|
GridTool: A Tool for Structured and Unstructured Grid Generation
Advancing Front Applications (VGRID System)
In this section,
using GridTool for VGRID system is described. VGRID system is a robust
and fast unstructured grid generator developed by VIGYAN Inc. for NASA
Langley Research Center. The VGRID code is fully functional and
supported and can be obtained from NASA Langley Research Center
(contact:
Dr. Shahyar Pirzadeh ). The VGRID system
is based on an advancing front technique, and readers are referred to
an excellent and detailed report by Parikh, Pirzadeh and Lö hner
VGRID[1]. A short description of advancing front
technique will be given here for the sake of completeness.
The advancing front method is an unstructured grid generation method
similar to parabolic and hyperbolic methods for structured grid
generation. Grids are generated by marching from boundaries (front)
towards the interior. First, the domain of interest is subdivided into
a set of patches which cover the entire domain. Next, these patches are
triangulated to form the "initial front". Finally, tetrahedral elements
are generated based on the initial front. As tetrahedral elements are
generated, the "initial front" is updated until the entire domain is
covered with tetrahedral elements, and the front is emptied. The above
process can be summarized in the following steps:
- subdivide the domain of interest using GridTool,
- specify grid spacings using GridTool,
- generate the "initial front" using VGRID,
- update the GridTool restart file to reflect the changes
from VGRID using GridTool,
- project the front onto the CAD surfaces using GridTool,
- generate the volume grid using VGRID,
- post-process the volume grid using VGRID.
The first step is to define the boundaries for the domain of interest.
These boundaries are then subdivided into smaller patches using
GridTool. In this paper, a patch is synonymous with a
three-dimensional polygon. In the VGRID system, three types of patches
are allowed: triangular Barnhill-Gregory-Nielson patches (three
arbitrary sides), bilinear transfinite Coon's patch (four arbitrary
sides), and planar patches (defined by an arbitrary number of sides,
all lying in one plane). Each patch consists of several sides, and
each side consists of several curves. In step 2, the grid spacing is
defined by nodal and linear sources. An excellent description of these
sources can be found in Ref. [10]. In
step
3, all patches are triangulated to form the "initial front" using the
VGRID system. In this step, VGRID may change the patch orientation. If
so, in step 4, the GridTool restart file must be updated to reflect the
changes. In step 5, the "initial front" is projected back onto the
original surfaces using GridTool. In step 6, the volume grid is
generated in one run or several restart runs using VGRID. In step 7,
the volume grid can be post-processed to enhance grid quality. The
details for steps 3, 5 and 6 can be found in
Ref. 1.
|
