Described here are the data exchange efforts and plans supported by the NASA Steering Committee for Surface Modeling and Grid Generation. Current methods for geometry data exchange between computer aided design (CAD) systems and NASA computational fluid dynamics (CFD) analysis systems are tedious and induce errors. A Geometry Data Exchange Standard is proposed, utilizing a subset of an existing national standard titled Initial Graphic Exchange Standard (IGES). Future plans for data exchange standardization include all aspects of CFD data. Software systems to utilize this NASA-IGES Geometry Data Exchange Specification are under development

Blake, Matthew W.

Chou, Jin J.

Kerr, Patricia A.

Thorp, Scott A.

English

NASA Technical Reports Server

N92-24398
THE NASA-IGES GEOMETRY DATA EXCHANGE STANDARD
Matthew W. Blake
NASA Ames Research Center
Moffett Field, CA
Jin J. Chou
Computer Sciences Corporation
NASA Ames Research Center
Moffett Field, CA
Patricia A. Kerr
NASA Langley Research Center
Hampton, VA
Scott A. Thorp
NASA Lewis Research Center
Cleveland, OH
SUMMARY
This paper describes the the data exchange efforts and plans supported by the NASA
Steering Committee for Surface Modeling and Grid Generation. Current methods for
geometry data exchange between Computer Aided Design (CAD) systems and NASA
Computational Fluid Dynamics (CFD) analysis systems are tedious and induce errors. A
Geometry Data Exchange Standard is proposed, utilizing a subset of an existing national
standard titled Initial Graphics Exchange Standard (IGES) (ref. 1). Future plans for Data
Exchange Standardization include all aspects of CFD data. Software systems to utilize this
NASA-IGES Geometry Data Exchange Specification are under development.
INTRODUCTION
The geometry data received by NASA scientists for analysis and modification is
currently supplied in numerous formats which often require hundreds of hours of
manipulation to achieve a format capable of being utilized by analysis software. This
modified data set usually has lost a level of accuracy from the original data and often may
not maintain the design intent of the original data as developed on the original designer's
system.
*Dr. Chou's participation is provided under NASA Contract NAs 2-12961
https://ntrs.nasa.gov/search.jsp?R=19920015155 2020-03-17T12:07:53+00:00Z
In thespringof 1991theNASA SteeringCommitteefor SurfaceModelingandGrid
Generation(ref.2) formedaGeometryDataExchangeSubcommitteeof technical
personnelfrom theNASA Ames,Langley,andLewisResearchCentersto developa
standardmethodfor transferringcomplexvehiclegeometriesbetweenvarioussoftware
systems.Followingananalysisof existingandproposedstandards,theGeometry
SubcommitteeselectedtheexistingIGESformatasthebasisfor aNASA standard.In the
UnitedStates,theIGESformatis themostwidelyusedproductdataexchangemethodfor
complexgeometries.Thelatestversionof theIGESspecification(version5.1)provides
anadequatesetof geometric entities to cover the current data transfer needs for CFD
research. A subset of the IGES document was selected and a draft NASA Technical
Specification was released in September of 1991 titled "NASA Geometry Data Exchange
Specification utilizing IGES" (ref. 3) commonly referred to as the NASA-IGES Geometry
Data Exchange Specification or simply NASA-IGES. A second draft of the document
should be available in early spring of 1992.
The NASA-IGES Geometry Data Exchange Specification is meant to provide a means
for rapid and accurate geometry data transfer for engineering analysis utilizing
Computational Fluid Dynamics and related methods. Specifically, it is to provide a method
for the transfer of data between CAD systems and Grid Generation Software. This will
greatly improve the ability of scientists and engineers to rapidly proceed with analysis of a
wide variety of vehicle geometries.
This document explains the IGES/PDES Organization (IPO), describes the contents of
the NASA-IGES Geometry Data Exchange Specification, discusses the NASA Steering
Committee for Surface Modeling and Grid Generation future plans for Data Exchange
Standardization, and describes some of the software under development to utilize the
NASA-IGES Geometry Data Exchange Specification.
IGES / PDES ORGANIZATION
The IGES/PDES Organization (IPO) is a body of volunteers from industry,
government, and academia who are dedicated to the development and implementation of
world-wide standards for the digital representation and communication of product data.
The IPO efforts focus in two main areas: IGES and PDES.
In the late 1970s and early 1980s The Initial Graphics Exchange Specification (IGES)
was designed and developed as a neutral data format to allow the digital exchange of data
among incompatible CAD systems. IGES Version 5.0 has been accepted by the American
