September 2015

NASA/TM–2015-218801

Pressure- and Temperature-Sensitive Paint at 

0.3-m Transsonic Cryogenic Tunnel

A. Neal Watkins, Bradley D. Leighty, and William E. Lipford

Langley Research Center, Hampton, Virginia

Kyle Z. Goodman

Analytical Mechanics Associates, Inc., Hampton, Virginia
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September 2015

NASA/TM–2015-218801

Pressure- and Temperature-Sensitive Paint at 

0.3-m Transonic Cryogenic Tunnel

A. Neal Watkins, Bradley D. Leighty, and William E. Lipford

Langley Research Center, Hampton, Virginia

Kyle Z. Goodman

Analytical Mechanics Associates, Inc., Hampton, Virginia
Available from: 

NASA STI Program/Mail Stop 148 

NASA Langlet Research Center 

Hampton, Virginia 23681-2199 

Fax: 757-864-6500 

Acknowledgments 

Funding for this work was provided by the NASA Aeronautical Sciences Project under 

the Fundamental Aeronautics Program. The authors would next like to thank Dr. Joel 

Everhart, Tom Jones, and Dr. Stan Smeltzer for their continued programmatic support. 

The authors would also like to thank the personnel at the 0.3-m Transonic Cryogenic 

Tunnel for their support for the tests described in this paper. These personnel are Clifford 

Obara, Wes Goodman, Michael Chambers, Karl Maddux, Tammy Price, Reginald 

Brown, and Richard Forrest. 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute an 

official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics 

and Space Administration.
Abstract

Recently both Pressure- and Temperature-Sensitive Paint 

experiments were conducted at cryogenic conditions in the 0.3-m 

Transonic Cryogenic Tunnel at NASA Langley Research Center. This 

represented a re-introduction of the techniques to the facility after more 

than a decade, and provided a means to upgrade the measurements using 

newer technology as well as demonstrate that the techniques were still 

viable in the facility. Temperature-Sensitive Paint was employed on a 

laminar airfoil for transition detection and Pressure-Sensitive Paint was 

employed on a supercritical airfoil. This report will detail the techniques 

and their unique challenges that need to be overcome in cryogenic 

environments. In addition, several optimization strategies will also be 

discussed.

1. Introduction

The accurate determination of spatially continuous pressure and temperature distributions on 

aerodynamic surfaces is critical for the understanding of complex flow mechanisms and for comparison 

with computational fluid dynamics (CFD) predictions. Conventional pressure measurements are based on

pressure taps and electronically scanned pressure transducers or embedded pressure transducers, while 

temperature measurements are usually conducted using mounted devices such as thermocouples, RTDs, 

or thin film gauges. While these approaches provide accurate pressure and/or temperature information, 

they are limited to providing data at discrete points. Moreover, the integration of a sufficient number of 

these devices on a surface can be time and labor intensive and expensive.

The Pressure-Sensitive Paint (PSP) and Temperature-Sensitive Paint (TSP) techniques allow for the 

accurate determination of pressure and temperature distributions over an aerodynamic surface and are 

based on an emitted optical signal from a luminescent coating. However, when full flight Reynolds 

number measurements are required, it is common to use a cryogenic facility, especially if an increase in 

model size is not a viable option. In this case, there are several challenges to overcome using both the PSP 

and TSP technique.

This report will detail the results of both a TSP test and a PSP test at the 0.3-m Transonic Cryogenic 

Tunnel (0.3-m TCT) conducted at NASA Langley Research Center. These tests were aimed at reintroducing the techniques into the facility after more than a decade. In addition, several areas of 

improvement have been identified and will be discussed.

2. PSP and TSP

Introduction to PSP

The PSP technique1-5 exploits the oxygen (O2) sensitivity of luminescent probe molecules suspended 

in gas-permeable binder materials. When a luminescent molecule absorbs a photon, it transitions to an 

excited singlet energy state. The molecule can then recover to the ground state by the emission of a 

photon of a longer wavelength, known as a radiative process. However, certain of these materials can also 
2

interact with an O2 molecule such that the transition back to the ground state is non-radiative in a process 

known as collisional quenching. The rate at which these two process (radiative vs. non-radiative) compete 

is dependent on the concentration of O2 present and can be described by the Stern-Volmer relationship6

2

/ 1 ( ) I0I   KSV T PO

(1)

where I0 is the luminescence intensity in the absence of O2 (i.e. vacuum), I is the luminescence 

intensity at some partial pressure of oxygen PO2, and KSV is the Stern-Volmer constant, which is 

dependent on temperature (T).

There are several issues with this relationship, especially in regards to wind-tunnel applications; first, 

it is a practical impossibility to measure I0 in a wind tunnel application. Second, the luminescent signal 

from the paint is not only a function of pressure; it also varies with factors such as illumination intensity, 

probe concentration, paint layer thickness, and detector sensitivity. These spatial variations typically 

result in a non-uniform luminescent signal from the painted surface. The spatial variations are usually 

eliminated by taking a ratio of the luminescent intensity of the paint at the test condition with the 

luminescent intensity of the paint at a known reference condition (usually wind-off). Thus Eq. 1 can be 

cast into a more suitable form

/ ( ) ( )*( / ) REF A T B T P PREF I I  

(2)

where IREF is the recovered luminescence intensity at a reference pressure, PREF. The coefficients A(T)

and B(T) are temperature dependent constants for a given PSP formulation and are usually determined 

beforehand using laboratory calibration procedures.

PSP measurements are difficult to make under cryogenic conditions for two reasons. First, the test gas 

is typically nitrogen, refrigerant, or some other medium which typically contains little or no oxygen. 

