I’m generating the geometry for an FSI case using CadQuery and Ansys.
The setup is:

• a 3D fluid volume
• an internal zero-thickness surface (the “flag”), lying exactly on a plane inside the fluid domain

I export both as a single multibody STEP and import it into ANSYS Mechanical.
The problem is Mechanical sees two separate bodies and produces two independent meshes.

If I manually select both bodies in DesignModeler and click Create → New Part, everything works: I get a true multibody Part and Mechanical produces a conformal interface.

I’m generating the geometry entirely via script, because I need to produce many different geometries automatically (randomized shapes and domains), so manual operations in DesignModeler are not an option.

What I’m trying to figure out is:
How can I achieve the same “New Part” result automatically?

In short:
Is there a reliable way to create a single Part between a fluid volume and an internal zero-thickness surface generated in CadQuery, without manual operations in DesignModeler?
I am asking both here and on the ANSYS forum, because I’m not sure whether this can be solved purely on the CadQuery/OCC side or if a solution exists inside DesignModeler.
I have already tried exporting the two bodies as a Compound in CadQuery, but ANSYS still treats them as separate bodies.

GEOMETRY_FLAG.py