~~~~~ ~ ~ ~~ 

NASA Technical Memorandum 10221 7 

Sensor Development Programs 

at NASA Ames Research Center 

Y.-C. Cho, Ames Research Center, Moffett Field, California 

August 1989 

National Aeronautics and 

Space Administration 

AmesResearchcentet 

Moffett Field, California 94035 
Sensor Development Programs at NASA Ames Research Center 

Y.-C. Cho 

* 

NASA Ames Research Center, Moffeu Field, California 94035 

ABSTRACT 

This paper presents a brief description of two sensor development programs at the Fluid Mechanics Laboratory, NASA Ames 

Research Center, one in progress and the other being initiated. The ongoing programs involve digital image velocimetry for 

velocity field measurements of time-dependent flows. The new program involves advanced acoustic sensors for wind tunnel 

applications. 

1. DIGITAL IMAGE VELOCIMETRY 

Digital image velocimetry @IV) is a new technique which is currently being developed for real-time measurements of 

instantaneous velocity fields of timedependent flows or of a collection of objects moving with varying velocities.1 Such 

measurements will yield infomation not possible to obtain using conventional point-measurement methods such as hot-wire 

anemometry and lascr Doppler velocimetry. This new information will provide a greater understanding of time-dependent 

flows a of objects moving with varying velocities. 

Recently, fluid dynamicists have exploited modern computer and optics technologies to develop methuds for measuring 

instantaneous velocity fields, including particle image velocimetry (PIV) and laser speckle velocimetry (LSV). With these 

methods, multiply exposad images of seed particles embedded in the flow or of moving objects arc caprured on a phom 

graphic film. The object travel distances betweem exposures are obtained hm the image separatons of the same seed particles or objects. The velocities arc then detamincd by dividing the travel distances with the timc interval between the exp 

sures. These methods are, however, subject to some serious restrictions, such as limited dynamic range of the velocity . 

measurements, ambiguity of the direction of the velocity vectors to be detarmned * , and difficulties of development into a realtime measurement capability. 

DIV, which is designed to eliminate these nsrrictions. exploits high-speed video photography, image processing. and digital 

Fourier transformation. With this technique, a time sequence of images of seed particles or moving objects are captured using 

a high-speed video camera, with cach frame containing a single-exposure image. A finite number of singleexposure images 

are sampled with a certain separation time. The sampled images are then digitized on the image pmxssor, enhanced. and 

linearly superposed to construct an image. This process is illustrated in Figure 1. 

The superposed image is equivalent to the multipleexposure image used in LSVPIV. However, there exists a subtle but 

important difference between the superposed image and the multiple-exposure image, especially with high concentrations of 

seed particles as in cases whue a derailed determination of velocity fields is required. With a high cancennation of seed particles. the different-time exposure images of individual objects can easily overlap. On the multipleexposure image in LSV or 

PIV, the intensity in the overlapped region may not differ much from that in the nonoverlapping images. Such overlapping 

will suppress the correlation between the images of the same objects, resulting in an increase of the uncatainty of the velocity determination. In contrast, on the linearly superposed image in DIV, the intensity of the image overlap is equal to the 

arithmetic sum of the intensities of the individual images, and thus the correlation between the images of the same objects is 

preserved. (See Figure 2.) 

The velocities are determined from the image separations recaptured on the supaposcd image. A single-exposure image is 

used to eliminate the uncertainty effects that are due to the finite image sizes and some interference effects caused by undesirable distributions of he seed particles or the objects. An example of this procedure. in which a Fourier-transform technique 

is used, is shown in Figures 3-5. The fringes shown in the figures are directly related to the magnitude and the orientation of 

the velocity. The direction of the velocity vector can be determined unambiguously using a singleexposure image, since this 

image contains information on the time history. 

It should be emphasized that the singleexposure images are not available in, nor can they be isolated from, the multipleexposure image with the photographic film technique used in LSV or PIV. 
2. ADVANCED ACOUSTIC SENSORS FOR WIND TUNNEL APPLICATIONS 

Acoustic measurements in wind tunnels are subject to certain interference effects not found in anechoic chambers. Such 

effects include wind noise. flow-sensor interaction noise, flow-induced sensor vibration, deflection of acoustic waves by 

sensor-induced boundary layers, and reflections fmm facility and sensor suppon components. Furthermore, it is often desirable to investigate specific source regions on the models being studied. Currently existing acoustic sensor techniques are not 

adequate to cope with these problems. 

Ames Research Center is initiating a program to develop advanced acoustic sensors to eliminate or minimize these interference restrictions. The program is based on new concepts for adaptive arrays using optical fibers as acoustic sensor elements. 

An ada tive array is a system composcd of a number of geometrically configured sensor elements and a real-time signal prospectral response for the acoustic signal. ”he directional properties of such a system allows discrimination against unwanted 

sound coming from directions other than the acoustic some under study. A proper adjustment of the spectral response discriminates noise of frequencies different from the signal frequencies. Another significant advantage of an adaptive amy can 

be derived from the &-time manipulation of the correlation of signals detected by various sen= elements. Such a manipulation can be used to discriminate random noise or prcssurc fluctuations generated locally on individual sensor elements. 

