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Hypersonic wind tunnel flow characterization by means of advanced laser spectroscopy

Giuseppe Ceglia and Antonio Del Vecchio

Dept. of Diagnostic Methodologies and Measurement Techniques, CIRA Italian Aerospace Research Center, Italy.

g.ceglia@cira.it

61st Course Hypersonic Meteoroid Entry Physics of the International School of Quantum Electronics Ettore Majorana Foundation and Centre for Scientific Culture・ERICE | ITALY ・

October 3 – 8, 2017

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OUTLINE

Non-Intrusive diagnostics:

• Optical emission spectroscopy;

• Laser induced fluorescence LIF.

Two-photon absorption LIF:

• density measurement.

Experimental setup;

Results:

• data reduction;

• calibration;

• atomic oxygen density.

Conclusions.

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NON-INTRUSIVE DIAGNOSTICS (1/5)

The characterization of arcjet flows involves the evaluation of the thermodynamic state of the test gas determining its distribution of energy in terms of kinetic, chemical, and thermal modes1.

Quantify the degree of departure from thermochemical equilibrium

1 Fletcher, Douglas G. Nonintrusive diagnostic strategies for arcjet stream characterization. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION MOFFETT FIELD CA AMES RESEARCH CENTER, 2000.

NO

A variety of different molecular-based optical measurement techniques are used to study aerospace flows:• Optical Emission spectroscopy;• Laser-Induced fluorescence;• Rayleigh and Raman scattering;• Coherent Anti-Stokes Raman Spectroscopy.

• Different temperatures:- Translational;- Rotational;- Vibrational;- Electronic.

• Number Density;• Pressure;• Velocity.

Use or disclosure of the information contained herein is subject to specific written approval from CIRA 4

SHS projectHot structure testing for the ESA Space Rider future vehicle

GHIBLI arcjet test

Optically based non-intrusive diagnostic strategies attempt to determine flow or thermodynamic properties from the analysis of the light that is emitted from either a thermally-induced or laser-induced process1.

1 Fletcher, Douglas G. Nonintrusive diagnostic strategies for arcjet stream characterization. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION MOFFETT FIELD CA AMES RESEARCH CENTER, 2000.

Optical Emission Spectroscopy:• suitable for studying luminous environment.

At modest temperatures (−𝐸𝐸𝑢𝑢/𝑘𝑘𝑘𝑘>1)

Measurements of two or more spectral features with different upper state provide the temperature.

Spectral radiance

𝐼𝐼𝜆𝜆 =𝑛𝑛𝐴𝐴𝑢𝑢𝑢𝑢ℎ𝑣𝑣𝑢𝑢𝑢𝑢

4𝜋𝜋𝜙𝜙 𝜆𝜆

𝑔𝑔𝑢𝑢𝑒𝑒−𝐸𝐸𝑢𝑢/𝑘𝑘𝑘𝑘

𝑄𝑄𝐿𝐿

𝑛𝑛𝐴𝐴𝑢𝑢𝑢𝑢ℎ𝑣𝑣𝑢𝑢𝑢𝑢4𝜋𝜋

radiant power density;

𝜙𝜙 𝜆𝜆 line shape function;

𝑔𝑔𝑢𝑢𝑒𝑒−𝐸𝐸𝑢𝑢/𝑘𝑘𝑘𝑘

𝑄𝑄Normalized Boltzmanndistribution (thermal equilibrium);

𝐿𝐿 optical detection length.

SHS projectHot structure testing for the ESA Space Rider future vehicle

GHIBLI arcjet test

NON-INTRUSIVE DIAGNOSTICS (2/5)

Use or disclosure of the information contained herein is subject to specific written approval from CIRA 5

SHS projectHot structure testing for the ESA Space Rider future vehicle

GHIBLI arcjet test

Optically based non-intrusive diagnostic strategies attempt to determine flow or thermodynamic properties from the analysis of the light that is emitted from either a thermally-induced or laser-induced process1.

1 Fletcher, Douglas G. Nonintrusive diagnostic strategies for arcjet stream characterization. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION MOFFETT FIELD CA AMES RESEARCH CENTER, 2000.

Optical Emission Spectroscopy:• suitable for studying luminous environment.

Drawbacks to emission-based property measurements:

• Assumption that the thermodynamic behavior of the populated excited electronic state is representative of the ground states;

• difficulty to obtain spatially resolved emission intensities.

