Frames¶

A frame says how the model you author maps onto the coordinates the solver backend, Elmer today, solves in. Solid solves the 3D model as it is. Axisymmetric solves the r-z half-plane of a model that is symmetric about an axis, so a rotationally symmetric part such as a coil or a tubular actuator solves as a 2D problem. You always author the full 3D model, and femforge sections it for the 2D solve. The frame is the frame field of SimulationConfig and defaults to Solid().

The authoring frame¶

You model in whatever frame suits the part, for example Z up in your CAD or in build123d. Every axis a frame names lives in that authored frame. An Axis is a line through origin (in mm) along direction; a scaled direction names the same axis. ff.Axis.X, ff.Axis.Y and ff.Axis.Z are the coordinate axes through the origin.

Solid¶

import femforge as ff

config = ff.SimulationConfig(
    frame=ff.Solid(),
    mesh=ff.MeshConfig(step="models/actuator.step", global_mesh_size=0.002),
    domain=ff.DomainConfig(
        materials=[ff.materials.Air(), ff.materials.N52()],
        tags=[
            ff.EntityTag(
                selector={ff.TagKey.MAGNETIZATION: "N"},
                conditions=[ff.Magnetization(direction=ff.Direction(1, 0, 0))],
            ),
            ff.EntityTag(selector={ff.TagKey.NAME: "mover"}, conditions=[ff.ForceComponent(label="mover")]),
        ],
    ),
)

print(config.frame.label, config.frame.model_dim)

A solid frame meshes the authored STEP itself when the config has no parametric geometry. Nothing is converted, so nothing is cached for the geometry stage.

Axisymmetric¶

import femforge as ff

config = ff.SimulationConfig(
    frame=ff.Axisymmetric(axis=ff.Axis.Z),
    mesh=ff.MeshConfig(step="models/tubular.step", global_mesh_size=0.0005),
    domain=ff.DomainConfig(
        materials=[ff.materials.Air(), ff.materials.N52()],
        tags=[
            ff.EntityTag(
                selector={ff.TagKey.MAGNETIZATION: "N"},
                conditions=[ff.Magnetization(direction=ff.Direction(0, 0, 1))],
            ),
            ff.EntityTag(selector={ff.TagKey.NAME: "mover"}, conditions=[ff.ForceComponent(label="mover")]),
        ],
    ),
)

print(config.frame.label, config.solved_physics())

femforge stores radial as its unit part perpendicular to the axis and rejects a radial parallel to the axis. With radial=None, the half-plane faces the world axis least aligned with the symmetry axis, X first: Axis.Z maps the authored X to the radial direction, and Axis.Y leaves the coordinates as authored. An unset radial is left out of the serialized config, so it adds nothing to the run identity.

For an axis that does not pass through the origin, give it an origin:

import femforge as ff

frame = ff.Axisymmetric(
    axis=ff.Axis(origin=ff.Vec3(10, 0, 0), direction=ff.Direction(0, 0, 1)),
    radial=ff.Direction(0, 1, 0),
)

print(frame.label)

How femforge sections the model¶

The driver converts the STEP in its first stage, prepare_geometry, through apply_frame:

1. It builds the parametric geometry when the config has one, else it reads the authored STEP.
2. It imports the STEP as tagged parts, one per named solid. Every body needs a key=value name, and a STEP that holds only faces is rejected: author the 3D model and let the frame section it.
3. axisymmetric_cross_section places the parts in the solve frame, then intersects each solid with the X >= 0 half of the Z = 0 plane. Each face keeps its solid's tags and color.
4. It writes the section to geometry.step in the run directory, and the mesher meshes it in 2D.

The section raises when a solid does not reach the half-plane, and when a section reaches X < 0, which Elmer's axisymmetric solve would reject silently. With an artifact store, the section is cached as a STEP artifact keyed by the geometry source and the frame (see Artifacts and retention).

cross_section_3D is the 3D counterpart for pictures: it cuts away a quarter wedge and keeps the tags.

Physics in 2D¶

SimulationConfig.solved_physics gives the physics a selection solves as under the frame. The config checks the frame against the physics and the conditions when you build it and when you assign a field.

Results in the solve frame¶

Results come back in the solve frame, and Result.frame records which one. Under Axisymmetric(), a vector's Y component is the axial one and its X component the radial one.

Elmer integrates per radian in its 2D symmetric coordinate systems. femforge converts only the typed force and torque fields to full-revolution totals:

Result.axial_force takes an axis in the authored frame. An axisymmetric run defaults to its symmetry axis and raises for an axis that is not parallel to it. A solid run has no default axis, so you pass one.

Limits¶

• Only magnetostatics and heat conduction have an axisymmetric form.
• An axisymmetric run computes no torque.
• femforge sections one half-plane and does not check that the model is symmetric.
• Every solid must cross the half-plane; a part off to the side of the section raises.