NASA/TM--2001-210698 AIAA-99-0641

Rayleigh Scattering Diagnostic for Dynamic

Measurement of Velocity and Temperature

R.G. Seasholtz

Glenn Research Center, Cleveland, Ohio

J. Panda

Modern Technologies Corporation, Middleburg Hts, Ohio

Prepared for the

37th Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 11-14, 1999

National Aeronautics and

Space Administration

Glenn Research Center

March 2001
Acknowledgments

We would like to acknowledge the diligent efforts of Mr. W. Trevor John, who was responsible for setting up and

aligning the optical system used for this work.

Available from

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Available electronically at http://gltrs._c.nasa.gov/GLTRS
Rayleigh Scattering Diagnostic for Dynamic Measurement of Velocity and Temperature

R.G. Seasholtz

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

J. Panda

Modem Technologies Corporation

Middleburg Hts., Ohio 44130

Abstract

A new technique for measuring dynamic gas

velocity and temperature is described. The technique is

based on molecular Rayleigh scattering of laser light, so

no seeding of the flow is necessary. The Rayleigh

scattered light is filtered with a fixed cavity, planar

mirror Fabry-Perot interferometer. A minimum number

of photodetectors were used in order to allow the high

data acquisition rate needed for dynamic measurements.

One photomultiplier tube (PMT) was used to measure

the total Rayleigh scattering, which is proportional to

the gas density. Two additional PMTs were used to

detect light that passes through two apertures in a mask

located in the interferometer fringe plane. An

uncertainty analysis was used to select the optimum

aperture parameters and to predict the measurement

uncertainty due to photon shot-noise. Results of an

experiment to measure the velocity of a subsonic free

jet are presented.

Introduction

A variety of nonintrusive, laser based flow

diagnostics (Rayleigh scattering, LDV, PIV, DGV, LIF)

are routinely being applied for time average and

instantaneous planar measurements of velocity, density,

temperature, and species concentrations. However, an

important class of measurements is not addressed by

current laser diagnostics. High frequency response

dynamic measurements of flow parameters cannot be

made with existing techniques. This type of time

history data is needed to determine, for example,

density and velocity spectra, density-velocity

correlations, and two-point correlations. Although

Laser Doppler Velocimetry (LDV) can typically

achieve data rates of a few tens of kHz, it is difficult to

achieve much higher rates without introducing larger

amounts of seed material into the flow. In practice,

LDV normally provides mean velocity and a measure of

turbulence intensity. Furthermore, the measurements

are random in time, which makes it difficult to obtain

time history data needed for power spectra. In turbulent

flows, LDV measurements are beset by a variety of so

called biasing errors, caused by correlations between

the measurement rate and flow properties. Planar

techniques, such as PIV, provide a large number of

simultaneous measurements in the measurement plane,

but are generally limited to low sampling rates,

determined by the pulse repetition rate of the laser and

by the time needed to transfer image data from the

camera.

New techniques are needed to provide

nonintrusive, dynamic measurements that can provide

data similar to that provided by hot wire anemometers.

Since it unrealistic to expect (at least at the present

time) to make measurements at a large number of

locations and at a high sampling rate, it seems prudent

to develop a laser diagnostic capable of point

measurements at a high sampling rate. Such time

resolved point measurements are in a form familiar to

fluid dynamics researchers.

In this paper, we describe a laser diagnostic for

dynamic measurement of gas properties based on

molecular Rayleigh scattering. Because the measurement

is based on molecular scattering, rather than particle

scattering, no seed material need be injected into the flow

under study. One difficulty with LDV and PIV is that

they require the flow to be "seeded" with micron sized

seed particles to provide a sufficient concentration of

scattering centers. The reliance on seed particles presents

a number of difficulties. It is difficult to inject a uniform

cloud of seed into many test facilities. And the seed

material must withstand the flow environment. For high

temperatures, refractory seed materials are necessary. In

addition, seed material can contaminate the facility,

coating surfaces and windows. Facility engineers are

often wary of the introduction of this foreign, often

abrasive material, into their equipment. A further

limitation of particle scattering methods is that the

particles, although small, may not be able to follow large

flow accelerations, thus introducing inaccuracy in the

prediction of the gas velocity.

