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Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.

JlfAAA98-32701

AIAA 98-2503Application of the PSP for Investigationof the Oscillating Pressure Fields

S. Fonov, V. Mosharov, andV. RadchenkoTsAGI, ZhukovskyMoscow, Russia

R. Engler and C. KleinDLRGottingen, Germany

20th AIAA Advanced Measurementand Ground Testing Technology

ConferenceJune 15-18, 1998/Albuquerque, NM

For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics1801 Alexander Bell Drive, Suite 500, Reston, VA 20191


AIAA-98-2503

APPLICATION OF THE PSP FOR INVESTIGATION OF THE OSCILLATING PRESSURE FIELDSS.Fonov*, V.Mosharovf, V.Radchenkof

TsAGI, Zhukovsky, Moscow reg., Russia

R.H.Engler', Chr.Kleint

DLR, Gottingen, Germany

Summary

The paper reviews the main problems encounteredduring investigations of the Oscillating PressureFields by the Pressure Sensitive Paint (PSP)technique. Measurement methodology, theoreticaland experimental estimations of the PSP responsetime are presented. It is shown that currentlyavailable pressure sensitive paint formulations can beused for oscillation frequencies up to 20Hz. The firstexperimental results obtained in the Transonic WindTunnel of DLR, Gottingen (TWO), on the pitchingwing model are presented.

1. Introduction

The Pressure Sensitive Paint (PSP) method provides agood opportunity for investigation of unsteady flows.Dynamic parameters of PSP (response time) andmeasurement system (time, amplitude and spatialresolutions) will determine the ultimate accuracy andshould be taken into consideration.

To measure periodical non-harmonic pressurefluctuations the PSP has to transfer several harmonics(about ten) of these fluctuations. Measurementsystem should provide luminescence acquisition inthe appropriate time intervals. Standard ImageDetector - digital CCD - camera can be used forperiodical pressure measurements, bound oscillationof a model, rotating machinery etc., in an integrationmode in combination with pulsed illuminatoroperating in stroboscopic mode synchronized with themain oscillating frequency. Maximal oscillatingfrequency in this case is restricted by the responsetime of PSP and excitation light pulse duration. If theinvestigated process is non-periodical, a fast ImageDetector can be used in combination with continuousor pulsed illumination of the model.

2. Background

PSP is two-dimensional array of oxygen sensors, eachof them consisting of the polymer layer with theluminophore molecules dissolved in it. Thesensitivity of PSP to the air pressure level is due toquenching of excited luminophore molecules by theoxygen of the air. Quenching of luminescence iscontrolled by the diffusion of the oxygen in thepolymer binder of PSP. Some PSP characteristics arediffusion-controlled, i.e. pressure sensitivity,temperature sensitivity, time response of PSP to thechange of external pressure, spatial resolution of PSP.

An analysis of these diffusion-controlledcharacteristics was presented by V.Mosharov1 et al.in 1994 and similar analysis was later performedindependently by M.Morris2 et al. To estimatediffusion-controlled characteristics of PSP weassume, that PSP has linear calibrationcharacteristics:

I0 =l/(a+bp); (1)

where lo and / are reference luminescence intensityof PSP at some reference pressure, and at air pressurep, a and b are coefficients of pressure sensitivity ofPSP. All the processes would be considered asisothermal to simplify analysis.

Luminescence output of PSP is an integral along thethickness of the PSP layer:

0£-c-lQ-aMn(10) f

•n(t,x,y,z)/Q.21a(2)

where Iex is excitation light intensity, 3>0 isluminescence quantum yield at the referencepressure, e is the extinction coefficient ofluminophore, c is the concentration of luminophore inPSP, a is the solubility of the oxygen in the PSPbinder polymer (0.21 is a percentage of the oxygen in

* AIAA Senior Member, Head of Image Processing Groupf Research Scientist'Head of PSP GroupCopyright ©1998 by the American Institute of Aeronautics and Astronautics, Inc.

1American Institute of Aeronautics and Astronautics


the air), n is the oxygen concentration at PSP point(x,y,z) at the moment t and h is the thickness of PSPlayer. Value x=0 corresponds to the PSP-airboundary, while x=h corresponds to the PSP-modelboundary.

As the optical density of PSP is d-ech andI0=Iex&(/(l-10~d), and substituting X=x/h, equation(2) may be rewritten as:

n(t,x, y,z) =

7 =<Mn(10)/ ~ 1-10-''

10,-rf-X

f, X,y,z)l 0.21(7

Integration of this equation for the constant oxygenconcentration with the Henry Law n = 0.2 lop resultsin equation (1). In the case of variable oxygenconcentration in the PSP layer, luminescenceintensity would depend on oxygen distribution in PSPbinder. Oxygen distribution in the polymer of PSPbinder arises in the case of transient or spatial airpressure changes outside PSP layer and is determinedby the mass transfer equation with appropriateboundary conditions:

*^n_dx2'

^n_V

d2n (4)

where D is the diffusion coefficient of the oxygen inthe polymer of the PSP binder. Boundary conditionfor PSP-model boundary is the condition of the zeronormal gradient:

.dx

The pressure on PSP-air boundary determines thesecond boundary condition.

