The application of color coding techniques used in processing remote sensing imagery to analyze and display fluid flow data is discussed. A minicomputer based color film recording and color CRT display system is described. High quality, high resolution images of two-dimensional data are produced on the film recorder. Three dimensional data, in large volume, are used to generate color motion pictures in which time is used to represent the third dimension. Several applications and examples are presented. System hardware and software is described

Giamati, C. C.

English

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Application of Image Processing
	
i .	 Techniques to Fluid Flow	 R
Data Analysis
	
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PHOCESSIOG TECHNIQUE S TG FLUID FLOW CATA
	
ANALYSIS (NA:3A)	 16 p HC AUl/nF AU1 CSCI UTA
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	Charles C. Giamati	 6	 ,
Lewis Research Center
Cleveland, Ohio
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Prepared for the
Eleventh AI ►nual Computer Output Microfilm
(COMtec) Confermce
Lincolnshire, 1hinois, February 24-26, 1581
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APPLICATION OF IMAGE PROCESSING TECHNIQUES
TO FLUID FLOW DATA ANALYSIS
By Charles C. Giamati
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
Image processing techn^,ques can be useful for analyzing fluid flow data
(in this case, calculated values of fluid flow parameters) because of the
nature of the data. The results of fluid flow calculations; (a) generate a
large number of data points, (b) represent several variables and implicitly
contain correlations between some variables, (c) can be usefully represented
as two dimensional contours. The technique which has been most useful for
representing fluid flow parameters is that of color coding, and that tech-
nique will be described here.
Figure 1 shows a conventional aerial photograph of the NASA facility
adjacent to Cleveland Hopkins International Airport, the Lewis Research
Center. For later reference note the aircraft hangar, the road network, the
O	 wind tunnels, cooling towers, a rio the river flowing around the perimeter of
7U-1 	 the installation. Figure 2 shows a black and white image reconstructed from
 a thermal scan of roughly the same scene taken a few hours after sundown.
The intensity of thermal radiation is proportional to the 'temperature of the
emitting structure, and in this scan higher temperatures are recorded as
lighter (or brighter) shades of gray. Again note the aircraft hangar, the
wind tunnels and piping, sheet metal roofed buildings, which are black of
cclaer than the surroundings, having absorbed less heat through the
reflective silver paint covering. The roaas, parts of the cooling towers,
and the river are hotter and appear brighter in the thermal image.
Since the human eye can discriminate only a relatively small number of
gray levels, 20 to 30, it is useful to extract more information from a
thermal image by color coding the data. Each shade or gray level is
assigned a distinct color, and the image constructed. Figure 3 shows the
r
	
	
same general area, taken from a somewhat greater altitude, and color coded.
The general progression of colors, from cold to hot is black, blue, Cyan,
green, magneta, red, yellow, and white. The coldest portions of the scene,
represented in black, are again easily distinguished, namely the lettering
on the hanger roof, the wind tunnels, piping, and sheet metal roofs. The
variations in temperature are more apparent in color then in black and
`
	
	
white, as shown by the magneta color of the runways, and the magenta,
orange, red colors in the river. An underground steam line down the middle
of a road is visible, and the heat leak from the open aircraft hangar doors
is seen to have easily aistinguishf^d contours. The presentation which
follows will describe how thermal images are senses and recorded in digital
form, how images are reconstructed from digital data and displayed, and how
`
	
