Development of a Sol-Gel Based Nanoporous ...

Development of a Sol-Gel Based Nanoporous Unsteady Pressure Sensitive Paint and Validation in the Large Transonic ONERA's S2MA Windtunnel Y. Michou1, B. Deléglise2, F. Lebrun3 ONERA, BP25, route départementale 215, 73500 Modane, France E. Scolan4, A. Grivel5, R. Steiger6, R. Pugin7 Centre Suisse d’Electronique et de Microtechnique, Neuchâtel, CH-2000, Switzerland and M.-C. Merienne8, Y. Le Sant9 ONERA, 8 rue des Vertugadins, 92190 Meudon, France

A new unsteady Pressure Sensitive Paint based on a nanostructured sol-gel coatings has been developped and tested on a steel model of a civil aircraft in the ONERA's S2MA transonic windtunnel. Two inserts were fitted in the model wing and coated with a sol-gel based nanostructured silica PSP and functionalized by soaking in a Ru(dpp)3 solution. A third insert was prepared with anodised-aluminum PSP for comparison. The test conditions were M = 0.875, Pi0 = 100 kPa and 185 kPa. Pressure sensitivity of the nanoporous PSP is lower than that of the AA-PSP, however its luminescence intensity is about five times higher than that of the AA-PSP, resulting in a favorable compromise. Images were acquired at camera frame rate up to 5000 fps for the nanoporous PSP and 2000 fps for the AA-PSP. The power spectral densities and coherence were determined. Results were in good agreement with those from dynamic pressure transducers.

Nomenclature AA-PSP fps Facq I IRef M p Pi0 pRef PSP PSD RMS α 1

= = = = = = = = =

= = = =

anodised Aluminum PSP frame per second PSP image acquisition rate intensity of luminescent light emitted by PSP intensity of luminescent light emitted by PSP at reference condition Mach Number surface pressure (Pa) stagnation pressure (Pa) surface pressure at reference conditions (Pa) Pressure-sensitive Paint Power Spectral Density (Pa²/Hz) Root Mean Square angle of attack of the model

Research Scientist, Wind Tunnel Division, [email protected] Research Technician, Wind Tunnel Division, [email protected]. 3 Research Scientist, Wind Tunnel Division, [email protected]. 4 Project Manager, Micro & Nano Systems division, Jaquet-Droz 1, Neuchâtel 5 Technician, Micro & Nano Systems division, Jaquet-Droz 1, Neuchâtel 6 Expert, Micro & Nano Systems division, Jaquet-Droz 1, Neuchâtel 7 Section head, Micro & Nano Systems division, Jaquet-Droz 1, Neuchâtel 8 Research Scientist, Fundamental and Experimental Aerodynamics department, [email protected]. 9 Research Scientist, Fundamental and Experimental Aerodynamics department, [email protected]. 1 American Institute of Aeronautics and Astronautics 2

