A high-speed wind tunnel has been used to experimentally simulate the flow experienced by a capsule entering a planetary atmosphere. High speed photography showed that a steady test time of approximately 50 μs existed in the facility. Holographic interferometry has been performed to measure the twodimensional density distribution around a cylinder in the flow. A peak density ratio (density normalised by the free-stream density) of about 14 was observed. Emission spectroscopy allowed the characterisation of the conditions along the stagnation streamline in front of the capsule model. The results showed a temperature that varied between 8,500 K and 11,000 K in this region

Brandis, A.

Eichmann, T.

Mallon, M.

McIntyre, T. J.

Morgan, R. G.

Potter, D.

Rubinsztein-Dunlop, H.

University of Queensland eSpace

16th Australasian Fluid Mechanics Conference
Crown Plaza, Gold Coast, Australia
2-7 December 2007
Radiation measurements in a simulated non-terrestrial atmosphere
T. N. Eichmann, D. Potter, T. J. McIntyre, A. Brandis, M. Mallon, R. G. Morgan, H. Rubinsztein-Dunlop
Centre for Hypersonics,
Schools of Physical Sciences and Engineering,
The University of Queensland,
Queensland, 4072
AUSTRALIA
Abstract
A high-speed wind tunnel has been used to experimentally sim-
ulate the flow experienced by a capsule entering a planetary
atmosphere. High speed photography showed that a steady
test time of approximately 50 µs existed in the facility. Holo-
graphic interferometry has been performed to measure the two-
dimensional density distribution around a cylinder in the flow.
A peak density ratio (density normalised by the free-stream
density) of about 14 was observed. Emission spectroscopy al-
lowed the characterisation of the conditions along the stagnation
streamline in front of the capsule model. The results showed a
temperature that varied between 8,500 K and 11,000 K in this
region.
Introduction
Aerocapture is a manoeuvre used to transfer a spacecraft from a
hyperbolic trajectory to an elliptical trajectory around an astro-
nomical body with an atmosphere, without the use of propellant.
The incoming spacecraft makes a single pass deep into the at-
mosphere where drag rapidly decelerates it into a captured orbit.
This is opposed to alternative techniques which require multiple
passes through the atmosphere and/or propellant before reach-
ing a stable orbit. The aerocapture technique can reduce the
mass of an interplanetary spacecraft by half compared to alter-
native techniques, allowing for a smaller cheaper vehicle or one
outfitted with a larger scientific payload. The design of an ae-
rocapture aeroshell requires reliable estimates of the total heat
transfer to the vehicle’s surface. At the conditions encountered
during an aerocapture manoeuvre, a large component of the heat
transfer to the vehicle will be radiative rather than convective in
nature. Present estimates arrived at by computational methods
vary greatly and necessitate the inclusion of a very large safety
margin (over 100%) in the design of the thermal protection sys-
tem [6].
Current experimental research of high-speed or super-orbital
flow at the University of Queensland is performed using the
X-series impulse facilities. These tunnels can generate high
speed gas flows simulating capsule entry into planetary atmo-
spheres. Previous studies have looked at entry into the Earth,
Martian, Titan and gas giant atmospheres. In general, gas con-
ditions and surface heat flux measurements have been examined
but few studies have addressed the radiative nature of the flow.
To investigate radiation, two approaches are possible. With the
test facility configured in the non-reflected shock tube mode,
the conditions behind the bow shock on a hypersonic vehicle
are directly simulated. These conditions are achieved behind
the inital shock wave as it propagates through the gas initially
placed in the test section. Free-stream densities and shock ve-
locities are directly recreated so that levels of radiation in the
test facility match those in flight. The alternative approach uses
a test model in the facility and operating conditions are adjusted
to preserve ρ-l or binary scaling where ρ is the density and l is
a length scale such as the diameter of the vehicle. This ensures
that non-equilibrium chemical processes such as ionisation and
dissociation proceed to the same level of completion along the
stagnation streamline in the test flow as for the flight vehicle.
A drawback of this approach is that radiative processes do not
scale correctly and the ratio of radiative to convective heat flux
to a vehicle is much lower than in flight conditions. However,
this configuration allows the implementation of other measure-
ment techniques to obtain important data about the conditions
in the flow field.
In the current work, the flow facility was operated taking the
latter approach to generate conditions that simulate entry into
a planetary atmosphere using binary scaling. A carbon diox-
ide/nitrogen test gas mixture was used to approximate the at-
mosphere of Mars. The test gas undergoes a series of chemical
reactions in the high temperature bow shock generated in front
of the capsule. One species that is formed in this region is CN
which is known to be a strong radiator [6] and hence is of great
interest when studying radiative heat flux. Several optical tech-
niques were used to study and measure conditions in the flow.
Experimental Facilities
Measurements were performed in the X2 super-orbital expan-
sion tube using three optical imaging techniques. High speed
