Geometry

The geometry is a sequence of features under Geometry 1, built from top to bottom with the OpenCASCADE kernel of gmsh. Every dimension can be an expression with parameters and units, so a parameter change rebuilds the geometry, including during a parametric sweep.

Geometry settings

Select Geometry 1 to set:

  • Length unit (m, mm, cm, µm, nm, km, in, ft). Numbers without a unit in geometry features are in this unit. A value with a unit (5[mm], or a parameter defined with one, directly or through other parameters in any row order) is converted.

  • Angular unit (degrees or radians) for rotation and revolution angles.

  • Default repair tolerance.

Building

Command Effect

Build Selected (F7)

Builds the sequence up to and including the selected feature. Use it to look at an intermediate step.

Build All (F8, Geometry  Build All)

Builds the whole sequence and finalizes it.

The last node, Form Union, glues all objects into one geometry with shared boundaries, so that fields are continuous across them. Its Action can also keep the objects separate. The Messages pane reports the number of domains, boundaries, edges and points.

Selecting a feature whose input you pick in the Graphics window (a face-parallel work plane, a fillet) shows the geometry built so far, with the entity labels you need. Selecting a feature inside a work plane shows the plane’s sketch.

Primitives

Space Primitives (Geometry  Primitives)

3D

Block, Sphere, Cylinder, Cone, Torus, Ellipsoid. Each has a position (corner or centre), an axis and a rotation angle.

2D, and inside a work plane

Rectangle, Circle, Ellipse, Polygon, Parametric Curve.
Polygon

A list of x and y coordinates. Its Type is Solid (a filled region), Closed curve or Open curve. Two points are enough for an open curve.

Parametric Curve

A curve x(s), y(s) for s from a minimum to a maximum, sampled at a number of points, for example a sinusoidal electrode x = lam*s, y = a0*cos(2*pi*s).

Convert to Solid

Joins curves (parametric curves, open polygons, lines) that form a closed outline into a solid region.

Circles are made of four arcs with vertices at 0°, 90°, 180° and 270°, as in COMSOL, so their points are numbered the same way.

Work planes, extrusion and revolution (3D)

A Work Plane (Geometry  Work Plane) holds a 2D sketch in 3D:

  • Plane type Quick: the xy, yz, zx, yx, zy or xz plane, at an Offset along its normal.

  • Plane type Face parallel: the plane of a planar face of the geometry built so far. Select the work plane, then click the face in the Graphics window. The plane’s origin is the face centre and its normal points out of the solid. Offset moves it along the normal.

Right-click the work plane to add 2D primitives, Booleans, transforms, Fillet and Chamfer to its sketch. Then:

  • Extrude turns the sketch into a solid along the plane normal, by one or more distances (a negative distance goes the other way).

  • Revolve rotates it about an axis in the plane by an angle.

Extrude and Revolve pick the last work plane as input by themselves.

Booleans and partitions

Union, Difference (objects to add, objects to subtract) and Intersection, with Keep input objects and, for Union, Keep interior boundaries. New Booleans start with sensible inputs (all objects for Union; the first object minus the others for Difference). Change them with the object lists in the Settings window.

Transforms

Move, Copy (move and keep the original), Rotate, Scale, Mirror and Array, each with Input objects (and, except for Copy and Array, Keep input objects).

Fillets and chamfers

  • In 2D and in work planes, Geometry  Fillet and Geometry  Chamfer round or cut vertices of the sketch. Select the feature, then click the corner points in the Graphics window (the sketch shows the point numbers), and set the radius or distance. Corners must be between two straight edges.

  • In 3D, Fillet and Chamfer act on edges of a solid.

Importing CAD

Geometry  Import reads STEP (.step, .stp), IGES (.iges, .igs) and BREP (.brep) files, with optional repair. Imported parts take part in Booleans and selections like any other object.

Form Union or Form an Assembly

The last node of every geometry, Form Union/Assembly, decides how the objects become domains:

  • Form a union (the default) fragments everything into one conforming geometry. Where objects touch or overlap they share boundaries, and the fields are continuous across them.

  • Form an assembly keeps the objects separate. Each object is meshed on its own, so the meshes on the two sides of a contact need not match. Where two objects touch, Elmer Studio makes an Identity Boundary Pair under the component’s Definitions (ap1, ap2, …), listing the source boundaries of the first object and the destination boundaries of the second. With Create pairs turned off, no pairs are made.

The physics keep their field continuous across every enabled pair, as across an interior boundary of a union, as in COMSOL. This applies to Heat Transfer, Electrostatics, Electric Currents and the Coefficient Form PDE. Elmer applies a mortar projection between the two sides. Disable a pair to leave the two sides unconnected (each then behaves like an insulated wall). A contact is found where a boundary’s middle point lies on the other object’s boundary, so touching faces, edges and curved surfaces (a shaft in a hole) are all found. Objects with a gap between them get no pair.

Box Selections

A Box Selection (Geometry  Box Selection) is a named geometry selection: all domains, boundaries, edges or points inside (or intersecting) a box. Leave a limit blank for no limit. Physics, mesh and results features can use it instead of a list of numbers. It stays right when a parameter change renumbers the geometry. See Selections.

Selections of geometry objects

Every geometry object that is still there at the end of the sequence offers its own selection, as COMSOL’s Resulting objects selection does. Its domains, and the boundaries of those domains, appear in every selection list as Block 1 (blk1). Such a selection follows the object through the union: if another object cuts it in two, the selection has both pieces.

How entities are numbered

Domains, boundaries, edges and points are numbered by position, in the order of their bounding boxes: smallest x first, then y, then z, then the box maxima. COMSOL numbers entities in a similar way, so the numbers in COMSOL instructions usually match. Numbers change only when the geometry changes. Turn on Show Labels in the Graphics toolbar to see them.

Limits

  • Fillets and chamfers in sketches work on corners between straight edges.

  • Identity pairs keep the field continuous; there is no contact, gap conductance or other pair condition yet.

Every geometry feature and its settings is listed in Reference › Geometry features.