Second, the diffusion of oxygen into the paint binder is highly temperature dependent, and at low 

temperatures, is practically nonexistent. Successful cryogenic PSP measurements have been conducted at 

0.3-m TCT,

7

the National Transonic Facility at 

NASA Langley,

8 as well as other facilities9,10

using a PSP binder that has a very large 

diffusion rate and bleeding in known amounts 

of oxygen into the flow stream. A typical 

calibration of a cryogenic PSP is shown in Fig. 

1.

Introduction to TSP

As with PSP, TSP is typically a polymerbased paint in which luminescent molecules are 

immobilized.5 However, as opposed to PSP, the 

binder in a TSP is typically chosen so that it is 

impermeable to O2. In addition, the luminescent 

molecules are typically chosen so that their 

quantum yield decreases with increasing 

Figure 1.Typical PSP response curve to increasing gas 

pressure. The gas used was 3000 ppm oxygen in nitrogen 

and the data was collected at 137K.
3

temperature (i.e. thermal quenching). The relationship between the luminescence of the probe molecule

and the absolute temperature generally follows Arrhenius behavior over a certain range5

















 

REF

NR

REF R T T

E

I(T )

I(T )

ln 1 1

(3)

where ENR is the activation energy for the non-radiative process, R is the universal gas constant, and 

TREF is the reference temperature. However, for some TSPs, Eq. (3) cannot fully describe the behavior, 

especially outside of temperature ranges where Arrenhius behavior occurs. Thus, it is also common to 

simply model the behavior of the TSP in a more generalized sense

I(T )/ I(T ) f (T / T ) REF

REF

(4)

where f(T/TREF) is a function that can be linear, polynomial, exponential, etc., to fit the experimental 

data over a suitable temperature range. This behavior is dependent on the nature of the probe, thus it is 

possible to select molecules that can lead to formulations that are temperature sensitive from cryogenic to 

473 K.5,11-15 The temperature response of several ruthenium-based luminophores as a function of 

temperature is shown in Fig. 2.

3. Experimental

0.3-m TCT

The 0.3-m TCT is a continuous-flow, single-return, fan-driven transonic tunnel which can employ 

either air (ambient temperature testing) or nitrogen (cryogenic temperature testing) as the test medium. It 

is capable of operating at stagnation temperatures from about 100 K (-280 oF) to about 322 K (120 oF) and 

stagnation pressures from slightly greater than 101 kPa (1 atm) to 607 kPa (6 atm). Test section Mach 

number can be varied from near 0 to 0.9. The ability to operate at cryogenic temperatures and high 

pressure provides an extremely high Reynolds number capability at relatively low model loadings. The 

test section has computer-controlled angle-of-attack and traversing-wake-survey rake systems. Two 

inches of honeycomb and five anti-turbulence 

screens in the settling chamber provide flow 

quality suitable for natural laminar flow testing. 

The relevant characteristics for the 0.3-m TCT 

are shown in Table 1, and additional design 

features and characteristics regarding the 

cryogenic concept in general and the 0.3-m 

TCT in particular can be found in works by 

Kilgore, Adcock, and Ray16 and Kilgore.17

Figure 2.Typical TSP response curves for three different 

ruthenium complexes.
4

Table 1. Relevant characteristics of the 0.3-m TCT

Test section dimensions 0.33 m by 0.33 m (13 in. by 13 in.)

Speed Mach 0.1 to 0.9

Reynolds Number 3.3 to 330 x 106 per m (1 to 100 x 106 per ft.) 

Temperature 100 to 322 K (-280 to 120 oF)

Pressure 101 to 607 kPa (1 to 6 atm)

Test gas Nitrogen or air

Models

There were two airfoil models employed in this work. For the TSP work, an airfoil that was designed 

for the NASA-funded Subsonic Ultra Green Aircraft Research (SUGAR) project. This project is funded 

by the Subsonic Fixed Wing Project in the Fundamental Aeronautics Program. The PSP test was 

conducted on an airfoil model designated as the NASA SC(3)-0712(B).18 The model is a supercritical 

airfoil with a 0.7 design lift coefficient and 12%-percent thick. The airfoil has a chord of 15.24 cm (6.0 

in) and is constructed from VascoMax C-200 steel. The airfoil also contained 74 pressure transducers on 

the upper surface, with approximately 40 visible in the PSP images.

Paint Formulations

This work encompasses two different wind tunnel entries. In the first entry, TSP was employed for 

transition detection, while PSP was employed in the second entry to measure the pressure distribution 

over an airfoil.

TSP: The TSP formulation used in this work was based on a formulation developed at NASA LaRC. 

Versions of this TSP have been used both in cryogenic conditions at the National Transonic Facility for 

transition detection19 and for the measurement of heating properties at hypersonic conditions.20 The 

formulation is based on a clear urethane sealant in which a ruthenium luminophore is dissolved. The 

urethane sealant acts as an oxygen impermeable binder, and the ruthenium-based luminophore can be 

easily excited using blue lights (e.g. blue LEDs) and exhibits a significant Stokes shift, emitting near 600 

nm. This allows for easy discrimination of the excitation light from the camera using optical filters.

