Experiments in Fluids 14, 33-41 (1993)

Experiments in Fluids �9 Springer-Verlag 1993

Surface pressure field mapping using luminescent coatings

B.G. McLachlan

NASA-Ames Research Center, Fluid Mechanics Laboratory (M/S 260-1), Moffett Field, CA 94035

J.L. Kavandi, J.B. Callis, M. Gouterman, E. Green and G. Khalil

Univ. of Washington, Dept. of Chemistry (BG-10), Seattle, WA 98195

D. Burns

Univ. of Washington, Center of Bioengineering (WD-12), Seattle, WA 98195

Abstract. In recent experiments we demonstrated the feasibility of using the oxygen dependence of luminescent molecules for surface pressure measurement in aerodynamic testing. This technique is based on the observation that for many luminescent molecules the light emitted increases as the oxygen partial pressure, and thus the air pressure, the molecules see decreases. In practice the surface to be observed is coated with an oxygen permeable polymer containing a luminescent molecule and illuminated with ultraviolet radiation. The airflow induced surface pressure field is seen as a luminescence intensity distribution which can be measured using quantitative video techniques. Computer processing converts the video data into a map of the surface pressure field. The experiments consisted of evaluating a trial luminescent coating in measuring the static surface pressure field over a two-dimensional NACA-0012 section model airfoil for Mach numbers ranging from 0.3 and 0.66. Comparison of the luminescent coating derived pressures were made to those ob- tained from conventional pressure taps. The method along with the experiment and its results will be described.

1 Introduction

Surface pressures in aerodynamic testing are currently mea- sured using pressure taps or pressure transducers. These convent ional methods are inherently limited to providing information at discrete points; in addit ion, their implemen- tat ion can be time consuming and expensive. We are propos- ing a new approach to these measurements. This approach is based on the observat ion that for many luminescent molecules the light emitted increases as the oxygen part ial pressure that the molecules see decreases. A phenomenon known as oxygen quenching. Based on this observat ion we felt it should be possible to use an oxygen sensitive lumines- cent coating or "paint" to map the continuous pressure field over an aerodynamic surface. The feasibility of this approach for static surface pressure field measurement was first demonst ra ted in experiments in the Fluid Mechanics Labo- ra tory at NASA-Ames in March 1989. A coating developed at the Universi ty of Washington was evaluated in measuring the static pressure field over a two-dimensional airfoil at subsonic speeds. This paper reports on this new method, the experiment, and its results.

In the l i terature on aerodynamic testing one finds de- scribed numerous flow visualization methods that allow in- formation to be gathered in a field sense on various fluid mechanical quantities over a surface. Examples are the use of oil flow and tuft methods for measurement of the surface flow direction pat tern; liquid crystals for measurement of surface temperature; and oil film interferometry for measure- ment of skin friction. A review of these and other conven- t ional field measurement techniques can be found in Merz- kirch (1987). To the authors knowledge there is no directly equivalent method, outside of the present work, that allows a direct measurement of the pressure field over a surface.

Oxygen quenching of luminescence has been proposed by Peterson and Fi tzgerald (1980) as the basis for a flow visual- ization method analogous to the oil flow method for surface flow direction pat tern recording. In this method jets of oxy- gen, or a gas such as nitrogen, are allowed to pass over a surface covered with a luminescent coat ing that is excited by ultraviolet light. Those por t ions of the surface where the oxygen or the nitrogen passes, will be darker or brighter, respectively, than that por t ion of the surface in contact with the normal composi t ion airstream. The resulting dark (oxy- gen) or bright (e.g. nitrogen) streak pat tern provides infor- mat ion on the direction of the surface flow.

Although only roughly connected to the present subject mention is warranted that an optical method, holographic interferometry in conjunct ion with tomographic techniques, can be used to measure the three-dimensional flow field a round a test object, information on the surface being a by-product . This method, however, is restricted to compress- ible flows as it measures refractive index changes resulting from flow induced density variations. The pressure, and oth- er flow quantities, are calculated from this measurement assuming isentropic flow. The test geometries that can be studied are limited due to the optical beam requirements of the method. Historically, these l imitat ions along with the complexity and time consuming nature of the method has severely restricted it's routine appl icat ion in aerodynamic testing. Reference may be made to Merzkirch (1987) for a description of this method.

