Mesh preparation¶

ACE3P reads meshes in its own NetCDF format (.ncdf). You produce them by exporting a Genesis file (.gen) from Cubit and converting it with acdtool. Building and meshing the model in Cubit is covered on the Cubit page; this page covers what the exported mesh must contain and how to check it.

Cubit export checklist¶

A typical meshing journal ends like this:

step2-Mesh-pillbox.jou

reset
import acis "pillbox4.sat"

volume all scheme Tetmesh
volume all sizing function type skeleton scale 5 time_accuracy_level 2 min_size auto max_size 12 max_gradient 1.2
mesh volume all

Sideset 1 surface 7                      # symmetry plane
Sideset 2 surface 1                      # symmetry plane
Sideset 6 surface all except 7 1         # exterior wall

block 1 volume all
block 1 element type tetra10             # quadratic tets, required

volume all scale 0.001                   # mm to m
export Genesis "pillbox4.gen" block all overwrite

Check each item before export:

Tetrahedral mesh with the Tetmesh scheme. Hex meshes are not supported.
Element type tetra10 on every block. Linear tetra4 elements lose the curved-surface information.
Every boundary surface in a sideset. Surfaces that belong to no sideset get boundary ID 0 and are treated as interior, which is almost never what you want. The idiom Sideset 6 surface all except ... catches everything you did not name explicitly.
Every volume in a block. Blocks become material attributes.
Scaled to meters.
Multiple volumes imprinted and merged so shared faces are a single surface. Otherwise the volumes are disconnected in the mesh. See the lossy load tutorial.

Sizing function¶

The skeleton sizing function adapts element size to local geometry. The parameters that matter most:

Parameter	Effect
`scale`	Overall density. Smaller is finer.
`max_size`	Largest edge length allowed, in model units
`max_gradient`	How fast element size may change between neighbors. 1.2 to 1.5 is typical.

Refine around small features such as coupler irises and rounded edges, and do a convergence study before trusting results. See the pillbox tutorial.

Convert with acdtool¶

Copy the .gen file to the HPC system and run:

acdtool meshconvert pillbox4.gen

This writes pillbox4.ncdf and prints statistics. It is a serial program and finishes in seconds for meshes with fewer than a million elements, so it can run on a login node or in a one-task batch job.

Read the statistics¶

Input: pillbox4.gen (contains quadratic 10-point elements)
Output: pillbox4.ncdf
TOTALS:
  elements: 13382
  coordinates: 2955
EULER CHARACTER:
  Surf Euler Char = 2
  Vol Euler Char = 1
  Euler Char is OK.
ASPECT RATIO:
  min      = 1.03213
  max      = 2.53273 <- GREAT
SHAPE MEASURE:
  min      = 0.26181 <- GREAT
Existing boundary IDs:
1 2 6
Material IDs     Number of Elems:
1:       13382
Total Number of invalid second order tetrahedral elements (ISOTE) is: 0

What to look for:

Euler characteristic OK. A closed, simply connected volume has surface Euler characteristic 2. Anything else means a hole or a disconnected piece.
Existing boundary IDs must match the sidesets you expect. A missing ID means a sideset was never created; an unexpected 0 means a surface was left out.
Material IDs must match your blocks.
Invalid second-order elements must be zero. A nonzero count means some mid-edge nodes fold the element inside out, usually from an aggressive sizing function on a curved surface. Refine the mesh or relax max_gradient.
Aspect ratio and shape measure are quality indicators. acdtool prints GREAT, GOOD, or a warning next to them. The CW23 slides give rough limits: aspect ratio above 20 or shape measure below 0.01 will slow the solvers noticeably.

Other acdtool mesh commands¶

acdtool mesh stats pillbox4.ncdf     # statistics only
acdtool mesh check pillbox4.ncdf     # validity check only
acdtool mesh fix in.ncdf out.ncdf    # replace invalid curved tets with straight ones
acdtool mesh deform in.ncdf out.ncdf 5000   # exaggerate a deformed mesh for viewing

The full list of acdtool mesh subcommands in the current source is: boundary, check, deform, fix, merge, partition, pick, rotate, scale, shuntimpedence, smooth, stats, straighten, summodes, surfsmooth, transform, untangle, warp, warpsurface. Most are undocumented beyond their source files; meshconvert already calls check and fix for you. The full acdtool command set, including every postprocess function and the resource tool, is in the acdtool command syntax.