Since this tunnel entry was conducted at a nominal temperature of ~230 K (-50 oF), the rutheniumbased luminophore used in the formulation was ruthenium bis(2,2’-bipyridine)(2,2’:6’,2”-

terpyridine)hexafluorophosphate, hereafter abbreviated Ru(bpy)2(trpy). The temperature sensitivity of 

Ru(bpy)2(trpy) is shown in Fig. 2, and displays excellent sensitivity in the expected temperature range. A 

typical synthesis of Ru(bpy)2(trpy) was accomplished by mixing 2 mmol cis-bis-(2,2’-bipyridine) 

ruthenium (II) chloride (Ru(bpy)2Cl2, GFS Chemicals), 4 mmol silver trifluoromethanesulfonate 

(Ag(CF3SO3), GFS Chemicals), and 2 mmol 2,2’,2”-terpyridine (trpy, GFS Chemicals) in 100 mL of a 

3:1 v/v solution of methanol (Aldrich, reagent grade):water. This mixture is then refluxed under nitrogen 

for 40 hours. After reflux, the solvents are removed and the residue is re-dissolved in a minimal amount 

of acetone (Aldrich, reagent grade). This is then added to a 100 mL aqueous solution containing 

approximately 6 mmol ammonium hexafluorophosphate (NH4PF6, GFS Chemicals), yielding 

Ru(bpy)2(trpy).

Previous cryogenic PSP7,8 and TSP13 tests have shown that highly polished stainless steel models must 

first be painted with a basecoat to enhance adhesion of the PSP or TSP to the model. For this work, the 

model was first coated with a self-etching primer layer (GBP 988 Self-Etching Primer, Sherwin-
5

Williams) and allowed to cure in air. Then a white basecoat created using the Spectra-Prime system 

(Sherwin-Williams) was applied using a conventional spray gun or airbrush. This layer is fully cured, 

either in air overnight or by heating to 60 oC for 1.5 hours. This layer is then wet-sanded using 2000 grit 

paper to achieve the desired finish. Finally, the urethane TSP solution is applied (again using 

conventional spraying techniques) and allowed to fully cure. The final result should be a paint layer 

between 25-50 μm (0.001-0.002 in.) thick, with a roughness (Ra) usually between 0.1-0.4 μm (5-15 μin.).

PSP: The PSP used in this formulation was one that was developed independently at NASA LaRC7

and the National Aerospace Laboratory in Japan.

9,10 This is based on using poly[1-(trimethylsilyl)-1-

propyne] (PTMSP) as the binder and applying a very thin coating of platinum mesotetra(pentafluorophenyl)porphine (Pt(TfPP)) as the PSP luminophore. PTMSP was chosen as the binder 

because it is a glassy polymer with a large free volume, enabling it to have a very high oxygen diffusion 

rate.21 This formulation has been used previously at 0.3-m TCT,

7 NTF8,22

, and other facilities9,10 because 

of its high oxygen diffusion at cryogenic temperatures as well as the fact that it can applied using 

traditional painting techniques. As with the TSP work, the model was initially painted with the same 

white basecoat.

Instrumentation

Illumination: Illumination was achieved using commercially available light emitting diode (LED) 

arrays. These arrays were designed specifically for PSP and TSP work, thus are capable of producing a 

very stable output of more than 3W with ~0.1% drift per hour after warm-up. The color of the LED arrays 

can also be changed depending on the experiment. For the TSP work, the LEDs were configured to emit 

at 460 nm (30 nm bandwidth at full width at half max (FWHM)). For the PSP w ork, the LEDs were 

configured to emit at 400 nm (20 nm FWHM). For this work, four LED arrays were used for illumination.

Image Acquisition: Images were acquired from a single camera that was placed coincident with the 

LEDs. The camera employed was a PCO.2000 camera from Cooke Corporation (now PCO-Tech Inc.). 

The PCO.2000 is a high resolution (2000 x 2000 pixel resolution) CCD camera operating with a 14-bit 

digital resolution. The camera is thermoelectrically cooled (to -50 oC relative to ambient) with a rated 

frame rate of up to 14.7 frames per second (fps) at full resolution. The camera was interfaced to the 

computer via Firewire (IEEE 1384) interface that was extended to the control room via fiber optic 

converter boxes.

Illumination and Image Acquisition 

Mounting: The optical access for the 0.3-m 

TCT consists of a “D-shaped” window that 

was originally designed for off-body flow 

visualization studies. The D-shaped 

window is constructed of Schlieren quality 

fused silica that is mounted in the upper 

half of the circular angle-of-attack 

turntables. For this work, the airfoil is 

centered horizontally in the test section 

with its center-line 1.9 cm below the lower 

edge of the window. As such, there is no 

direct optical access to the surface. A 

Figure 3. Geometry of the “D-shaped” window with a generic 

airfoil showing approximate location. From Ref. 3. 
6

diagram of the D-shaped window with a 

generic airfoil is shown in Fig. 3.23In 

addition, the D-shaped window (and test 

section) is separated from the outside of 

the tunnel by a rectangular pressure 

plenum. To facilitate illumination and 

image acquisition, a pair of mirrors were 

deployed as a periscope to allow optical 

access to the upper surface of the model. 

This periscope was attached to the test 

section door and inside the plenum. A 

photograph of the optical setup is shown 

in Fig. 4.

Optical access from outside of the 

plenum is provided by a window placed 

in the plenum wall. This window is also 

of Schlieren quality fused silica with a 

diameter of 22.9 cm. To keep the outer 

window clear of condensation (due to the 

large temperature difference on either side 

of the window), a large canister with a 

purge ring is connected to the plenum. 

The camera and the LEDs were placed in 

this canister. The canister mounted to the 

plenum is shown in Fig. 5.

Oxygen Monitoring: To facilitate 

calibration of the paint, the oxygen 

concentration in the flow must be 

measured accurately. This was done by 

interfacing an oxygen monitoring system into the tunnel just aft of the test section and just before the first 

turn in the tunnel through an existing feed-through. The oxygen sensor system (Thermox TM2000)

employs a zirconium oxide sensor with a time response of less than 10 s. The system is also equipped 

with an in situ calibration option allowing it to maintain linearity and repeatability of less than 2% of 

reading or 0.5 ppm O2 absolute. The unit is controlled by a personal computer via RS-232 protocol.