Time Response.

If at the moment t-0 the pressure changes from thevalue po to the value p0+pj in all points of the PSP-air boundary (x=0,y, z):

n(t,0,y,z) =0.2lap0 ;t<0. (6)

oxygen concentration inside the PSP binder begins tochange in time. This is one-dimensional case asnothing depends on y and z coordinates. Solving theequation (4) with the boundary conditions (5) and (6),one can get oxygen concentration distribution in thePSP binder on time:

0.21CT 7>o + Pi 1-1n x—) —)•exp2 h V

//J

.(7)

So, characteristic time of relaxation of oxygenconcentration in polymer layer is:

T = -4h2

(8)

To get luminescence intensity relaxation, i.e. PSPtime response, equation (3) must be integrated usingsolution (7). Results of computational integration fordifferent optical densities of PSP d and po-0bar,Pl=lbar and bxpj=3a are presented in Fig.l, whereT=t/T(i is determined by (8)) and A=(I-I (po+p ]))/(!(P0)-I(P0+P1»-

As the optical density of real PSP is within the range0.3-^1.0, 99% relaxation of luminescence intensity ofreal PSP occurs in (2.5^-3)t. The thickness and thediffusion coefficient of binder polymer determineresponse time of concrete PSP.

Amplitude-Frequency and Phase-FrequencyCharacteristics

Periodical pressure component with the frequency codetermines the next condition on PSP-air boundary:

(9)

Solution of equation (4) with the boundary conditions(5) and (9) is:

n(t,x, y,z)= (jQ)0.21(7 [po + Pi (A(co, x) • sin(ot) + B(a), x) • cos(aK))]

A(a>, x) = - I2D

B(a>,x) = -IsmlJ——x 1 + s.nU—,

,. , 2m,ch\.\-—h +cos , — h

To get Amplitude-Frequency Characteristics (AFC)and Phase-Frequency Characteristics (PFC) thepressure value of PSP pi must be calculated:

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n - - -Pl~bi ~b~1-10"

(MO"™ -In(lO)I a + bp0 + bpt [A(a>, Xh) • sin(«) + B((O, Xh) • cos(fflf)]

(11)

Since the periodical component is usually small(Pl«p0), this equation may be simplified:

PI =

Po+Pi

(12)_

1-10o, Xh) • sia(ox) + B(co,Xh)- cos(of)]dX

One can see that small periodic pressure componentdoes not cause any change of the average value of thepressure measured with PSP.

This equation allows determining AmplitudeFrequency Characteristics (AFC) and PhaseFrequency Characteristics (PFC) of PSP. Let

(13)

then AFC may be written as A=^O1+I32 (Fig.2)and PFC - as (p-arctg (ft/a) (Fig.3) (on these figuresW=2coh2/D=amf/2 (t is determined by (8)).

3. Measurement Concept

To measure periodical non-harmonic pressurefluctuations the PSP has to transfer several harmonics(about ten) of these fluctuations. Estimations basedon Amplitude-Frequency and Phase-FrequencyCharacteristics for polymer-based PSP show thatAFC=0.99 with PFC=-5° occurs at frequencyca=\.2ln trei while at frequency 10<w- AFC=0.68 and

PFC=-35°. Thus, for example, the fastest OPTROD'sPSP F2 (T=5msec) can be used for measurement ofperiodical non-harmonic pressure fluctuation with themain frequency up to WHz only4.

A test setup assembled in a wind tunnel to fulfill PSPmeasurements on oscillating model is presented inFig.4. Pulsed light source is used for PSP excitationto acquire luminescence images at certain phase ofmodel position. To compensate model movement andnon-stability of excitation light distribution from one

flash to another the paint, having additional(reference) non-sensitive to the pressure luminescent

a_ output is used. Flash duration of l+3ms is enough forb measurements at the models oscillating frequencies

up to 20-J-30//Z. The flash of the lamp is initiated at acertain phase of the model position.

The luminescence of Active and Referencecomponents of PSP are acquired simultaneously toexclude an effect of instability of excitation lightenergy and distribution.

4. Investigation of PSP Dynamic Characteristics

Experimental investigation of PSP reaction toperiodic pressure change was performed at the end of1990 for PSP L2 and PSP Rl (produced by OPTRODLtd., Russia) that are based on the similar polymerbinders3.