D 

. 

cessor. f The signal processor is designed to automatically adjust the array sensitivity for enhancing the directivity and the 

i 

The technology of fiba-optic acoustic setlsocs has matured in undenvata awustics applications, and is readily available for 

implementation to aero-acoustics? Fiber-optic acoustic sensors offer a number of advantages. The sensor element package 

can be very compact, thus reducing flow-sensor interactions. The optical fibers arc geometrically versatile and can therefore 

be configured with gnat flexibility as extended (or distributed) sensar elements. Such configurations would generate a great 

variety of array sensitivity patterns. Fu~thmore, the optical fib can be molded on the surface of a solid body. With the 

sensor elements molded on an aerodynamically smooth body, the flow interaction can be made negligible. the aerodynamically induced vibration Will be minimized, and effects of the boundary layer will be made more tractable. 

An adaptive array with fiberqtic intcrfaomeuic acoustic sulsors is schematically displayed in Figure 6. The system is 

composed of three elements: a light source, fiber optic interferometric sensor elements, and a phouxWct0p intufaced with a 

signal processor. Light, transmitted from the light some into the optical fik (waveguide), will be split at the first coupling 

branch. One part will propagate through the reference 8nn without expaiaing any cxtcmal effect The otha pars which 

will propagate through the sensing ann, will be af€ccted by the external acoustic signals. The exation of the acoustic pressure, Ap, on the optical fibex will result in the phase shift, At), of the light (or the optical wave). In other words, the acoustic 

erence light at the second coupling branch. The interference will then be recurded on the photodetector, and processed on the 

signal processor to extract information on the acoustic signal. 

I signal modulates the phase of the optical wave. The phasc-modulated optical wave will be combined with the unaffected refI 

3. REFERENCES 

‘Y.-C. Cho. “Digital image velocimefry,” App. Op. 28,740-748 (1989). 

2R. A. Monzingo and T. M. Mille.r,Introduction to Adaptive hays, John Wiley & Sons, New York (1980). 

3T. G. Giallorenz, J. A. Bucaro, A. Dandridge, G. H. Sigel, Jr., J. H. Cole, S. C. Rasbleigh, and R. G. Priest, “Optical fiber 

sensor technology,” EEE J. Quantum Electronics, QE-18,626-665 (1982). 

2 
t=O 

la 

t=T 

r 

t = 2T 

A 

A 

Figure 1. Construction of a multiple-exposure image as a superposition of singlecxposure images. 

I 

XO 

PHOTOGRAPH IC IMAGE 

IN LSV OR PIV 

II 

x1 x2 

x1 x2 

X 

I 

x3 

SUPERIMPOSED IMAGE 

IN DIV 

xO x1 x2 x3 

Figure 2. Intensity of multiple-exposure images in overlap region. 

3 
I 1 

(b) (8) 

Figure 3. (a) Triple-exposure image and (b) the modulus of the FoUria transformed image. 

e 

A e 

mA 

e e A 

c 

a 

Figure 4. (a) Single-exposurc image which is contained in the image in Figurc 3(a) as a component detected at the initial 

exposure and (b) the modulus of the Fourier-uansfomed image. 

, 

4 
f 

5 0 

(h1 

1 

f 

Figure 5. (a) Fringe paetcm. which is obtained by dividing the Fourier-transformed image in Figure 30) 

with that in Figure 4(b); and (b) magnified fiinge pattan. 

LIGHT OPTICAL FIBER 

SOURCE 

t 

SIGNAL 

Ap - An -. 69 

n: REFRACTIVE INDEX OF FIBER 

+: PHASE OF OPTICAL WAVE 

Figure 6. Schematic of an adaptive array with fibu-optic interfernmetric sensor elements. 

5 
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

Y.-C. Cho 

NASA TM-102217 

4. Title and Subtile 

Sensor Development Programs at NASA Ames Research Center 

7. Author@) 

A-89167 

10. Wok Unit No. 

5. Report Date 

August 1989 

6. Performing Organization Code 

8. Performing Organization Report No. 

505-61-01 

9. Performing Organization Name and Address 

11. Contract or Grant No. 

Ames Research Center 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

Moffett Field, CA 94035 I 

Technical Memorandum 

14. Sponsoring Agency Code 

13. Type of Report and Period Covered 

19. Security Classif. (of this report) 20. Security Chssif. (of this page) 

Unclassified Unclassified 

21. No. of Pages 22. Price 

6 A02 

17. Key Words (Suggested by Author(s)) 1 18. Distribution Statement I' 

Digital image velocimetry 

Sensors 

Fiber optics 

Unclassified-Unlimited 

Subject Category - 01 

I 

1 I I I 1 

NASA FORM 1626 OCT 88 

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