National Standards Institute (ANSI). Version 5.1 was released in September of 1991.
The Product Data Exchange using STEP (PDES) effort is the United States activity
which supports the development of the International Standard referred to as the Standard
for the Exchange of Product Model Data (STEP). The first version of STEP is yet to be
released.
The IPO is headquartered at the National Institute of Standards and Technology (NIST)
in Washington D. C. The IPO has regular quarterly meetings. IPO membership is in
excess of 600 persons.
For further information on the IPO, see reference 4.
2
SPECIFICATIONDESCRIPTION
TheNASA-IGESGeometryDataExchangeSpecificationdiffers from standardIGES
in oneimportantaspect.StandardIGESfilesaretypicallyvery largeandareanattemptto
representeveryaspectof productdata. NASA-IGESis intendedto provideavery small
subsetof thiscapability.NASA-IGESis designedsolelyto provideadataexchange
formatfor transferringcomplexvehiclegeometriesbetweensoftwaresystems.Very little
otherproductdatais includedin theNASA-IGESSpecification.Thisminimal approach
shouldprovidesmallorganizationsandindividualresearchscientistswith adatatransfer
methodtheycanaffordto utilize in theirparticularsoftware.TheNASA-IGES
Specificationalsowill providesoftwaredeveloperswith adatatransfermethodthatNASA
hascommittedto utilizing for theforeseeablefuture. Thiscontinuityshouldallowgreater
communicationbetweenfuturesoftwarepackages.
TheNASA-IGESGeometryDataExchangeSpecificationincludesseveralgeometry
entitiesandafewotherentities.Thegeometryentitiesincludesimpleshapessuchas
circlesandconicsandacomplexrepresentationmethodutilizing RationalB-Splines.
Additionalgeometryrelatedentitiesareincludedto provideinformationsuchascoordinate
systemtransformation.Thenon-geometricentitiesincludedweretheminimumdeemed
necessarytoconveygroupingandvisualizationinformationabouttheobjectaswell asa
brief descriptioncapability.
Thefollowing is a list, orderedbyIGESEntity typenumber,of thespecificentities
includedin theNASA-IGESSpecification.Foradescriptionof eachentity seeeitherthe
NASA-IGESGeometryDataExchangeSpecificationor theIGESVersion5.1document.
IGES Entity Type Number Entity Name
Entity 0:
Entity 100:
Entity 102:
Entity 104:
Entity 106:
Entity 108:
Entity 110:
Entity 116:
Entity 124:
Entity 126:
Entity 128:
Entity 141:
Entity 142:
Entity 143:
Entity 212:
Entity 314:
Entity 402:
Entity 406,
Entity 406,
Form 1:
Form 15:
Null Entity
Circular Arc
Composite Curve
Conic Arc
Copious Data
Plane
Line
Point
Transformation Matrix
Rational B-Spline Curve
Rational B-Spline Surface
Boundary
Curve on a Parametric Surface
Bounded Surface
General Note
Color Definition
Associativity Instance
Definition Levels
Name
3
It is expected that most grid generation and analysis software that utilizes NASA-IGES
data will represent the geometry internally as Non-Uniform Rational B-Splines (NURBS).
Because of this, it is desired that CAD systems be able to provide geometry in a NURBS
based form (entities 126 and 128) rather than as a Circular Arc, Line, Copious Data, Plane,
Point, or Conic Arc. Also, Entity 104 (Conic Arc) was included so simple CAD systems
utilizing only this geometric modeling capability could be utilized and to allow development
of simple shapes when desired for a specific purpose. Due to accuracy problems in the
method IGES uses to represent conics, it is highly desirable to always convert conics to B-
Splines before generating a NASA-IGES file.
Existing CFD grid generation programs often input a mesh of points to represent the
geometry. Because of this, the geometry data is often represented solely as a mesh of
points. This mesh is often redistributed several times before achieving the desired grid for
CFD. This meshing and re-meshing of the vehicle surface geometry induces and
propagates errors. For this reason, the NASA-IGES Specification emphasizes the use of
the original design curves and surfaces with continuous derivatives as the correct way to
represent geometry rather than utilizing a mesh of discrete points. The intent is to transfer
the highest order surface representation available.
To summarize,
entities:
Entity 126:
Entity 128:
Entity 141:
Entity 142:
Entity 143:
it is desirable to represent all geometric objects utilizing the following
Rational B-Spline Curve
Rational B-Spline Surface
Boundary
Curve on a Parametric Surface
Bounded Surface
The following geometry related entities are also needed:
Entity 102:
Entity 124:
Composite Curve
Transformation Matrix
The following geometric entities are allowed but discouraged for most purposes:
Entity 100: Circular Arc
Entity 110: Line
Entity 106: Copious Data
Entity 108: Plane
Entity 116: Point
The following geometric entity is allowed but discouraged at all times:
Entity 104: Conic Arc