	
L.hese techniques are adapted to the display and analysis of compressible
fluid flow through aircraft components.
The essentials of recording a digital image are shown in Fig. q . The
line of flight of the aircraft (or satellite in other cases) is from right
to left. The instantaneous field of view of the optical system is identi-
fied here as a ground resolution element, a rectangular (often square) patch
of ground. The data from such a ground resolution element is usually re-
ferred to as a pixel, a picture element. The radiation from the ground
resolution element is reflected from the rotating scan mirror, through the
optics system to a thermal detector. The electronic output of the thermal'
detector is sent through a sample and ho ► d circuit, whose output is
digitized and recorded on magnetic tape. The rotating mirror provides a
sequence of GRE's presented to the digitizer, and scans a swath perper--
dicular to the line of flight. One rotation of the mirror generates a
sequence of pixels which are put on magnetic tape as one record. The pro-
gression of the aircraft along the line of flight results in the next scan
being offset a'ionq the line of flight, so that the continuous series of tape
records represen^ a rectangular scene of the ground underneath the aircraft.
Once a scene has been recorded on magnetic tape, the user must recreate
an imag- or series of images from the tape. A schematic of the hardware
system used to recreate an image is shown in Fig. 5. In order to quickly
preview or screen a scene, a CRT display is necessary. For hard copy, a
film recorder or a printer/plotter is requireo. 'rhe system includes a mini-
computer with main memory, several removable disk cartridges containing
system softw&;-, - : applications programs, magnetic tape units to read and
write tapes, and typewriter consoles for user commands. The image output
hardware, the film recorder and the CRT display contain limited memory and
processor capability and can accept both data and corranand words from the
mini—computer. To create a CRT image the following steps are necessary, as
shown on Fig. 6. A record is read from magnetic tape into the mini,.
computer memory, at a location called the input buffer. As we have seen,
this uata corresponds to a scan line of pixels from -c.he original scene. The
data in the input buffer is reformatea and stored along with suitable com-
mands into another mini—computer memory location designated as the output
buffer for eventual transfer to the CRT display unit. These activities
occur under control of the mini—computer CPU, which finally initiates a
transfer of the output buffer contents to the display processor. The net
result is that a scan line of up to 512 pixels are stored in the display
memory. Repetition of this process can store up to 512 scan lines, usually
in a one—to—one correspondence with the records on the magnetic tape. The
display memory locations in turn are in an one—to—one correspondence with a
square array of locations on the CRT screen, so one can consider the CRT
screen image to be made up of 512 x 512 colored dots or pixels. The display
processor contains electronics which continuously reads the contents of the
display memory and generates a T.V. raster with appropriate red, green, and
blue signals for the color CRT (a stanoard commercial T.V. monitor). The
screen is refreshed 30 times per second which is much faster than an image
can be reap
 into the entire display memory (about 10-15 seccnds).
The color coding for an entire image can be changed much more rapidly
then the image because of this method of refreshing the display at 1/30
second intervals. The pixel or data values in the display memory are ^Jseo
as a pointer, or index into a color translation table which contains the
red, green, blue information used to generate the T.V. signals. Figure 7
i
shows a schematic of the color translation table stored in auxiliary
read/write memory of the display processor. For 8 bit data (0-2551256
different colors can be stored. The unit can store 1024 colors if 2^ bit
data is to ba represented, but in most remote sensing work 24 colors are
more than adequate. This scheme restricts the CRT display to 16 gray levels
(4 bits) since a gray color requires equal RBG intensity. As stated, the
pixel value at each display memory location is used as a pointer to the
color information in the color translation table prior to the generation of
the T.V. raster.
A color translation table in analogue form is shown in Fig. 8. The
gray scale at the top is linear, and contains 16 levels, hence each level
represents 16 data values to provide 16x16 - 256 data values. The color
scheme contains 12 colors and again is linear in that the same number of
data values are assigned to each color, except for th extreme colors black
and white. All data values below a threshold are assigned black and all
values above an u pper limit are assigned white. The threshold can be chosen
to suit the specific data, since the gain of the scan system will vary from
scene to scene. This amounts to sliding and stretching a color scheme up,
down, and along the maximum data range of 0-255. The next two viewgraphs
show an application of color coding for a thermal image. Figure 9 shows
another aerial photograph of the Altitude Wind Tunnel of the Lewis Research
Center. The points of interest are the wind tunnel, the refrigeration
building, the cooling tower, and the water settling tanks adjacent to the
cooling tower. Figure 10 shows a color-coded thermal image of the area.
The images here represent a refrigerration building associated with a wind
tunnel and cooling tower at Lewis. Figure 10(a) at the top was scanned from
a higher altitude then Fig. 10(b), so the displayed image was expanded
electronically by doubling or quadrupling each pixel, hence the obvious
digital nature of the image. The relatively hot water in the settling
basins adjacent to the cooling tower is obvious in both pictures, as are the
vents on top of the cooling towers.
As shown in Fig. 11, characteristics of the CRT displays to keep in
mind are: medium resolution, quick image change, very fast color change,
geometric distortion., The geometric distortion results in a square image in
memory being displayed as a rectangle wider than high.
The method of forming an image on the film recorder is different in one
important aspect, namely the digital data is supplied to a high resolution
black and white CRT. The white light emitted from the CRT face is passed
through a color filter, then focused on color film. To record a full color