NASA/TM--2001-210698 1
Theseproblemsassociatedwith particle

scatteringmeasurements areeliminatedif molecular

scatteringisused,sincethegasmoleculesthatconstitute

theflowunderstudyareusedasthescatteringcenters.

Thesimplestmolecularscatteringbaseddiagnostics

Rayleighscattering. ThefrequencyspectrumofRayleigh

scatteringiscloselyrelatedtothevelocitydistributionof

thescatteringgas.Thespectrumaybeanalyzedto

determinetemperature, dnsity, andvelocity.Densityis

simplyproportionalto the total scatteredlight;

temperatureisobtainedfromthewidthof theRayleigh

spectrum; andonecomponent of velocityisproportional

to theshiftof theRayleighspectralpeakfromthe

frequencyof theincidentlight. Becausethespectral

widthisalsoafunctionof themolecular weightof the

gasesin theflow,knowledgeof thegascompositionis

generallyrequired.This,however, isnotaconcernithe

proposedwork,whichisdirectedtowardairflowswhere

thecompositionis well defined.However,because

velocityisdeterminedfromthepeakof spectrum, it is

independent ofthegascomposition. Rayleighscattering

isparticularlysuitableformeasurement ofsupersonic and

hypersonicvelocities, wherethemeanmolecular velocity

(flowvelocityJis greaterthantherandommolecular

velocity(temperature).Onequantityof particular

interestinthestudyofaerodynamically generatedsound

that can be obtainedfrom Rayleighscattering

measurements is pV (the product of gas density and

velocity).

The key to obtaining high frequency response

data is to combine a high power, continuous wave laser,

or a high repetition rate pulsed laser, with high efficiency

collection optics. Density measurements are relatively

straightforward, since the Rayleigh scattered light is

directly proportional to the density. Velocity and

temperature measurements are more challenging because

they must be determined from the spectral characteristics

of the Rayleigh scattered light.

Both atomic/molecular vapor absorption filters

and Fabry-Perot interferometers can be used to measure

spectral characteristics that are related to the gas

velocity and temperature. Iodine vapor is commonly

used as an absorption medium because it has a rich

absorption spectrum in the green wavelength region t.

Use of absorption filters requires a tunable laser that

can be tuned to a suitable absorption line. One

approach for obtaining real-time velocity measurements

is to use an absorption filter to analyze frequency

modulated Rayleigh scattered light 23. A Faraday cell

has been used as a frequency discriminator for seeded

flows 4. Fabry-Perot interferometers, on the other hand,

act as bandpass filters. They are quite flexible in that

they can be used at any laser frequency, and the width

of their passband can be easily adjusted. However, the

transmission is also a function of the angle of the light

beam in the Fabry-Perot interferometer cavity relative

to the optical axis. Fabry-Perot etalons have been

proposed for analyzing Rayleigh scattering from the

atmosphere to obtain wind velocity and temperature -_.

The approach taken in the present work is to

use a Fabry-Perot interferometer with fixed mirror

spacing. Characteristics of the transmitted light are

measured using a minimum number of fast photo

detectors. This allows a high sampling rate to achieve

the desired dynamic response.

We first establish the lower bounds for

uncertainty in the measurements of velocity and

temperature caused by photon statistical noise. We then

examine a particular scheme based on a Fabry-Perot

interferometer and three photomultipler tubes and

determine the lower bounds for measurement errors. A

similar approach has previously been used to determine

lower bounds for temperature and velocity uncertainties

for measurements obtained with a scanning Fabry-Perot

interferometer 6, as well as for an imaging system 7. The

results of a preliminary experiment to evaluate this

technique are then presented.

Theor_

For the purposes of this paper we consider

Rayleigh scattering from a low density gas, which has a

Gaussian spectrum given by

24__ [ r27r(f-f )- L

where fo is the laser frequency and u is the mean gas

velocity. Note that the spectral peak is shifted by a

frequency proportional to the component of the bulk

velocity in the K direction. The spectral width is

proportional to the square root of the gas temperature.