Selective ability to probe ground

state populations

High spatial resolution

measurements

Laser-Induced process

NON-INTRUSIVE DIAGNOSTICS (3/5)

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Optically based non-intrusive diagnostic strategies attempt to determine flow or thermodynamic properties from the analysis of the light that is emitted from either a thermally-induced or laser-induced process1.

Laser-Induced Fluorescence LIF is based on the excitation of a species with laser light, which matches the possible energy differences of its energy levels.

After the excitation process, the species returns to its ground state by emitting red shifted radiation or through thermal relaxation due to collisions with other neighbor ones2.

• hv energy of the absorbed and emitted photons;• B12 and A21 probabilities for absorption and emission;• Q21 collisional quenching rate;• Iv laser’s spectral irradiance.

2Tropea, C., Yarin, A. L., and Foss, J. F. Springer handbook of experimental fluid mechanics (Vol. 1). Springer 2007

1

2

hv hvB12Iv Q21 A21

Two-level energy model of single-photon fluorescence

For arcjet flows, useful information would be obtained from LIF measurements in air for the main species:• Atomic Oxygen O;• Atomic Nitrogen N;• Nitric oxygen NO;• Argon.

NON-INTRUSIVE DIAGNOSTICS (4/5)

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The calibration is based on reference measurements with noble gas at known concentration.

Xenon can be used as reference gas:

• Spatial, temporal and spectral intensity distribution as the atomic oxygen3;

• Simpler arrangement of the experimental setup.

3Niemi, K., V. Schulz-Von Der Gathen, and H. F. Döbele. Absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence spectroscopy in an RF-excited atmospheric pressure plasma jet. Plasma Sources Science and Technology 14.2 375 (2005)

Unfortunately, single-photon transition for N and O requires excitation source at Vacuum Ultraviolet wavelengths making difficult the implementation in large-scale facilities.One solution is to use two-photon absorption LIF technique TALIF.

TALIF measurements for atomic oxygen density estimation

NON-INTRUSIVE DIAGNOSTICS (5/5)

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LASER INDUCED FLUORESCENCE: DENSITY MEASUREMENT

The calibration is performed by comparing the fluorescence signal of the xenon at known density withrespect to that of the atomic oxygen.

For unsaturated TALIF applicable for both xenon and atomic oxygen using the same experimentalsetup, the ratio of the oxygen number density nO to xenon one nXe reads as

𝑛𝑛𝑂𝑂𝑛𝑛𝑋𝑋𝑒𝑒

=𝐸𝐸𝑋𝑋𝑒𝑒2

𝐸𝐸𝑂𝑂2𝐴𝐴21𝑋𝑋𝑒𝑒𝐴𝐴21𝑂𝑂

𝜏𝜏𝑋𝑋𝑒𝑒𝜏𝜏𝑂𝑂

𝜎𝜎𝜔𝜔,𝑋𝑋𝑒𝑒(2)

𝜎𝜎𝜔𝜔,𝑂𝑂(2)

𝑔𝑔(Δ𝜔𝜔𝑋𝑋𝑒𝑒)𝑔𝑔(Δ𝜔𝜔𝑂𝑂)

𝑆𝑆𝑂𝑂𝑆𝑆𝑋𝑋𝑒𝑒

𝜂𝜂𝑋𝑋𝑒𝑒𝜂𝜂𝑂𝑂

Measured parameters

From literature𝐴𝐴21𝑋𝑋𝑋𝑋𝐴𝐴21𝑂𝑂

= 0.85 by Niemi et al.3

𝜎𝜎𝜔𝜔,𝑋𝑋𝑋𝑋(2)

𝜎𝜎𝜔𝜔,𝑂𝑂(2) = 1.9, ~20% of uncertainty (Niemi et al.3)

Goal of the present

measurements

3Niemi, K., V. Schulz-Von Der Gathen, and H. F. Döbele. Absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence spectroscopy in an RF-excited atmospheric pressure plasma jet. Plasma Sources Science and Technology 14.2 375 (2005)

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EXPERIMENTAL SETUP (1/4)

Output laser beam with a maximum energy of ~500 µJ per pulse (10Hz) and a measured linewidth of2.3±0.05 pm.The final wavelength ranges between 224.000 nm and 224.020 nm for the xenon investigation and225.340 nm and 225.360 nm for the atomic oxygen investigation. The step width is ~0.5 pm.

