Grid Generation

This tutorial is intended to take a simple geometry from start to finish through the grid generation process.  It is also intended to illustrate some of the special functionality such as precedence and periodic surfaces. Start by downloading the file pinfins.igs.gz (right click on the filename and Save As).  Occasionally, the first step in grid generation is building the actual geometry, but for this tutorial the geometry will be given.  Also, many times geometry from other CAD systems may not be of the quality required for grid generation, and in that case, geometry clean-up would be required (clean-up will be covered in another tutorial).  The geometry provided is ready for gridding.

1.  Read in pinfins.igs.gz  (for help on fileio see ) with no glue, no trim (-ng -nt). The geometry is two pin fins on a flat plate.  The brown fin is intended to be connected to the plate and will be used to illustrate  (so don't worry that the base of the fin doesn't lie in the plane of the fin).  The green fin is a closed cylinder set slightly above the plate and will be used to illustrate techniques needed for gridding tight areas.
2.  Before any gridding can be done, the geometry has to be trimmed to produce a topologically valid model.  To start, a circular piece where the brown pin connects to the plate needs to be trimmed out.  Select the curves shown and the plate and . Repeat for the other pin.  NOTE: The display resolution of the surface being projected on affects the accuracy of the projection.  You may want to increase the resolution of the surface when projecting.  See  for help.
3.  Select the plate and hit  and  to work with the plane by itself.
4.  Select the surface and  but don't trim. There needs to be a hole at the left circle because the brown fin is intended to connect to the plate, but the green fin sits above the plate.  It's not physically attached to the plate, and there will be no hole in the yellow surface like with the other fin.   Instead, a surface trimmed into a circle will fill the second hole.  This will become clearer as you progress.
5.  A duplicate of the the yellow surface is needed to create the circular trimmed surface mentioned above.  Select the surface, click , and set the angle of rotation to 360o as shown .  Now, click  to create the copy (you could also translate with a distance of 0).  There will be no visible change; the screen will look just like the image above.  Also, select one of the surfaces (doesn't matter which one, they are identical) and .  The result will be one trimmed surface on top of a surface like the one shown in step 4.  Select the trimmed surface by hitting the key "p", positioning the pointer over one of the points on one of the circles, and clicking the far right mouse button.  Turn the surface off.  This will leave only the untrimmed surface and its parametric curves.
6.  Select the curves shown and leaving only the curves that would be below the green fin.  This eliminates the other valid trimming loops leaving only the loop under the green pin.
7.  Select the surface and .  Hit   to view all the surfaces.  Creating a round trimmed surface under the green fin will help to line up the edge grid between the fin and the plate.  Also this allows more control of the surface grid point density below the green pin. This will help to ensure a good quality volume grid.
8.  Click  then  to see any problems with the topology.  This will cause the curves shown to be picked as problems.  The four problem curves at the boundary will be fixed when the outer boundary is added, so these are not really problems.  The other four curves selected are at the base of the brown fin.
9.  Hit the "Esc" key or click to clear the pick list and zoom on the base of the brown fin.  Normally, a gap this large would not be acceptable and clean-up would be required to make sure the fin connects to the plate within tolerance, but to illustrate the effects of , increase the glue tolerance to 0.0065 and.  The boundary curve of the fin will disappear (gluing has made it topologically the same as the parametric.), and now a  only picks the curves on the boundary of the plate.
10.  The only thing left to do is create the volume grid's outer boundary and set point spacings, but to see the effect of , set all of the point spacings to 0.0125 and select one of the brown fin surfaces and apply  (click the button by precedence in the menu and click ).
11.  Now, .  As you can see,  the points on the curve that are shared between the two surfaces are forced to attach to the surface with .  For two surfaces where neither has precedence the points may lie on either of the two surfaces.  Precedence forces the points to lie in the plane of the surface with precedence.  In this case, the plate is intended to be a plane of symmetry (i.e. only half of the geometry is being modeled), so all points on the symmetry plane should lie in the yellow plane.  For this case, the plate should have precedence.
12.  Select the surface you selected in step 10 and remove precedence by unselecting  in the  menu and clicking .  Now, select the plate (yellow surface) and apply  and .  As you can see, all of the points attached themselves in the plane of the plate.  For  now, use the visible graphics list toggles and turn the surface grid and edge grid off.
13.  The outer boundary for this case will be like a shoe box covering the pin fins and plate.  The five additional surfaces that make up the outer boundary can be created several ways.  One way is to create the four corners and click .  To do this select the corner shown (the lower red point, it is best to use the POINT pick limit to ensure the vertex is picked and not an interior point on one of the curves) and .  This places the coordinates of the point selected in the X,Y,Z fields in the points application.  Enter 0.5 in the Z field and click .
