Several types of meshes are used in MCNPX and MCNP. Superimposed meshes were introduced in MCNP4C for grid-based weight windows. Tally meshes are used for the collection and display of various types of data on a grid. The MCNPX and MCNP5 tally meshes are similar but different in detail. All mesh types may be rectangular (“REC” or “XYZ”) or cylindrical (“CYL” or “RZT”). (MCNPX also defines a spherical mesh that is not yet supported by Moritz.) Meshes can be displayed in the Moritz 2D and 3D graphics windows. Superimposed and mesh tally mesh specifications can be read from MCNPX and MCNP input files. The mesh definition(s) contained in a mesh tally file (MCNPX mdata file or MCNP5 meshtal file) is imported when reading the data file. New meshes can be created with the Mesh property sheet. A new mesh may be copied from an existing mesh. Aligned meshes, with axes (or the axis of cylindrical meshes) parallel to the X, Y, or Z coordinate axes, can be created interactively by drawing in the 2D windows. An automatic mesh generation algorithm creates rectangular or cylindrical meshes enclosing a model geometry. The automatic meshes can be uniform or based on the extent of geometry cells. The volume fraction feature calculates the distribution of materials in mesh cells. The Figures on this page show 2D and 3D views of a rectangular and a cylindrical mesh.
Coarse and Fine Meshes. The superimposed mesh and MCNP5 tally mesh consist of a coarse mesh with an arbitrary number of fine mesh lines evenly spaced between each pair of coarse mesh lines. MCNPX tally meshes do not define separate coarse and fine mesh values. Many MCNPX users, however, implicitly define a fine mesh by use of the interpolation syntax. When reading an MCNPX tally mesh specification from an input file, the interpolation syntax is used to define a fine mesh in Moritz. For example,
-10 20i 10produces 20 fine mesh lines between the coarse mesh lines at -10 and 10. Fine meshes can be used in Moritz to aid in the definition of an MCNPX tally mesh. When writing an MCNPX mesh tally input, Moritz converts fine mesh values to interpolation items. The coarse and fine meshes carry separate visualization settings. One may, for example, wish to see the coarse mesh outline of the grid but hide a dense fine mesh that would obscure objects within the mesh. Meshes read from a mdata or meshtal data file contain only coarse values. If these data meshes match an existing mesh (as described below), the coarse/fine structure of the presumed original mesh will be used.
Meshes are limited to 1,000 coarse mesh values in each direction. The limit does not include additional mesh lines introduced by the fine mesh.
Multiple Meshes. Any number of meshes can be displayed in Moritz. MCNP4C allows only a single superimposed mesh that must cover all of the geometry. Moritz does not (yet) check if more than one superimposed mesh is present and if it is large enough. In some cases, display of multiple meshes is appropriate, such as when grid tally meshes cover different portions of a model. This Figure shows a model with 3 tally meshes present. In other cases it is not appropriate, such as when the same mesh is used multiple times for mesh tally data with different particles (each particle type requires a distinct mesh tally mesh). The Mesh Style property page contains a setting to permit only a single mesh to be visible at a time.
Mesh Visualization Settings. A number of user choices affect the mesh appearance. The colors used for the coarse and fine mesh lines are specified on the Mesh Style property page and on the Component Colors property page. The coarse and fine colors are the same in the 2D and 3D plots. In the rectangular mesh Figures, the coarse mesh is gray and the fine mesh is red. The coarse mesh lines in the 3D mesh Figures are brown while the fine mesh is orange.
Four independent line widths are used for the coarse and fine meshes in 2D and 3D. The coarse mesh only, without the fine mesh lines, can be drawn in 2D, 3D, or both. Mesh visibility (i.e., whether not the mesh is shown) can be set separately in the 2D and 3D plots. The widths and visibility are set on the Mesh Style property page. In the case of multiple meshes, the 2D and 3D visibility of each can be specified on the Mesh Tally property page.
The 2D mesh lines are not very useful when the mesh cells are so small that the lines are crowded together. The Mesh Style property page contains a setting to prohibit drawing of the lines when the mesh cell dimensions are smaller than some specified number of pixels. This feature only works for XYZ meshes at present.
Automatic Mesh GenerationThe Auto Mesh property page of the Mesh property sheet controls program generation of a rectangular or cylindrical superimposed mesh covering all of the geometry. A uniform mesh can be generated or an adaptive algorithm used. The uniform mesh has constant mesh line spacing in each direction; the spacing is determined either by a specified distance between mesh lines or the number of mesh lines in each direction. The adaptive mesh algorithm places coarse mesh lines based on the bounding boxes of the geometry cells with a specified number of fine mesh intervals between the coarse mesh lines. The coarse mesh cells coincide with as many cell boxes as possible.
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