34

The luminescent coating method contains the positive attributes of the above methods: it provides a quantitative field measurement of the flow quantity of interest in a non- intrusive manner that is simple and can be routinely imple- mented.

2 D e s c r i p t i o n o f m e t h o d

2.1 General concept

A schematic representation of how the concept works is shown in Fig. 1. In practice the surface to be observed is coated with a suitable luminescent molecule dissolved in a oxygen permeable plastic resin and illuminated with ultra- violet radiation. The resulting luminescence intensity de- pends on the oxygen partial pressure seen by the coating molecules. Knowledge of the oxygen partial pressure allows the air pressure to be readily calculated since the mole frac- tion of oxygen in air is a known constant. The result of this oxygen dependence is that the airflow induced surface pres- sure field, and the corresponding oxygen partial pressure field, is accompanied by a luminescence intensity distribu- tion. As in the present case this intensity field can be imaged using a video camera, the image digitized, stored and pro- cessed on a computer to produce a map of the surface pres- sure field. The processing relies on a calibration curve for the coating material previously obtained under controlled static conditions.

2.2 Theory

Photoluminescence is a process whereby the molecules of a substance absorb light of a particular wavelength and re- emit it as light of a different, usually longer, wavelength. Oxygen quenching of luminescence is the result of collision deactivation of the emission process. This phenomenon can be described using the Stern-Volmer relation which is usual-

Cam IO'0"'zerll IC~

~\\ , . ~ ring emits light (luminesces) (--

�9 Luminescence intensi ty depends on oxygen partial pressure

�9 Mole fract ion of oxygen in air is known

�9 V ideo/photo + image processing to measure intensiW,

thus mapping surface pressure

Fig. 1. Schematic representation of pressure sensitive luminescent coating concept

ly written (see e.g. Parker 1968) in the following form

])max 4) - 1 + K P o 2 (1)

where 4) is the quantum yield, 4)max is its maximum possible value that occurs in the absence of quencher, K is the Stern- Volmer quenching constant characteristic of the molecule, and Po2 is the oxygen partial pressure. It must be noted that the Stern-Volmer relation is strictly valid only for free molecules in solution. It will be seen, however, that it is an accurate description of the quenching process in the present situation.

The response of the luminescence imaging device, in the present case a video camera, and the final digitized grey level is proportional to the intensity of the light received. By definition (see Parker 1968) the luminescence intensity, I, is linearly proportional to the quantum yield for a given quenching condition, i.e. I = l a b s 4), where l ab s is the excita- tion light intensity absorbed by the luminescent species with- in the coating and can be treated as a constant for a given fixed geometry of the experimental arrangement. It is clear that

4)max /max - - - - (2)

4) i

Thus,

Im,x I - I + K P o 2 . (3)

Equation 3 can be manipulated into a form more convenient for aerodynamic testing purposes by taking the ratio of the measured luminescence intensity for two different flow, and hence quenching conditions, one with flow, and the other for a no-flow condition. Noting, in addition, that the oxygen partial pressure, Po2, is related to the air pressure, P, by Po2 = XP, where X = mole fraction of oxygen in air, Eq. (3) can be replaced by

Io = A P + B - - . (4) I Po

Where

1 K X P o A - and B =

I + K X P o ' I + K X P o "

Here the subscript "0" denotes the value for no-flow condi- tions; the quantities I, Io, and P are functions of the spatial coordinates in the recording plane, (x, y); Po is known to be a constant. While in general I, I o, and P are functions of time, here they are considered as time averaged quantities. The coefficients A and B are the coating sensitivities and are determined from an experimental calibration plot. Note that A + B = I .

Equation 4 explicitly indicates the form of the relation- ship between the relative intensity, Io / I , and the relative pressure, P/Po and that the form of this relationship is linear.

