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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.
 
Screenshots
Unstructured Meshing
Volume Grid Generation



Outline of Capabilities

  • Convenient generation of tetrahedral grids for the computation of viscous and inviscid flows around complex configurations.
  • Capable of generating anisotropic stretched grids for improved efficiency of the grid clustering.
  • Smooth grids by means of the source concept and solving an elliptic PDE on a `transparent' Cartesian background grid.
  • User control over grid distribution through simple adjustment of source parameters such as spacing, intensity, and direction of source propagation.
  • Graphics capabilities for convenient visualization of surface and volume grids and interactive surface mesh edge swapping.
  • Restart capability through an efficient recurrent local/global renumbering system to reduce computer memory requirement for generating large grids on small workstations.
  • Grid quality improvement by a combination of the Laplacian ("spring") smoothing and local remeshing.
  • Grid movement capability.
  • 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, click here.
 
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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)
 
 Top | Background | Capabilities | Description | Performance | Examples | Manual | Code Request | References



References

  1. 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.
  2. Pirzadeh, S., "Three-Dimensional Unstructured Viscous Grids by the Advancing-Layers Methods," AIAA Journal, Vol. 34, No. 1, 1996, pp. 43-49.
  3. Pirzadeh, S., "Structured Background Grids for Generation of Unstructured Grids by Advancing Front Method," AIAA Journal, Vol. 31, No. 2, 1993, pp. 257-265.
  4. Pirzadeh, S., "Progress Toward a User-Oriented Unstructured Viscous Grid Generator," AIAA Paper 96-0031, 1996.





NASA Official Responsible For Content: Dr. Shahyar Z. Pirzadeh
Site Curator: Dr. Neal T. Frink
Last Updated: August 8, 2003

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