I

Introduction

P

ressure Sensitive Paint (PSP) is a global surface pressure measurement technique that has long been used at ONERA as a routine technique, in both research and large scale windtunnels, for a large domain of velocities from low speed to supersonic steady state flows. The extension of the PSP technique to unsteady measurements that would provide precious informations on different phenomena like buffeting onset. It is a new challenge that requires the development of new paints with response time several orders of magnitude below that of conventional PSP. Several ways have been tested in order to reduce the response time of the paint while maintaining a high pressure sensitivity and a high intensity of luminescence. One way consists of increasing the porosity of the binder by adding hard ceramic particles in the polymer1. A second solution is to remove the binder and to fix the luminophore directly on the model surface. Asai et al.,2 and Sakaue et al.,3 have developed a fast responding PSP based on porous anodized aluminum (AA-PSP) with the dye adsorbed on the surface, achieving a very short response time of the order of few tenths of microseconds. Spectral analyses, including coherence, from unsteady PSP measurements for the study of transsonic buffet have been reported.4,5 An overview of the state-of-art describing development of PSP technique for unsteady application is given by Gregory et al.6 In a previous study by Merienne et al.,7 an anodized-aluminum PSP functionalized with a ruthenium based luminophore was used to investigate transonic buffeting effects on a civil aircraft in the ONERA's S2MA windtunnel. Acquisitions up to a frame rate of 1000 fps were achieved. Due to the low intensity of luminescence image, only results from 500 fps were analyzed: due to increased noise level, the higher acquisition rate did not bring more information. The power spectral densities obtained from PSP were in good agreement with those from unsteady pressure sensors and allowed the analysis of unsteady pressure phenomena in the shock area. However, the limited bandwith and the low luminescent image intensity were the limiting factors for the study of buffeting. In order to address those issues, a new way has been investigated at ONERA, in collaboration with the C.S.E.M., to develop a sol-gel based nanoporous pressure sensitive paint (nano PSP) with short response time and high luminescent intensity, that would provide an increased frequency bandwidth (> 2kHz), thus extending the capability of this technique to the study of buffeting. Nanoporous metal oxide films have already been successfully used as gas optical sensors.8,9 In this frame, the influence of some nanoporous film features on the sensing performances has been established.10 More specifically, the pore diameter and the total pore volume have a decisive effect on the sensitivity of dye loaded films for O2 concentration measurement. Moreover, the high film transparency enables optical measurements. Finally, the films contain a crosslinked polymer that improve their mechanical resistance. All these characteristics make these nanoporous films relevant candidates for the optical measurement of O2 pressure in a transonic windtunnel. However, the design of the film nanostructure to match the specifications of the unsteady PSP in terms of sensitivity, response time, emission intensity and mechanical resistance has not been established. More specifically, the scale-up of the deposition process on curved metallic inserts with dm2 scale has to be developed to get homogeneous PSP layers. In this study, a newly developed sol-gel based nanostructured unsteady PSP is tested on the same test model and windtunnel conditions as the previous study. For this purpose, a few challenges have been addressed. First, the nanoporous PSP composition has been designed in order to obtain suitable pore sizes enabling low response times, film thickness for an optimal luminescence intensity and mechanical stability to support the wind impact. Secondly, the functionalization process with the luminescent dye has been modified to improve the sensitivity to O2, the luminescence intensity and homogeneity over the whole surface area. Finally, nanoporous layer deposition and functionalization processes have been scaled up to cover non-even substrates with a homogeneously distributed luminescence and O2 sensitivity. The first generation of this nanoporous unsteady PSP is tested in the S2MA windtunnel. The power spectral densities and coherence are determined and compared with those from dynamic pressure transducers. Finally, ongoing developments of a second generation nanoporous PSP are discussed.

II

Experimental Methods

A. Development of sol-gel based nano-structured PSP A nanoporous coating is being developped for fast PSP application. It is obtained by sol-gel deposition technnique: first, a waterborne stable dispersion of metal oxide nanoparticles is prepared. Then this dispersion is deposited on the aluminum insert by a dip-coating process. The dimensions of the pores are directly related to the diameter of the nanoparticle size, whereas the layer thickness is controlled by the deposition process and drying conditions in the 1-20 µm range. Contrary to the anodised aluminum layer, the pores shape is not tubular but related to the shape of the nanoparticles. The pores are interconnected into a 3D network which facilitates the diffusion of 2 American Institute of Aeronautics and Astronautics