35

It is the operational form of the Stern-Volmer relation used by the authors to map the measured luminescence intensity field into the surface pressure field. The advantage of this form of the Stern-Volmer relation is that in a practical test- ing situation it is far easier to measure the luminescence intensity, I o, resulting from a known pressure, Po than it is to determine the maximum luminescence intensity in the absence of quencher, /max"

An important benefit arises from taking the ratio of the luminescence intensity field, I, with respect to the lumines- cence intensity field at a reference condition, I o. The result of this ratioing is the elimination of the effect of surface spatial non-uniformities in excitation light intensity, coating thickness, and concentration distribution of the molecule in the coating. This can be shown by examining in more detail the quantities that make up, l a b s , the excitation light intensi- ty absorbed by the coating. Assuming the Beer-Lambert law of absorption is valid l ab s is given by

labs = l.v (1 - lO-~Ca), (5)

where I,v is the excitation light intensity, e is the absorption coefficient of the luminescent molecule, c is the concentra- tion of the molecule in the coating, and d is the coating thickness. Equation (5) holds for one wavelength of light, the total l ab s being given by integration over the bandwidth of the exciting light. It is obvious from Eq. (5) that for a given experimental arrangement, even if P is constant over the surface, I in general will be a function of spatial position due to spatial variations of I,v, c, and d. e can be considered a constant. Noting from before that I = l a b s ~b it is clear that the result of creating Io/I is cancellation of l a b s and consequently the spatial dependence effect of the quantities making up lab s . This is the case so long as those quantities do not change between the two quenching conditions used in the formulation of the relative intensity, Io/I.

2.3 History - Biomedical application

The method described here for the measurement of static pressure was first hypothesized by one of the authors, J. B. Callis, as an extension of a method developed by Khalil et al. (1989) to measure the oxygen content of blood. In the meth- od of Khalil a metalloporphyrin (see Gouterman 1978) is embedded in a oxygen permeable plastic matrix and placed onto the end of an optical fiber. The optical fiber is used as a catheter in patients. Exciting light is sent down the fiber causing the metalloporphyrin to luminesce. Oxygen reaching the metalloporphyrin through the plastic matrix quenches the luminescence. The luminescence intensity can be mea- sured and correlated to the amount of oxygen present.

2.4 Coating

In this section a description of the coating, it's chemistry, application, calibration, and characteristics, is provided in

sufficient detail for it's operational use. For a more exhaus- tive treatment of this subject reference may be made to Ka- vandi (1990).

The choice of molecule for the present investigation was based on the biomedical work of Khalil et al. Platinum octa- ethylporphyrin, PtOEP, was the molecule chosen (Fig. 2). From the work of Khalil et al. the molecule is known to exhibit a high luminescent quantum yield (~90%), short (~ 100 txsec) triplet lifetime, and a suitable oxygen quenching sensitivity. The molecule was selected based on these proper- ties. The excitation-emission spectrum for PtOEP is dis- played in Fig. 3. The molecule luminesces at a wavelength of approximately 650 nm (red) when exposed to wavelengths of light in the 360 to 560 nm (blue-green) range.

The coating mixture is prepared by dissolving the por- phyrin in a room temperature curing silicone resin solution. Proportions in the present case were 9.4 mg PtOEP to 100 ml of resin solution (Genesee Polymers Corp. GP-197). The resin solution's carrier solvent is mostly l,l ,l-trichloro- ethane. This mixture is suitable for spraying onto a surface. This can be accomplished using apparatus as simple as a hobby type paint airbrush. After the solvent evaporates, typically in approx, five minutes, a thin smooth hard uni- form film of PtOEP dissolved in oxygen permeable polymer remains. The coating thickness for the test described was on the order of 5 to 15 p.m. Coating thickness variations over this range have been shown by Kavandi (1990) to have no effect on the relative intensity, Io/I .

It was discovered that first painting the surface white, providing a white backing to the luminescent coating, signif-

CH2CH 3 CH2CH 3

N~pt/N

N / ' "~N

C H 3 C H / ~ C H 2 C H 3 CH2CH 3 CH2CH 3

Fig. 2. Platinum octaethy|porphyfin (PtOEP)

Excitotion spectrum

m " '

300 400 500 600 700 Wovelength ( nm )

Fig. 3. Excitation and emission spectra of PtOEP

36

icantly increases the luminescence intensity for a given UV excitation intensity level. Although the cause of this en- hanced luminescence is not understood, it is hypothesized that the white backing scatters unabsorbed exciting light back through the coating allowing it a further chance to excite porphyrin molecules, and that it also reflects the emit- ted luminescence providing a further contr ibut ion to the detected intensity. The white backing serves the addi t ional role of promot ing adhesion of the luminescent coating to the surface.

