Acoustic velocity measurements were taken using Particle Image Velocimetry (PIV) in a Normal Incidence Tube configuration at various frequency, phase, and amplitude levels. This report presents the results of the PIV analysis and data reduction portions of the test and details the processing that was done. Estimates of lower measurement sensitivity levels were determined based on PIV image quality, correlation, and noise level parameters used in the test. Comparison of measurements with linear acoustic theory are presented. The onset of nonlinear, harmonic frequency acoustic levels were also studied for various decibel and frequency levels ranging from 90 to 132 dB and 500 to 3000 Hz, respectively

Blackshire, James L.

English

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NASA Contractor Report 201664
Analysis of Particle Image Ve!ocimetry
(PIV) Data for Acoustsc Velocsty
Measurements
James L. Blackshire
ViGYAN, Inc., Hampton, Virginia
Contract NAS1-19505
January 1997
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23681-0001
https://ntrs.nasa.gov/search.jsp?R=19970013638 2020-06-16T03:02:39+00:00Z
Analysis of Particle Image Velocimetry (PIV)
Data for Acoustic Velocity Measurements
Abstract
Acoustic velocity measurements were taken using Particle Image Velocimetry (PIV) in a
Normal Incidence Tube configuration at various frequency, phase, and amplitude levels. This
report presents the results of the PIV analysis and data reduction portions of the test and details
the processing that was done. Estimates of lower measurement sensitivity levels were determined
based on PIV image quality, correlation, and noise level parameters used in the test. Comparison
of measurement results with linear acoustic theory are presented. The onset of nonlinear,
harmonic frequency acoustic levels were also studied for various decibel and frequency levels
ranging from 90 to 132 dB and 500 to 3000 Hz, respectively.
Analysis of Particle Image Velocimetry (PIV)
Data for Acoustic Velocity Measurements
Introd nction
Measurements were taken of acoustic velocities using Particle Image Velocimetry (PIV)
as part of the Advanced Subsonic Technology (AST) program at Langley Research Center. A
normal incidence tube configuration was used to set up standing wave acoustic fields which were
examined using PIV. The PIV technique, which is typically used to study global velocities in
aerodynamic and fluid dynamic flows at Langley, was used to measure acoustically driven
displacements on the order of 1-50 microns. The technique will be further refined to measure
global acoustic displacement levels in advanced acoustic liner concepts for flow enhancement and
noise reduction research efforts.
Normal Incidence Tube (NIT) Theory/Description
In order to test the feasibility of making acoustic velocity measurements with the PIV
technique, a series of tests were conducted using an acoustic chamber in a normal incidence
configuration. In such a configuration, an acoustic driver is placed at one end of the test cell and
a solid wall is placed at the other. The normal incidence tube design provides stationary and well
behaved acoustic standing waves according to the equations:
,,(x) =  [[V(L)Ie sin(k(L-x))] (1)
4(x) :  u(x) (2)
(3)
where u(x) is the displacement induced by the acoustic field, 9 is the gas density, c is the speed of
sound, co = 2rcf, fis the frequency, k is the wavenumber (c0/c), P(L) is the rms pressure at L, P0 is
the reference pressure of201LtPa, SPL is the measured sound pressure level in dB in the chamber,
and the e +i_t sign convention is being used. A vibrating displacement field is set up under these
conditions representing a sinusoidally varying standing wave pattern along the tube length. This
situation is depicted in figure 1.
Particle Image Velocimetry (PIV) System Description
Particle Image Velocimetry (PIV) is a non-intrusive global velocimetry/displacement
measurement technique that is well established. _ The technique typically uses a double pulsed
2
light sheet configuration to illuminate a seeded flow field of interest. Insertion of a precise time
delay between the two laser pulses produces temporally displaced particle images that can be
recorded on photographic or digital recording media. The temporally displaced particle images
are recorded as pairs of images, which can be exploited using image correlation techniques to
evaluate localized mean displacement/velocity levels in the flow with measurement accuracies
approaching 1% and measurement scales on the order of 1mm 2.
PIV measurement sensitivity levels are determined and limited by a number of system
parameters related to the imaging, laser illumination, seed, and data reduction techniques used.
The accurate measurement of acoustic displacement levels on the order of 1 micron in this test
required refinements of the existing Langley PIV system. 2,3 In particular, near-diffraction limited
imaging, synchronized (conditionally sampled) phase laser illumination, specialized seeding, and
enhanced data analysis and validation schemes were developed and implemented in this effort.
The acquisition system used in this test is depicted in figure 2. Two 580mj YAG lasers
were synchronized with the acoustic driver system, and provided light sheets 1 mm thick by 50
mm wide. Images of 2.3 micron hollow silica microsphere seed were captured using a near-
diffraction limited imaging system, and a Kodak Megaplus 1.4 digital camera system. An
approximate 1: 1 magnification level was used which provided for an approximate 7.23mm x
5.78mm field of view. Images were captured and transferred in binary 8-bit format using custom
interface and control software at a rate of approximately 7 images per minute.
PIV Analysis Process
A custom and automated PIV analysis system was used in this effort based on a PC/AT
486/120 platform incorporating an Alacron/i860 array processing system. The system is currently
capable of processing 5 autocorrelation displacement measurements per second, and completes an
entire image field in 1 minute with 50% oversampling A 128x128 pixel correlation size was used
to provide the needed accuracy levels. This translates to an approximate 0.7232mm spatial
resolution capability, and a 0.3616mm measurement resolution size. Near-diffraction limited
image sizes ranged from 4-15 microns which covered 1-4 pixels on the digital camera. This,
p.
along with excellent seed density levels of 10-30 image pairs per mm 2, resulted in correlation