It is convenient to introduce the velocity component uK

= K. u/K, which represents the measured velocity

component. The interaction wave vector is K = ks-ko

(with i% and ks being the wave vectors of the incident

and scattered light), and a = (2x'l"/m) _/'- is the most

probable molecular speed (with _c being Boltzmann's

constant, m the molecular mass, and T the gas

temperature). (Note that a = (2/7) 1_ c_, where c_ is the

speed of sound and 7is the ratio of specific heats.).

The assumption of a Gaussian shaped Rayleigh

scattering spectrum is only valid if

y=_P << 1

rIKa

where p is the gas pressure and r/ is the shear viscosity.

For higher density gases O' - 1), a more detailed kinetic

NASA/TM--2001-210698 2
theorymodel,suchastheTenti $6 model s is required to

describe the Rayleigh scattering spectrum. To simplify

the analysis presented here, we use the Gaussian

spectral model. However, as shown in Figure 1, the

Tenti spectrum differs significantly from the Gaussian

spectrum for the conditions encountered in this work (3'

= 0.79). A more accurate analysis would use the Tenti

spectral model.

Since Rayleigh scattering is a relatively weak

process, the uncertainty in the measurements often is set

by the photon statistical noise (shot noise), which

determines the lower bound on measurement

uncertainty. For example, the variance in the number of

photoelectron counts for a Poisson process is equal to

the mean number of counts. Thus the lower bound for

the relative uncertainty in the measurement of gas

density p, is equal to the square root of the variance

divided by the mean number of counts. The relative

uncertainties can be va'itten 69

o,o, P tN_) ' o,w, r = , aO,_)= a (2) (2NR)'l"

These relations provide a lower bound for the

measurement uncertainties, which can only be

approached if the shot noise is the dominant noise and if

an ideal instrument is used. Note that the lower bounds

for the uncertainties of density, temperature, and

velocity are all inversely proportional to the square root

of the number of detected photons.

Estimates of the measurement uncertainty for

the technique described here, where the Rayleigh

scattered light is analyzed with a planar mirror FabryPerot interferometer, are obtained by numerically

calculating the Cramer-Rao lower bound j°. The

variance of the estimate of a parameter a, (e.g.,

temperature or velocity) is given by

],,

where, for Poisson statistics, F is the Fisher information

matrix with elements

and the summation is over the number of

photodetectors. For example, in previous work tl using

a CCD array detector, the summation would be over the

pixels in the image being analyzed. For this work,

however, a CCD camera cannot be used because of its

slow readout rate (typically on the order of I second for

scientific grade cameras). To achieve the high data

acquisition rate desired for dynamic measurements, we

use photomultipliers tubes (PMTs), which combine

reasonable quantum efficiency with high gain and lownoise. One PMT is used to measure the total Rayleigh

scattered light (i.e., the spectrally integrated light),

which gives a measurement of the gas density. Two

additional PMT's are used to detect the Rayleigh

scattered light that is transmitted through a planar

mirror interferometer.

Consider an experiment with the following

parameters: a laser (532 nm) with output power Po = 4

W, air at STP, probe volume length L, = 1 mm, f/4

collecting optics (i.e., solid collection angle f_ = 0.05

sr), and efficiency factor e = 5 %. The rate of detected

photons, given by

Ne e_PonL,_'_( dtY lsin"

= hc -d-dJ -

= 39 million counts / sec

Z

(5)

In this equation, n is the gas number density, d_/d..Q is

the differential scattering cross section, Z is the angle

between the electric field vector of the (linearly

polarized) incident light and the direction of the

scattered light, h is Pianck's constant, and c is the

velocity of light. If we wish to sample at a 10 kHz rate,

the total number of detectable photons in each sampling

period would be 3900. Equation 2 gives the lower

bound for measurement of density, temperature and

velocity for each sample period:

a (P)-1.6% ' a(r) - 2.3 % ' a(UK)=3.6m/sec

p T

This shows that high sampling rate Rayleigh scattering

(3) measurements are at least feasible. It must be

emphasized, however, that these values represent the

best possible measurements given an ideal instrument.