The pressure dependence of the luminescent coating de- posited on a white surface was obtained by placing samples in a chamber where static pressure could be varied in a control led manner. At a given pressure the luminescence intensity over an area of the sample composed of 100 pixels was summed and time averaged over 5 video frames. By repeating this process for a range of pressures a cal ibrat ion curve could be established. The UV excitation, imaging, and image processing appara tus used during this cal ibrat ion procedure were closely similar to that used during the wind tunnel experiment. Figure 4 shows the resulting cal ibrat ion curve of luminescence intensity as a function of pressure. The da ta is presented in the normalized form, Io/I versus P/Po, suggested by the Stern-Volmer relation. The subscript "0" denotes the no-flow reference condition, in this case atmo- spheric conditions. The data displayed was taken at a tem- perature of 23 ~ and is for the coating mixture used in the wind tunnel demonstra t ion experiment. Clearly evident in Fig. 4 is a linear relationship between Io/I and P/Po. This linearity can be taken as an indicat ion of the validity of the Stern-Volmer relation in describing the quenching process in the present case. The coating sensitivity coefficients, A and B, were found using least-squares curve fitting to be 0.32 4-0.01 and 0.70_+ 0.01, respectively.

It is appropr ia te here to note that the present coating formulat ion does display some undesirable characteristics. The first of these is that the paint photodegrades, it's re- sponse decreasing with time of exposure to UV light. An il lustration of the form of this degradat ion is shown in Fig. 5 where luminescence intensity is plot ted as function of time of exposure to 380 nm light. Here the coating loses 46% of it's response after one hour of exposure. This data should only be taken as a representative example of this phenomenon as it depends on a number of parameters, such as the intensity and wavelength of the incident UV light, and the coating thickness. While we had no way of quantifying the incident UV intensity used in Fig. 5, the condit ions can be taken as a worst case situation, the UV intensity being large in com- parison to what was used in the experiment. We have not found a detailed quantification of the photodegrada t ion phenomenon to be necessary, as it is not a serious l imitat ion in the use of the method, since only during data acquisit ion is the coating exposed to UV excitation. At other times the transmission of UV light to the model is blocked. Opera- tionally, at some point after extended use a new coating will need to be applied to the model surface. This is a simple

1.0

0.8

~o 0.6

0.4

I I I i i I i I

0.2 0.z~ 0.6 0.8 1.0 PIP0 - "

Fig. 4. Calibration curve, luminescence intensity ratio (1o/I) as a function of pressure ratio (P/P0), of coating containing PtOEP on a white background at 23 ~

1.0

0.9

t 0.8 0.7

0.6

0.5 i I i I i 0 10 20 30 Z~O 50 60

Time (min)

Fig. 5. Representative example of photodegradation of coating re- sponse after exposure to 380 nm light for one hour at 24 ~C and at atmospheric pressure (760 torr)

140

120

100

80

60

~0

2 0 I I r I r I I I T ~ q ~ l ~

10 20 30 40 50 Tempera tu re (~

Fig. 6. Luminescence intensity as a function of temperature of coat- ing containing PtOEP on a white background at atmospheric pres- sure (760 torr)

procedure as the new coating can be applied over the old one.

The second undesirable characteristic diplayed by the paint is that it exhibits a temperature dependence. The char- acter of this dependence is displayed in Fig. 6 where lumines- cence intensity is plot ted as a function of temperature for a constant pressure of one atmosphere. As can be seen the response of the coating decreases in a non-l inear fashion as