signals with SNR in excess of 100:1. A weighted mean centroiding scheme was used, and
provided measurement sensitivity levels of 0.01 to 0.1 pixels, which corresponds to a lower limit
of approximately 0.2 to 0.3 microns.
PIV Data Validation
Data validation and post processing efforts also involved using custom and automated
algorithms based on a PC/AT 486/120 computer platform. Measurement validation rates in
excess of 99% percent were achieved based primarily upon the high image quality available in the
raw data. A four-tiered validation routine was used to reduce the analyzed data. This involved a
bandpass validator operation, followed by a local median validator, a 3x3 local mean filter, and
finally an image shift subtraction operation. The validation process was automated, relying only
minimally on operator input, and took approximately 1 second per analyzed data file. Following
data validation, the ensemble mean/standard deviation, and data convergence algorithms were
applied to the various data sets.
Results
Preliminary data convergence tests indicated an ensemble file number of 20 files to be
adequate for this preliminary phase of testing. A 500 Hz driver signal was studied first at
increasing decibel levels. Figures 3 and 4 show the results of these measurements. Ensemble
average values were obtained for each condition at each position of the flow. This represented a
19x 15 grid of spatially resolved displacement measurements for each condition. The mean and
standard deviation of the 19x15 grid values were then computed and are plotted in figures 3 and
4. Theory predicts a logl0 increase in relative displacement for increasing decibel level which is
shown in the logl0 plot of figure 4. A 6 dB increase effectively doubled the relative displacement
which agrees well with theory. This onset of harmonic frequency, and nonlinear influences was
expected at decibel levels above approximately 125 dB for 500 Hz, but no deviation away from
the linear fit was seen up to 132 dB levels.
A study of the phase dependence of the signal level was also done for 500 Hz, the results
of which are shown in figure 5. In this study the laser pulse separation timing was set to 1/2 the
wavelength of the driving function or 1/2*l/500Hz = 1/2*2ms = lms. This ensured maximum
displacement levels for all measurements. The relative phase of the driver and laser pulsing
systems were then varied at 1/16th wave increments (0.125ms). As expected, the sinusoidally
varying driver function resulted in a sinusoidally varying displacement trace with a phase shift
introduced by the speaker driver system. A sinusoidal fit to the data showed a Chi 2 value of
0.11959. An additional set of phase dependence data was analyzed for 1500 and 3000 Hz driver
p.
frequencies at 118 dB each. These results are shown in figures 6 and 7. Theory predicts an
inverse relationship between driving frequency and displacement amount which agrees well with
the results. A doubling in frequency effectively halved the amount of vibratory displacement. The
relative phases are also seen to be in synchronization with the driving frequency for both cases.
Summary
Over 1500 Particle Image Velocimetry (PIV) images were analyzed as part of a NASA
Langley effort make acoustic displacement measurements. To test the feasibility of the technique,
a series of measurements were taken in a Normal Incidence Tube (NIT) configuration at
frequencies between 500 Hz and 3000 Hz, and decibel levels from 90 dB to 132 dB. Custom PIV
analysis algorithms were developed and implemented in the test for data analysis, validation, and
post processing efforts. Estimates of measurement sensitivity were also made based on image
quality, data convergence, and ensemble statistic evaluations. Several examples of the amplitude,
frequency and phase dependencies of the standing wave patterns were presented.
Acknowledgments
The author wishes to thank W.M. Humphreys and S.M. Bartram of the Measurement
Science and Technology Branch for providing the PIV images, and for assistance and guidance in
development of the various algorithms used in this effort. Thanks are also extended to Dr. T.
Parrot of the Aero-Acoustics Branch (AAB), and Mr. Mike Jones of Lockheed for their guidance
and assistance with the Normal Incidence Tube, acoustic field theory/characteristics, and
microphone/tube calibration.
References
[ 1] Humphreys, W.M., Bartram, S.M., and Blackshire, J.L., "A Survey of Particle Image
Velocimetry Applications in Langley Aerospace Facilities," 31st Aerospace Sciences Meeting
& Exhibit, AIAA 93-0411, Reno, NV, January 11-14, 1993.
[2] Adriane, R. J., "Scattering Particle Characteristics and Their Effect on Pulsed Laser
Measurements of Fluid Flow: Speckle Velocimetry vs Particle Image Velocimetry," Appl.
Opt. 23, 1690 (1984).
[3] Lourenco L., and Krothapalli, A., "On the Accuracy of Velocity and Vorticity Measurements
with PIV,", Experiments in Fluids, 18, 421-428 (1995).
5
_(x) wall
I//
[//"
I/ /
"l
L
>
x
Laser
AD¢1°ustic I rl Light Sheet
__iiii::ii_ _t_ Vector Field
_:ili33:: Digital
/'_:::m .......Camera
_:. ;:'_::"!: :::::::::::::::::::::::::::::: _j
 2ji "
Cell Image !:_:!i!i!!iiii!!_!i_
_.,z.
_PC Computer
Autocorrelation
Analysis System
Figure 1. Acoustic Field incident on hardwall. Figure 2. Experiment layout.
5O
4O
3O
_2
1)
r-
2o
?5
10
500 Hz
7_12 phase
Decibel Level (dB)
Figure 3. Variation of Acoustic Displacement with increased Decibel Levels at 500 Hz.
?:5
500 Hz
X/2 phase
" i ...... i...... i .... i .... ----*-- Raw Data .... i...... !.... : -_ .... !..... i
.... ;...... _..... _ .... ] ', " " " 2"" " - ...... 19x15 e_b._nTt_leave
.... i ..... :.... i..I..i-. .:...... i.... i .................. ! .... _-,_id •
96 99 102 105 108 111 114 117 120 123 126 129 132 135 138
Decibel Level (dB)
Figure 4. Variation of Acoustic Displacement with Increased Decibel Level (Logl0 Scale).
10
5£
.o
ID
°
o
-5
-10
\J
0.2 0.4
500 Hz I0.05v => 115 dB
/
/
i I
0.6 0.8 1.0 1.4
Ensemble Ave
of ZO files each
w/averag© of all
285 vectors in
-- 19x15 ensenlhle Ave
v_ec_ i
t grd
\
i i
1.2
Relative Phase
Figure 5. Variation of Acoustic Displacement with Driver/Laser Pulsing Sy,'nchronization Phasc.
7
oe-,(D
¢3
e3
10
-5
-10
I Phase Dependence
, , , ,. ,. ,
i _ '. i ' '
L i i /
i ' i : iJ 4 ,,'_x- Z
'(/ i ....
_ _ 500 Uz _ l lS dg I I i
1
--_-- 3000 Hz/118 dB • : :
i
0.2 0.4 0.6 0.8 1.0 1.2 1.4
Relative Phase
Figure 6. Variation of Acoustic Displacement with Driver/Laser Pulsing Ss,nchronization Phasc for
1500 Hz and 3000 Hz at 118 dB.
8