In practice, we are limited to instruments, such as the

Fabry-Perot interferometer used here, which result in

significantly higher uncertainties, as described below.

(4) Achievement of the uncertainties given above will

require a photoelectron detection rate considerably

above those given in this example. This can be

achieved by using higher laser power, larger collection

f-number, or higher quantum efficiency photodetectors.

Optical configuration

In order to perform an uncertainty analysis, we

first describe the setup (fig. 2) used for the experimental

work. Light from a 5W, 532 nm, single-frequency,

NASA/TM--2001-210698 3
Nd:VanadateCWlaserisfocusedbylensLI (300mm

focallength)to a 150_tmdiameterbeamattheprobe

volume.The90° Rayleighscatteredlightiscollected

andcollimatedbylensL2(250mmfocalength)andis

thensplitintotwopathswithanuncoatedopticalfiat.

About10%of thelightisreflectedandfocusedbylens

L3ontoPMT1(typeR3896).Thisdetectedsignalis

proportional togasdensity.Lightinthesecondpathis

directedthrough a planar mirror Fabry-Perot

interferometer (70mmdia.mirrors,90%reflectivity,

15 GHz free spectral range). The light exiting the

interferometer is focused by the fringe forming lens, IA

(135 mm focal length, f/2). An addition lens L5 (50

irma focal length, f/1.2) is used to form a magnified

image of the fringe pattern. The effective focal length

of combination of lenses L4 and L5 is 1823 mm. The

fringe pattern formed at the focal plane of this lens pair

is analyzed to determine the velocity and temperature.

Typical calculated images of the inner fringe of

Rayleigh scattered light from a gas with zero bulk

velocity and a temperature of 300 K are shown in figure

3. One image shows Rayleigh scattered light from an

extended region, which results in the full circular fringe;

the other image shows the image resulting from a line

source as was used in the experiment.

Because, in the experiment, the laser beam is

locused to a line in the probe volume, the fringe exists

only along that line. Also, to simplify the setup and to

use a minimum number of photodetectors, only one half

of the fringe pattern is utilized. A mask having two

apertures, shown as white rectangles in figure 3 and

referred to as the "inner" and "outer" regions, is located

in the fringe plane. Light passing through the inner and

outer regions is directed to two PMT's (type 8645). As

the velocity and temperature of the gas change, the

relative distribution of the light detected by the two

PMT's changes. Note that a flow in the direction

opposite the K vector results in a negative frequency

shift. Use of additional detectors would improve the

nleasurement uncertainty, but at an increase in the

system complexity.

Additional optics were included to provide for

a reference image consisting of light at the unshifted

laser frequency. To accomplish this, several

components could be placed in the optical path. When

placed in the beam, mirror M directed laser light

through neutral density filter NDF onto a diffuser

assembly that could be inserted at the measurement

location. This assembly consisted of a diffuser that

scattered light into the collection optics and an iris to

limit the spatial extent of the light. Finally, a prism

assembly could be placed in the light path between the

Fabry-Perot interferometer and the PMT's. This served

to direct light into a standard video camera. The video

signal from this camera was digitized by a frame

grabber card in a 486 PC. A computer program

analyzed this image and generated signals to control the

Fabry-Perot mirror alignment. The mirror M, diffuser

assembly, and prism assembly were mounted on

remotely controlled pneumatic linear actuators.

Uncertainty analysis

We now calculate the Cramer-Rao lower

bounds for velocity and temperature measurements

based on the experimental setup described above. This

allows us to conduct parametric studies to determine the

optimum configuration for Fabry-Perot interferometer

and for the light detection system. The observed fringe

pattern for Rayleigh scattered light can be expressed as

< Noq > = II ARSR(f)IFp (f' O) df dr2 (6)

where <NDq> is the expected number of detected

photons in the q" area, AR is the number of photons that

would be detected if the Fabry-Perot interferometer

were removed from the system, and Irp is the FabryPerot instrument function given by l"

Ire(V ) II + Fsin"( I#'/] -t

= (7)

