Meshing Fundamentals

It is very convenient that NX NASTRAN has a built-in meshing tool. Other programs require the model to be meshed outside of the software and then imported to the solver.

Essentially, all the dimensions on the geometry to be meshed should be bigger than the size of the smallest mesh element. Meshing is often regarded as a hard task, given the multitude of problems that can occur during the process. If errors are encountered, the user should use the auto heal geometry tool or the other manual tools on the same menu to repair the geometry. However, it is noted that it is easy to get lost in the menus of the healing tools. If the user is confident that his geometry is ready to be meshed, it is now time to actually select the mesh parameters.

The main meshing control is over the mean element size. The element should not be big enough so that it doesn’t accurately represent the features of the model, but it also shouldn’t be smaller than necessary to achieve the desired accuracy because this would result in additional calculation time for no reason. Therefore, experimentation will tell the user which mean element size is optimal. In addition, experience goes a long way in speeding up the process of achieving a proper mesh. Although NX NASTRAN has a feature to automatically calculate the mean element size based on a built in algorithm taking into account properties of your part, it is always a good idea to check over NASTRAN’s guess if time permits by performing a mesh test.

The next control defines how much the element size will vary. The maximum variation of 100% requires most computer power/time, but also yields better results. The variation of element sizes is important because it allows less relevant areas to have coarser mesh, while important regions have a finer grip. It is also important to define which type of element to use. In solid mesh, tetrahedrals of both 4 and 10 nodes are available. As the displacement is measured at each node, the 10-node option represents a better resolution on the results, while taking more time to mesh.

Another tool that has to do with the accuracy of the solution is the Jacobian size. It sets a maximum allowable value for elements. If the mesh elements tend to elongate (as a deformed square) and surpass the threshold value set, the mesh fails and new initial conditions for the mesh must be specified. The reason this is important and convenient is because while meshing a 3-d part thousands if not millions of cells are generated. In order for the solution to be as correct as possible elongated cells cannot exist because they cause the final solution to the problem to be incorrectly calculated.