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Dipankar Choudhury, Chief Technology Officer, Fluent Inc.
Ted Blacker and his winning surface mesh
The Tenth International Meshing Roundtable conference was held last fall
in Newport Beach, CA. One of the highlights of this annual meeting is
the naming of the Meshing Maestro, a coveted award that is given to a
conference poster presenter who has generated a mesh that exhibits innovative
technology, and is both eye-catching and technically sound. Ted Blacker,
the project leader for GAMBIT, was last year’s winner of this prestigious
award. His poster also won the “Best Technical Poster” award.
The clown grid, one of the examples submitted by Blacker, made use of
technology that was developed at Fluent Inc. by Blacker and his colleagues
Richard Smith, Yongheng Shao, and Jin Zhu. In particular, new advances
in mesh density controls were used that are now available in GAMBIT.
These controls, called size functions, are aimed at eliminating automation
obstacles during meshing, particularly when generating a tetrahedral mesh.
Historically, most volume meshing problems are related to a bad surface
mesh. The problematic surface mesh typically doesn’t capture the
geometry well, or isn’t sized appropriately for thin regions of the
geometry. It is also particularly important in CFD analysis that the gradation
of the mesh be tightly controlled. This control limits transition rates
from small to large elements, allowing capture of the boundary layer phenomena
as well as control over solution accuracy.
Although density control is not new in the meshing community, this technique
is unique in how grading controls radiate or propagate to surrounding
regions in a tightly controlled manner. For example, the eyebrows on
the clown have a tight curvature, which is captured through a curvature-based
size function. Not only is the eyebrow adjusted, however, but portions
of the geometry in close proximity are included in the sizing effects
as well. The forehead near the eyebrow attachment and even the interior
of the eyelid show a graceful, controlled gradation of size. This ensures
that the volumetric tet mesher can successfully fill this region with
wellshaped elements, with minimal intervention by the user. A simple
size function was defined to capture the curvature and set the gradation
rate. This size function was attached to the volume and the meshing initiated.
The software then generated the needed octree background grid and automatically
guided the meshing based on these controls. (An octree is a hierarchical
structure used in certain grid generation algorithms. It begins with a
coarse background grid that is recursively divided until the desired grid
density is achieved.)
The technical advance that is central to the new controls in GAMBIT
is accomplished by imposing individual size functions (such as the curvature
of individual surfaces) on the underlying octree-based background grid.
The octree depth (the number of levels in the hierarchy, which corresponds
to the grid density) adjusts automatically to capture regions of importance
in the size function. With the aid of the octree background grid, the
size functions can then radiate beyond the regions where they are defined
to accomplish the control and effects as desired. Three types of size
functions are available, and these can be specified individually by the
user. The edge, face and volume meshing tools then obtain sizing information
directly from the background in a highly efficient manner.
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