L2K operating conditions at 1MW/m2

Airmass flow rate [g/s] 36

reservoir pressure [Pa] 105

reservoir temperature

[K] 4218

reservoir density [kg/m3] 0.0702total enthalpy [MJ/kg] 8.4

Simulated parametersmass fraction N2 0.7433mass fraction O2 0.0223mass fraction NO 0.0439mass fraction N 0.0017mass fraction O 0.1889

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EXPERIMENTAL SETUP (2/4)

Optical setup arrangement outside of the L2K test chamber consists of: (A) Laser Exit;(B) Variable Attenuator;(C) Beam splitter;(D) Ultrafast photodiode.

Variable attenuator allows to vary the energy of the laser radiation and prevent saturation effects.

Photodiode monitors the amount of the laser energy during its time evolution and it allows for time resolved measurements, for the evaluation of the terms ⁄𝐸𝐸𝑋𝑋𝑒𝑒2 𝐸𝐸𝑂𝑂2.

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EXPERIMENTAL SETUP (3/4)

Optical detection system consists of: (A) PhotoMultiplier Tube;(B) Interference filter;(C) Fused-Silica lens L1, f=-50 mm;(D) Fused-Silica lens L2, f=100 mm.

The fluorescence radiation is observed transverse, i.e. ~90°, with respect to the direction of the laser beam.

L1 and L2 with diameter of 1’and 2’ and with -50 mm and 100 mm of focal length, respectively, are disposed in series at relative distance of L~54 mm.

L2Object plane

Imageplane

L1

LInterference

filter

37%Xenon 30%

atomicoxygen

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EXPERIMENTAL SETUP (4/4)

L2K test chamber: nozzle exit

Maximum Laser Energy at nozzle centreline: 180µJ.Reference Gas Cell:• 4 optical windows installed on opposite faces;• Quartz lens;• Pressure sensor;• Vacuum pump connection.

Laser Energy [µJ] Atomic oxygen Xenon

Laser exit

155mm from

Nozzle exit

Freestream

Shock layer

Pressure [Pa]

139 246 344 698

200 x x x x x x

300 100 x x x x x x

400 140 x x x x x x

500 180 x x x

600 x x

Wavelength [nm] (0.5pm step)

225.340÷225.360 224.000÷224.020

10 uncorrelated samples are acquired for each investigated wavelength.

400 samples are gathered by ranging the wavelength in a 10-neighbourhood of 224.010 nm and of 225.350 nm for xenon and oxygen, respectively.

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RESULTS (1/8): DATA REDUCTION

Data reduction methods: phase-shift and vertical-shift

Raw data: 4 uncorrelated signals

𝑆𝑆𝑆𝑆𝑔𝑔𝑛𝑛𝑆𝑆𝑆𝑆𝑁𝑁𝑁𝑁𝑆𝑆𝑁𝑁𝑒𝑒

The signal of the laser light represented by the positive peaks is phase-shifted in time with respect to each other, because the timing system is triggered on the signal of the pump laser.

This can affect the averaging process causing the reduction of the signal/noise ratio.

Home-built software based on cross-correlationalgorithm with zero-padding procedure is applied to reduce the phase-shift of the uncorrelated data.

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RESULTS (2/8): DATA REDUCTION

𝑆𝑆𝑆𝑆𝑔𝑔𝑛𝑛𝑆𝑆𝑆𝑆𝑁𝑁𝑁𝑁𝑆𝑆𝑁𝑁𝑒𝑒

Data reduction methods: phase-shift and vertical-shift

Raw data: 4 uncorrelated signals

The signal is also affected by a vertical shift due to the gain of ~4.4V applied on the Photomultiplier device.

This vertical shift is estimated in statistical sense by averaging over time each data with respect to the wavelength and by calculating the parameters that characterize the fitting curve.

Averaged signal over time with respect to the wavelength for xenon at 698Pa: raw data (dots), Voigt fitting (continuous blue line).

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RESULTS (3/8): CALIBRATION

To investigate the fluorescence signal quantitatively as concentration measurements

Linear dependency of the fluorescence signal with respect to the square of the laser energy has to be verified.

Integrated fluorescence signal over time with respect to the square of the laser intensity and linear fitting for atomic oxygen (a) and for xenon (b).

