OVERVIEW OF COMPUTER GRAPHICS FOR CFD AT NASA AMES (June 1986)


GRAPHICS HARDWARE

The main graphics device used for computational fluid dynamics at NASA Ames is
the Silicon Graphics IRIS 2500 workstation.  The IRIS is notable for its fast
3D transformation and display capability (50,000 points per second) and large
disk (370 MBytes formatted).  Still picture hardcopy is
obtained using a Dunn Camera (model 635).  This system can take 8"x10"
polaroids and transparencies, 4"x5" polaroids, or 35mm slides.  For recording
movies on 16mm film, another Dunn Camera (model 632) is used.  A videotape
system, consisting of a Lyon-Lamb VAS IV animation controller, Lyon-Lamb
ENC-VI sync generator & color encoder, Sony RS170 RGB & NTSC monitor, FOR-A
420 time-base corrector, and Sony 5850 Umatic (3/4") videotape recorder,
allows frame-by-frame control for animation sequences (when it's working).


MAJOR GRAPHICS CODES USED FOR CFD

Three major pieces of graphics software are being used by the Computational
Fluid Dynamics Branch and the Applied Computational Fluids Branch at NASA Ames
for CFD research.  These are PLOT3D, GAS, and ARCGraph.  All were written at
NASA Ames, and to varying degrees are tailored to the existing computer
environment there.  In combination, they cover the graphics needs of a large
percentage of the CFD work being performed, and provide an example of a
graphics environment for engineering research.  Many peripheral tools have
also been developed for specific tasks, such as binary data conversion between
dissimilar machines (e.g., CRAY 2 to IRIS workstation).  Commercial graphics
packages which are in use at Ames are DISSPLA and DI-3000.

Interactive grid generation programs which are coupled with graphics have yet
to be developed at Ames.  This weakness in the CFD effort is mainly due to the
emphasis on developing algorithms rather than the application of those
algorithms to complex engineering problems.

PLOT3D

PLOT3D is an "applications-level" program, in that it deals specifically with
CFD grids and data.  The idea here is that the interactive examination of
computed results (or grids) is one of the primary tasks which occupies
the researcher.  In that one really doesn't know ahead-of-time what one will
find in the CFD solution, PLOT3D takes as input the original CFD grid and
solution; plots are produced directly on the computer graphics terminal.  The
program allows the selection of different subsets of the original data, the
plotting of a variety of functions based on the solution variables (such as
Mach number, pressure,
vorticity, etc.), changing the number and color of contours, and, on the IRIS
workstation, the real-time rotation and translation of the picture on the
screen.  Some flow visualization techniques are also built into PLOT3D: a
rudimentary scheme to find and display shock waves, and particle tracing to
simulate smoke in a wind tunnel or oil smears on the surface of a wind tunnel
model.  PLOT3D does NOT show motion of the particle traces (but does save
(x,y,z) and time information for further processing), nor will it display a
series of frames in a "movie mode".  These can be done using GAS.

While the expected CFD solution file and computed functions are based on the
Euler or Navier-Stokes equations, the code can, with some effort, be modified
for other equation sets.  This is on the basis that, independent of the
equations, most quantities to be plotted are either scalar or vector fields.
The interface to the graphics library (DISSPLA or the IRIS library) can be
modified as well, to acommodate different graphics calls.

PLOT3D is written in FORTRAN 77, and is available for the following systems:
. VAX/VMS (V4.2) and DISSPLA (10.0)
. IRIS 1500 (W2.3?-GL1) and IRIS (GL1) Graphics Library
. IRIS 2500 (W2.3-GL2) and IRIS (GL2) Graphics Library
. IRIS 2500 (W2.3-GL2) and ARCGraph
. CRAY/COS and DISSPLA (10.0)
It may be obtained by sending a magnetic tape with a letter of request to
Pieter G. Buning, Computational Fluid Dynamics Branch, MS 202A-1, NASA Ames
Research Center, Moffett Field, CA 94035, phone (415) 694-5194.

GAS

GAS (Graphics Animation System) is a program to interactively set up, view,
and record on videotape or film, an animation sequence on the IRIS.  An
animation sequence implies moving the image around on the screen (3D
translation, rotation, zooming, etc.), and/or looping through a series of
frames, representing (for instance)
a time-varying process or a sequence of slices through a 3D grid.  When a
researcher wants to see a movie-mode playback of an animation sequence, or
record a movie to take to a presentation, he uses GAS.  Input to GAS is in the
form of .GRA files, generated using the ARCGraph GRAFIX library or by any
program following the opcode and data format of ARCGraph.  These files contain
3D graphical information.  Output of GAS is pictures or movie sequences on the
screen, videotape, or 16mm movie.

Due to the flexibility of dealing with purely graphical information, multiple
plots can be overlaid, allowing the comparison or correlation of different
functions or datasets.  An interactive titling and text overlay capability,
including several fonts, is available in GAS as well.  These types of features
are very valuable for both day-to-day research and final copy/presentation
preparation.

GAS is written in C, specifically for the Silicon Graphics IRIS 2400/2500 or
3020/3030, and requires 32 bit-planes, near/far clipping, and z-buffering.  In
addition, videotape output requires the RS170 output option.  GAS is available
by sending a letter of request and tape to Fergus J. Merritt or Gordon V.
Bancroft, Workstation Applications Office, MS 202A-14, NASA Ames Research
Center, Moffett Field, CA 94035, phone (415) 694-4052.

ARCGRAPH

ARCGraph is a collection of graphics libraries and utilities, installed on
many computers at NASA Ames.  Of particular interest is the GRAFIX library,
which supplies the graphics calls to generate .GRA files, the graphics files
which can serve as input to GAS.  A version of PLOT3D is available which
generates .GRA files, and can thus be used to make sequences of frames to be
"animated" by GAS.

No applications code such as PLOT3D handles all types of computations,
however.  In addition, flow solutions being generated on the CRAY 2 may be too
large to store on the workstation (or even the CRAY!), or post-processing may
be too time-consuming on the workstation.  ARCGraph allows the researcher to
write a problem-specific program for graphics generation, which calls routines
in the GRAFIX library to create a .GRA file.  These files contain purely
graphical information (opcodes and data for 3D moves, draws, color changes,
etc.), and thus
are not restricted to use with any specific engineering application or code.
ARCGraph .GRA files are in binary form, and are machine independent.  Thus a
file generated on the CRAY 2 or other machine can be copied (with care) and
displayed on an IRIS workstation.

This mode of operation is not interactive in that it requires one program to
generate the graphics file and another to view it, but clearly allows
much-needed flexibility in specifying what graphical information is presented,
as well as where that information is generated.

ARCGraph is written in FORTRAN 77, and has been installed on the following
systems:
. VAX/VMS (V4.2)
. IRIS 2400/2500 (W2.3-GL2)
. CRAY/COS
. CRAY 2/UniCOS
More information and documentation is available from Eric Hibbard, Computer
Graphics Research Group, MS 233-14, NASA Ames Research Center, Moffett Field,
CA 94035, phone (415) 694-6629.


SUMMARY

This environment thus allows the quick-look or interactive examination of a
CFD database without extensive post-processing; the generation of animation
sequences for viewing or recording; preparation of more complex images and
text overlays; and the creation of unique or problem-specific graphical
displays.