2 )j

where _ is the phase change (neglecting any phase

change on reflection) of the light between successive

reflections given by

(f ,O_)- 47r f lt dcosO_ (8)

C

Here, _ is the refractive index of the medium in the

Fabry-Perot cavity, d is the Fabry-Perot mirror spacing,

0, is the angle between the ray and the optic axis, and F

= 1/(sin'-(rt/2ND where AlE is the effective finesse. In

general, the image of a monochromatic extended source

located in the object plane consists of a series of

unequally spaced concentric rings. In this work,

however, the field of view is restricted to include only

the inner fringe as shown in figure 3.

We now calculate the lower bounds for

velocity and temperature measurement uncertainties

based on the detected light from the inner and outer

regions. The uncertainties are evaluated as a function

of two parameters no and nf, which characterize the two

active regions in the fringe plane. Because the number

of photoelectron counts from the two regions is a

function of both temperature and velocity, a

NASA/TM--2001-210698 4
measurement ofthephotoelectroncountsfromasingle

regioncannotbeusedtodeterminevelocity.

Foragivenwavelength_o,theorderofinterference atthe

centerofthefringepatternisgivenby2/.td=(n+n,)/2_,

where n and n,. represent the integral and fractional parts

of the order, respectively 12. A bright fringe is located on

the optical axis if n, = 0. Off axis, the order of

interference at angle 0 is given by 2/.td cosO = (n + n,-

n_)/_, where n_ represents the change in the order of

interference between optical axis and the angular radius

0. (Note that a bright fringe occurs at n_ = n, ). For

small angles, nk = /ado "/2o • The spectral interval

included in the region from the optical axis out to 0 is

n_"FSR, where the free spectral range FSR = c/2d. It

follows that the spectral interval between orders nkl and

nk: is (n_ - nk_ )FSR. For the purpose of analysis, it is

convenient to use the fractional order n_ to describe

locations in the Fabry-Perot fringe plane. Because we are

working in air, we assume that p =1 in the remainder of

this analysis.

We can now define the boundaries of the

fringe plane mask, as shown in figure 3, in terms of

fractional orders: no is the fractional order of the fringe

corresponding to the laser wavelength; (no- nf) and (no

+ nf) correspond to the inner boundary and outer

boundaries of the central

corresponds to the outer

(Here we assume that the

located at fringe position.)

stop, respectively; and n,,

boundary of the aperture.

central stop of the mask is

Note that the actual radius

of a fractional order n_ in the focal plane of the fringe

forming lens is given by the product the angle 0_ = (_.o

n_/d )_/'_ and the focal length of fringe forming lens.

Also note that the image of the fringe pattern projected

back to the probe volume is similarly given by the

product of the angle and focal length of the collimating

lens. For the mask used in this work, the active area of

the probe volume is about 1 mm long.

The lower bounds for velocity and temperature

were numerically evaluated as a function of the

parameters no and nf, which characterize the dimensions

of the two active areas. The results are shown in figure

4. These plots are for a particular velocity (0 m/sec)

and temperature (300 K). The order of the outer

boundary of the outer aperture was n,, = 0.341. Note

that the minimum uncertainties for velocity and

temperature occur at different values of no and nf. It can

be seen that the values of no and nf that result in a

minimum velocity uncertainty (about 13 rn/sec) are

0.087 and 0.027, respectively. On the other hand, the

minimum temperature uncertainty (about 34 K)

corresponds to no and nf values of 0.14 and 0.08,

respectively. Other temperatures and velocities would

result in other optimum values of no and rtf. For this

study, we chose the configuration that results in the

minimum velocity uncertainty at the expense of higher

temperature uncertainty. We show in figure 5 the

velocity and temperature uncertainties as a function of

velocity and temperature for no = 0.087 and nf = 0.039

(close to values that result in minimum velocity

uncertainty at 0 m/sec and 300 K). The minimum

velocity uncertainty is about a factor of four greater

than that calculated above for an ideal instrument, and

the minimum temperature uncertainty is about a factor

of five greater than that of an ideal instrument. (Recall,

however, that the minimum velocity and temperature

uncertainties do not occur for the same values of no and

l_f. ).