oxygen within the layer. By reducing the layer thickness, gas diffusion within the layer can be accelerated thus reducing the response time but also reducing the porous volume which impede the capacity to hold luminophore and thus reduce the pressure sensitivity of the active layer. The mesoporous layer have been subsequently functionalized by soaking in a Ru(dpp)3 solution. Many parameters from the formulation of the metal oxide aqueous dispersion (composition and concentrations) and the dip-coating process (speed of withdrawal, drying scheme) to the functionalization step (soaking time, dye concentration) have been varied to get a suitable compromise between the resulting PSP response time, sensitivity, luminescence intensity and their homogeneity over the sample area. Important aspects like sensitivity to temperature, photodegradation or homogeneity were also investigated during the development process. The two configurations which resulted in the best compromise were used for the windtunnel test. B. Test Object A steel model of a civil aircraft at a 1/21 scale, with a total wingspan of 1.5 m, is used for this study. The right wing is equipped with a replaceable aluminum-alloy insert thus allowing the testing of several configurations of PSP coatings (Figure 1). The insert is equipped with two tubular Kulite pressure sensors for comparison with PSP. Two temperature sensors, one near the wing root, one near the wing tip, are used to characterize the temperature gradient across the wing span. The left wing is equipped with pelicular pressure sensors which can be used for pressure comparison with PSP measurements at corresponding locations on the insert on the right wing. The transition is artificially triggered by CadcutTM strips.

Temperature sensors

Pressure sensors (Kulite)

130 mm

Figure 1 Sketch of the model left wing, with pellicular pressure sensors, and right wing, with an insert.

One insert (Insert #1) was prepared by ONERA with anodised-aluminum PSP following the same technique as described in 7. This insert is intended for comparison with the sol-gel based nano-structured PSP. Two inserts (Inserts #2 and #3) were coated at C.S.E.M. with a sol-gel based nanostructured silica PSP. The luminophore used is the organometallic complex Ru(dpp)3 dissolved in dichloromethane CH2Cl2: it is excited by wavelengths close to 337 nm and 457 nm and desexcited with a luminescence peak around 630 nm. The two nanoporous layer coated inserts differ only by the dip-coating process resulting in different layer thicknesses: around 16.5 µm and 10 µm, respectively for the inserts #2 and #3 (Figure 2 and Figure 3). Insert #2 has a head to tail two-pass dipping sequence whereas insert #3 is a single pass only.

3 American Institute of Aeronautics and Astronautics

Insert #3 Figure 2 SEM micrograph of the nanoporous silica layer cross-section of a sample similar to insert #3 (10 µm thick).

Insert #2

Figure 3 Fluorescence pictures (excitation 365 nm) of nano-PSP homogeneously treated inserts #2 and #3.

C. Wind Tunnel Configuration The test was conducted in the transonic S2MA windtunnel. It has a rectangular test section of 1.77 m by 1.75 m and is equipped with perforated floor and roof (global porosity of 2.6%). The wind tunnel can be pressurized up to 250 kPa. The test conditions are the following: M = 0.875, Pi0 = 100 kPa and 185 kPa. For each condition (M, Pi0) a continuous polar with an angle of attack varying from α = 0° to 4° is performed. Then several stabilized points are chosen at the relevant angle of attack such that the chock lies on one of the two pressure sensors of the PSP insert. D. PSP Measurement system The excitation of the PSP is provided by two LED lamps (HARDsoft IL-106B), each having a peak wavelength centered at 462 nm and a maximum continuous power of 7.2 W (400 lumen). Each lamp is connected to an optical fiber of 5 m length and 5 mm core diameter that guides the light to illuminators installed at the test section ceiling. The illuminated area on the right wing has a diameter around 20 cm centered on the insert. For this test, the lamps were used in continuous mode at their maximum power. The camera used is a Phantom V710 monochrome from Vision Research equipped with a 1280x800 pixels CMOS sensor of 12 bits dynamic range and 7000 ISO sensitivity. The camera was installed at the ceiling of the test section and equipped with a 50 mm focal length lens set at an aperture of f1/1.2. A high pass filter (ANDOVER 590 FG 05-50) was placed in front of the lens. The images were cropped around the PSP insert, resulting in a size of 208x400 pixels. The pixel resolution is 0.35 mm on the model. E. Dynamic Pressures E.1 Kulite sensors The acquisition and processing of dynamic pressures from the Kulite sensors are as follow. Processing during a continuous polar acquisition rate = 10000 Hz low pass filter: Fc = 1500 Hz FFT on 512 points per blocks, 33 blocks (50% overlapping) spectral resolution, df = 19.53 Hz Processing during stabilized point acquisition rate = 5000 Hz, low pass filter: Fc = 1500 Hz FFT on 512 points per blocks, 33 blocks (50% overlapping) spectral resolution, df = 9.77 Hz The position of the Kulite sensors are presented in Figure 4. 4 American Institute of Aeronautics and Astronautics

