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Added description of universes and lattices.

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Paul Romano 2012-08-02 17:26:27 -04:00
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docs/source/methods/geometry.rst
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@ -67,9 +67,47 @@ can be modeled in OpenMC. For example, the equation for a sphere centered at
Universes
---------
OpenMC supports universe-based geometry similar to the likes of MCNP_ and
Serpent_. This capability enables user to model any identical repeated
structures once and then fill them in various spots in the geometry. A
prototypical example of a repeated structure would be a fuel pin within a fuel
assembly or a fuel assembly within a core.
Each closed volume, or cell, in OpenMC can either be filled with a normal
material or with a universe. If the cell is filled with a univese, only the
region of the universe that is within the defined boundaries of the parent cell
will be present in the geometry. That is to say, even though a collection of
cells in a universe may extend to infinity, not all of the universe will be
"visible" in the geometry since it will be truncated by the boundaries of the
cell that contains it.
When a cell is filled with a universe, it is possible to specify that the
universe filling the cell should be rotated and translated. This is done through
the ``rotation`` and ``translation`` attributes on a cell (note though that
these can only be specified on a cell that is filled with another universe, not
a material).
It is not necessary to use or assign universes in a geometry if there are no
repeated structures. Any cell in the geometry that is not assigned to a
specified universe is automatically part of the "base" universe whose
coordinates are just the normal coordinates in Euclidean space.
Lattices
--------
Often times, repeated structures in a geometry occur in a regular pattern such
as a rectangular or hexagonal lattice. In such a case, it would be cumbersome
for a user to have to define the boundaries of each of the cells to be filled
with a universe. Thus, OpenMC provides a lattice capability similar to that used
in MCNP_ and Serpent_.
The implementation of lattices is similar in principle to universes -- instead
of a cell being filled with a universe, the user can specify that it is filled
with a finite lattice. The lattice is then defined by a two-dimensional array of
universes that are to fill each position in the lattice. A good example of the
use of lattices and universes can be seen in the OpenMC model for the `Monte
Carlo Performance benchmark`_.
------------------------------------------
Computing the Distance to Nearest Boundary
------------------------------------------
@ -556,3 +594,6 @@ form of the solution:
.. _constructive solid geometry: http://en.wikipedia.org/wiki/Constructive_solid_geometry
.. _surfaces: http://en.wikipedia.org/wiki/Surface
.. _MCNP: http://mcnp.lanl.gov
.. _Serpent: http://montecarlo.vtt.fi
.. _Monte Carlo Performance benchmark: https://github.com/paulromano/benchmarks/tree/master/mc-performance/openmc