37

the temperature increases. The effect of temperature on the coating calibration curve is shown in Fig. 7 a. As with the previous calibration curve, Fig. 4, the data is presented in the Stern-Volmer normalized form of Io/I versus P/Po for three different temperatures 50, 23.7, and 6 ~ Note that in Fig. 7 a the reference condition denoted by subscript "0" was taken at a temperature of 23.7 ~ It is clear from the three curves of Fig. 7 a that the slope displays a significant variation with temperature, while the intercept is by comparison relatively temperature invariant. An interesting property displayed by the curves of Fig. 7 a is that the coating sensitivity coeffi- cients add to unity, i.e. A + B = 1, only for the 23.7 ~ curve, while for the 50 and 6 ~ curves A + B r 1. This occurs be- cause I and I 0 for those two curves were not taken at the same temperatures. If they were this condition of coefficient addition to unity would be satisfied. A demonstration of this is exhibited in Fig. 7 b where the data of Fig. 7 a has been replotted using the appropriate I 0 for each curve. In this form both the slope and intercept of the three curves changes significantly with temperature, while Io/I is forced to unity for P/Po = 1. The coating's temperature dependence may prove a limitation on it's quantitative use in flows where compressibility and heat transfer effects are a dominant fac- tor. It must be added, however, that even though displaying the above characteristics, the described coating formulation proved adequate in measuring static surface pressures over the conditions of the present test. A detailed characterization of the temperature dependence along with the photodegra- dation phenomenon is presently underway.

4 9 .

C(deg)

t 3 zx 6.o o 23.7

o

1

0 ~ I I I I r I i I i

1.0

0.8

0.6

.~ 0.4 o

0.2

0 I I i I t r t I r

0 0.2 0.4 0.6 0.8 1.0 P/Po "

Fig. 7 a and b. Effect of temperature on coating calibration curve: a I o for each curve was measured at 23.7 ~ b I o for each curve was taken at its respective temperature

3 Experiment - Description

The experiment was carried out in the NASA Ames Fluid Mechanics Laboratory 25 x 35 cm indraft wind tunnel. The model tested was a steel 7.62 cm (3 in) chord, 25 cm (10 in) span, two-dimensional airfoil of NACA-0012 section. It hori- zontally spanned the test section and was fixed at 5 deg geometric angle of attack. The Mach number ranged from 0.3 to 0.66, corresponding to Reynolds numbers, based on chord, of 5.0 to 9.5 x 105, respectively. No boundary layer trip was installed for the experiment.

Figure 8 shows the experimental arrangement. The lu- minescent coating completely covered the surface of the model exposed to airflow. Ultraviolet illumination for exci- tation of the coating was provided by a xenon light source covered with a blue filter. Only one light source was avail- able and was used to excite the coating on the upper surface of the model. A video camera was used to image the lu- minescing upper surface. Two types of video cameras were employed, CCD and vidicon, at different times during the test. Each camera was adjusted such that gamma = 1 and the automatic gain control (AGC) was disabled so that the cam- era output would be linearly proportional to light intensity received. The video camera was mounted above the test section looking directly down upon the model and posi- tioned such that the wing chordwise axis was aligned with the horizontal axis of the video frame. A bandpass filter (650 nm) was placed over the camera lens, effectively block- ing all light other than that from the luminescing coating from reaching the camera. The camera analog signal corre- sponding to the video image of the model's luminescing upper surface was digitized by an image digitizer (frame grabber board) with 256 (8 bit) grey level resolution and 512 x 512 pixel (picture elements) spatial resolution. This digitizing process was controlled by an AT PC host com- puter where the data was stored. The video camera signal was also recorded on a typical commercial grade u (3/4 in. format) for later analysis.

For comparative purposes the model was equipped with static pressure holes or taps. The taps were distributed in a midspan chordwise row with 16 on the upper surface and 9 on the lower surface. The model static pressure distribution, test section static and total pressure, were measured using a 2 transducer (24 port/transducer) scanivalve coupled to a minicomputer. In addition, ambient pressure, temperature, and scanivalve transducer calibration pressures were simul- taneously recorded for each test case. The luminescent derived pressures used for comparison to the conventional pressure tap data were obtained by spanwise spatial averag- ing the luminescence intensity distribution from 5 adjacent chordwise lines next to the row of pressure taps, each line being 1 pixel wide, and the result of averaging together 100 video frames.