Oxygen Addition to the Tunnel: As mentioned above, PSP actually measures the partial pressure of O2. 

Thus, O2 must be present in the flow for PSP to be effective. However, in cryogenic conditions, the test 

medium is typically liquid nitrogen, which generally contains a very small amount of O2 (less than 50 

ppm depending on grade). Therefore, to use PSP at cryogenic conditions, some O2 must be injected into 

the tunnel to increase its concentration. For this test, O2 was introduced using cylinders of air. The air was 

introduced to the flow just prior to the test section (through another pre-existing feed-through). To keep 

the air from freezing at the feed-through, the line was heated using a heater tape. The flow was activated 

remotely from the control room using a solenoid valve. The air insertion equipment is shown in Fig. 6. 

With this configuration, the amount of O2 in the flow could be increased from ~20 ppm (no air flow) to 

500-1000 ppm, depending on tunnel conditions.

Figure 4. Optical setup showing the “D-shaped” window and the 

periscope assembly.

Figure 5. Canister mounted onto the side of the tunnel containing 

the camera and LED illumination sources.
7

Data Acquisition

For both the TSP and PSP experiments 

the standard radiometric method for 

acquiring data was employed. In this 

method a reference image at a known 

condition is acquired followed by an 

image at the condition to be tested. In 

these cases, conversion of the ratioed data 

to temperature or pressure can be 

accomplished using Eqs. (1)-(4) as 

described above. However, for cryogenic 

testing, several minor tweaks to the data 

acquisition procedures need to be made 

and these will be described below.

TSP: For transition detection using TSP it is generally desirable to enhance the natural transition 

temperature difference on the surface by introducing a step change in temperature to the flow. For 

cryogenic operations, this is typically dine by either temporarily increasing (to lower the temperature) or 

decreasing (to increase the temperature) the nitrogen injection rate into the facility. For this work, the 

temperature was lowered by increasing nitrogen injection. The typical data acquisition procedure 

generally followed the following paradigm:

1. The tunnel was cooled and the flow was established.

2. After the temperature had stabilized, a series of images was acquired to act as a reference image.

3. The nitrogen flow rate was increased to lower temperature. Meanwhile, image collection from the 

camera was begun.

4. Images were collected for several seconds throughout the temperature step (which generally 

resulted in a temperature decrease of ~10 K).

5. The image where the temperature change was greatest was generally selected as the image for 

ratioing with the reference image.

PSP: For the PSP work, care must be taken to ensure that the reference images are acquired under 

similar oxygen concentrations as the run images. To ensure that this occurs, the reference images were 

taken at Mach 0.2. This was done as there is very little pressure changes occurring on the surface at these 

speeds, but it also ensures that the oxygen that was injected will mix in the flow. In order to make more 

efficient use of the wind tunnel time, the general data acquisition procedure for the PSP work was as 

follows:

1. Flow was established in the tunnel and stabilized at the desired speed.

2. Air was injected into the tunnel and allowed to equilibrate as measured by the oxygen sensor 

system.

Figure 6. Air insertion line showing heating tape (to keep air 

from freezing upon injection) and solenoid for remote 

operation.
8

3. Wind-on images were acquired at various angles of attack. The oxygen concentration at each image 

was noted.

4. The speed of the tunnel was decreased to Mach 0.2, leaving the temperature constant.

5 Air was again introduced and metered to match the wind-on condition.

6. Wind-off images were taken at the same angles of attack as the wind-on condition.

4. Results and Discussion

TSP Test Results

Transition from laminar to turbulent flow over a surface is generally indicated by a small change in 

temperature on the surface. However, at transonic conditions, the adiabatic wall temperature difference 

for transition is on the order of 0.5 K,

12 which, under steady conditions, can be overwhelmed by factors 

such as heat conduction through the model surface. One method to amplify the temperature change is

artificially increasing the temperature difference between laminar and turbulent flow by introducing a 

rapid step change of temperature to the flow.24 Because the convective heat transfer coefficient of 

turbulent flow is much higher than that of laminar flow, the temperature change in the flow is more 

rapidly transferred to the surface in turbulent flow, leading to an increased change in the TSP response in 

a turbulent boundary layer. The transition line between laminar and turbulent flow can thus be detected as 

the borderline between light and dark areas in a TSP image, as shown in Fig. 7. In this work, the 

temperature step was introduced by increasing the injection of liquid nitrogen into the flow, thus creating 

a negative temperature step (temperature rapidly decreases). In Fig. 7, the light areas indicate a higher 

surface temperature, and thus laminar flow, as the temperature decrease in the flow is not efficiently 

transferred to the surface in a laminar boundary layer. Likewise, the darker areas indicate regions of 

turbulent flow.

Fig. 7 also shows one of the effects of the paint on the surface. There seems to be a multitude of 

turbulent wedges forming on the surface. 

One of the causes of these wedges is most 

certainly due to the roughness of the 

paint. While the TSP can be worked to 

improve its surface roughness, it can only 

partially correct for any surface 

roughness. In addition, the TSP coating 

can also act as an anchor for small bits of 

debris to stick to the surface. This can in 

turn also cause additional roughness, 

equivalent to small randomly placed trip 

dots. This effect can be implied by 

examining two different runs, as shown in 

Fig. 8. It is readily apparent that the 

wedges are originating from different 

points on the surface. If this was due 

simply to the inherent roughness of the 

Figure 7.Representative TSP image for transition detection. 