The following non-geometric entities are available:
Entity 0:
Entity 212:
Entity 314:
Entity 402:
Entity 406, Form 1:
Entity 406, Form 15:
Null Entity
General Note
Color Definition
Associativity Instance
Definition Levels
Name
4
FUTURE PLANS
The NASA Steering Committee for Surface Modeling and Grid Generation has started a
coordinated effort to standardize the exchange of geometry, grid, and solution data used in
the analysis of computational aemphysics problems. The initial purpose is to provide rapid
and accurate geometry data exchange between CAD systems and grid generation software.
The long term goal is to provide a rapid, accurate, and stable data exchange method for use
in comprehensive engineering design and analysis in support of such fields as
Computational Fluid Dynamics (CFD), Computational Electro-Magnetics (CEM), and
Finite Element Method (FEM) Structural Analysis.
The following timetable has been agreed to by the NASA Steering Committee for
Surface Modeling and Grid Generation. Several items relate to national standards
organization efforts, and therefore the time frame for completion is dependent upon
performance by those organizations.
NASA-wide consensus on the NASA-IGES
Geometry Data Exchange Specification: - 4th quarter 1992
Approval of an IGES Application Protocol
following the NASA_IGES Geometry Data
Exchange Specification format: - 4th quarter 1993
Approval of a STEP Application Protocol
providing the geometry data exchange
capability desired for CFD: - estimate 1993- 1994
NASA-wide consensus on a NASA
specification describing grid and solution
data for structured and unstructured methods: 2nd quarter 1993
Approval of an IGES Application Protocol
for grid and solution data for structured
and unstructured methods: 2nd quarter 1994
Approval of a STEP Application Protocol
for grid and solution data for structured
and unstructured methods: - estimate 1994- 1995
SOFTWARE UTILLZING NASA-IGES
The three NASA Research Centers are currently developing grid generation software
capable of utilizing geometry data in the form specified in the NASA-IGES Geometry Data
Exchange Specification. In direct support of this effort, personnel at the Numerical
Aerodynamic Simulation Facility at NASA Ames are developing a reader-viewer-translator
for IGES data files (ref. 5). NASA personnel at all three Research Centers are planning to
modify software to utilize NURBS based NASA-IGES geometry data. A few of the
programs include S3D - an interactive surface grid generation tool (ref. 6), VGRID - an
unstructured grid generator and flow solver developed by ViGYAN under contract to
5
NASALangley(ref.7), GRIDGEN- aslructuredgridgenerationsystembeingenhanced
byMDA Engineeringundercontracto ComputerSciencesCorporationfor NASALangley
(ref. 8),anda turbomachineryanalysiscodeunderdevelopmentNASA Lewis.
SPECIFICATIONAUTHORS
Theauthorsof theNASA-IGESGeometryDataExchangeSpecificationincludemorethan
15engineersandscientistsfrom NASA Ames,Langley,andLewisResearchCenters.The
coordinatorsof theprojectarelistedasauthorson thispaper.
REFERENCES
°
.
o
.
5.
°
°
.
Initial Graphics Exchange Specification (IGES) Version 5.1, distributed by the
National Computer Graphics Association (NCGA) Technical Services and Standards,
IPO Administrator, 2722 Merrilee Drive, Suite 200, Fairfax, VA 22031, telephone
703-698-9600 extension 325
NASA Steering Committee for Surface Modeling and Grid Generation, organized
through the NASA Office of Aeronautics and Space Technology (OAST), contact
Pare Richardson, NASA Headquarters
Geometry Subcommittee, NASA Steering Committee for Surface Modeling and Grid
Generation, Matthew W. Blake Project Lead, NASA Geometry Data Exchange
Specification Utilizing IGES (draft), September 30, 1991. To obtain a copy, contact
Matthew Blake, MS258-5, NASA Ames Research Center, Moffett Field, CA 94035
or e-mail blake@nas.nasa.gov
IPO, National Institute of Standards and Technology, Gaithersburg, MD 20899
Blake, Matthew W.; Jasinskyj, Lonhyn; Chou, Jin: NASA-IGES Geometry Data
Visualizer. NASA Workshop on Software Systems for Surface Modeling and Grid
Generation, NASA CP- 3143, April 1992.
Luh, Raymond C.-C.; Pierce, Lawrence E.; Yip, David: S3D - An Interactive
Surface Grid Generation Tool. NASA Workshop on Software Systems for Surface
Modeling and Grid Generation, NASA CP-3143, April 1992.
Pafikh, Paresh; Pirzadeh, Shahyar: Recent Advances in Unstructured Grid
Generation Program VGRID. NASA Workshop on Software Systems for Surface
Modeling and Grid Generation, NASA CP-3143, April 1992.
Steinbrenner, John; Chauner, John: Recent Enhancements to the GRIDGEN
Structured Grid Generation System. NASA Workshop on Software Systems for
Surface Modeling and Grid Generation, NASA CP- 3143, April 1992.
6


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