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VGRID Unstructured Grid Generator
Background
VGRIDns is a computer software for generating unstructured tetrahedral
grids for the computational fluid dynamics (CFD) and other computational
applications. The base version of the code (VGRID) was developed
by the ViGYAN Inc. under a NASA-sponsored Small Business Innovation Research
Program and has been released to the public domain. The current version
of the code (VGRIDns) has been developed at the NASA Langley Research Center
and includes extensions such as the capability of generating thin-layered
"viscous" grids and multidirectional anisotropic grid stretching. VGRIDns
is a key component of the NASA Langley's Tetrahedral Unstructured-grid
Software System (TetrUSS),
a winner of the NASA
1996 Software of the Year Award. For more information on the
background of VGRIDns, please refer to the corresponding Grid
Generation Methodology page.
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Screenshots
Unstructured Meshing
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Volume Grid Generation
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Outline of Capabilities
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Convenient generation of tetrahedral grids for the computation of viscous
and inviscid flows around complex configurations.
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Capable of generating anisotropic stretched grids for improved efficiency
of the grid clustering.
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Smooth grids by means of the source concept and solving an elliptic
PDE on a `transparent' Cartesian background grid.
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User control over grid distribution through simple adjustment of source
parameters such as spacing, intensity, and direction of source propagation.
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Graphics capabilities for convenient visualization of surface and volume
grids and interactive surface mesh edge swapping.
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Restart capability through an efficient recurrent local/global renumbering
system to reduce computer memory requirement for generating large grids
on small workstations.
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Grid quality improvement by a combination of the Laplacian ("spring") smoothing
and local remeshing.
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Grid movement capability.
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User-friendly; widely used by engineers and researchers from industry,
universities, and government organizations.
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Code Description
and Grid Generation Process
The complete package of the grid generation
codes consists of VGRIDns (for the generation of triangular surface and
tetrahedral volume grids), POSTGRID (for post processing of volume grids),
and GridTool (for geometry
surface construction/manipulation, surface mesh projection, etc.)
A geometry, usually defined in a standard surface definition such as IGES,
is first transformed into a number of contiguous surface patches with GridTool.
The union of all patches, including the outer boundaries, forms a "solid"
surface which is used by VGRIDns for triangulation. The grid characteristics
such as spacing, grid stretching, rate of growth, etc. are also prescribed
by the user with GridTool to complete the VGRIDns input file. The processes
of geometry preparation and grid parameters setup with GridTool constitutes
50-90% of the total grid generation time depending on the complexity of
the geometry and the "cleanliness" of the surface definition.
The grid generation technique used in VGRIDns is based on the Advancing-Front
(AFM) and the Advancing-Layers (ALM) methods, and produces tetrahedral
meshes for both inviscid and viscous flow computations. The generation
of a complete grid is divided into three main steps: (1) generation of
triangular surface grid by the ALM and/or AFM, (2) generation of thin tetrahedral
cells in the boundary layer by the ALM, and (3) generation of regular (inviscid)
tetrahedral grid outside the boundary layer by the AFM. Although the entire
process is completed in separate stages with this approach, it is performed
in a single run with automatic transitions from one stage to another. Generation
of "inviscid" grids is accomplished in a similar fashion by simply skipping
the second step. For more information about the grid generation methodology
used in VGRIDns, .
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Computing Environment
& Code Performance
VGRIDns is written in FORTRAN and C languages. It runs on most UNIX-based
platforms in the batch mode. A graphical version runs on the Silicon Graphics
workstations, Mac computers running OS X, and PCs running Linux. VGRIDns generates approximately 1450 tetrahedral cells
per second with the Advancing-Front and about 3900 cells per second with
the Advancing-Layers method using a Silicon Graphics Octane workstation
with an R10000 processor. (The numbers are approximate and problem-dependent.)
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Selected Examples
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User's Manual
A comprehensive user's manual for VGRIDns is yet to be prepared. At present,
we rely on scattered notes and technical papers for training. A preliminary
(draft) version of a VGRIDns Reference Document is also available on line.
(pdf 145k | PostScript
715k)
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| Capabilities | Description |
Performance
| Examples | Manual | Code
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References
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Lohner, R. and Parikh, P., "Generation of Three-Dimensional Unstructured
Grids by the Advancing Front Method," International Journal of Numerical
Methods in Fluids, Vol. 8, 1988, pp. 1135-1149.
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Pirzadeh, S., "Three-Dimensional Unstructured Viscous Grids by the Advancing-Layers
Methods," AIAA Journal, Vol. 34, No. 1, 1996, pp. 43-49.
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Pirzadeh, S., "Structured Background Grids for Generation of Unstructured
Grids by Advancing Front Method," AIAA Journal, Vol. 31, No. 2, 1993, pp.
257-265.
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Pirzadeh, S., "Progress Toward a User-Oriented Unstructured Viscous Grid
Generator," AIAA Paper 96-0031, 1996.
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