Speaker
Description
-
INTRODUCTION
The complexity of the flowfield encountered around reentering
vehicles poses significant problems to the design
of spacecraft thermal protection systems. One
large source of uncertainty is linked to thermochemical
non-equilibrium. Ground testing is conducted to generate
flows similar to those encountered in flight, replicating
the essential features of non-equilibrium flows.
One such canonical experimental setup is the shock
tube, where a normal shock transiently passes through a
straight tube [CB17]. Although subject to several facilityrelated
artefacts [CMM23, CSMM22], this setup represents
one of the most fundamental fluid mechanical
processes, which allows the isolation of thermochemical
non-equilibrium from other aspects of complex flowfields.
As such, shock tubes provide the opportunity of
studying fundamentals of thermochemical reactions for
flight-relevant enthalpies.
Optical diagnostics provide the ideal vehicle to interrogate
these flows with respect to their thermochemical
state. As one such technique, laser absorption spectroscopy
(LAS) is seeing a rise in use due to the recent
improvement in high-speed tunable diode lasers and
quantum cascade lasers [SKW+19, GGD+24]. LAS targets
the lower state of radiative transitions and can therefore
provide absolute number density measurements of
low-energy quantum states. If ground states are measured,
absolute particle densities of the probed species
can be inferred with high accuracy. Scanning absorption
spectroscopy techniques utilise a spectrally narrow light
source whose central wavelength is changed very rapidly.
By scanning over an absorption line in this fashion and
recording the transmitted light with a detector, the line
profile and absolute absorbance are measured and can be
utilised to infer the translational temperature and lower
state density. Broad band light sources can be utilised as
well, however, the absorption features need to be spectrally
resolved by a spectrometer. In the current work, a
broad band light source is used, as it provides an instantaneous
snapshot of the passing shock wave and does not
require tuning of the wavelength during the experiment. -
METHODOLOGY
Shock tube flows are generated in the Oxford T6 Stalker
Tunnel in aluminium shock tube mode [GCM22]. Optical
measurements are taken through windows set in the
shock tube wall, and acquisition is triggered to record
as it passes this location. Flow conditions investigated
in this work will consist of a set of velocities between
5.5 km.s1 and 6.5 km.s1 [GCM22]. The laser absorption
spectroscopy system utilises a bespoke modeless
laser based on the work by Ewart [Ewa85]. A pulsed
laser source provides a beam with a high degree of collimation
that is advantageous for propagation over a long
path, accurate steerability through the region of interest
and efficient illumination of the spectrometer for recording
the absorption spectrum. Laser sources, however,
are usually characterized by a spectrum of longitudinal
modes with frequency separation related to the laser cavity
length. The spectral gaps between modes and their
fluctuation in amplitude and frequency leads to difficulties
in recording absorption spectra consisting of narrow
spectral features. Some molecular absorption lines that
may fall between the modes will not be recorded or distorted
as a result of the amplitude and frequency fluctuations.
The modeless laser, since it operates without a resonant
cavity, is free from longitudinal mode-structure and
provides an essentially continuous spectrum with spectral
noise determined basically by quantum fluctuations in
the amplified spontaneous emission from the amplifying
medium. The unique advantage of this system is that it
provides a tunable centre wavelength with variable bandwidth
and a continuous spectrum that eliminates mode
noise [SSE91, EAB+05, KE97].
A flashlamp-pumped nanosecond Surelite I-10 Nd:YAG
laser is used in its third harmonic mode producing radiation
at 355 nm which is passed through a set of lenses
to control the beam size. The Nd:YAG fundamental and
second harmonic (1064 nm and 532 nm) are dumped in
an enclosure outside of the Nd:YAG laser head, with
only the third harmonic propagating into the modeless
laser system. The third harmonic beam is separated into
four beams by a four-faceted prism which are each absorbed
by a dye cell at different heights [Ewa85]. The
dye cell features a continuous flow of ethanol containing
0.28 g.L1 of Coumarin dye. The dye produces a spectrally
broad output at each of the four pumped locations.
