HOW TO GENERATE UNSTRUCTURED GRID FOR CFD-PLASMA

Assuming that the user knows the block-structured grid generation and geometric manipulation in CFD-GEOM, the user will start his unstructured mesh generation from PLASMA1.GGD (load from ftp.cfdrc.com, pub/tutorials/plasma/tutorial0 ). Following are he steps to create the unstructured portion of the mesh.

Define Loops: Loops in GEOM mean closed surface boundaries. Loops can be defined using either curve/lines or edges or a combination of curve, lines and edges. If an edge is used, its node distribution is used in the subsequent unstructured mesh generation.

To create a loop:

Go to Geom/Loop/Add a loop: You would be prompted to pick edge/curve/line. You have to pick them in a sequence to complete a "loop".

Note: Make sure that when picking the edge, the edge is picked and not the underlying curve/line

Create a loop for the "case" and one each for each of the coil

Create a surface to be trimmed by these loops: A surface slightly larger than the case is to be created. This surface will be trimmed by the loops which will be explained later.

To create the surface to be trimmed the easiest way is to create a vertical line slightly larger than the case and slightly outside of it, as shown to the left of the case in PLASMA1.GGD. Then

Go to Geom/Surface/Extrude (Vector). Pick the line and extrude by 0.5 m

Trim Surface with Loops: Go to Geom/Trim/Trim Surface with a Loop. Pick the surface and then the loop for the case. Hid middle mouse. Now you would not see the surface. Go to View(top menu bar)/Surface Display Options. And turn on the grid (only outline is displayed by default). Now you can again see the trimmed surface. Now pick this surface and then pick all the coil loops and hit middle mouse. The surface is now trimmed with all the loops. A trimmed surface has the information necessary for the grid generator to know which portions are to be meshed and which not.

Note: See the small (light blue) arrows from the coil loops. These arrows should point outwards indicating that mesh would be generated on the outside of these coils. If the arrows are pointing inwards Go to Geom/Trim/Reverse a loop. As you pick each loop, it would be reversed (arrow directions would be reversed).

Now you are ready for unstructured mesh generation

Create an Open Surface: Go to U-Grid/U-Grid/Surface Defn./Open Surface (meaning a 2D grid). Pick the trimmed surface and an open surface is created. Note that a small arrow pointing in the z-direction is created (difficult to see) which is the hanlder for this surface-set.

Define Domain: Go to Geom/U-Grid/Domain Defn./Create Domain. Pick the surface set you created in the last step. An unstructured domain wold be created with magenta color. Its handle is a larger size arrow (more visible) in the z-direction.

Grid Domain: Go to U-Grid/U-Grid/Grid Domain/Surface. Pick the unstructured domain you created in the previous step. A triangular mesh would be automatically generated.

Boundary Conditions: Boundary conditions for the 2D unstructured grids are specified on edges or curves that were used in the created on loops

Volume Condition: Volume conditions for 2D unstructured grids are applied on loops.