Experiment

The optical system described above and shown

in figure 2 (with the laser output power equal 5 W) was

used to measure the flow in the potential core of a

subsonic free jet (15.9 mm exit diameter). The

laboratory compressed air supply used for the jet was

passed through a 0.2 lam filter to remove particles. As

shown in figure 2, the jet axis bisected the incident and

scattered light directions. With this configuration, the

measured velocity component was the jet axial

component. Since the flow velocity was opposite the K

vector, the frequency shift was negative. Thus the

Fabry-Perot interferometer fringe diameters of the

Rayleigh scattered light decreased as the flow velocity

increased. That is, as the velocity increased, the signal

from the inner region increased, while the signal from

the outer region decreased. An electronic pressure

gauge was used to measure the total pressure in the

nozzle plenum. These pressure measurements were

used to calculate the flow velocity and temperature

using the isentropic flow relations.

Signals from the three PMT's were digitized at

a 50 kHz rate with a digital oscilloscope. For this work,

the PMT signals were fed directly into the 1 MI2

oscilloscope inputs. The data record length was 5 sec,

which corresponded to 250,000 samples on each of the

three channels. These data were stored on the

oscilloscope's hard disk for later processing.

Data processing

The data processing scheme was based on the

model described above. To account for gas density

changes, the signals from PMT2 and PMT3 were first

normalized by the signal from PMTI. The next step

was to find the gain factor that related the measured

signal voltages to the number of photoelectrons counts

obtained from the model. To do this, an initial data set

was taken at a known flow condition, usually zero, or

close to zero, velocity and a known temperature. The

NASA/TM--2001-210698 5
gain factors were obtainedby matchingthe

photoelectroncountsfor thetwo channelsfromthe

modelwith the measuredvalues. Notethat this

calibrationonlyrequiresameasurement atasingleflow

condition.Thedatatakenatotherflowconditionswere

thenanalyzedusingthetheoretical modeltoobtainboth

velocityandtemperature. Figure6 showsthegridof

predictedvelocityandtemperatureasafunctionofthe

signalsfrominnerandouterregions.Themeasured

valuesoftheinnerandoutersignals(exampleshownas

squaresonfigure6) wereconvertedto velocityand

temperatureby interpolatingbetweenthe constant

temperatureandtheconstantvelocitycontours.Note

thatthemeasuredvaluesgenerallyfollowthemodel.

As discussed above, the system was optimized for

velocity measurement near zero velocity.

Experimental results

A typical time record of the digitized signals

from the three PMT's is shown in figure 7. The

bandwidth of the signals was set by the RC time

constant of the PMT's and associated coaxial cables'

capacitance and the 1 M_ oscilloscope input

impedance (1.12 ms for PMT 2 and PMT 3, determined

using a pulsed LED as the light source). In figure 8, we

show an example of a time record obtained while the jet

total pressure was varied. The inner and outer signals

are shown along with the velocity obtained from them.

This illustrates the real-time capability of the technique.

In figure 9, we show two sets of velocity data taken at

different fixed jet velocities. The velocity obtained

from the Rayleigh scattering measurements is shown

along with the velocity obtained from the measured

total pressure and isentropic flow relations. The flow

velocity range was from zero to about -100 m/sec

(negative because the frequency shift is negative). The

point used to obtain the calibration is shown on each

set. It should be noted that the total pressure

measurement only had a resolution of 0.01 psia, thus the

isentropic velocities could have a larger error at low

velocities. These results are based on the average time

record over the 5 second time interval. The first set

shows good agreement, including the last point, which

was taken at the same flow condition as the first point.

(The fn-st point was used as the calibration point.) The

second set of measurements shows poor agreement if

the first point is used as the calibration point. If the last

point, however, is used as the calibration point, better

agreement is obtained. The disagreement at the low

velocity may be a result of the inaccuracies in the

velocity obtained from the total pressure at low

velocity.