The spontaneous emission from the four pumped strips
in the dye cell are amplified as a travelling wave by refection
at two totally internally reflecting (TIR) prisms
with apexes slightly displaced relative to each other. The
dispersing prism selects a band of wavelengths from the
fluorescence spectrum of the dye. The orientation of the
right-hand TIR prism is used to select a band centred on
452 nm. The output beam is subsequently frequency doubled
in a critically phase-matched crystal of BBO (Beta
Barium Borate) to 226 nm with a bandwidth (FWHM)
of approximately 2 nm. This ultra-violet beam is separated
from the fundamental at 452 nm using a Pellin-
Broca prims and directed to the shock tube.
Once the beam is produced with the aforementioned
spectral properties, it is passed through a system of turning
mirrors and relay lenses to the side of the T6 tunnel.
At this location it is expanded and collimated by a twolenses
system containing a cylindrical and a spherical
lens, resulting in a laser sheet. This sheet is aligned with
the field of view of a telecentric imaging system. As the
shock wave travel through this field of view, the laser is
activated and partially absorbed by the nitric oxide in the
flow. The laser sheet will be arranged in such a way that it
covers the freestream, non-equilibrium region and equilibrium
region behind the shock front. The transmitted
radiation of the sheet is imaged onto the entrance slit of
the spectrometer. Hence, the resulting spectral image corresponds
to a spatially resolved image along one dimension.
This way, the absorbance can be simultaneously
measured at different locations across the shock wave, resulting
in a resolution of the non-equilibrium layer. The
data acquired in this way will be used to infer NO number
densities and excitation temperatures through spectral
fitting methods. -
RESULTS
The full paper will present the data of the currently ongoing
test campaign and will contain setup, calibration and
post-processing. Raw absorbance data will be shown, as
well as the post-processed properties of nitric oxide in the
shock layer. The post-processing will be undertaken by
comparing the measured absorbance to a computational
model which simulates the absorption through a hightemperature
gas. This will allow the determination of
nitric oxide ground-state densities, as well as vibrational
and rotational temperature.
Summary
A flashlamp-pumped nanosecond Surelite I-10 Nd:YAG
laser is used in its third harmonic mode producing radiation
at 355 nm which is passed through a set of lenses
to control the beam size. The Nd:YAG fundamental and
second harmonic (1064 nm and 532 nm) are dumped in
an enclosure outside of the Nd:YAG laser head, with
only the third harmonic propagating into the modeless
laser system. The third harmonic beam is separated into
four beams by a four-faceted prism which are each absorbed
by a dye cell at different heights [Ewa85]. The
dye cell features a continuous flow of ethanol containing
0.28 g.L1 of Coumarin dye. The dye produces a spectrally
broad output at each of the four pumped locations.
The spontaneous emission from the four pumped strips
in the dye cell are amplified as a travelling wave by refection
at two totally internally reflecting (TIR) prisms
with apexes slightly displaced relative to each other. The
dispersing prism selects a band of wavelengths from the
fluorescence spectrum of the dye. The orientation of the
right-hand TIR prism is used to select a band centred on
452 nm. The output beam is subsequently frequency doubled
in a critically phase-matched crystal of BBO (Beta
Barium Borate) to 226 nm with a bandwidth (FWHM)
of approximately 2 nm. This ultra-violet beam is separated
from the fundamental at 452 nm using a Pellin-
Broca prims and directed to the shock tube.
Once the beam is produced with the aforementioned
spectral properties, it is passed through a system of turning
mirrors and relay lenses to the side of the T6 tunnel.
At this location it is expanded and collimated by a twolenses
system containing a cylindrical and a spherical
lens, resulting in a laser sheet. This sheet is aligned with
the field of view of a telecentric imaging system. As the
shock wave travel through this field of view, the laser is
activated and partially absorbed by the nitric oxide in the
flow. The laser sheet will be arranged in such a way that it
covers the freestream, non-equilibrium region and equilibrium
region behind the shock front. The transmitted
radiation of the sheet is imaged onto the entrance slit of
the spectrometer. Hence, the resulting spectral image corresponds
to a spatially resolved image along one dimension.
This way, the absorbance can be simultaneously
measured at different locations across the shock wave, resulting
in a resolution of the non-equilibrium layer. The
data acquired in this way will be used to infer NO number
densities and excitation temperatures through spectral
fitting methods.
