Body Fitting algorithm generates hexahedrons of a Cartesian grid in
the internal part of geometry and polyhedrons and other types of
elements at the intersection of Cartesian cells with the geometrical
-boundary.
+boundary. The algorithm supports construction of
+:ref:`viscous boundary layers<cartesian_VL_anchor>` (use
+:ref:`Viscous Layers hypothesis <viscous_layers_anchor>` to define them).
.. image:: ../images/cartesian3D_sphere.png
:align: center
.. centered::
A sphere meshed by Body Fitting algorithm
-.. note:: The algorithm creates only 3D elements. To add 2D elements use :ref:`Generate boundary elements <make_2dmesh_from_3d_page>` operation.
-
The meshing algorithm is as follows.
#. Lines of a Cartesian structured grid defined by :ref:`Body Fitting Parameters <cartesian_hyp_anchor>` hypothesis are intersected with the geometry boundary, thus nodes lying on the boundary are found. This step also allows finding out for each node of the Cartesian grid if it is inside or outside the geometry.
* add a hexahedron in the mesh, if all nodes are inside
* add a polyhedron or another cell type in the mesh, if some nodes are inside and some outside.
-To apply this algorithm when you define your mesh, select **Body Fitting** in the list of 3D algorithms and add **Body Fitting Parameters** hypothesis. The following dialog will appear:
+.. _cartesian_VL_anchor:
+
+**Viscous boundary layers** are constructed as follows:
+
+ * create an offset geometry with offset value equal to full layers thickness by using BRepOffset_MakeOffset OCCT class;
+ * mesh the offset geometry with the Body Fitting algorithm;
+ * create prisms of the layers by projecting boundary nodes of offset geometry onto the boundary of initial geometry.
+
+.. note:: Viscous layers can't be constructed on geometry with **shared/internal** faces.
+
+To apply the algorithm when you define your mesh, select **Body Fitting** in the list of 3D algorithms and add **Body Fitting Parameters** hypothesis. The following dialog will appear:
.. _cartesian_hyp_anchor:
* **Create Faces** check-box activates creation on mesh faces.
* **Consider Shared and Internal Faces** check-box activates treatment of faces shared by solids and internal. By default the algorithm considers only outer boundaries of the geometry.
* **Apply Threshold to Shared / Internal Faces** check-box activates application of **Threshold** to cells cut by shared and internal faces, that can cause appearance of holes inside the mesh.
+* **Set Quanta** check-box activates application of **Quanta Value** to replace **polyhedrons** by hexahedrons at the boundary of the geometry.
+* **Quanta Value** the relation between the volume of a polyhedrons and the equivalent hexahedron at the solid boundary. When **Set Quanta** is checked, those elements are replaced by hexahedrons if the volume of the polyhedron divided by the equivalente hexahedron is bigger than **Quanta**.
* **Definition mode** allows choosing how Cartesian structured grid is defined. Location of nodes along each grid axis is defined individually:
* You can specify the **Coordinates** of grid nodes. **Insert** button inserts a node at **Step** distance (negative or positive) from the selected node. **Delete** button removes the selected node. Double click on a coordinate in the list enables its edition. **Note** that node coordinates are measured along directions of axes that can differ from the directions of the Global Coordinate System.