KDEX K41

KDIN

wind

K38 K37 K36 K35 K34 K33 K32 K31

Tubular pressure sensor on the right wing insert (KDIN and KDEX) Pellicular pressure sensor of the left wing at its corresponding position on the right wing (K31 to K41) ° Screw hole

Figure 4 Geometry of the insert and position of the pressure sensors (left and right wings).

E.2

PSP Processing of the PSP during a a continuous polar acquisiton rates: 2000 fps FFT on 512 points per blocks on 3 blocks (50% overlapping) spectral resolution: df = 3.91 Hz Processing of the PSP during stabilized point acquisiton rates: up to 2000 fps for AA-PSP, and 5000 fps nano-PSP number of images per point: 8704 FFT on 512 points per blocks, 33 blocks (50% overlapping) spectral resolution: df = 3.91 Hz (2000 fps) and 9.77 (5000 fps)

F. PSP Data Processing The large amount of images acquired at each run requires fast and accurate tools to perform the data reduction. All data processings were done with an in-house software based on the AFIX_2 in-house software 11 for the optical methods and adapted for use in the industrial windtunnels. The implementation of the software on GPU (Graphic Processor Unit) using the CUDA language from NVIDIA reduces significantly the computing time of two orders of magnitude. The maps of the average pressure, of the pressure fluctuations, as well as of the power spectral denstity (PSD) and coherence are computed. The extraction at a given pixel is performed on a region of interest of 5x5 pixels, over which the pressure is averaged.


III Results A. PSP Characteristics A.1 Pressure and Temperature Sensitivities The pressure sensitivity of the PSP paints has been evaluated in a pressurized chamber for each insert. Figure 5 presents the calibration curves for the AA PSP (insert #1) and the nano PSP (insert #3). The pressure sensitivity at reference pressure and temperature are 0.55%/kPa for the AA-PSP and around 0.33%/kPa for the two nanoporous PSP coated inserts. The temperature sensitivity of the paints are -1%/K and -1.5%/K, respectively for the AA-PSP and the nanoporous paints. 1.4 1.2

Iref/I

1.0 0.8 AA-PSP

0.6

(insert #1)

Nano PSP (insert #3)

0.4 0.2 0.2

0.4

0.6

0.8

1.0

1.2

1.4

P/Pref

Figure 5 Calibration curve for AA-PSP and nano PSP (inserts #1 et #3) A.2 Response time The response time of uPSP samples has been assessed by using an unsteady calibration chamber designed at Onera,10. The chamber is able to create a pressure jump with a maximum amplitude of 25kPa. The slope of the pressure jump is limited by the opening time of the valve in the calibration chamber. We define the response time as the settle time (t99) which is the time necessary to reach the final pressure level within a gap of ±1 %. It has been has been evaluated below 1 ms, which is suitable for the intended purpose. A.3 Luminescent Intensity The nanoporous PSP has a lower sensitivity to pressure than the AA-PSP, however the intensity of luminescence measured on both the inserts #2 and #3 coated with the sol-gel based nanoporous PSP was found to be about 3 to 5 times higher than that of the AA-PSP under the same conditions (Table 1). This is an important results since the intensity level obtained in the images at a short exposure time is a limiting factor for increasing the acquisition frequency. Thus the lower pressure sensitivity of the nanoporous PSP might as well be partially compensated by the higher luminescent intensity and lead to a useful compromise.