The slopes of the linear fitting curves for the freestream and for the shock layer investigations are 2.01±0.16 and 2.11±0.20, respectively.This discards the presence of saturationor ionization processes which lead to a reduced fluorescence signal.

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RESULTS (4/8): CALIBRATION

Slope 2.0

To investigate the fluorescence signal quantitatively as concentration measurements

Linear dependency of the fluorescence signal with respect to the square of the laser energy has to be verified.

Integrated fluorescence signal over time with respect to the square of the laser intensity and linear fitting for atomic oxygen (a) and for xenon (b).

The quadratic dependency is not satisfied for all investigated xe 6p’[3/2]2 transitions at different pressure values.

For xenon at 139 Pa, the slope is equal to 1.81±0.09.

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RESULTS (5/8): CALIBRATION

𝜏𝜏𝑋𝑋𝑒𝑒=28.2 ns 𝜏𝜏𝑂𝑂𝑓𝑓𝑓𝑓𝑋𝑋𝑋𝑋𝑓𝑓𝑓𝑓𝑓𝑓𝑋𝑋𝑓𝑓𝑓𝑓 =25.8 ns 𝜏𝜏𝑂𝑂𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑘𝑘 𝑢𝑢𝑓𝑓𝑙𝑙𝑋𝑋𝑓𝑓=11.4 ns

Averaged fluorescence signal of the xenon (a) at the pressure of 139 Pa and atomic oxygen in the freestream (b) and in the shock layer (c); the exponential fitting curve is superimposed.

Exponential function F=Ze− ⁄t τ

• Z represents the amplitude of the signal, is superimposed in order to evaluate the measured lifetime 𝜏𝜏 of the species.

• fitting procedure is based on a Levenberg-Marquardt algorithm.

The fluorescence signal of the atomic oxygen increases in the shock layer respect to the freestream investigation.

𝜏𝜏𝑂𝑂𝑓𝑓𝑓𝑓𝑋𝑋𝑋𝑋𝑓𝑓𝑓𝑓𝑓𝑓𝑋𝑋𝑓𝑓𝑓𝑓 > 𝜏𝜏𝑂𝑂𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑘𝑘 𝑢𝑢𝑓𝑓𝑙𝑙𝑋𝑋𝑓𝑓𝑝𝑝𝑒𝑒𝑆𝑆𝑘𝑘𝑓𝑓𝑓𝑓𝑒𝑒𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑒𝑒𝑓𝑓𝑓𝑓 < 𝑝𝑝𝑒𝑒𝑆𝑆𝑘𝑘𝑆𝑆ℎ𝑆𝑆𝑆𝑆𝑘𝑘 𝑢𝑢𝑓𝑓𝑙𝑙𝑒𝑒𝑓𝑓

atomic oxygen concentration increases in the shock layer

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RESULTS (6/8): CALIBRATION

Inverse lifetime of xenon 6p’ [3/2]2 TALIF measurements for different laser energies, linear fitting curves superimposed.

3Niemi, K., V. Schulz-Von Der Gathen, and H. F. Döbele. Absolute atomicoxygen density measurements by two-photon absorption laser-induced fluorescence spectroscopy in an RF-excited atmospheric pressure plasma jet. Plasma Sources Science and Technology 14.2 375 (2005)

The fitting curve with linear regression calculated from the inverse lifetime:

• at laser energy ~300 µJ, the intercept is equal to ~2.4×107 s-1 in agreement with Niemi et al2 (𝐴𝐴21𝑋𝑋𝑒𝑒= 2.45×107 s-1);

• at laser energy of ~400 µJ, the fitting curve is translated in vertical direction and it is characterized by an intercept equal to ~2.7×107 s-1 in agreement with that found by Eichhorn et al4.

4Eichhorn C. Dissertation, Über die Mehr-Photonen-Spektroskopie an Xenon zur plasmadiagnostischen Untersuchung elektrischer Weltraumantriebe, Uni. Stuttgart (submitted)

There is not presence of ionization or inter-level cross processes that could compromise the calibration procedure.