Periodic pressure component was created byharmonically moving piston, driven by electric motor(Fig.5). Frequency of periodic pressure componentwas changed by changing rotation frequency of themotor in a range of 0.2-40/fe. As the periodicpressure component was small enough P}~0.1po,periodic pressure component was also practicallyharmonic.

PSP sample was installed in a vicinity of movingpiston in hermetic cell. PSP sample was continuouslyexcited through a fused silica window by well-stabilized Xe-arc lamp. Excitation light of a necessaryspectral range was selected by appropriate opticalglass filter. Luminescence light was acquired byphotomultiplyer tube through the same quartzwindow. Luminescence light was separated fromexcitation light by carefully chosen optical glassfilter. To exclude any additional light themeasurement volume was protected from anyexternal light. An ordinary pressure sensor measuredpressure in a cell.

Multiplexed analog-digital converter installed inpersonal computer measured luminescence intensityand pressure. Conversion rate was changeddepending on rotation frequency of a motor. Series of1000 pairs of luminescence intensity and pressurevalues were saved. Pressure values ps were used todetermine mean pressure value ps0, first harmonicfrequency / and amplitude psl. PSP luminescenceintensity was recalculated to a pressure value pi usingstatic calibration characteristic and its mean value p[0,first harmonic amplitude pn and phase shift df of PSPvalue from the real pressure were determined.Obtained results allow determining Amplitude-Frequency Characteristics (AFC) as Pu(f)/psl(f) andPhase-Frequency Characteristics (PFC) as df (f).



Differences between mean pressure values plo andPSQ was below measurement resolution.

Experimentally determined AFC and PFC of PSP L2with the thickness h=20\\m and optical density d-0.2are presented in Fig.6 (characteristics of PSP Rl aresimilar since it contains similar polymer binder).Experimental results are described well enough bythe theoretical equation (12), (13) with thecharacteristic time T=0.2s (see equation (8)).

Experimental investigation was fulfilled for the set ofPSP samples with different thickness in the range of5H-60|0,m. All the results show good correlation withthe theoretical model, that proves the validity of thetheoretical model.

As the theoretical analysis of time response andspatial resolution of PSP was based on the sametheoretical model, obtained results allow determiningresponse time of investigated PSPs. Since the opticaldensity of PSP L2 and PSP Rl of 20|im thickness is0.2, their response time is O.Ssec for the 99%relaxation of luminescence intensity.

Unfortunately, this experimental setup did not allowinvestigating fast PSP due to restricted range ofpressure frequencies. In 1997 a laboratory setup forinvestigation of PSP response to the step pressurechange was assembled in TsAGI. To provide fastpressure change the cell of this setup (Fig.7) wasmade as fused silica tube of 20mm internal diameterand 40mm long. PSP sample was installed verticallyon the bottom of the cell on the aluminum plate Immthick to minimize an effect on air movement insidethe cell. An input of the tube was closed by cellulosemembrane. After vacuumization of the cell themembrane was destroyed by a needle that initiate themovement of shock wave inside the tube. Normally,the membrane is destroyed absolutely and does notresist to air movement that causes very fast pressurechange inside the cell. Pressure relaxation in this cellwas not measured, but theoretically was estimatedshould to be in a range of 5CM-200|isec, while somesonic waves can also appear in this cell with the mainfrequency about 2000//Z that is determined by thelength of the tube. The main disadvantage of this cellis that the small pieces of destroyed membrane aremoving with the air and produce optical distortionsfor the excitation and emission light causingadditional aperiodic noise of luminescence signal.

The excitation of PSP sample was performed bysmall continuous luminescent UV lamp withappropriate UV glass filter. The luminescence wasacquired by photomultiplyer tube, the signal wasdigitized by oscilloscope 'Nicolet 2090' and obtaineddata were transferred to the personal computer forfurther processing.

Fig. 8 presents time response of PSP L4 paint(improved version of PSP L2, OPTROD Ltd.) to thestep pressure change. This response corresponds tothe theoretical predictions with characteristic time (8)T=0.172s' (this correspondence is especially evidentvia comparison of logarithmic amplitude values A).This characteristic time corresponds well to the oneobtained by investigation of PSP response toharmonic pressure change for PSP L2. The differenceof obtained values can be explained by differentthickness of PSP polymer layer. Fig.9 presents timeresponse of fast PSP F2 paint (OPTROD Ltd.) to thestep pressure change. This response corresponds tothe theoretical predictions with characteristic time (8)T=2.6m.y. This characteristic time determinesResponse Time of 99% relaxation of luminescenceintensity equal to 6.5ms, that corresponds well totheoretical prediction. Also this characteristic timedetermines Amplitude-Frequency and Phase-Frequency Characteristics of PSP (AFC and PFC).AFC of this PSP is equal to 0.99 at frequency IHzand 0.7 at JOHz, thus this PSP can be used forunsteady pressure measurements at the frequency upto few dozens Hz.

