determine whether nodes are connected

[Marie Backman]

Hi all,

For a mesh with 8-node bricks, is there a way to determine whether two nodes are connected, i.e. share an element edge? I can use _mesh.nodeToElemMap() to find out whether two nodes belong to the same element, but I would need information about the edges.

Thanks!

Marie

[Cody Permann]

There are lots of useful tools for getting information about neighbors in at least two places in the libMesh API:

First on the "Elem" objects themselves:

http://libmesh.github.io/doxygen/classlibMesh_1_1Elem.html

Looks for all the "neighbors" methods: "find_point_neighbors, find_edge_neighbors, etc"

Secondly, look at the tools in the MeshTools namespace:

http://libmesh.github.io/doxygen/namespacelibMesh_1_1MeshTools.html

The method I suggest that you take a look at is MeshTools::find_nodal_neighbors(). It'll give you back all of the nodes that connect to the one you pass in. The only tricky part to using that method is that you need to build a "nodes_to_elem_map" first that you have to supply as a parameter. You can do that with MeshTools::build_nodes_to_elem_map().

I've implemented all of this in the FeatureFloodCount object if you want to take a look:

modules/phase_field/src/postprocessors/FeatureFloodCount.C

Hopefully that helps,

Cody

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[Peterson, JW]

On Thu, Apr 23, 2015 at 2:04 PM, Marie Backman <marie....@gmail.com> wrote:

Hi all,

For a mesh with 8-node bricks, is there a way to determine whether two nodes are connected, i.e. share an element edge? I can use _mesh.nodeToElemMap() to find out whether two nodes belong to the same element, but I would need information about the edges.

Hmm... if you have an Elem* and two local node numbers, n1 and n2, then for each edge_num you can check:

elem->is_node_on_edge(n1, edge_num) && elem->is_node_on_edge(n2, edge_num)

Alternatively, you could build a std::set of valid (n1,n2) pairs ahead of time (see Hex8::edge_nodes_map in libmesh/src/geom/cell_hex8.C) and search in that if you find the linear search is too slow. 

-- 

John

[Marie Backman]

Thanks for the suggestions, Cody and John! This should get me started.

Marie

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[Derek Gaston]

Be aware that build_nodes_to_elem_map() is very slow and uses a lot of memory. Also, if you are storing its output it's up to you to make sure to keep it in sync with the adapting mesh.

I would recommend staying away from that if you can.

Derek

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