14.  Clear the pick list and select the other corner (the lower red point) and repeat step 13.
15.  Select the four points from the above two steps and.
16.  Repeat steps 13 and 14 to create the surface opposite the one just created.
17.  The procedure above could be used to create the remainder of the surfaces, but for illustrative purposes, the other surfaces will be created differently.  Select the curves shown and .  This creates a surfaces with straight edges between the two selected curves.  Creating a ruled surface here would not have been appropriate if the plate had not been flat.  If the plate had any curvature, the curve where the new surface and the plate meet would not match.
18.  Repeat step 17 for the surface opposite the one just created.
19.  To create the last surface, select the curves shown and .  This creates a TFI surfaces from the selected curves.  Typically, a TFI surface is more "expensive" in terms of control points than the other types (ruled, four point, ...) but is used here for illustration.
20.  Select the 5 surfaces (shown in purple) created and create a new group named OuterBoundary.  Click then .  A message should appear saying:  Integrity check passed!  0 Edges Picked.  The model is ready for gridding.
21.  The point spacing applied earlier should be all right for the fins but may be too fine for the edges of the plate and outer boundary.  Select the corners of the plate and the points at the corners of the outer boundary and apply a 0.025 spacing.
22.  This spacing is probably OK for the plate but is still too fine for the outer boundary.  Select the points at the corners of the outer boundary and  with a ratio of 2.  This doubles the point spacing of the selected points.
23.  Hit  to generate the surface grid.  Use the visible graphics list toggles to turn the surface grid on and the surfaces off.  Click the quality toggle in the visible graphics list toggles.  This calculates the surface angles of the triangles in the surface grid.  For a good quality grid, maximum angle should be less than 120o.  You can see the grid is more coarse at the top of the outer boundary where lots of points are not needed.  Also if you look at the surface grid on the plate the points are more dense around the pin fins to better resolve the geometry.  In real cases, the points spacing will be dictated by the geometry being modeled, but it is good practice to increase the point spacing in areas where little activity is expected.  This saves points and computation time.  Also, usually, the outer boundary is much farther from the geometry than shown.  The small package was necessary for illustrative purposes.
24.  For this model,  the fins will have viscous packing, so select groups PinFinGrid, and PinFinPrecedence.  Click , click the  toggle, and .  Also, select the Plate group and click the  toggle on and .  This applies the normal growth boundary condition to the two fins and rebuild to the plate.  Select the outer boundary and apply .  This will allow the volume generator to reconnect the triangles on the outer boundary as necessary to maintain grid quality.  FYI:  Click on the group name Plate in the group editor.  This selects the surfaces in that group and displays the applied boundary conditions in the information window.  Hit "Esc" to clear the pick list or click .
25.  Since the Rebuild boundary condition has been applied to the Plate group, that surface will be rebuilt in the volume grid generator to "match" the boundary layer grid (normal growth) built on the pin fin.  This means that the entire group grid for Plate will be destroyed and rebuilt using the edge grid at it's boundaries.  Basically, for the way the surfaces are currently grouped, this will undo the work done to control the spacing below the floating pin. To control this, select the circular surface below the floating pin and create a new group named BelowFloatingFin containing only this surface.  Now when the Plate surface gets rebuilt, the edge grid of the surface below the floating pin will be used also.  Hit  again to update the surface grid.
26.  In the Initial Spacing field of the  menu,  enter 2e-6 (this is the packing normal to the solid surfaces and the value depends on the geometry and run conditions). Leave all the other options as the default.  Click .  This lunches the volume grid generator.  Save your changes (igs file) and exit.
27.  Once the volume generator has finished read the resulting grid into SolidMesh (sm -grid  gridfilename).  Now, create a cutting plane to view the volume of the grid.  Click  then click the .  Click the button beside the YZ to change the plane to a YZ cutting plane.  Enter 1.075 in the X field and  (the Add button in the viz panel not the group Add).  Turn the OuterBoundary group off and click  to turn the preview plane off.  Zooming on the corner below the fin not attached to the plate, you can see the viscous packing go underneath the fin through the gap.  Also, notice how the circular surface in the plate helps to refine the surface grid under the fin.  Later, you might try skipping step 25 (leaving the circular surface in the Plate group) to see what differences in the surface and volume grid result.  (Surfaces and cutting planes are shown Wire/Shad)
28.  Create another cutting plane like above but this time with an X value of 0.1.  Looking at the surface grid of the Plate you can see the results of the Symmetry boundary condition.  The Plate group grid has been rebuilt to "match" the boundary layer.  Also, you can see the boundary layer grid on the pin fin reaches the plate and stops.
29.  Click the toggle beside Volume Weather Map.  Change the max value from 160 to 150.  Click    A message appears in the info window with the maximum surface and volume angles and the number of elements drawn.  Every element out side the Min-Max range is displayed (in this case 0-150).  This function can be used to located problem areas in the volume grid.