Some experimental samples of very thin PSP F2 wasprepared and tested in this cell. Their characteristictimes were of the order of 100-̂ 300|is, as a result aharmonic component of the pressure caused by thesonic waves inside the cell was detected with thefrequency about 2000% and first maximumamplitude about Q.lbar that decays in five periods toinsignificant value.

5. Pressure field investigation on the pitching model

These tests were conducted in the Transonic WindTunnel of DLR Gottigen (TWO) and were aimed toexplore measurement technique and PSP potentialityto give acceptable results for the pressure field on thesurface of the oscillating model. The model of winghaving NLR 7301 profile was painted by L4 PSP.Maximum pitching frequency was up to 20Hz. Due tosmall model displacement (oscillation amplitude wasonly 0.6°) and good stability of the light source,standard paint was used. To minimize response timeof this paint it was applied as very thin layer, so itcharacteristic response time was by estimation about0.05-0.1 sec. To obtain larger SNR level and tominimize spread in the illumination intensity the eachimage integrates up to 20 flashes in stroboscopicmode. Plots and pictures in Fig. 10 showing pressuredistributions obtained with single flash (gray line)and 20 flashes (black line) demonstrate increase ofSNR without offset excluding the region near theleading edge.



Model was instrumented with internal pressuremeasurement system providing possibility todetermine static component of the pressuredistribution. The plots in Fig. 11 show a goodagreement between PSP and Kulite measurements.

Descreapency at Ma=0.82 can be attributed to 3Dstructure of the pressure distribution as it shown inFig. 12.

References.

1. V. Mosharov, A. Orlov, V.Radchenko, M.Kuzmin,N.Sadovskii, "Luminescent Pressure Sensors forAerospace Research: Diffusion-ControlledCharacteristics." 2nd European Conference onOptical Chemical Sensors and Biosensors, Firenze,Italy, April 19-21, 1994.

2. B.F.Carroll, J.D.Abbitt, E.W.Lukas, M.J.Morris,"Pressure Sensitive Paint Response to a Step PressureChange." AIAA Paper 95-0483, Jan. 1995.

3. OPTROD Ltd, Dugin str. Zhukovsky, Moscowreg., 140160 Russia. Fax: 07 095 939 0290, e-mailaddress: [email protected] Attn. Mosharov



0.5 1.5 2.5

T,s

3.5

Fig. 1. Theoretical time relaxation of PSP luminescence intensity after the pressure change for different opticaldensities d.

20 40 60

W

80 100 120

Fig.2. Amplitude-Frequency characteristic of PSP for different optical densities d.



20 40 60

W

80 100 120

Fig.3. Phase-Frequency characteristic of PSP for different optical densities d.

Pulsegenerator

Flashlamp

Phasegauge

CCD cameras

Fig.4. Schematics of Test Setup for PSP Measurements on Oscillating Model

Harmonically PSP sample Excitationmoving piston I Optical filter

Xe-arc lamp

EmissionOptical filterPhotomultiplyer

Personal computer withAnalog to digital converter

Fig.5. Schematic of laboratory facility for investigation of PSP reaction to periodic pressure changes.



10f.Hz

15 20

Fig.6. Experimentally determined Amplitude-Frequency and Phase-Frequency Characteristics of PSP L2

with the thickness h-20\\.m and optical density d=0.2; characteristic time is T=0.2sec.

Initialization needle

Cellulosemembrane

PSP sample

To the vacuum pump

Fused silicatube

Viewingdirection

Fig.7. Schematic of a cell for investigation of PSP response to the step pressure change.



0.1 0.2 0.3t, s

ExperimentCalculation

0.4 0.5 0.6

Fig.8. Time response of PSP L4 to the step pressure change, i=Q.\12sec('A' - the same as in fig.l.).

ExperimentApproximation

ms10

Fig.9. Time response of PSP F2 to the step pressure change, i=2.6msec.



20 flashes with 25 us pulse duration single flash with 25 us pulse duration

20 single flashes with 25us puls duration 1 flash with 25us puls duration

0.5

-2.0

—— 20 single flashes with 25 us puls duration—— 1 flash with 25 us puls duration

0.2 0.4 0.6

Cp-distribution for Ma =0.7 , a = 3.0 °,Aa = ±0.6 °, aeff =2.4 °, f= 20Hz

Fig. 10

0.8 , 1.0x/c



0.2Cp

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

— 20 single flashes with 25 Us puls durationkulite measurement Ma= 0.80

* kulite measurement Ma= 0.82

0.0 0.2 0.4 0.6 0.8 x/c 1-0

Fig.l 1. Comparison with kulite measurements.

Fig. 12. Pressure field for Ma=0.82, cc=0.24°


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