Skin extraction with elemental averaging fails with mixed models

[janvonrickenbach]

Before submitting the issue

• I have checked for Compatibility issues
• I have searched among the existing issues
• I am using a Python virtual environment

Description of the bug

Skin extraction for a model that contains both solid and shell elements returns too many elementary data for elemental skin results. I think the root cause is that the ShellLayers property of a field has a single value (derived from the original shell elements). For a mixed model the skin elements have only a single value whereas the original shell elements have multiple shell layers. The to_elemental operator therefore assumes the skin elements have also three shell layers and returns three values per element instead of one.

Note: We could integrate the shell layer selection in the result extraction workflow and force the user to select a shell layer when shell data is combined with skin data.

Steps To Reproduce

Run the following snippet. The assertion in the end fails, we get 3 elementary data.

Use: examples.download_all_kinds_of_complexity_modal()

from ansys.dpf import post

rst_path = r"modal_allKindOfComplexity.rst"
sim = post.StaticMechanicalSimulation(rst_path)
fc_all = sim.stress_elemental()._fc

all_elements = fc_all[0].meshed_region.elements

solid_elements = [e for e in all_elements if e.type.name == "Hex20"]

fc_skin = sim.stress_elemental(skin=True)._fc

skin_mesh = fc_skin[0].meshed_region
skin_node_ids = skin_mesh.nodes.scoping.ids

# Find a skin node (fc_skin contains skin as well as shell elements)
skin_node_id = None
for solid_element in solid_elements:
    for node_id in solid_element.node_ids:
        if node_id in skin_node_ids:
            skin_node_id = node_id
            break

assert skin_node_id is not None

# Elements attached to skin node
skin_element_ids = skin_mesh.elements.scoping.ids[skin_mesh.nodes.nodal_connectivity_field.get_entity_data_by_id(node_id)]

# Get data of first skin element
skin_element_data = fc_skin[0].get_entity_data_by_id(skin_element_ids[0])
assert skin_element_data.shape[0] == 1, skin_element_data.shape[0]

Which Operating System are you using?

Windows

Which DPF/Ansys version are you using?

DPF Server 2025.1.pre0

Which Python version are you using?

3.10

Installed packages

[janvonrickenbach]

@FedericoNegri