image, three separate passes are necessary, one each through a red, blue,
and green filter. This requires that for each one pixel image, three
"separation" images need to be created, as indicated in Fig. 12. This
method creates a very high resolution film image, limited by the deflection
capabilities for the electron beam. The film image is formed dot by dot,
with the exposure regulated by the amount of time the electron beam is
allowed to remain at one place, and hence is highly accurate and repro-
ducible. An example of the quality of image possible with this process is
the thermal scam of Fig. 3. The requirements for high resolution and
accurate exposure level lead to long times to produce a film negative, of
the order of 3-15 minutes, depending on the size of the image.
7
In Fig. 13 the characteristics of the film recorder image are shown:
high resolution, slow image change, slow color scheme change due to
sequential color generation, no geometric distortion.
Having discussed the characteristics of the image output hardware, and
seen some examples of remote sensing products, we can turn to the discussion,
of applying these techniques to fluid flow.data, resulting from theoretical
calculations. The data represent fluid flow parameters such as pressure,
temperature, velocity, vorticity as a function of three spatial dimensions
or as a function of two spatial dimensions and time. The calculations are
implemented by choosing a grid of points (rectangular, square, cylindrical)
and assigning boundary conditions and/or initial conditions. Numerical
calculations fill in the 3-D spatial grid, or extend a 2-D grid in the time
e	 dimension. The final results represent thevalues of one or more ,parameters
at each grid point, and are often presented as page upon page of tabulated
data.
Figure 14 shows a configuration of a curved duct, of square cross
section. The elementary 2-D grid is a slice perpendicular to Vie duct
axis. The characteristics of the data are: a large volume of data (but
smallEr than scenes from remote sensing), a sequence of two-dimensional data
arrays, typically 250 stations, low resolution (32x32 or 64x64) at each
station.
Any one 2—D slice can be represented as an image by using the grid
coordinates of the calculations for the image coordinates, and taking a
parameter value such as pressure to be a pixel value. The number of gria
points is small so it is usual to interpolate between grid points to obtain
a smoothly varying image. The resultant data valucs are scaled between 0
and 255 to form a data set for an image. The interest in such a configura-
tion is in the three dimensional properties of the flow. One way to grasp
these properties is to project the sequence cf 2—D slices in a motion
picture, and imagine that the viewpoint is that of an observer moving along
the flow axis. The movie which will be shown next was generated by
displaying calculated values of flow parameters for one frame (a,t one axial
station) on the CRT screen, and photographing the CRT screen with a 16 mm
movie camera, driven in single—frame mode by the computer system. Each film
clip corresponds to about 250 axial stations. For animation purposes, 4
frames on the film were photographed at each axial station.
The next configuration to be shown is a mixer nozzle for the outlet
from a jet engine turbine, Fig. 15. In jet engines, cold air is bypassed
around the outside of the combuster, and mixea with the hot combustion
products flowing through the engine core. Nozzles at the combuster outlet
are arranged in a sinusoidal or wavy fashion to promote mixing of the hot
air from the combuster core with the cola air from the bypass. Figure 16
shows the total temperature profile at the final calculation station for a
12 lobe mixer configuration. The temperature scale is shown at the lower
right, blue representing colder temperatures, red and yellow representing
warmer temperatures. The outline of the mixer nozzles is still apparent
showing that the mixing is not complete. The next Fig. 17 shows the same
axial station for an 18 lobe mixer. Here the mixing has been more complete
a; shown by a more uniform terperature across the image. The traces of the
18 nozzles are still visible. The next two figures (18, 19), show the
4
variations of toal pressure through the same configuration. The film clips
to be shown next pi• asent temperature and pressure parameters for a variety
of mixer nozzle configurations.
The movie clips shown present an approach to analyzing or screening
large amounts of data. In this system, the flow calculations and grid inter-
polations were performed on a large computer whose output included magnetic
tape containing the data for 256x256 pixel images. To minimize intra-record
gaps on the tape, each image was written in only 4 records of about 16 K
Byte length each. The tapes were later played on the mini-computer image
processor. A run containing 45U axial stations can be displayed on the CRT
in about 30 minutes. In order for the research engineer to review and
asimilate the data in image form, further data compression is useful, and
the 16 mm movie technique was selected. The method of photographing the CRT
display with a single-frame motion picutre camera was chosen because it was
fast, because the resolution was more than adequate for the data, and
because the hardware and film processing capabilities were on hand and
available. The computer based system for controll i ng the CRT and movie
camera produced a film clip of 1500 frames in about three hours. At 24
frames per second viewing speed, that film clip runs in just over a minute.
To summarize, thase movies have been extremely useful to the research
engineers for analyzing data and gaining insight to the fluid flow phenomena
under investigation. The usefulness of the technique does not depend on the
hardware used, since other systems of hardware/software could be devised.
Figure 20 highlights the benefits from this techn i que, Homely to al?ow
assimulation of large amounts of data relating to complex flow phenomena, to
use the human eye-brain to integrate and recognize patterns, and the fact
that lengthly or complex computer data processing is not required.
{ORIGINAL PAGE
HI ACK AND WHITE PHOTOGRAPH
Aerial photograph of Lewis Research Center.
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Figure 20. - Benefits of graphic display of fluid flaw data.


Application of image processing techniques to fluid flow data analysis

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