Run 8256 8258 8259

Paint / Insert #

Freq [fps]

Texpo [µs]

AA - PSP Insert #1 Nano PSP Insert #2 Nano PSP Insert #3

1000 2000 2000 5000 2000 5000

972 486 495 198 494 198

Reference Image Average Intensity [DL] 1028 530 2700 1100 3500 1400

Table 1 Comparison of the intensity of luminescence from AA-PSP and Nano PSP at various acquisition rates. 6 American Institute of Aeronautics and Astronautics

A.4 Homogeneity and local correction Homogeneity of the pressure sensitivity depends mainly on the homogeneity of the coating thickness and of the fluorophore concentration. Prior to the windtunnel test, the a priori calibration law of the PSP is determined globally for each insert in a calibration chamber with controlled pressure and temperature. A second calibration is conducted at the beginning of each run by acquiring images at various pressures. This second calibration is used to determine a "pixel by pixel" correction in order to account for the inhomogeneities of the paint. Figure 6 shows the effect of these local corrections the RMS map of the pressure for a calibration image. Inhomogeneities are visible at the edges of the inserts and around the screws on the results without pixel by pixel correction. They are reduced by the correction although not on all the surface.

Figure 6 Effect of pixel-by-pixel correction of the calibration law on the pressure rms of nano PSP (insert #3). Calibration image at Pi0 = 130 kPa (wind-off) B. Windtunnel test results B.1 Test matrix The test conditions are presented in Table 2. Run

Paint

8256

AA PSP - Insert #1

8258

Nano PSP - Insert #2

8259

Nano PSP - Insert #3

Mach

0.875

Pi0 [kPa] 100 185 100 185 100 185

α [°] 0.62, 1.02, 1.99 1.03 0.62, 1.02, 1.99 0.64° ,1.11° 0.62, 1.02, 1.99 0.64°, 1.11°

Table 2 Test Conditions


Facq [fps] 1000, 2000 1000, 2000 2000, 5000 2000, 5000 2000, 5000 2000, 5000

B.2 Average pressure, RMS and PSD The average pressure maps from AA-PSP (Insert #1) at an increasing angle of attack between 0.62° and 1.99° are presented in Figure 7 for a stagnation pressure of 100 kPa. The corresponding RMS maps for the AA-PSP and nano PSP are presented Figure 8. A low pressure area is visible which is the supersonic area before the shock.The pressure increases then after the shock. When the incidence increases, the shock first moves downstream up to around α = 1.23°, then moves upstream at higher incidences. In the low pressure area before the shock, the pressure decreases when the incidence angle increases because the Mach number increases before the shock. A second shock downstream the main shock is visible in the rms images at α = 0.62°. It is likely related to the lambda shock structure that appears when entering buffeting. At higher angles of attack, this second shock moves upstream and vanishes.

wind

α = 0.62°

α = 1.02°

-1.3

α = 1.97° +0.5 kP

Figure 7 Average pressure maps from the AA-PSP (Insert #1). M = 0.875, Pi0 = 100 kPa, Facq = 2000 fps

α = 0.62°

α = 1.02°

α = 1.97°

α = 0.62°

α = 1.02°

α = 1.99°

500

4000 Pa

Figure 8 RMS maps from the AA-PSP (Insert #1 - top row) and from the nano PSP (Insert #3 - bottom row). M = 0.875, Pi0 = 100 kPa , Facq = 2000 fps 8 American Institute of Aeronautics and Astronautics

