smesh/doc/salome/gui/SMESH/input/prism_3d_algo.doc

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/*!
\page prism_3d_algo_page Extrusion 3D meshing algorithm
Extrusion 3D algorithm can be used for meshing prisms, i.e. 3D shapes
defined by two opposing faces having the same number of vertices and
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edges. These two faces should be connected by quadrangle "side" faces.
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The prism is allowed to have sides composed of several faces. (A prism
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side is a row of faces (or one face) connecting the corresponding edges of
the top and base faces). However, a prism
side can be split only vertically as indicated in the
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picture below.
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\image html prism_ok_ko.png "A suitable and an unsuitable prism"
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In this picture, the left prism is suitable for meshing with 3D
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extrusion algorithm: it has six sides, two of which are split
vertically. The right prism cannot be meshed with this
algorithm because one of the prism sides is split horizontally (the
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splitting edge is highlighted).
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The algorithm can propagate 2D mesh not only between horizontal
(i.e. base and top) faces of one prism but also between faces of prisms
organized in a stack and between stacks sharing prism sides.
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\image html prism_stack.png "Prism stacks"
This picture shows four neighboring prism stacks, each comprising two prisms.
The shown sub-mesh is used by the algorithm to mesh
all eight prisms in the stacks.
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To use <em>Extrusion 3D</em> algorithm you need to assign algorithms
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and hypotheses of lower dimensions as follows.
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(A sample picture below shows algorithms and hypotheses used to
mesh a cylinder with prismatic volumes).
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\image html prism_needs_hyps.png
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The \b Global algorithms and hypotheses to be chosen at
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\ref create_mesh_anchor "Creation of a mesh object" are:
<ul>
<li> 1D algorithm and hypothesis that will be applied for meshing
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(logically) vertical edges of the prism (which connect the top and the
base faces of the prism). In the sample picture above these are
"Regular_1D" algorithm and "Number of Segments" hypothesis named "Vertical
Nb. Segments".</li>
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</ul>
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The \b Local algorithms and hypotheses to be chosen at
\ref constructing_submeshes_page "Construction of sub-meshes" are:
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<ul>
<li> 1D and 2D algorithms and hypotheses that will be applied for
meshing the top and the base prism
\ref submesh_shape_section "faces". These faces can be meshed
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with any type of 2D elements: quadrangles, triangles, polygons or
their mix. It is enough to define a sub-mesh on either the top or
the base face. In the sample picture above, "NETGEN_1D2D"
algorithm meshes "bottom disk" face with triangles. (1D algorithm
is not assigned as "NETGEN_1D2D" does not require divided edges to
create a 2D mesh.)
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</li>
<li> Optionally you can define a 1D sub-mesh on some vertical
\ref submesh_shape_section "edges" of stacked prisms, which will
override the global 1D hypothesis mentioned above. In the <b>Prism
stacks</b> picture, the vertical division is not equidistant on
the whole length because a "Number Of Segments" hypothesis with
Scale Factor=3 is assigned to the highlighted edge.
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</li></ul>
If <em>Extrusion 3D</em> algorithm is assigned to a sub-mesh in a mesh
with multiple sub-meshes, the described above approach may not work as
expected. For example the bottom face may be meshed by other algorithm
before <em>Extrusion 3D</em> have a chance to project a mesh from the
base face. This thing can happen with vertical edges as well. All
these can lead to either a meshing failure or to an incorrect meshing.
In such a case, it's necessary to explicitly define algorithms
that <em>Extrusion 3D</em> implicitly applies in a simple case:
- assign \ref projection_1D2D algorithm to the top face and
- assign a 1D algorithm to a group of all vertical edges.
\image html image157.gif "Prism with Extrusion 3D meshing. Vertical division is different on neighbor edges because several local 1D hypotheses are assigned."
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\sa a sample TUI Script of
\ref tui_prism_3d_algo "Use Extrusion 3D meshing algorithm".
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*/