Concluding remarks

Molecular Rayleigh scattering appears to be a

feasible technique for the dynamic measurement of gas

density, velocity, and temperature. There are, however.

several requirements for obtaining successful

measurements. First, a high power laser source is

needed to generate a sufficiently large Rayleigh

scattering signal to overcome the inherent statistical

photon noise. Second, the maximum amount of the

Rayleigh scattered light must be collected and detected,

which means fast collection optics coupled with large

diameter Fabry-Perot mirrors should be used with lownoise, high quantum efficiency photodetectors. The

Fabry-Perot interferometer is extremely sensitive to any

outside disturbances, including acoustic noise,

mechanical vibration, and temperature changes. It is

highly desirable to locate the Fabry-Perot in a quiet,

stable environment, with the Rayleigh scattered light

transmitted through fiber optics from the test facility.

An automatic system for maintaining the interferometer

alignment is also essential.

Two requirements must be met for the

successful implementation of the technique described.

One is the elimination of particles in the flow. The

other is preventing any stray scattered laser light from

reaching the photodetectors. These requirements will

probably restrict this technique to unconfined flows

unless extraordinary measures are taken. It should be

noted that particle and stray light can be handled if

more details of the spectrum are measured, which

means the use of additional detectors. The system

described here was designed to use the minimum

number of detectors.

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Academic Press, New York, 1971, pp. 324-231.

1_Seasholtz, R.G., Rayleigh scattering diagnostic for

measurement of temperature and velocity in harsh

environments, AIAA 36 th Aerospace Sciences

Meeting, AIAA-98-0206, Reno, NV, Janl2-15,

1998.

i,-Vaughan, J.M., The Fab_y Perot lntetferometer,

Histol S, Theot3,, Practice and Applications, Adam

Hilger, Bristol, 1989, pp. 89-112.

NASA/TM--2001-210698 7
06

05

-2

<e04

03

N

02

E

Z Ol

o0

Gaussian

/

_ Tenti $6

, \,\

,, \

\\,

1 2

Frequency, GHz

Fig. 1 - Comparison of Gaussian and Tenti $6 Rayleigh scattering spectra; y = 0.79.

PMT 2

PMT 3 MASK

LASER

VIDEO

CAMERA D

PMT 1

%

NDF

NOZZLE

L5 L4 _ FPI BS L2

PRISM

ASSEMBLY

=_ L1

DIFFUSER

L ASSEMBLY

LIGHT

TRAP

Fig. 2 - Setup for Dynamic Rayleigh scattering experiment.

NASA/TM--2001-210698 8
0 50 1 O0 150 200

0 50 I00 150 200

/ '\

Fig. 3 - Fabry-Perot interferometer fringe pattern with mask showing two active areas; no = 0.087, nc = 0.039, nrn =

0.341; left image shows full fringe for extended source; right image shows fringe for line source used in experiment.

NASAJTM--2001-210698 9
¢-

0040 ,I / ,/ ,/ ,fJ-X _" ,\

f ,,'I/,/,,'" \ ,, \

_133 \ \ "i_ 12.6 --13.4\' _,

0025" _\_ /

0080 0085 0.090 0.095

(a)

0.020"

'k

/

,! i

1,3/

/135

13.6

1

0100

n o

(b)

o lO

009

0.08'

007 "

006

"-,,.'I

_-_ "23 ........_-J

_ 18-.

,5.,.

012

=

0 14 0 16 018

(c)

O04O

0 035

0030

0020 '

x

0 080 0 085 0 090 0 095 0 t OG

(d)

010

0.09

0.08 '

007 "

006 "

0 12 014 016 018

,'1o

Fig. 4 Lower bounds for uncertainties for velocity and temperature as a function of no and n_ (a) and (b) show

contours of velocity uncertainty in rn/sec; (c) and (d) show contours of temperature uncertainty in K" x on (a) and

(c) at values of no and nt; used for experiment

_o. _ _ \

\

14

o- "'_12

_ -50"

_ 15 _

(a)

150 24/11/27//"I"

50-

0  ̧

E

. -50"

>

-100"

(b)

45

50

55

60 65

90

85

75

"- " " - .80 "

i i r ,

220 240 260 280 300 320 220 240 260 280 300 320

Temperature, K Temperature, K

Fig. 5 - Uncertainty for velocity (a) and temperature (b) as a function of velocity and temperature for no= 0.087 and

nr = 0.039.