At a stagnation pressure of 185kPa, two incidences where studied, α = 0.64° and α = 1.11°. At an incidence of α = 0.64°, the main shock lies on the KDEX sensor and at α = 1.11°, the main shock lies on the KDIN sensor. The rms map and the psd at selected positions are presented in Figure 9 for α = 0.64° and Figure 10 for α = 1.11°, both for a PSP acquisition rate of 5000 fps. In the center of the figure, the RMS map is shown and the position of the two pressure sensors on the insert are materialized by two black dots. The black square indicates the corresponding position of a pressure sensor from the left wing. The plus signs indicate the position of the pixels from the PSP image chosen for the comparison with the pressure sensors. The pixel used for the comparison of the PSP with the pressure sensors must be carefully selected because it may have a significant impact on the results, especially in areas of high gradients. In the case of the two pressure sensors on the insert of the right wing, the pixel cannot be selected at the sensor position since there is no PSP on it: therefore the comparison is made between two signals taken at different positions. However, the pixel is selected such as to best match the flow condition on the corresponding pressure sensor. For the comparison with a pressure sensor on the left wing, the pixel is chosen at the symetrical position of the sensor therefore the comparison is made under the hypothesis that the flow conditions are the same on both wings. In Figure 9, the RMS image reveals the position of the main shock that passes over the KDEX sensor, while a second shock appears downstream the KDIN sensor, close to the K37 sensor. On the main shock or the second shock where the rms is high, the PSD from the PSP shows a good agreement with the PSD from the pressure sensor until it reaches a level of 2000 Pa²/Hz. Downstream the main shock and upstream the main shock in Figure 10, in areas where the rms is low, the PSD from the PSP starts do diverge from the pressure sensor spectra: it does show the good trend but does not reproduce well the slope of the decrease. It can be noted that in the low rms areas, the PSP generaly has peaks above the PSD of the pressure sensor at frequencies below 50 Hz. Considering all cases, a ground level due to noise appears between 1000 and 2000 Pa²Hz-1 depending on the flow condition. The effect of the acquisition rate on the PSD is presented Figure 11. For the comparison, the RMS is integrated on the frequency range 0-1000 Hz. The RMS map at Facq = 5000 fps shows more noise than at 2000 fps but globally reveals the same shock pattern. The PSD does not show significant differences with respect to the PSP acquisition rate.

+

PSP Pressure sensor Kulite sensor on right wing insert Kulite sensor on left wing

+ KDEX

+

+

KDIN

pixel from from the right wing

PSP map on

K36 1000

6000 Pa

Figure 9 RMS pressure map and PSD from nano PSP (Insert #3). Pi0 = 185 kPa, M = 0.875, α = 0.64°, Facq = 5000 fps 9 American Institute of Aeronautics and Astronautics

+

PSP Pressure sensor Kulite sensor on right wing insert Kulite sensor on left wing

+ KDEX

+

pixel from PSP map right wing

+

KDIN 1000

6000 Pa

K36

Figure 10 RMS map and PSD from nano PSP (Insert #3). Pi0 = 185 kPa, M = 0.875, α = 1.11°, Facq = 5000 fps


KDEX

KDIN

Facq = 2000 fps

Facq =5000 fps

K36

500 6000 Pa Figure 11 Effect of PSP acquisition rate on rms map and psd for Facq = 2000 fps and 5000 fps. Left: PSD from PSP Pi0 = 185 kPa, M = 0.875 and α = 0.64°. For the comparison, the RMS is integrated on the frequency range 0-1000 Hz The short term repetability, i.e. within the same run, is good and the long term consistency of the results across the two inserts with nano PSP is also good. Figure 12 presents the comparison of the PSD obtained from the nano PSP with inserts #2 and #3.

KDEX KDIN K36 Figure 12 PSD from nano PSP (inserts 2 and 3). Pi0 = 185 kPa, α = 1.12°, M = 0.875 , F = 5000 fps 11 American Institute of Aeronautics and Astronautics

B.3

Coherence The coherence provides useful information on the flow for the study of the buffeting onset for example 11. The PSP allows to determine the coherence for each pixel with respect to a reference pixel. PSP provides a spatial information of high density that would be difficult to achieved with pressure sensors. The coherence, γxy, between two signals x and y is defined in Equation 1.