It should be pointed out that the luminescence intensities, I and I o, used in Eq. (4) to derive the pressure, and in the coating calibration curve, are noise corrected values ob-

38

Fig. 8. Photograph of experimental set- up

tained by subtracting the camera dark current noise from the measured coating intensities. Dark noise is defined here as the output value of the camera when no light is incident on the sensing array. It was measured when the camera lense was covered and before each set of no-flow and flow coating intensity measurements. The dark noise values used in the correction process were time and spatial averaged in a pro- cess identical to that for the measured coating intensities.

4 Exper iment - Results

At M~ = 0.66 a shock wave exists on the model upper sur- face at approx. 30% chord. The surface pressure map for this condition is shown in Fig. 9. Displayed is a plan view of the upper surface midspan region of the wing. Here color de- notes pressure level. The shock wave is indicated by the red/yellow boundary. Note that the two-dimensional char- acter of the flow in the midspan region is quite evident. The conventional pressure taps are indicated by the chordwise row of light color dots.

The pressure map of Fig. 9 was computed from the Stern- Volmer relation Eq. (4). The no-flow, Io(x,y ), and flow, ! (x, y), intensity fields used in this computation are shown in Fig. 10. Color is used to denote luminescence intensity level and graphically illustrates the non-uniformity effects de- scribed previously, that arise, for example, from the spatial non-uniformity of the UV light excitation intensity.

Quantitative assessment of the luminescent method was made by comparing the luminescent derived pressures to those obtained from the conventional pressure taps. Lu- minescent data on a chordwise line next to the line of static pressure taps was used. This comparison for the M| = 0.66 case is shown in Fig. 11 a. The luminescent coating accurate- ly captured the shock location, pressure rise through the shock, pressure recovery aft of the shock, and the low pres- sure distribution ahead of the shock, including a laminar separation bubble just upstream of the shock. The agree- ment between the luminescent and conventional data is poor, however, right at the nose region. The reason for this difference is unknown at present.

Comparisons of luminescent to conventional measured pressure distributions for the other Mach numbers exam- ined, ranging down to M~=0.30, are displayed in Fig. 11 b-f. They display the same degree of correlation as the above M~ = 0.66 case and provide a further illustration of the results that can be achieved. Evident in these compari- sons, especially in the Moo = 0.30 case, is an increased noise in the luminescent trace as the Mach number decreases. The source of this "noise" is strongly suspected of arising fom a signal-to-noise problem in the imaging instrumentation sys- tem: at lower Mach numbers the luminescence intensity sig- nal becomes smaller while the imaging system noise remains constant. The novelty and newness of the experimental methodology, along with limited tunnel access time which was provided on short notice, did not allow us to preplan the

Fig. 9. Map of upper surface pressure field, mid-span region, for M| The flow is from left to right. Color de- notes pressure level. The shock location is denoted by the red/yellow boundary

Fig. 10. Upper surface luminescence in- tensity field maps for ! and I o used to create pressure map of Fig. 9. Color de- notes the luminescence intensity level, or grey scale. The correlation of color to intensity level is indicated in the vertical bar key between the maps

39

Fig. 9

Fig. 10

40

-3

-2

tl

1 -3

-2

-1 (,3

1 -3

-2

I -1

o =

M ~ = 0.66

\

M ~ = 0.49

1 3 \ \

D %

0 _ j i i E

M ~ = 0.36

M ~ = 0.58

M ~ = 0.40

D'~%

Ct _ d

M ~ = 0.30

i i i ~ _ _ i I i _ _ I i

0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 x/c - ,...- x /c ~

Fig. 11 a - f . Comparison of luminescent coating derived pressures ( - ) to conventional pressure tap measurement (=). Midspan chordwisc pressure distribution for M~ = 0.66; 0.58; 0.49; 0.40; 0.36; and 0.30

experiment in an optimal way, and masked this and other limitations in the data that did not become apparent until the experiment was over. Even so, the reported data are of sufficient quality to demonstrate the feasibility of the method and its capabilities.