NASA/TM--2001-210698 l0
275

250

225

09

I-- 200

z

0

ro 175

Lu

I"-" 150

O

125

10o

75

T K

35O

"__ V = -100 m/sec

, I , , , , I , , , , I , , , , I , , , i I , , i , I i i , , I , , i , I , , , , I , ,

350 400 450 500 550 600 650 700 750

INNER COUNTS

Fig. 6 - Plot of signal from inner and outer active regions as a function of velocity and temperature for no

and nc = 0.039. Squares mark location of experimental measurements.

=0.087

Inner

Density

Fig. 7 - Typical PMT signals (density is divided by l 0), sampling rate = 50 kHz.

NASA/TM--2001-210698 11
600

55O

u_

oO 450

t-- 400

350

"_ 300

0

250

,.C

13_ 200

150

OUTER

.,00 ....... /

__ • • • , I , , , , I , , , I , , , , I , , , , I

> 0 1 2 3 4 5

Time, sec

Fig. 8 - Time history of signal from two PMT's and velocity obtained from these signals. Velocity was manually

changed over the time period.

E

'G

o

>

20-

0"

-20"

_10"

-60-

-80-

-100

/ Calibration point

o-- tsentropic

_> Rayleigh

cz /

...LS: ;/

L_Io

"'0 (

B

!

/

i J i _ i i

1 2 3 4 5 6

Reading Number

0

-20

E

- -40

>

-8o

-lOO

(:,

--o-- Isentropic

o_ --o-- Rayleigh

I I I I I I I I I

10 11 12 13 14 15 16 17 18

Reading number

Calibration point

Fig. 9 - Comparison between time average velocity obtained from isentropic flow relations and from Rayleigh

scattering measurements.

NASAfI'M--2001-210698 12

REPORT DOCUMENTATION PAGE FormApprove,J

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated lo average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

March 2001 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Rayleigh Scattering Diagnostic for Dynamic Measurement of

Velocity and Temperature

6. AUTHOR(S)

R.G. Seashoitz and J. Panda

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-274-00-00-(O

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12627

10. SPONSORING/MONITORING

AGENCY REPORTNUMBER

NASA TM--2001-210698

AIAA-99--0641

11. SUPPLEMENTARY NOTES

Prepared for the 37th Aerospace Sciences Meeting and Exhibit sponsored by the American Institute of Aeronautics and

Astronautics, Reno, Nevada, January, 11-14, 1999. R.G. Seasholtz, NASA Glenn Research Center; J. Panda, Modem

Technologies Corporation, Middleburg Hts, Ohio 44130. Responsible person, Richard G. Seasholtz, organization

code 5520, 216--433-3754.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 35 Distribution: Nonstandard

Available electronically at htm://gltrs.m'c.nas_,gov/GLTRS

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390.

13. ABSTRACT (Maximum 200 words)

12b. DISTRIBUTION CODE

A new technique for measuring dynamic gas velocity and temperature is described. The technique is based on molecular

Rayleigh scattering of laser light, so no seeding of the flow is necessary. The Rayleigh scattered light is filtered with a

fixed cavity, planar mirror Fabry-Perot interferometer. A minimum number of photodetectors were used in order to

allow the high data acquisition rate needed for dynamic measurements. One photomultiplier tube (PMT) was used to

measure the total Rayleigh scattering, which is proportional to the gas density. Two additional PMTs were used to detect

light that passes through two apertures in a mask located in the interferometer fringe plane. An uncertainty analysis was

used to select the optimum aperture parameters and to predict the measurement uncertainty due to photon shot-noise.

Results of an experiment to measure the velocity of a subsonic free jet are presented.

14. SUBJECT TERMS

Rayleigh scattering; Fabry-Perot interferometers

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

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Unclassified

NSN 7540-01-280-5500

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OFPAGES

18

16. PRICE CODE

A03

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102