γ xy =

Pxy Pxx Pyy

Equation 1 Coherence where Pxy , Pxx and Pyy are respectively the cross and auto power spectral densities of the two signals. The coherence has been computed for all the pressure sensors of the left wing with respect to the reference sensor K38, which is the sensor closest to the trailing edge that also overlaps the insert area on the right wing. For the PSP insert, the reference pixel was chosen to best match the position of K38. Figure 13 shows the coherence spectra of 4 pressure sensors of the left wing (K32, , K34, K36, and K41) compared to the coherence obtained from the nano PSP (Insert #3) at the corresponding pixel positions for the condition Pi0 = 185 kPa, M = 0.875 α = 0.64°, Facq=5000 fps. The coherence and PSD maps from PSP are presented Figure 14 at selected frequencies. Figure 15 and Figure 16 are presenting the coherence spectra and coherence and psd maps for the incidence α = 1.11°. Looking first at the coherence spectra from the pressure sensors in Figure 13, the coherence level increases up to a peak around 120Hz, followed by a decrease whose slope depends on the sensor position with respect to the reference sensor K38. The coherence determined by PSP gives the correct tendencies. At an incidence of α = 0.64°, sensors K32 and K34 lie on the same side of the main shock as the reference sensor, while K41 is on the main shock. K36 is on the second shock which seems to be related to lambda shock structure that appears when entering buffeting. It can explain that K36 is the only sensor that maintains a high level of coherence ( γ > 0.5) with K38 at frequencies higher than 200 Hz with a maximum around 520Hz. This information is outlined in the coherence maps (Figure 14) obtained with PSP where a pocket of coherence appears in the trailing edge area around 520Hz. In Figure 15, at an incidence of α = 1.11°, the situation is different. PSP gives the correct tendencies except for K32 and K41 which both lie upstream the main shock while reference sensor K38 remains downstream. As mentioned in §B2, Figure 11, PSP shows its limitations to measure correct pressure level in low RMS areas especially upstream the main shock. It can explain the lack of results for K32 and K41. Results are much better for K34 and K36 which respectively lie on the shock and downstream. Overall, the coherence determined by PSP do not reproduce the absolute levels obtained with the pressure sensors but can provide the correct tendencies as long as the sensors lie on the same side of the main shock.


K32

K34

K36

K41

Figure 13 Coherence spectra from sensors K32, K34, K37 and K41 of left wing and from nano PSP (Insert #3). Pi0 = 185 kPa, M = 0.875, α = 0.64°, Facq = 5000 fps.


68.3 Hz

78.1 Hz

97.6 Hz

107.4 Hz

117.1 Hz

126.9 Hz

136.7 Hz

146.4 Hz

156.2 Hz

166 Hz

517.5 Hz

527.3 Hz

537.1 Hz

546.8 Hz

556.6 Hz

566.4 Hz

576.1 Hz

.......

......

175.7 Hz

185.5 Hz

.......

Coherence : 0 PSD

0

1 50000 Pa²Hz-1

Figure 14 Coherence and psd maps from nano PSP (Insert #3) at selected frequencies. Pi0 = 185 kPa, M = 0.875, α = 0.64°, Facq=5000 fps. 14 American Institute of Aeronautics and Astronautics

K32

K34

K36

K41

Figure 15 Coherence spectra from sensors K32, K34, K37 and K41 of left wing and from nano PSP (insert #3). Pi0 = 185 kPa, M = 0.875 α = 1.11°, Facq=5000 fps.


f = 68.3 Hz

f = 78.1 Hz

f = 97.6 Hz

f = 107.4 Hz f = 117.1 Hz f = 126.9 Hz

136.7 Hz

146.4 Hz

156.2 Hz

166 Hz

556.6 Hz

566.4 Hz

576.1 Hz

585.9 Hz

.......

.......

175.7 Hz

185.5 Hz

527.3 Hz

.......

537.1 Hz

546.8 Hz

Coherence : 0 PSD

: 0

1 50000 Pa²Hz-1

Figure 16 Coherence and psd maps from nano PSP (insert #3) at selected frequencies. Pi0 = 185 kPa, M = 0.875, α = 1.11°, Facq = 5000 fps.