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
January 1997 Contractor Report
;4. TITLE AND SUBTITLE
Analysis of Particle Image Velocimetry (PIV)
Data for Acoustic Velocity Measurements
6. AUTHOR(S)
James L. Blackshlre
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Vlgyan, Inc.
30 Research Dr.
Hampton, VA 23666
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Langley Research Center
Hampton, VA 23681-0001
5. FUNDING NUMBERS
C NASI - 19505
WU 538-03-12-04
8. PERFORMING ORGANIZATION
REPORT NUMBER
10. SPONSORING / MONITORING
AGENCY REPORT NUMBER
NASA CR-201664
11. SUPPLEMENTARY NOTES
Langley Technical Monitor : Richard K. Antcliff
Final Report
12a. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified - Unlimited
Subject Category 36
12b. DISTRIBUTION CODE
13. ABSTRACT(Maximum200words)
Acoustic velocity measurements were taken using Particle Image Velocimetry (PLY)
in a Normal Incidence Tube configuration at various frequency, phase, and
amplitude levels. This report presents the results of the PlY analysis and
data reduction portions of the test and details the processing that was done.
Estimates of lower measurement sensitivity levels were determined based on PIV
image quality, correlation, and noise level parameters used in the test.
Comparison of mesurements with linear acoustic theory are presented. The onset
of nonlinear, harmonic frequency acoustic levels were also studied for various
decibel and frequency levels ranging from 90 to 132 dB and 500 to 3000 Hz, p
respectively.
14. SUBJECT TERMS
Particle Image Veloclmetry, Acoustic Velocity, Normal Incidence
Tube, Phase Matched Detection
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Analysis of Particle Image Velocimetry (PIV) Data for Acoustic Velocity Measurements

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