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++++
+date = "2019-03-19T09:56:57+01:00"
+title = "Submeshes for Boundary Conditions, Source Terms, and Flux Calculations"
+author = "Thomas Fischer"
+weight = 1
+
+[menu]
+  [menu.tools]
+    parent = "Meshing: Submeshes"
++++
+
+## Motivation
+
+Boundary conditions are defined on the boundary of a given domain. Source terms
+may exist on subdomains of the domain. Sometimes the flux across a subdomain
+is interesting for the modeller.
+
+Consequently, we need a way to specify such subdomains for the Finite Element
+simulation. There are several possibilities to define a subdomain. One
+possibility is to use a geometry via a gli- or gml-file. Since the geometry
+often doesn't match exactly on the domain mesh, one has to specify a search
+radius to find nodes, elements, or faces in the neighborhood of the geometry.
+It can be difficult to find an appropriate search readius for adaptive refined
+domain meshes. Finally, with the geometry and a suitable search radius the
+associated domain elements and domain nodes are searched for. Since this happens
+during the simulation, this approach is not very robust.
+
+Another possibility, avoiding the search during the simulation and thus more
+robust, is to precompute the subdomains as meshes. These precomputed subdomains
+are now passed to the OGS-6 simulator in the same format as the bulk mesh, the
+vtu format. The subdomains additionaly contain information to identify the
+corresponding bulk mesh entities like nodes, elements and faces of elements.
+
+### A simple example
+
+In the next figures it is explained how a boundary condition can be set to the
+top surface of a quad domain for a OGS-6 simulation. The first figure depicts
+the bulk mesh and the id's of the nodes on the top.
+![](01_bulk_mesh_with_top_node_ids.png)
+The differently coloured top surface mesh can be obtained for instance by the
+[ExtractSurface]({{< ref "extract-surface" >}}) tool.
+![](02_bulk_mesh_top_surface.png)
+For visualisation purposes the top surface is translated upwards. The picture
+shows the bulk mesh and the id's of the nodes on the top of it. Furthermore,
+the translated top surface mesh with the corresponding bulk node id's are
+represented.
+![](03_bulk_mesh_with_ids_top_surface_bulk_node_ids.png)
+