[doc] Add wiki page on gmsh with ALUGrid.

doc/doxygen/DoxygenDumuxLayout.xml

@@ -35,6 +35,7 @@ SPDX-License-Identifier: GPL-3.0-or-later
       <tab type="user" url="@ref building-the-documentation" visible="yes" title="Build doxygen documentation" intro=""/>
       <tab type="user" url="@ref changing-property-name" visible="yes" title="Changing property name" intro=""/>
       <tab type="user" url="@ref custom-input-data" visible="yes" title="Custom input data" intro=""/>
+      <tab type="user" url="@ref gmsh-with-alugrid" visible="yes" title="Gmsh with ALUGrid" intro=""/>
     </tab>
     <tab type="user" url="@ref citelist" visible="yes" title="Bibliography" intro=""/>
     <tab type="namespaces" visible="yes" title="">

doc/doxygen/pages/gmsh-with-alugrid.md

+# Gmsh with ALUGrid
+
+If you are using Gmsh to create more complex grids and are using "Physical groups" to mark different boundary segments, you can normally access these inside your definition of the `neumann` function (solution dependent von Neumann boundaries) in your problem file by using something like the following:
+
+```c++
+NumEqVector neumann(const Element& element,
+                    const FVElementGeometry& fvGeometry,
+                    const ElementVolumeVariables& elemVolVars,
+                    const SubControlVolumeFace& scvf) const
+{
+    ...
+    const auto boundaryId = gridData_->getBoundaryDomainMarker(scvf.boundaryFlag());
+    ...
+}
+```
+where `gridData_` should be a `GridData` instance and a member of your problem class. (Add the grid data as argument in your problem constructor, it can be obtained via the grid manager: `gridManager.getGridData()`.)
+
+Additionally, you have to specify to use boundary segments and domain markers in your `params.input` file:
+
+```ini
+[Grid]
+File = mygrid.msh
+DomainMarkers = 1
+BoundarySegments = 1
+```
+
+Unfortunately `ALUGrid` uses boundary flags differently, therefore you have to adapt your `FVGridGeometry` to use the class `BoundarySegmentIndexFlag` instead of the normal `BoundaryFlag` class.
+This can be done in your property settings, where you need to add the following:
+
+```c++
+#include <dumux/common/boundaryflag.hh>
+
+namespace Properties {
+
+// other property settings
+...
+
+// custom grid geometry traits that enable you to use another boundary flag class
+template<class GridView>
+struct MyGridGeometryTraits : public BoxDefaultGridGeometryTraits<GridView>
+{
+    struct MyScvfTraits : public BoxDefaultScvfGeometryTraits<GridView>
+    {
+        // use BoundarySegmentIndexFlag
+        using BoundaryFlag = BoundarySegmentIndexFlag;
+    };
+
+    using SubControlVolumeFace = BoxSubControlVolumeFace<GridView, MyScvfTraits>;
+};
+
+// custom FVGridGeometry that uses your specified boundary flag implementation
+template<class TypeTag>
+struct FVGridGeometry<TypeTag, TTag::MyProblem>
+{
+private:
+    static constexpr bool enableCache = getPropValue<TypeTag, Properties::EnableFVGridGeometryCache>();
+    using GridView = GetPropType<TypeTag, Properties::GridView>;
+    using Scalar = GetPropType<TypeTag, Properties::Scalar>;
+public:
+    using type = BoxFVGridGeometry<Scalar, GridView, enableCache,
+                                   MyGridGeometryTraits<GridView>>;
+};
+...
+} // end namespace properties
+```
+This was how to do it when using the Box discretization, for CCTpfa it works similarly:
+
+```c++
+namespace Properties {
+
+// other property settings
+...
+
+template<class GridView>
+struct MyGridGeometryTraits : public CCTpfaDefaultGridGeometryTraits<GridView>
+{
+    struct MyScvfTraits : public CCTpfaDefaultScvfGeometryTraits<GridView>
+    {
+        // use BoundarySegmentIndexFlag
+        using BoundaryFlag = BoundarySegmentIndexFlag;
+    };
+
+    using SubControlVolumeFace = CCTpfaSubControlVolumeFace<GridView, MyScvfTraits>;
+};
+
+template<class TypeTag>
+struct FVGridGeometry<TypeTag, TTag::MyProblem>
+{
+private:
+    static constexpr bool enableCache = getPropValue<TypeTag, Properties::EnableFVGridGeometryCache>();
+    using GridView = GetPropType<TypeTag, Properties::GridView>;
+    using Scalar = GetPropType<TypeTag, Properties::Scalar>;
+public:
+    using type = CCTpfaFVGridGeometry<GridView, enableCache,
+                                   MyGridGeometryTraits<GridView>>;
+};
+...
+} // end namespace properties
+```
+
+Then you can access the boundary marker/physical group in the same way as shown above
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