Estimate a wheel rotation axis with an OBB

Estimate a wheel rotation axis with an OBB#

This example shows how to use the draft context to fit an oriented bounding box (OBB) to a set of wheel surfaces, then derive the rotation axis and radius of the wheel from that box. These values can be used to configure a rotating wall boundary condition or a rotating reference frame without measuring the geometry by hand.

import flow360 as fl

project = fl.Project.from_cloud("PROJECT_ID_HERE")
geometry = project.geometry

with fl.create_draft(
    new_run_from=geometry,
    face_grouping="face_grouping_tag",
) as draft:

    # Select the surfaces that make up the wheel.
    wheel_surfaces = draft.surfaces["wheel_*"]

    # Fit the oriented bounding box to those surfaces. The result holds the
    # box geometry only: center, principal axes and half-extents.
    obb = draft.compute_obb(wheel_surfaces)

    # Derive the rotation axis and radius from the box. Pass a known
    # direction with rotation_axis_hint (or an explicit axis_index). When
    # neither is given the axis is inferred from the most circular
    # cross-section and a warning is emitted.
    rotation = obb.get_rotation_axis_and_radius(rotation_axis_hint=[0, 1, 0])

    print("center:", obb.center)
    print("rotation axis:", rotation.axis_of_rotation)
    print("averaged radius:", rotation.averaged_radius)

    # Printing the result also shows how the radius was averaged.
    print(rotation)

Notes#

  • compute_obb is available only for drafts created from a Geometry asset. Drafts created from a surface mesh or volume mesh do not carry the tessellation data required to fit the box.

  • entities accepts a single surface, a list of surfaces, or a surface view (e.g. draft.surfaces[...]). Non-surface entities (for example mirrored surfaces) are skipped with a warning.

  • compute_obb returns an OBBResult that describes the box geometry only: center, axes (the principal axes as row vectors) and extents (the half-extents along each axis). When the project has a length unit, center and extents carry that unit.

  • The rotation axis and radius are obtained separately via OBBResult.get_rotation_axis_and_radius(), which returns a RotationAxisAndRadius with axis_of_rotation (a unit vector) and averaged_radius. The averaged radius is the mean of the two box half-extents perpendicular to the rotation axis, and inherits the project length unit from those extents. Printing the RotationAxisAndRadius shows the values that were averaged.

  • Select the rotation axis with either rotation_axis_hint (the principal axis most aligned with the given direction is used) or axis_index (0, 1 or 2). Passing both, an out-of-range index, or a zero hint raises Flow360ValueError. If neither is given, the axis is inferred from the most circular cross-section and a warning is emitted, since this is an inference rather than a known geometric property.

Example use cases#

  • Automatic rotation setup for wheels and other cylindrical components

  • Estimating component size and orientation directly from geometry

See also

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