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Exodus Node and Element Maps

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Greg Sjaardema edited this page May 23, 2022 · 2 revisions

An exodus file can have optional node and/or element maps. These maps provide a mapping from "file implicit" ids to "global" ids.

Assume we have an exodus file with 3 element blocks:

  • block 100 has 5 hex elements
  • block 200 has 10 hex elements
  • block 300 has 5 hex elements

If the elements are laid out in a line:

+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
|  1 |  2 |  3 |  4 | 5  |  6 |  7 |  8 |  9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 |  18| 19 | 20 |
+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
     block 100           |                   block 200                     |          block 300     |

The mesh has:

  • 3 element blocks
  • 20 total elements
  • 42 total nodes

The ids in the center of the elements above are the implicit (1..#element_in_file) ids.

Case 1: No node or element maps

In the case that there are no node or element maps, a call to ex_get_id_map(exoid, EX_NODE_MAP, *node_map) [Assume node_map of at least size 42] would return node_map containing the values 1...42.

Similarly, a call to ex_get_id_map(exoid, EX_ELEM_MAP, *elem_map) [with elem_map of size 20 or more] would return elem_map containing the values 1...20

We could also call ex_get_block_id_map(exoid, EX_ELEM_BLOCK, 200, elem_map) which would return the implicit element ids for the elements in block 200. This would be the values 6..15

Case 2: File contains node and element map.

Assume that the file has the following node and element maps:

  • node id map: 11, 22, 33, 44, 55, 66, ...., 442. Or, in other words, the global id of each node is its implicit id multiplied by 11.
  • element id map: 10, 20, 30, 40, ..., 200. Or in other words, the global id of each element is its implicit id multiplied by 10.

The call to ex_get_id_map(exoid, EX_NODE_MAP, *node_map) [Assume node_map of at least size 42] would return node_map containing the values 11...442.

Similarly, the call to ex_get_id_map(exoid, EX_ELEM_MAP, *elem_map) [with elem_map of size 20 or more] would return elem_map containing the values 10...200.

And the call to ex_get_block_id_map(exoid, EX_ELEM_BLOCK, 200, elem_map) which would return the implicit element ids for the 10 elements in block 200. This would be the values 60..150

Other info:

  • All node and element numbers stored in the exodus file are implicit ids. The nodal connectivity, the sideset elements, the nodeset nodes are all implicit ids.
  • To convert the implicit ids to global ids, you read the node or element id map(s) and use it to convert the implicit to global.
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