Meshing advanced features

This course is focused in exploring the advanced meshing features present in GiD. It is recommended to follow the basic meshing course before this one.

Skip entities

Using classical surface meshers, all the lines of the model are meshed, so the user is not allowed to generate elements larger than the surface they belong to.
Using GiD, it is possible to skip the inner lines between surfaces in contact. This option is very interesting in the cases were the geometrical definition of the model use high distorted surfaces, or very small ones in comparison with the elements size required for the simulation.
In this course we will use the model model_skipentities.gid, which is a geometrical model of a mechanical part depicted in the following picture:

View of the model of a mechanical part used in this course.

Mesh using automatic parameters

First of all we are going to generate a mesh with all the default parameters in GiD.

Select the Reset mesh data option from the Mesh menu.
Open the Preferences window and set the Default values in the meshing branch. To set the Default values in all the branches of Meshing, you should click the right mouse button, and set the Default values on selection option. Then click on Apply and close the window.
Generate the mesh (for instance using the shortcut Control-g) setting to 9 the general mesh size. The generated mesh should look like this:

View of the mesh generated with the default meshing parameters of GiD.

It can be seen that the regions where the original surfaces are very thin make the mesher generate elements fitted inside those surfaces' contour lines. In the following figure some of these surfaces and its related elements are highlighted in red:


Some thin surfaces highlighted in red.	The elements corresponding to the thin surfaces highlighted.

Skip entities automatically

If a mesh with a more uniform element size distribution is required, the Skip entities automatically (Meshing branch in the Preferences window) option should be set. With this option, following entities will be skipped when meshing:

points belonging to two lines which are tangent enough at that point.
lines belonging to two surfaces which are tangent enough at that line.

The tangent enough concept is defined by a maximum angle formed by the lines (in case of points) or the normals of the surface (in case of lines). The default angle is 10 degrees, and can be set as well from the preferences window.
Other criteria are applied in order to allow or not an entity to be skipped:

Entities with some material, condition or group assigned are never skipped.
Entities which parents have different groups, materials, or conditions applied are never skipped.

Let's see how it works.

Open the Preferences window, and set the Skip entities automatically group option in the Meshing --> Structuration type branch from the Preferences window:
Generate the mesh with the same size as before (size equal to 9). The resulting mesh should look like the following one:

Mesh of the model using the 'Skip entities automatically' option.

As can be seen, now the elements are larger than in the mesh generated with the default options. In this case, the mesher is not forced to mesh all the lines and points of the model, so the triangles can be larger than the surfaces they belong to.
Note that the lines which form a sharp edge between the surfaces they belong to are not skipped.
It has to be pointed out that the nodes of the mesh are exactly placed onto the surfaces.
Often when meshing with the skip the entities automatically option enabled is more difficult than using the conventional approach. In some complex geometrical models, sometimes the mesher cannot skip the desired entities, so GiD may generate the meshes without skipping any entity.

Skip entities automatically: Assign sizes

Although this option allows to generate mesh patches of surfaces together, user can assign different element sizes to the different surfaces or lines, and these sizes will be considered during the mesh process. Let's see it in an example:

Assign 3 as unstructured size of the surfaces highlighted in the following figure (select Mesh --> Unstructured --> Assign sizes on surfaces from the top menu bar, enter 3 in the window that appears and select these two surfaces, and close the window):

Surfaces which unstructured size must be 3.

Generate again the mesh with 9 as the general size. The resulting mesh should look like the following one:

View of the mesh skipping entities automatically and assigning a size of 3 in some surfaces

As it can be seen, the size of the elements in the region of the selected surfaces has been modified, although the lines between 'tangent enough' surfaces are still skipped.

Skip specific entities

As it has been seen in the previous points, skipping entities automatically all the lines and points accomplishing the given tangency criteria are skipped. However, the user may want not to skip some entities, or skip just a specific line or point (not all of them). For this purpose the user may use the Skip options in the Mesh criteria part of the Mesh menu.
Accordingly to the easier way of setting the parameters for meshing, user may be interested in one of these two options:

Skip almost all the entities accomplishing the automatic skip criteria except some line or point. In this case, Skip entities automatically option must be set in the Preferences window, and user may select manually the entities not to be skipped ( Mesh --> Mesh criteria --> No Skip --> Points / Lines).
Only skip some specific entity (and don't skip the major part of entities of the model). In this case, Skip entities automatically must be unset from the Preferences window, and the entities to be skipped must be selected manually ( Mesh --> Mesh criteria --> Skip --> Points / Lines).

Let's see an example of the second case. Imagine we only want to skip lines number 253 and 254 and points number 37 and 184. In the following figure the region were these entities are is highlighted in red, as well a zoom of the zone is presented with the corresponding labels in this area.


Entities to be skipped highlighted in red.	labels of the entities to be skipped.

In the following figure a detail of the mesh in that region is shown, when the option Skip entities automatically is not set (don't skip any entity).

Let's try to skip these entities:

Select Mesh criteria --> Skip --> Lines in the Mesh menu, and select the lines 234 and 235. Click ESCAPE to end the selection.
Select Mesh criteria --> Skip --> Points in the Mesh menu, and select the points 37 and 184. Click ESCAPE to end the selection.
The Draw --> Skip entities option of the Mesh menu can be used in order to see graphically the entities that will be skipped or not when meshing.
Ensure the option Skip entities automatically is not set in the preferences window, and generate the mesh with 9 as the general mesh size. A zoom of the resulting mesh in the area of interest should be like the following one.


Detail of the mesh in the zone of interest without skipping entities automatically.	View of the mesh generated skipping only the selected lines and points.