The lighter areas represent laminar flow while the darker areas 

represent turbulent flow.
9

Figure 8. Comparison of two different TSP runs showing that there is variability in the turbulent wedges in 

both number and origination points.

TSP, then the wedges should all be originating from the same places on the surface (there is no access to 

the model once it is cooled down, so there was no opportunity to re-work the surface). These wedges are 

numerous enough that for most cases, it was difficult to determine the exact point of transition, though 

there are some cases where the transition point can be seen.

A sample of the TSP images collected for an entire run is shown in Fig. 9, detailing the evolution of 

the temperature step throughout a single run. From Fig. 9, it can be seen that once the injection rate of 

nitrogen is increase (corresponding to t = 0s), it takes several seconds for the temperature change to really 

begin. However, once it begins (in this case, at t = 7s), the visualization of the transition can be easily 

seen. In this data set, the temperature has begun to stabilize at t > 15s as evidenced by smaller temperature 

differences occurring on the model causing the visualization of the transition areas to fade. If this data set 

had been continued, the image would have shown similar features as seen at t = 0s. For the remainder of 

the report, results will be presented using the images that provided the highest contrast between laminar 

and turbulent flow (i.e. the point of highest temperature change in the tunnel).

The transition location on the airfoil surface was investigated as a function of both Mach number and 

angle of attack. The results for Mach 0.5 are shown in Fig. 10. The approximate location of the transition 

is depicted by the dotted line. For the lowest angle of attack (AOA = -1.75o), this is difficult to identify 

due to the large number of turbulent wedges. However, for the other angles of attack, it is readily apparent 

where the transition front occurs. Furthermore, this seems to move to the leading edge as one increases 

the angle of attack. Unfortunately, the rest of the images did not provide as clear of a transition front, 

again, due to excessive turbulent wedges. The results for Mach numbers 0.67, 0.74, and 0.76 are shown in 

Figs. 11, 12, and 13, respectively. While the determination of the transition front is difficult to determine, 

there is evidence of a shock location near the trailing edge of the model. These are highlighted in Figs. 12 

and 13. It is unclear whether this is a shock on the top of the model (and visualized due to refractive index 

changes in the air during the temperature step, similar to a shadowgraph-type of effect), or transmitted as 

a temperature change through the model from the underside. There was no computational results to 
10

compare these results to see if and where a shock occurs. 

PSP Test Results

For PSP testing, data was taken at several angles of attack and Mach numbers of 0.7 or 0.5. The 

majority of the data was taken under full cryogenic conditions (116 K or -250 oF) though a set of data was 

taken at a warmer condition (250 K or -10 oF) to more closely match on of the conditions acquired in the 

first cryogenic PSP test conducted in the late 1990s.7In addition, a second cryogenic PSP formulation 

was also applied to the model in an attempt to demonstrate a more sensitive formulation. Unfortunately, 

due to an issue in the application, this formulation did not perform adequately in the tunnel. A line on the 

images that follow is used to demarcate the area that has the second PSP formulation. For this discussion, 

all data will be presented based on the Pt(TfPP) in PTMSP formulation described above.

A similar test on the same airfoil was conducted at the 0.3-m TCT in the 1990s, as mentioned above.

7

A sample of the data is shown in Fig, 14. Unfortunately, the data is currently available only as an image, 

so no manipulation of that data is possible. However, it does show that the PSP is capable of visualizing 

pressure gradients at cryogenic temperatures, with better results as the temperature is warmed (as should 

Figure 9. Series of TSP images from one run showing the evolution of the temperature step. The rate of 

temperature change is greatest at t = 11.5s.
11

Figure 12. TSP transition images at M = 0.74. Possible shock locations are also indicated.

be expected).

Results for the case at M = 0.5 taken at a tunnel temperature of 116 K is shown in Fig. 15. For this run, 

the O2 concentration could only be increased from ~18 ppm (native to the liquid N2) to ~550 ppm using 

the air bottles. While the data is fairly noisy, the general trend of the PSP does qualitatively agree with the 

pressure tap data (as shone in the comparison plots below each image). However, there are cases where 

the PSP begins to significantly deviate from the pressure taps. This, as well as the increased noise in the 

data, is most likely due to the low concentration of O2 in the flow. Previous testing7 have shown that 

Figure 11. TSP transition images at M = 0.67.

Figure 10. TSP transition images at M = 0.5. The red line demotes approximate transition location.
12

Figure 13. TSP transition images at M = 0.76. Possible shock locations are also indicated.

Figure 14.Representative PSP images acquired from the 0.3-m TCT from Ref. 7. (A) Data acquired at T = 

168 K, M = 0.7, Re = 12.3 x 1066/ft, AOA = 2o; (B) data acquired at T = 251 K, M = 0.7, Re = 3.78x106/ft, 

AOA = 2o; (C) pressure tap measurements from (B).

ideally, a concentration of more than ~1500 ppm is ideal.

When the Mach number is increased to 0.7, the results become better due to the larger pressure 

gradients on the model. The results are shown in Fig. 16. In these cases, the noise is approximately half 

that of the M = 0.5 cases and the PSP data does agree more closely with the pressure taps. The separation 

bubble near the leading edge of the airfoil also becomes very pronounced at the higher angles of attack.

When the temperature of the tunnel is increased to 250 K, it would be expected that the PSP would 

have better results. This would be mostly due to the temperature dependence of the O2 diffusion in the 
13

Figure 15. PSP data acquired at T = 116 K, M = 0.5, Re = 9.7 x 106/ft. The line in the upper regions denotes 

the location of a second PSP formulation which exhibited a different calibration and sensitivity.

