.. _introduction-exodus:

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Exodus II Database Naming Conventions
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When the mesh file has an Exodus II format, there are three basic conventions that apply to user input for various command lines. The conventions concern side sets (surfaces), node sets and blocks.

First, for a mesh file with the Exodus II format, the Exodus II side set is referenced as a surface. In SIERRA, a surface consists of element faces plus all the nodes and edges associated with these faces. A surface definition can be used not only to select a group of faces but also to select a group of edges or a group of nodes that are associated with those faces. In the case of boundary conditions, a surface definition can be used not only to apply boundary conditions that typically use surface specifications (pressure) but also to apply boundary conditions for what are called nodal boundary conditions (fixed displacement components). For nodal boundary conditions that use the surface specification, all the nodes associated with the faces on a specific surface will have this boundary condition applied to them. The specification for a surface identifier in the following chapters is ``surface_name``. It typically has the form ``surface_integerid``, where ``integerid`` is the integer identifier for the surface. If the side set identifier is 125, the value of ``surface_name`` would be ``surface_125``. It is also possible to generate an alias for the side set\ [#f1]_ and use this for ``surface_name``. If ``surface_125`` is aliased to ``outer_skin``, then ``surface_name`` becomes ``outer_skin`` in the actual input line. It is also possible to name a surface in some mesh generation programs and that name can be used in the input file. 

Second, for a mesh file with the Exodus II format, the Exodus II node set is still referenced as a node set. A node set can be used only for cases where a group of nodes needs to be defined. The specification for a node set identifier in the following chapters is ``nodelist_name``. It typically has the form ``nodelist_integerid``, where ``integerid`` is the integer identifier for the node set. If the node set number is 225, the value of ``nodelist_name`` would be ``nodelist_225``. It is also possible to generate an alias for the node set and use this for ``nodelist_name``. If ``nodelist_225`` is aliased to ``inner_skin``, then ``nodelist_name`` becomes ``inner_skin`` in the actual input line. It is also possible to name a nodelist in some mesh generation programs and that name can be used in the input file. 

Third, an element block is referenced as a block. The specification for an element block identifier in the following chapters is ``block_name``. It typically has the form ``block_integerid``, where ``integerid`` is the integer identifier for the block. If the element block number is 300, the value of ``block_name`` would be ``block_300``. It is also possible to generate an alias for the block and use this for ``block_name``. If ``block_300`` is aliased to ``big_chunk``, then ``block_name`` becomes ``big_chunk`` in the actual input line. It is also possible to name an element block in some mesh generation programs and that name can be used in the input file.

A group of elements can also be used to select other mesh entities. In SIERRA, a block consists of elements plus all the faces, edges, and nodes associated with the elements. The block and surface concepts are similar in that both have associated derived quantities. :numref:`conditions` and :numref:`contact` show how this concept of derived quantities is used in the input command structure.

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.. rubric:: Footnotes

.. [#f1]

    See the ``ALIAS`` command in :numref:`elements-fem-alias`.
