Creating a Simple Structure In GID

This section will describe how to create a simple structure, the planar cell, in GID. Firstly you must ensure the GID problem type has been placed in the main GID directory.

GID Notes

The initial viewport is odd in GID I usually change the setting to the following
1) Utilities -> Graphical ->Perspective
2) Tick ‘Axonometric Perspective’
3) Move the slider to about -0.6
4) Click ‘Accept’ and then ‘Close’

Here is a description of some of the items on the left hand menu

Creating the points, surfaces and volumes

Now the structure can be entered into GID. All structures should be defined starting at the origin, and extending in the positive x, y and z direction.

Create the points:
1) Geometry -> Create -> Point
2) Type “point 0,0,0” then enter
3) Type “xMax,0,0” then enter
4) Type “0,yMax,0” then enter
5) Type “xMax,yMax,0” then enter
6) Type “0,0,zMax” then enter
7) Type “xMax,0,zMax” then enter
8) Type “0,yMax,zMax” then enter
9) Type “xMax,yMax,zMax” then enter

Create the lines:
10) Click on the create line button on the left hand tool bar
11) Join up all the points to form a box

Create the surfaces:
12) Click on from the left hand menu.
13) Surfaces can be created in 2 ways, either by clicking on the lines that make up the surface and then hitting ESC, or automatically; right click on the background and select Contextual -> Automatic then type in ‘4’ and click ok. This creates surfaces automatically when 4 connected lines are found. The surfaces should now appear.
Create volumes:
14) Click on from the left hand menu.
15) Again volumes can either be created by clicking on the surfaces that make up the volume or automatically; right click on the background and select Contextual -> Automatic struct.

Boundary Condition Assignment

For this simple example we want to assign an electrode and fixed directors along the upper surface. The lower surface is ground and also has fixed directors. The four sides have the Neumann condition applied. Only one domain is present.

Select Problem type:
1) Data -> Problem type -> problem_type_LC2
Bring up the material dialog
2) Data -> Materials
3) Assign FixLC-2_Electrode-1 to the upper surface
4) Assign FixLC-1_Ground to the lower surface
5) Assign Neumann to the four sides
6) Assign Domain-1 to the volume

Periodic Conditions

Currently the periodic boundary condition has been fully tested, and so its use is encouraged on the sides of the structure. The Neumann condition for the potential has been verified for the potential solution, but the surface integral required for the director has not yet been implemented, so its use is NOT recommended if a change in the director is expected in the plane of the boundary.

If periodic conditions are required a more complex procedure is required in addition to setting the sides to Periodic in the Material Dialog. A nodal equivalence scheme is used in the program, and so it is advantageous to ensure that the same surface mesh is used on the periodic faces. This requirement is not essential, and if the meshes differ interpolation is used on one surface mesh to eliminate nodes on the opposing side. If the meshes are significantly different then the condition will not be properly applied.

To make the mesh the same on the two opposing surfaces where this condition is applied. This is set using the instructions below.
1) Label all the points
2) View -> Label -> All in ->
3) Deal with the first pair of conditions:
4) Geometry -> Create -> Contact -> Separated volume
5) Select the opposing faces on the x-axis
6) The program will pick a point on one of the surfaces and ask for a point on the opposing surface to connect it to. A list of possible points will be given.
7) Click or type in the number of the opposing point
8) Repeat for the faces on the y-axis
It is easy to make mistakes using this procedure for complex structures, and it can be quite time consuming. Such mistakes give rise to elements with zero volume, which will cause the simulation program to fail. This problem can be checked using the following command:
Utilities -> Collapse -> Elements
Then select all elements and press ESC. A report given in the status bar indicates if any elements or nodes were eliminated, and if this is the case, then the Separated volume contacts have been improperly created. It is necessary to delete the volumes and start again.

For complex structures this constraint can be overly restrictive, causing the mesh generator to fail. My advice would be to avoid create these contact volumes unless the surface mesh on opposing surfaces is found to differ significantly.

Meshing

1) You must ensure a surface mesh is output in the mesh file:
2) Meshing -> Mesh criteria -> Mesh -> Surfaces
3) Draw a box around all surface and then press ESC
4) You can then create the mesh
5) Meshing -> Generate
6) You will be asked for the size of the elements to be generated, it tries to make the element sides this long. Enter a suitable value.
7) It will give you a summary of the mesh, look at the number of elements, see if this is enough.
8) When you click ok you will see a view of the mesh, if it is too coarse, just click generate again and enter a smaller value.
9) This button toggles the mesh view on and off
10) If you are happy with the mesh you can export it
11) File -> Import/Export -> Write mesh
12) Type in ‘mesh.txt’ as the filename, this is what the modelling program expects
13) You can look at the surface mesh using:
14) File -> Postprocess
15) To get back to the normal mode use:
16) File -> Preprocess

This page last modified 2 June, 2006 by r.james