Figure 16. PSP data acquired at T = 116 K, M = 0.7, Re = 12.6 x 106/ft. The line in the upper regions 

denotes the location of a second PSP formulation which exhibited a different calibration and sensitivity.

polymer. Furthermore, with the lower nitrogen injection rates (to achieve the higher tunnel temperature), 

the O2 concentration could be increased to almost twice the previous test. The results obtained at M = 0.5 

are shown in Fig. 17, and it is readily apparent that the PSP is operating better than at 116 K. The noise in 

the images have decreased significantly, though there is still some disagreement between the PSP and the 

taps, especially nearer the leading edge of the model. The cause of this is still under investigation. 
14

Figure 17. PSP data acquired at T = 250 K, M = 0.5, Re = 3.3 x 106/ft. The line in the upper regions 

denotes the location of a second PSP formulation which exhibited a different calibration and sensitivity.

Figure 18. PSP data acquired at T = 250 K, M = 0.7, Re = 4.2 x 106/ft. The line in the upper regions denotes 

the location of a second PSP formulation which exhibited a different calibration and sensitivity.

Regardless, this does show that PSP can be used in a nitrogen tunnel at lower Mach numbers.

The results are significantly improved as the Mach is increased to 0.7, as shown in Fig. 18, with much 

greater agreement between the pressure taps and PSP (with one notable exception at the most negative 

angle of attack). The separation region is also well defined. In addition, it can be seen that the second PSP 

formulation is actually beginning to show response as well, though it does have a different sensitivity 

(accounting for the apparent differences in pressure in this region).
15

Figure 19. PSP data acquired at T = 250 K with no injection of air. The line in the upper regions denotes 

the location of a second PSP formulation which exhibited a different calibration and sensitivity.

Figure 20. PSP data acquired at T = 250 K with no injection of air. This is calibrated using the second 

PSP formulation (below the dotted line).

The greater sensitivity of the PSP at the higher temperatures is almost completely due to the increased 

diffusion rate of O2 in the PTMSP binder, with the increased O2 concentration having a secondary effect. 

This can be proven as a setoff PSP runs was collected at the higher temperature without any addition of 

O2 to the flow. In this case, the native O2 concentration 35-70 ppm (some change was noticed, most likely 

due to the final bleed-out of the air from a previous run), and the PSP should have shown little, if any, 

response. However, as can be seen in Fig. 19, there is a significant PSP response. While it is higher in 

noise than the previous runs, the separation bubble is evident at both speeds, and the PSP data has a fair 
16

qualitative agreement with the taps. One other observation to note is that the second PSP formulation 

seems to have a significantly different response. To investigate this, the images were calibrated with 

regards to the second PSP formulation, and the results are shown in Fig. 20. While noisier than the first 

PSP formation, the results are fairly similar.

There is one final observation that needs to be made. There is a significant difference in the airfoil 

performance between the 1990s and this test. This is readily apparent by simply observing the pressures 

measured by the taps in Fig. 14c and comparing with the pressures measured in this experiment. 

Therefore, a meaningful direct comparison between this data and the previous data cannot easily be made. 

Regardless, this work has shown that PSP has been successfully reintroduced to the 0.3-m TCT.

5. Conclusions and Future Work

This report has presented results from two recent tests at the 0.3-m TCT in order to re-introduce the 

optical paint techniques back into the facility. This required essentially re-learning the techniques due to 

several factors, including the loss of experience and the need to implement newer technologies. These 

factors were overcome and the results of the tests were both successful. Hopefully, this report will serve 

as a means to convey the knowledge and experience gained in these tests for possible future work 

conducted in this facility.

For the TSP test, the detection of transition points had some success but also had several issues. These 

issues involved the generation of an excessive number of transitional wedges on the surface which made 

the accurate determination of the transition point difficult to accurately determine. Transitional wedges 

are usually formed by surface roughness, and this could very well be one of the causes of their formation, 

even though the surface of the TSP was worked to as smooth a finish as possible. There were no edges on 

the leading edge, as the TSP was applied around the leading edge to a point ~20% of the chord on the 

underside of the airfoil. Another possibility could be debris from the tunnel flow, and there is evidence 

that this indeed caused a significant amount of the wedges, mostly from the fact that the wedge 

origination points are not consistent from one run to another. If this was truly due solely to the finish of 

the paint, then it could be assumed that the vast majority of the wedge origination points would be 

consistently in the same place, especially given the fact that there was no access to the model between 

runs. Regardless, this test showed that the mounting strategy for the lights and camera was viable, the 

methodology to produce the temperature steps was correct, and the overall performance of the TSP for 

capturing the temperature changes was sufficient to easily visualize the flow phenomena on this surface.

Extension of this testing to PSP was then done using a supercritical airfoil shape. For the PSP, the 

addition of a suitable means to introduce O2 to the flow as well as measure the concentration was 

required. This was done using existing plumbing connections in the tunnel (i.e. no physical modification 

of the facility was needed), and seemed to provide an adequate solution to the problem. One issue that 

was encountered was that using the air injection system designed for this experiment, the overall 

concentration of O2 that could be introduced was significantly less than ideal (at most about half of what 

was used in previous testing in the 1990s.

7 This resulted in some added noise to the images (especially in 

the Mach 0.5 cases), but there was a generally good agreement between the pressure taps and PSP. For 

the faster speeds, the PSP data was less noisy (due to the larger pressure gradients on the model). When 

the temperature was increased from 116 K to 250 K, the results were much better due to the increased rate 

of diffusion of O2 in the PTMSP binder. This was proven as a set of data was acquired at 250 K that had 

less than 100 ppm O2 in the flow, and adequate results could still be obtained.
17

Figure 21.LED-based arrays custom designed for use in the 

National Transonic Facility. From Ref. 8.

