LcXU-MuxRj4sOwDRyWqvsdGdrEd$)__;JS8v!!G_uCfscf7eWS!cn6JRmJG7QhdyZz0lvZaYRtKqRQvyvTdVQ3gxFv0ygegKzmT7TIhIGJY1-3Lqsx-WYQr8TFguDTc80rdQ59W2YsgwLc6X6ro$ ) 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!cn6JRmJG7QhdyZz0lvZaYRtKqRQvyvTdVQ3gxFv0ygegKzmT7TIhIGJY1-3Lqsx-WYQr8TFguDTc80rdQ59W2YsgwMAHwbRI$ 
>>>
>>> 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/*5D(http:/*www.cse.buffalo.edu/*knepley/)__;fiUvfg!!G_uCfscf7eWS!cn6JRmJG7QhdyZz0lvZaYRtKqRQvyvTdVQ3gxFv0ygegKzmT7TIhIGJY1-3Lqsx-WYQr8TFguDTc80rdQ59W2YsgwP9eqEp3$ 
>
> --
>
> 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/*5D(http:/*www.cse.buffalo.edu/*knepley/)__;fiUvfg!!G_uCfscf7eWS!cn6JRmJG7QhdyZz0lvZaYRtKqRQvyvTdVQ3gxFv0ygegKzmT7TIhIGJY1-3Lqsx-WYQr8TFguDTc80rdQ59W2YsgwP9eqEp3$ 
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