The consistency of the coherence results must be assessed. The effect of the acquisition frequency on the coherence spectra determined from the nano PSP is presented in Figure 17 for PSP acquisition rates of 2000 fps and 5000 fps. The tendencies are the same for the two acquisition frequencies.

Figure 17 Coherence spectra from pressure sensor K36 of left wing and from nano PSP (insert #3) at α = 0.64° and α = 1.11°. Pi0 = 185 kPa, M = 0.875, Facq = 2000 fps and 5000 fps. B.4 Mechanical resistance The nanoporous coating showed a good mechanical resistance to the wind tunnel conditions, allowing to perform several hours of test. C. Nano PSP development Since the windtunnel test was conducted, a second generation of nanoporous PSP is under development. The pressure sensitivity, which was quite low with the first generation tested in the windtunnel (0.35%/kP) has been improved up to around 0.8 %/kPa with a better linearity. This increased sensitivity has been achieved while maintaining the high luminescence intensity found also with the first generation. Figure 18 presents the calibration curves of this 2nd generation of nano PSP detemined on a sample before and after it has been submitted to the wind in supersonic conditions for a duration approximately of 1.5 hr. The sample was placed in such a position as to best reproduce as much as possible the conditions it would have on a model wing. No PSP measurement could be made on it during this test as the time slot did not allow to. The two calibrations are almost identical, there has been no change in the luminescent intensity either. The nanoporous coating also showed a very good mecanical resistance to those high Mach conditions. However, the new formulation has not improved the temperature sensitivity which remains at around -1.2%/K. Nonetheless, the accomplished improvements should yield better accurracy of the unsteady PSP measurements in the future test that is scheduled later in 2015.


30000

1.8 Before Windtunnel Test

1.6

Before Wind Tunnel Test

25000

After Windtunnel Test

After Wind Tunnel Test

1.4 Signal [DL]

20000

Iref/I

1.2 1

15000

10000 0.8 5000 0.6 0

0.4 0.4

0.6

0.8

1

1.2

1.4

1.6

0

1.8

20

40

60

80

100

120

140

160

Pressure [kPa]

P/Pref

Figure 18 Calibration curve for the new nano PSP before and after 1.5h wind tunnel test (Mach 2.4)

Figure 19 Luminescence intensity for the new nano PSP before and after 1.5h wind tunnel test (Mach > 2)

IV Conclusions and Perspectives A new unsteady Pressure Sensitive Paint based on a nanostructured sol-gel coatings has been developped and tested on a steel model. The first generation of this PSP has a pressure sensitivity around 0.35 %/ kPa which is lower than that of the AA-PSP, however its luminescence intensity is about three to five times higher than that of the AAPSP, resulting in a favorable compromise for fast acquisition rate which require short exposure time. This first generation of nanoporous PSP has been tested on a civil aircraft in the ONERA's S2MA transonic windtunnel. Two inserts were fitted in the model right wing and coated with a sol-gel based nanostructured silica PSP and functionalized by soaking in a Ru(dpp)3 solution. The test conditions were Mach = 0.875, Pi0 = 100 kPa and 185 kPa. PSP measurements where achieved at a frame rate up to 5000 fps. Useful results as RMS, PSD and coherence maps were obtained and compared favorably with those of pressure sensors. PSP measurements provide a high density of spatial information over a large area that is difficult to achieve with pressure sensors. However, this first generation of nanoporous PSP has still ways of impovement and the ONERA and its partner C.S.E.M have maintained the development effort. Since the test was conducted, a second generation of the nanoporous PSP has been under development. The pressure sensitivity has been improved while maintaining the high luminescence intensity of the first generation. A wind tunnel test of this new generation is scheduled later this year. Future developments will aim at reducing the temperature sensitivity and improving the usability on larger 3D surfaces by applying the paint by spraying instead of by dip-coating.

Acknowledgments This work takes part in a research Carnot contract under French governmental fundings which concerns the quality of flow in industrial wind tunnels and the associated measurement techniques. The authors would like to thank all the members who operate the transonic wind tunnel.

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