Even with these results, there are several areas that can be further optimized for improved results. 

These include the need to improve the final TSP coating, investigate some new lighting techniques, trying 

to increase the O2 concentration in the tunnel, and the development of newer, more sensitive cryogenic 

PSP formulations. Some ideas for each of these areas will be briefly discussed below.

Further development of the TSP application and treatment of the final coating will continue to obtain 

as smooth a coating as possible. However, this will most likely not completely alleviate the transitional 

wedge problem if there is debris or contamination from the tunnel flow. The TSP formulations that are 

currently available have good sensitivity throughout the possible temperature ranges of the 0.3-m TCT, so 

no further development of these needs to be done.

One of the issues that needs to be addressed is the illumination of the model. While the LED-based 

lighting that was used in these tests is significantly better in terms of brightness, stability, and energy 

consumption than the quartz-halogen lamps used in the previous tests, the quartz-halogen lamps had a 

significant advantage in that they could be placed in tunnel plenum and able to illuminate the airfoil 

surface from the top. This allowed for a much brighter illumination field on the model surfac e compared 

with this testing, which had the LEDs imaging the surface through the periscope assembly. In their 

current configuration, the LED lights cannot be placed in the cryogenic environment inside the plenum 

for several reasons. First, mechanically, they are not designed for this environment, and several 

components that are epoxied will most likely delaminate. Second, LEDs do not operate natively in 

cryogenic environments. Light generation by an LED is accomplished through the interaction of electrons 

with electron-holes that are generated in a semiconducting material with impurities. As the temperature is 

lowered, the current required to generate the photons (through these electron and electron-hole 

interactions) is generally increased to overcome the increasing band gap. Eventually, this band gap 

becomes too large and no photons are generated. This issue was solved in the NTF using specially 

designed LED arrays, as shown in Fig. 21.

8,25 Briefly, these arrays consist of 80 individual LED elements 

arranged on a 12.7 cm diameter 4.5 m thick aluminum substrate. The aluminum substrate is also equipped 

with an RTD sensor to monitor temperature as well as resistive heaters on the back for cryogenic 

operation. Employing these above the test section should allow a better illumination field as well as 

provide more freedom to position the camera to conceivable image more of the surface.

If the current generation of cryogenic PSP formulations are employed, the O2 concentration in the 

tunnel will need to be increased to improve 

results. There are several possible solutions 

to this. First, the tube connecting the air 

bottle to the injection point could be 

increased to increase the flow rate. This 

would necessarily increase the O2

concentration (by a factor of ~4 if the tube 

diameter is doubled), though the air bottles 

would be used at a much faster rate (by the 

same factor of ~4). Another alternative 

would be to replace the air cylinders with 

cylinders of O2 gas. This would increase the 

concentration of O2 be a factor of ~5 without 

any other modifications. However, this does 

introduce potential safety issues that will 
18

need to be addressed at both the injection point as well as the storage location for the cylinders. Finally, 

instead of using gas cylinders, the injection could be tied directly to the air line from the NASA steam 

plant. This method was recently used at the National Transonic Facilit8 without any direct evidence of 

frost generation (from water introduced with the air). 

Finally, while the current generation of cryogenic PSP formulations can provide suitable data, 

formulations that have a higher sensitivity to O2 will probably need to be developed. This could involve 

optimization of two components of the PSP. First, the development of a binder that has an even greater 

permeability to O2 can be realized to enhance sensitivity. However, the PTMSP has been shown to have 

one of the highest O2 permeability currently, so this could be a difficult task to accomplish. Alternately, 

the Pt(TfPP) dye could be replaced with another dye that would show greater sensitivity. This would most 

likely require the new dye to have a significantly longer lifetime than Pt(TfPP). Several candidate 

luminophores have been identified and testing of these is currently underway.

References

1. Kavandi, J., Callis, J., Gouterman, M., Khalil, G., Wright, D., Green, E., Burns, D., and 

McLachlan, B., “Luminescent Barometry in Wind Tunnels,” Review of Scientific 

Instrumentation, Vol. 61, No. 11, 1990, pp. 3340-3347.

2. Morris, M.J., Benne, M.E., Crites, R.C., and Donovan, J.F., “Aerodynamic Measurements 

Based on Photoluminescence,” 31st Aerospace Sciences Meeting, AIAA, Reno, NV, 1993,

Paper 93-0175.

3. McLachlan, B., and Bell, J., “Pressure-Sensitive Paint in Aerodynamic Testing,” Experimental 

Thermal and Fluid Science, Vol. 10, No. 4, 1995, pp. 470-485. 

4. Liu. T., Campbell, B., Burns, S., and Sullivan, J., “Temperature- and Pressure-Sensitive 

Luminescent Paints in Aerodynamics,” Applied Mechanics Reviews, Vol. 50, No. 4, 1997, pp. 

227-246.

5. Liu, T., and Sullivan, J., Pressure and Temperature Sensitive Paints (Experimental Fluid 

Dynamics), Springer-Verlag, Berlin, 2004.

6. Lakowicz, J., Principles of Fluorescence Spectroscopy, 2nd ed., Kluwer Academic/Plenum 

Publishers, New York, 1999, pp. 239-242.

7. Upchurch, B.T. and Oglesby, D.M., “New PSP Developments at NASA Langley Research 

Center – Low Temperature PSP,” Proceeding of the 6th Annual Pressure Sensitive Paint 

Workshop, Seattle, Washington, October 6-8, 1998, pp. 10-1 – 10-24.

