# Introduction to Preprocessing

This Page refers to Preprocessing in the Finite Element Analysis Domain. If you are looking for a more general explanation of Preprocessing please refer to the Preprocessor Page.
Preprocessing also called Meshing is the first step in solving a problem in Finite Element Analysis Here the entire domain is discretized (divided) into meaningful divisions often called "Elements". These elements form the building block on which the Boundary conditions and external effects are specified.
Many software exist to create a useable Mesh, Some of the popular ones are
1. Hypermesh from Altair
2. Medina from GRM Consulting
3. ICEM from ANSYS
Meshing a domain would consist of the following tasks
1. Defining a domain to Mesh
    1.1 Importing a CAD Geometry (Computer Aided Geometry) OR
    1.2 Creating your own Geometry OR
    1.3 Creating node based elements
2. Selecting the analysis type (Ex: 3-Dimensional, 2-Dimensional, 1-Dimensional)
3. Creating the mesh
4. Choosing the element type
5. Validating the mesh. Check for Errors, Connectivity, Quality etc.

## Importing CAD Geometry

CAD standing for Computer Aided Geometry. It is a procedure of generating Virtual parts using Computers. It dates back to (almost) the beginning of computers . In 1960s when digitizing Drawings, Surfaces, Generating Numeric Codes for automatic machining of metals gained popularity. As Finite Element Analysis became popular during the 1970s, Meshing a Native CAD geometry got importance.
Today many native CAD geometry can be imported into a meshing software for the purpose of creating a finite element mesh. There are many neutral geometry formats available such as the popular IGES (Inter Graphical Exchange Specification) Some of the CAD geometries that could be imported and meshed in commonly used meshers (Not exhaustive list..) are;
1. Pro-Engineer/Creo
2. CATIA
3. STL (Stereo Lithography Format Tessalation)
4. IGES (Initial Graphics Exchange Specification)
5. DXF (Data Exchange Format)
6. Parasolid
7. Unigraphics
8. ACIS (Andy Charles Ians System solid modelling geometric engine) etc.

## Node

Nodes forms the building block of any mesh. An element is specified through it's connectivity to a set of nodes. Nodes can be simply thought of as a coordinate which has a label to it. So that when you refer to this label, you mean the coordinate. A sample node may look like this.
Node Label 1,X Coordinate,Y Coordinate, Z Coordinate
Notes: Node has to be defined according to the Coordinate system to which it refers. (In majority of the solvers the default would refer to a Global Cartesian Coordinate System) Most of the example defined may refer to Abaqus neutral and hypermesh software. But an attempt to generalise the examples is also on.
Example of a Node defined in Cylindrical coordinate system;
Node label 1, R Coordinate, Theta Coordinate, Z Coordinate

## Steps in Meshing

1. The first step in meshing would be to select a suitable element to mesh your domain.
2. Let's continue by saying that you have selected a 2Dimensional mesh of 1st order (corner nodes only) mixed mesh containing rectangles and triangles.
3. You could then define an element as the element type that you want to fill the nodes with (say trias) as follows
3.1 Example with trias: <element no>, <connecting node 1>, <connecting node 2>, <connecting node 3> 3.2 Example with quads/rectangles: <element no>, <connecting node 1>, <connecting node 2>, <connecting node 3>, <connecting node 4>

## Element Quality

Element quality is very important for a solver to provide you with a reasonable result.
Element quality is most often defined as a deviation from a perfect element.
It's important for the user to understand what is Iso-Parametric element
Order of element. An element can be of either 1st order
Elements can be classified into many kinds, but the most important classification would be; Stuctural and Continuum.
Some of the basic element quality checks;
3 Dimensional Hexahedral element
1. Jacobian - It's the deviation from a perfect element
2. Warpage - It refers to the deviation
3. Aspect Ratio - It refers to the ratio of largest side to the smallest side
4. Skew -
3 Dimensional Tetrahedral element
1. Aspect Ratio
2. Skew
3. Collapse ratio
4. Jacobian
Shell element
1. Min Quad Angle (min tria angle in case of a tria)
2. Max Quad Angle (max tria angle in case of a tria)
3. Jacobian
4. Warpage
5. Aspect Ratio
6. Skew

## Advanced Topics

1. Element checks
    1.1 Fixing element cracks
    1.2 Checking element volume
    1.3 Element normal check
2. Element technology - Selection of elements
3. Continuum Vs Structural elements
4. Special elements
    4.1 Incompatible element
    4.2 Hybrid element
    4.3 Other special element types
5. Order of elements
6.

## See Also

1. http://en.wikipedia.org/wiki/Adaptive_mesh_refinement#Introduction_to_adaptive_mesh_refinement
2. http://en.wikipedia.org/wiki/Polygon_mesh