Creating membrane elements

1

Hi,

I need to perform FSI simulations on a lid-driven cavity (here an example of the configuration). I use CalculiX to compute the structure part of the problem, preCICE for the coupling and an in-house code for the CFD part.

To model the lid, I wish to use a *MEMBRANE SECTION. The initial mesh is generated using the following script using cgx:

# Define side length
valu c 1.0

# Define points and line to generate the geometry
pnt p1 0.0 0.0 0.0
pnt p2 0.0 0.0 c
line l1 p1 p2 1
swep all new tra c 0 0

# Group all sides together
seta sides l l1 L001 L002 L003

# Generate the mesh
div sides 32
elty all qu4
mesh all

# Group all nodes together
seta nodes n all

# Group nodes by boundary conditions
enq nodes right rec c _ _ 0.0001
enq nodes bottom rec _ _ c 0.0001
enq nodes left rec 0.0 _ _ 0.0001
enq nodes top rec _ _ 0.0 0.0001
enq nodes fsiinterface rec _ 0.0 _ 0.0001

# Write all nodes to their respective files
send all abq
send right abq nam
send left abq nam
send top abq nam
send bottom abq nam
send fsiinterface abq nam

# Inspection with cgx
view elem
plot f all

The mesh is fed to ccx using another script and I define the element type here:

********************************************************
** MATERIAL MODEL
********************************************************
*MATERIAL,NAME=LID
*ELASTIC, TYPE=ISO
250.0,0.3
*DENSITY
500.0
*ELEMENT, TYPE=M3D4, ELSET=Eall
*MEMBRANE SECTION, ELSET=Eall, MATERIAL=LID
0.0002

This code creates the following error:

 *ERROR reading *MEMBRANE SECTION: *MEMBRANE SECTION can
        only be used for membrane elements.
        Element            1  is not a membrane element.
 *INFO reading *STEP: nonlinear geometric
       effects are turned on
 
 *ERROR in gen3delem: first thickness
        in node            1  of element            1
        is zero

When checking the .msh file, the element type chosen by cgx is TYPE=S4. When replacing, the S4 type with M3D4, I don’t get any errors. The mesh generation script works fine if I use *SHELL SECTION as S4 elements are used for shells.

I can’t find any information on how to generate membrane elements from cgx in the documentation. What is the usual procedure to define a membrane properly?

Feel free to ask for additional information if you need it. I will continue to search for a solution.

Thank you in advance for your help.

2

it seems setting as membrane element type not yet supported in CGX, maybe by adding ‘m’ for this purpose like elty all qu4m

2025-04-15 23_36_17-Window
2025-04-15 23_36_17-Window604×380 47 KB

3

Thanks for the answer, however there is no such thing as a qu4m element and cgx returns an error. I forgot to mention that I use cgx/ccx v2.20.

Note that setting M3D4 manually in the .msh file works fine, but I would like to not myself.

4

Since membrane elements are not listed under elty in the manual:

at last an attribute describing the mathematical formulation or other features (c: axisymmetric, e: plain strain, s: plain stress, u: unstructured mesh, r: reduced integration, i: incompatible modes, f: fluid element for ccx, t: initial temperatures are interpolated linearly within the tet element (ccx:C3D10T))

we can assume they aren’t currently supported. You could ask @klaus.wittig to make this improvement in cgx.

5

Alright thank you for the very quick answer.