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RESULTS (7/8): ATOMIC OXYGEN DENSITY

Absorption profile of xenon (6p’ [3/2]2) at cold gas cell condition P=139 Pa, Voigt fitting (continuous line) is superimposed

Voigt function is based on the convolution of a Gaussian function with a Lorentzian function:• 𝑉𝑉 = 𝐺𝐺 ⊗ 𝐿𝐿;

• 𝜆𝜆𝐺𝐺 , 𝑋𝑋𝑒𝑒= 8𝑘𝑘𝑘𝑘 𝑢𝑢𝑛𝑛2

𝑓𝑓𝑆𝑆2𝑓𝑓0. 𝜆𝜆𝐺𝐺 , 𝑋𝑋𝑒𝑒

= 0.12 pm

Cold condition

𝜆𝜆𝐿𝐿, 𝑆𝑆𝑛𝑛𝑁𝑁𝑖𝑖𝑖𝑖= 3.1 pm

Voigt fit𝜆𝜆𝐺𝐺 , 𝑋𝑋𝑒𝑒

locked5

Voigt fit to atomic oxygen data

Voigt fit𝜆𝜆𝐿𝐿, 𝑆𝑆𝑛𝑛𝑁𝑁𝑖𝑖𝑖𝑖

locked5

𝜆𝜆𝐺𝐺 , 𝑂𝑂 𝑓𝑓𝑖𝑖𝑒𝑒𝑒𝑒𝑁𝑁𝑖𝑖𝑖𝑖𝑒𝑒𝑆𝑆𝑓𝑓= 1.3 pm

𝜆𝜆𝐺𝐺 , 𝑂𝑂 𝑁𝑁ℎ𝑁𝑁𝑠𝑠𝑘𝑘 𝑆𝑆𝑆𝑆𝑙𝑙𝑒𝑒𝑖𝑖= 2.3 pm5Marynowski T, Löhle S and Fasoulas S (2014) Two-Photon Absorption Laser-Induced Fluorescence Investigation of CO2 Plasmas for Mars Entry. Journal of Thermophysics and Heat Transfer, 28(3): 394-400.

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RESULTS (8/8): ATOMIC OXYGEN DENSITY

Freestream

Laser energy [µJ] 200 300 400

Pressure [Pa] Atomic oxygen concentration ×1023 [m-3]

1393.33 3.41

246

3443.44 2.28

6983.17 3.60 2.73

Average3.46 2.81

Standard deviation 0.135 0.572

Shock layer

Laser energy [µJ] 200 300 400

Pressure [Pa] Atomic oxygen concentration ×1023 [m-3]

13915.0 18.7

246

34415.5 12.5

69816.0 16.2 14.9

Average15.6 15.4

Standard deviation 0.609 3.13

𝑛𝑛𝑂𝑂 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑘𝑘 𝑢𝑢𝑓𝑓𝑙𝑙𝑋𝑋𝑓𝑓

𝑛𝑛𝑂𝑂 𝑓𝑓𝑓𝑓𝑋𝑋𝑋𝑋𝑓𝑓𝑓𝑓𝑓𝑓𝑋𝑋𝑓𝑓𝑓𝑓= 4.5

𝑛𝑛𝑂𝑂 𝑓𝑓𝑓𝑓𝑒𝑒𝑒𝑒𝑓𝑓𝑓𝑓𝑓𝑓𝑒𝑒𝑓𝑓𝑓𝑓 = 3.46 ± 0.47 ×1023 [m-3] 𝑛𝑛𝑂𝑂 𝑆𝑆ℎ𝑆𝑆𝑆𝑆𝑘𝑘 𝑢𝑢𝑓𝑓𝑙𝑙𝑒𝑒𝑓𝑓 = 15.6 ± 0.94 ×1023 [m-3]

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CONCLUSIONS

Absolute atomic oxygen concentration measurements using two-photon absorption laser induced fluorescence experimental technique have been carried out in high-enthalpy plasma flow.

The measurements have been conducted in L2K hypersonic wind tunnel at DLR.

The test campaign is performed at 600 A of arc current in air with a mass flow rate of 36 g/s and a stagnation pressure of 105 Pa. The averaged total enthalpy is ~8.4 MJ/kg and the temperature is assumed to be equal to 4218 K.

• Energy sensitivity analysis for the characterization of the experimental setup;• Absolute atomic oxygen concentration measurements in the freestream and into the shock layer,

i.e., 5 mm upstream from the model surface.The estimation of the atomic oxygen concentration is performed by using relative fluorescence signalsconsidering xenon as reference gas. The number density of the atomic oxygen is equal to ~3.46 × 1023

m-3 in the freestream and ~15.6 × 1023 m-3 into the shock layer.

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Thank you for your attention!Any

questions?