8. Watkins, A. N., Leighty, B.D., Lipford, W.E., Oglesby, D.M., Goodman, K.Z., Goad, W.K., 

Goad, L.R., and Massey, E.A., “The Development and Implementation of a Cryogenic 

Pressure Sensitive paint System in the National Transonic Facility,” 47th AIAA Aerospace 

Sciences Meeting including The New Horizon Forum and Aerospace Exposition , AIAA, 

Orlando, FL, 2009, Paper 2009-421.

9. Asai, K., Amao, Y., Iijima, Y., Okura, I., and Nishide, H., “Novel Pressure-Sensitive Paint for 

Cryogenic and Unsteady Wind Tunnel Testing,” 21st AIAA Aerodynamic Measurement 

Technology and Ground Testing Conference, Denver, Colorado, June 19-22, 2000, Paper 

2000-2527.
19

10. Asai, K., Amao, Y., Iijima, Y., Okura, I., and Nishide, H., “Novel Pressure-Sensitive Paint 

for Cryogenic and Unsteady Wind-Tunnel Testing,” Journal of Thermophysics and Heat 

Transfer, Vol. 16, No. 1, 2002, pp. 109-115.

11. Gallery, J., Gouterman, M., Callis, J., Khalil, G., McLachlan, B., and Bell, J., “Luminescent 

Thermometry for Aerodynamic Measurements,” Review of Scientific Instrumentation, Vol. 

65, No. 3, 1994, pp. 712-720.

12. Fey, U., Engler, R.H., Egami, Y., Iijima, Y., Asai, K., Jansen, U., and Quest, J., “Transition 

Detection Using Temperature Sensitive Paint at Cryogenic Temperatures in The European 

Transonic Wind Tunnel (ETW),” 20th International Congress on Instrumentation in 

Aerospace Simulation Facilities, IEEE, Gottingen, Germany, 2003, pp. 77-88.

13. Hamner, M. P., Popernack, T. G., Jr., Owens, L. R., and Wahls, R. A., “Using Temperature 

Sensitive Paint Technology,” 40th Aerospace Sciences Meeting, AIAA, Reno, NV, 2002, 

Paper 2002-0742.

14. Liu, T., Campbell, B.T., Sullivan, J.P., Lafferty, J., and Yanta, W., “Heat Transfer 

Measurement on a Waverider at Mach 10 Using Fluorescent Paint,” Journal of 

Thermophysics and Heat Transfer, Vol. 9, No. 4, 1995, 605-611.

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R.L., “Assessment of the National Transonic Facility for Laminar Flow Testing,” 48th AIAA 

Aerospace Sciences Meeting including The New Horizon Forum and Aerospace Exposition , 

AIAA, Orlando, FL, 2010, Paper 2010-1302.

20. Watkins, A.N., Buck, G.M., Leighty, B.D., Lipford, W.E., and Oglesby, D.M., “Using 

Pressure- and Temperature-Sensitive Paint in the Aftbody of a Capsule Vehicle,” AIAA 

Journal, Vol. 47, No.4, 2009, pp. 821-829.

21. Masuda, T., Isobe, E., and Higashimura, T., “Poly[1-(trimethylsilyl)-1-propyne]: A New 

Polymer Synthesized With Transition-Metal Catalysts and Characterized By Extremely High 

Gas Permeability,” Journal of the American Chemical Society, Vol. 105, No. 25, 1983, pp. 

7473-7474.

22. Watkins, A.N., Goad, W.K., Obara, C.J., Sprinkle, D.R., Campbell, R.L., Carter, M.B., 

Pendergraft, O.C., Bell, J.H., Ingram, J.L., Oglesby, D.M., Underwood, P.J., and Humber, 

L.R., “Flow Visualization at Cryogenic Conditions Using a Modified Pressure Sensitive Paint 

Approach,” 43rd AIAA Aerospace Sciences Meeting and Exhibit, AIAA, Reno, NV, 2005, 

Paper 2005-456.

23. Burner, A.W. and Goad, W.K., Flow Visualization in a Cryogenic Winf Tunnel using 

Holography. NASA-TM-84556, 1982.
20

24. Asai, K., Kanda, H., Kunimasu, T., Liu, T., and Sullivan, J.P., “Boundary-Layer Transition 

Detection in a Cryogenic Wind Tunnel Using Luminescent Paint,” Jounal of Aircraft, Vol. 

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Results From a Pressure Sensitive Paint Test Conducted at the National Transonic Facility on 

Test 197: The Common Research Model. NASA/TM-2011-217065, 2011.
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Pressure- and Temperature-Sensitive Paint at 0.3-m Transonic Cryogenic 

Tunnel 

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 6. AUTHOR(S)

Watkins, A. Neal; Leighty, Bradley D.; Lipford, William E.; Goodman, 

Kyle Z.

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NASA Langley Research Center

Hampton, VA 23681-2199

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14. ABSTRACT

Recently both Pressure- and Temperature-Sensitive Paint experiments were conducted at cryogenic conditions in the 0.3-m 

Transonic Cryogenic Tunnel at NASA Langley Research Center. This represented a re-introduction of the techniques to the 

facility after more than a decade, and provided a means to upgrade the measurements using newer technology as well as 

demonstrate that the techniques were still viable in the facility. Temperature-Sensitive Paint was employed on a laminar airfoil 

for transition detection and Pressure-Sensitive Paint was employed on a supercritical airfoil. This report will detail the 

techniques and their unique challenges that need to be overcome in cryogenic environments. In addition, several optimization 

strategies will also be discussed. 

15. SUBJECT TERMS

Cryogenic testing; Global surface measurements; Pressure sensitive paint; Temperature sensitive paint

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