Creation and meshing of a surface

We will now continue with the creation and meshing of a surface.
First, we will create a second line between points 1 and 3.

We will now generate the second line. We will now use again the Coordinates Window to enter the points. (Utilities->Tools->Coordinates Window)
Select the line creation tool in the toolbar.

Enter point (0,10,0) in the Coordinates Window and click Apply.

With option Contextual->Join Ctrl-a (mouse menu) click over point 1. A line should be created between (0,10,0) and (0,0,0). Press Escape twice.
With this, a right angle of the square has been defined.
Center the image in the screen with View->Zoom->Frame.

Finish the square by creating point (10,10,0) and the lines that join this point with points 2 and 3.

Now, we will create the surface that these four lines define. To do this, access the create surface command by choosing: Geometry->Create->NURBS surface->By contour. This option is also available in the toolbar:

GiD then asks the user to define the 4 lines that describe the contour of the surface. Select the lines using the cursor on the screen, either by choosing them one by one or selecting them all with a window. Next, press Escape twice.
As can be seen below, the new surface is created and appears as a smaller, magenta square drawn inside the original four lines.

Once the surface has been created, the mesh can be created in the same way as was done for the line. From the Top Menu select: Mesh->Generate mesh.
A window appears asking if the previous mesh should be eliminated. Click Yes.

Another window appears which asks for the maximum size of the element, in this example defined as 1.

We can see that the lines containing elements of two nodes have not been meshed. Rather the mesh generated over the surface consists of planes of three-nodded, triangular elements.
 NOTE: GiD meshes by default the entity of highest order with which it is working.

GiD allows the user to concentrate elements in specified geometry zones. Next, a brief example will be presented in which the elements are concentrated in the top right corner of the square.

This operation is realized by assigning a smaller element size to the point in this zone than for the rest of the mesh. Select the following sequence: Mesh->Unstructured->Assign sizes on points. The following dialog box appears, in which the user can define the size:

Enter 0.1 and click Assign.
Select the upper-right corner point and press Escape. The same window comes up again, click Close.
We must now regenerate the mesh, erasing the mesh generated earlier, and we obtain the following:

As can be seen in the figure above, the elements are concentrated around the chosen point. Various possibilities exist for controlling the evolution of the element size, which will be presented later in the manual.

To generate a surface mesh in which the elements are presented uniformly, the user can select the option for a structured mesh. This guarantees that the same number of elements appears around a node and that the element size is as uniform as possible. To generate this type of mesh, choose: Mesh->Structured->Surfaces->Assign number of divisions to surface lines.

Using this command, the user should first select the 4-sided NURBS surface that will be defined by the mesh and press ESC.
Then a window appears where the number of subdivisions for the surface limit lines should be entered.

Enter 10 and click Assign and select one of the horizontal lines, the parallel line is also selected. Press ESC.
The same window appears again, click Assign and select one of the vertical lines, the parallel one is also selected. Press ESC.
Click Close when the window appears again.

The number of divisions can be checked selecting Mesh->Draw->Num of divisions. To exit this visualization mode press ESC.

 NOTE: GiD only generates structured meshes for surfaces of the type 4-sided surface or NURBS surface.

When this has been done, the mesh is generated in the same way as the unstructured mesh, by choosing Mesh->Generate mesh. Erase the old mesh and assign a general element size of 1, though in this case it is not necessary.

 NOTE: Another way to get the same result is using the option Mesh->Structured->Surfaces->Assign size to surface lines. With this option we set the element size. If we want to get 10 elements per line and the line measures 10 units, we should set 1 as size.
If we don't know how much measures a line we can use the option Utilities->Distance and select the 2 points defining the line.

We can see here that the default element type used by GiD to create a structured mesh is a square element of four nodes rather than a three-nodded, triangular element. To obtain triangular elements, the user can specifically define this type of element, by choosing Mesh->Element type->Triangle, and selecting the surface to mesh as a triangular element. Regenerate the mesh, and the following figure is obtained:

GiD also allows the user to concentrate elements in structured meshes. This can be done by selecting Mesh->Structured->Lines->Concentrate elements.

First, we must select the lines that need to be assigned an element concentration weight. The value of this weight can be either positive or negative, depending on whether the user wants to concentrate elements at the beginning or end of the lines. Next, a vector appears which defines the start and end of the line and which helps the user assign the weight correctly.

We want to concentrate the elements in the left zone of the square.
Select both horizontal lines and press ESC. A window appears to enter the weights values.
Both lines should have the same direction so enter a weight of 0.5 to the beginning of the line and click Ok. Press ESC again to leave the function.

If lines have different direction, to obtain the same result, we should assign the weight for one line to the beginning and to the end for the other line.

From these operations, we obtain the following mesh: