Meshing Can Be An End Unto Itself

A while back I wrote something to the effect that “no one gets an award for meshing.” This is one of those times when it’s nice to be proven wrong.

We are very happy and proud that a mesh generated by Travis Carrigan and Carolyn Woeber was recognized at the 22nd International Meshing Roundtable with the IMR Meshing Maestro award and the first IMR Meshing Contest award.

Novelty, Complexity, Quality, and Usability

The 22nd IMR was the first at which a meshing contest was held. Steering committee chairperson Josep Sarrate told me that their hope was for the contest to attract student participation as a means of getting new, younger people involved in mesh generation. He also confided that their biggest fear was they would receive absolutely no entries. Neither their hope nor their fear materialized. Instead, seven industry and government organizations submitted meshes.

Our participation was never in doubt. Sometimes a commercial organization gets trapped by the fear of losing in public. (“What might that imply about our product?”) But this competition hits three components of our mission statement: the quality of our product, the skills of our engineers, and our advocacy for the profession.

We also believe in “eating our own dog food” meaning that using our own software to make meshes for external use and evaluation is often a great exercise in identifying things that need to be fixed or changed. In the end, the only issue was finding the time to do the work.

The contest’s requirements were very straightforward. A CAD model was provided in several formats. Contestants could mesh all or part of the model using any technique as long as the mesh was limited to 20 million cells and the mesh file was smaller than 1 GB.

The IMR steering committee judged each contest entry based on

  • novelty of the approach,
  • the complexity of the discretization
  • the quality of the mesh and
  • the usability of the proposed solution.

The Preliminaries

The geometry represented a small unmanned aerial vehicle (UAV) driven by an electric brushless motor without propeller (Figure 1). Carolyn and Travis decided that a cruising speed of 40 miles per hour was appropriate and set a y+ value of 100 in order to not exceed the cell count limit.

Figure 1: The IMR Meshing Contest involved this CAD model of a UAV.

Figure 1: The IMR Meshing Contest involved this CAD model of a UAV.

CAD: Neutral Import, Cleanup, and Solid Modeling

Pointwise’s CAD readers could import all CAD formats provided. Our engineers chose, somewhat by accident, to use both the IGES and STEP files. The STEP file was used by one engineer to mesh the wing and fuselage geometry whereas the IGES file was used by the other engineer to mesh the motor and electronics package. Despite using this style of parallel processing, the two separate grids and their related geometry were assembled into a single watertight surface mesh – something you might not expect when using geometry from two different sources. This is doubly true when you consider that the motor geometry (Figure 2) was incomplete and inaccurate.

After geometry cleanup, the entire UAV was a single, closed, manifold, water-tight, solid model. Our engineers created quilts, topological entities from Pointwise’s solid meshing feature suite. A quilt is a group of surfaces to be meshed as a single region. Use of quilts vastly simplified the topology of the CAD model, making it easier to mesh.

Figure 2: The CAD geometry of the UAV's motor required some cleanup before meshing could begin.

Figure 2: The CAD geometry of the UAV’s motor required some cleanup before meshing could begin.

Advancing Front Surface Meshes

An advancing front algorithm was applied to generate the triangular surface mesh (Figure 3) with Pointwise’s T-Rex technique (anisotropic tetrahedral extrusion, an advancing layer method) applied to resolve the wing leading edges. T-Rex extrudes layers of high aspect ratio, right angle triangles normal to user defined edges until the cells achieve isotropy or collide with another extrusion front or boundary. For the UAV, high aspect ratio triangles were grown off the leading edges of the wings and interfaced with an advancing front mesh for the remainder of the wing surface meshes.

Figure 3: This view shows the surface meshes on the motor and a cut through the volume mesh colored by cell aspect ratio.

T-Rex for Hybrid Volume Mesh

An unstructured volume mesh was created and was defined by a spherical farfield and the UAV’s watertight aircraft surface mesh. T-Rex was used to generate high aspect ratio, anisotropic tets normal to the body of the aircraft, transitioning to isotropic tets in the farfield. T-Rex allows extruded cells to stop locally if there was a collision with another extrusion layer, if quality criteria (e.g. skewness, volume) were violated, or if isotropy was achieved (Figure 4).

T-Rex tags tets in the near-wall extrusion layer if they are candidates for combination into prisms. These tagged cells are evaluated during mesh export and stacks of three anisotropic tets are agglomerated into a prism (again, according to cell quality criteria). Creation of prisms reduced the overall cell count by over 60 percent.

Volume-Cut

Figure 4: This fuselage station cut through the mesh shows the geometry model and the volume mesh cells colored by cell volume.

Export to OpenFOAM

The final mesh was exported to the OpenFOAM format after flow solver boundary conditions had been specified within Pointwise. Cell counts for the final mesh are listed in the table below.

Tetrahedra 2,792,530
Prisms 10,331,317
Pyramids 158,971
Total 13,282,818

Beauty is Only Mesh Deep

Unlike the judging for the meshing contest, the IMR’s annual selection of a Meshing Maestro is voted on by all those attending the conference’s poster session. Three awards come out of the poster session: best technical poster, best student poster, and Meshing Maestro. While the first two awards have strict criteria (technical issue, creativity of solution, impact of the solution, quality of the presentation), the selection of Meshing Maestro is based only on aesthetics and “striking visuals.”

006

Figure 5: IMR attendees review Pointwise’s poster.

There were 11 participants in this year’s poster session. Awards were given to

  • Best Technical Poster: “Gamma 3 Meshing Technologies” by P.L. George, F. Alauzet, H. Borouchaki, P. Laug, A. Loseille, L. Maréchal, N. Barral, E. Mbinky and V. Menier. (This poster was excellent and got my vote.)
  • Best Student Poster: “Parallel Mesh Adaption” by Victorien Menier and Adrien Loseille.

Our poster (Figure 5) presented our meshing contest grid in a different light for a larger audience. As luck would have it, our good fortune continued as it was awarded the Meshing Maestro (Figure 6).

013

Figure 6: Awards were presented at the conference banquet in the China pavilion at Disney’s Epcot. The trophies for the Meshing Contest and Meshing Maestro are shown here.

Other IMR Winners

The best paper award was won by Frédéric Alauzet and David Marcum: “A closed advancing-layer method with changing topology mesh movement for viscous mesh generation.”

And Dr. Stephen J. Owen was named an IMR Fellow.

The 23rd Meshing Roundtable

The IMR community will gather next year in London for the 23rd International Meshing Roundtable (October 2014). We look forward to seeing you all there.

And we thank the IMR for giving us the opportunity to participate in their event and the IMR community for recognizing our work.

This entry was posted in Uncategorized. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s