It must be noted that all the luminescent coating pressure distributions presented have been offset vertically to mini- mize the difference with the conventional data. Thus, when viewing the pressure distribution comparisons it is only proper to evaluate the relative pressure changes in the lu- minescent versus conventional distribution curves. The rea- son for this offset procedure is that in the present experiment it was not possible to measure the absolute pressure using the luminescent coating. This difficulty was a result of the output intensity of the UV excitation light source varying with time. This was noticed as a significant variation in the no-flow intensity distribution, I o (x, y), from one test case to another. Changes in the UV excitation intensity between the time no-flow and flow images are taken would, for the pres- ent test arrangement, result in the addition of a constant level change to the ratio of the luminescence intensity distri- butions and thus the perceived level of the pressure distribu- tion along a chordwise line. A proposed solution to this problem is to monitor the intensity of the UV light source (e.g. using a photo-diode) and appropriately correct the data.

5 Summary (Discussion and conclusions)

The results presented demonstrate the feasibility of using the luminescence phenomenon of oxygen quenching to measure surface pressure fields in aerodynamic testing. Considering the novelty of the method it is important to provide a brief assessment of it.

The most significant feature, and advantage, of the lu- minescent coating method is that it allows the mapping of the continuous pressure field over a surface in a non-intru- sive manner. This provides information of greater spatial resolution than conventional methods which only provide information at discrete points. The luminescent coating method also allows the measurement of the surface pressure at locations where it is difficult for conventional methods to do so. This surface location limitation of conventional meth- ods arises from the installation size of the required taps or transducers, which restricts where they can be placed in the model, e.g., near the trailing edge of an airfoil. With the luminescent method the entire surface can be coated and any part of the surface accessible to UV illumination and the camera can be mapped. A benefit not to be overlooked is that the luminescent method avoids the cost of installing conventional pressure taps and transducers in a model. For aircraft development wind tunnel testing this can be a con- siderable expense. An exciting aspect of the method is that it

41

offers the possibility of measuring the fluctuating surface pressure field. In principle the time response of the coating to pressure transients may be as fast as the decay of lumines- cence. For platinum octaethylporphyrin this is roughly 0.1 milliseconds. However, with the present polymer coating the response to pressure fluctuations will be slowed by the rate of oxygen diffusion through the polymer. Extending the method to the study of unsteady pressure fields is one of the next aspects of our studies.

Even from a qualitative standpoint the information pro- vided by the luminescent coating can be useful. In the engi- neering design process it would allow the rapid evaluation of relative changes in the surface pressure field between differ- ent test geometries. It would also allow the determination of unusual features of the surface pressure field that require further investigation by other means. An example of interest to the aircraft design engineer would be the locations of surface pressure maximums and minimums.

Acknowledgements

The authors wish to thank J. Espina, M. Fidrich, and D. Yaste for their outstanding and indispensable technical support of the exper- iment. Thanks are also due to C. Boswell and J. Ma for creation of the color graphics, and Y. C. Cho for the use of his image processor in that endeavor. Appreciation is also extended to D. Wright for developing the practical method for preparing the coating. We also wish to thank J. Crowder of the Boeing Co. for the loan of his UV lamp during the experiment. The financial support of the Ames Directors Discretionary Fund is gratefully acknowledged. Finally, it is a pleasure to thank D. Cooper, head of the Ames Basic Research Council, who realized the significance of this work and enthusiasti- cally supported it from the very beginning.

References

Gouterman, M. 1978: Optical spectra and electronic structure of porphyrins and related rings. In: The Porphyrins, Physical Chemistry, Part A (edited by D. Dolphin). New York: Academic Press

Kavandi, J. L. 1990: Luminescence imaging for aerodynamic pres- sure measurements, Doctoral Thesis, Chemistry Department, University of Washington. Summarized in Kavandi et al., November 1990: Luminescent barometry in wind tunnels, Rev. Sci. Instrum. 61, 3340-3347

Khalil, G. E.; Green, E.; Gouterman, M., March 1989: Method and composition for measuring oxygen concentration. U.S. Patent No. 4810655

Merzkirch, W. 1987: Flow visualization. 2nd edn. Orlando: Aca- demic Press

Parker, C. A. 1968: Photoluminescence of solutions. Amsterdam: Elsevier

Peterson, J. I.; Fitzgerald, R. V., May 1980: New technique of surface flow visualization based on oxygen quenching of fluorescence. Rev. Sci. Instrum. 51,670 671

Received March 11, 1992