photography was used to study the steady test time of the flow.
Holographic interferometry has been used to visualise the shock
shape and to determine the overall density distributions in the
shock layer. Spectroscopic methods have been employed to de-
tect CN along the stagnation streamline and to infer the flow
temperature. The following sections describe the various as-
pects of the experimental approach.
Test gas 96%CO2 + 4%N2
Total enthalpy (MJ/kg) 25
Velocity (km/s) 6.4
Mach number 12
Density (kg/m3) 0.0018
Temperature (K) 880
Table 1: Calculated free-stream conditions.
The X2 super-orbital expansion tube
Flow was generated using the X2 super-orbital expansion tube
[5] shown in figure 1. To achieve the test section conditions,
a piston was used to compress a helium/argon driver gas mix-
ture to burst a steel diaphragm. The resultant shock wave com-
pressed and heated the test gas which burst a second, thinner
diaphragm. The test gas then underwent an unsteady expan-
sion accelerating to the free-stream conditions observed in the
test section. Steady flow was expected to last for around 50 -
100 µs. The calculated operating conditions are given in table
1. These values were determined from shock velocity measure-
ments in the shock and accelerator tubes, and from the initial
461
Reservoir
Piston Vacuum
Pump
Compression
Tube
Primary
Diaphragm
Shock
Tube
Secondary
Diaphragm
Accelerator
Tube
Test
Section
Figure 1: A schematic diagram of the X2 super-orbital expansion tube.
pressures in each section. All measurements were carried out
using a cylindrical test model that had a 25 mm diameter and
was 100 mm in length. Previous studies [2] have shown that
flow around such a body is quasi-two-dimensional, at least for
the shock stand-off and density distribution.
High speed photography
High speed photography was implemented to identify the pe-
riod when steady conditions were established in front of the
cylinder. A Shimadzu HPV-1 high speed camera was used to
image the natural luminosity present in the flow. The region be-
tween the shock wave and the cylinder is at high temperature
and species present in the flow give strong radiation, particu-
larly in the blue region of the spectrum. The camera was placed
adjacent to the test section and recorded images in tandem with
the spectroscopic measurements. The layout for these tests is
shown in figure 2(a).
The camera began recording before arrival of the incident shock
wave in the test section and was set to take 100 frames at 2
µs intervals spanning the test time. After recording, the shock
stand-off along the stagnation streamline was measured by an
automated process and plotted as a function of time.
Holographic Interferometry
The flow around the cylinder was imaged using holographic
interferometry as shown in figure 2(b). The laser and optics
were mounted on stable optical tables placed either side of the
test section of the facility. Dust is a significant problem in this
environment and the equipment was protected by large covers
placed on top of the tables. To record an image, the 532 nm
output from an injection-seeded pulsed Nd:YAG laser was used
to illuminate the test section of the facility. The laser was trig-
gered by a pressure transducer or photodiode mounted upstream
in the expansion tube which detected the arrival of the initial
shock wave generated from diaphragm rupture. The trigger
time was controlled by a programmable timing system which
ensured that the laser pulse occurred during the steady test time
of the facility. Holographic images of the model were recorded
on a single holographic plate for both no-flow and with-flow
conditions. The holographic plate was later used to reconstruct
the two recorded images which were then overlapped yielding
interference fringes. Further details can be found in [4].
The interferograms were post processed using an interactive
analysis package developed by Bishop [1]. Fast-Fourier tech-
niques were used to remove noise followed by an unwrapping
routine which extracts the phase shift imparted on the light as it
passes through the test flow. This phase shift is dependent on the
density of the flow and the results can be used for comparison
with numerical simulations either as phase or density.
Emission Spectroscopy
The characteristics of the radiation present in the hot test gas
were measured using emission spectroscopy. A gated, inten-
sified CCD camera (Princeton Instruments) was connected to
a 300 mm focal length grating spectrometer (Acton Research)
and placed adjacent to the test section as shown in figure 2(a).
The flow along the stagnation streamline was imaged onto the
entrance slit of the spectrometer via a 90o beam rotator yielding
an image on the camera that had one spatial dimension (along
the flow) and one spectral dimension. A 1200 lines per millime-
tre grating was used allowing a spectrum that was about 140 nm
wide to be recorded. The camera was triggered as for the inter-
ferometry measurements and delayed to record an image in the
test time.
In the analysis, the region between the shock and the body was
divided into bins each about 0.1 mm wide. For every bin, the
spectrum was determined by averaging across the section of the
flow covered by the bin. For the current work, no attempt was
made to calibrate the absolute intensity of the measurements.
Instead, the relative intensities of the CN molecular bands were
compared with simulations using the LIFBASE program [3].
In determining a simulated spectrum, the flow was presumed
to be in chemical and thermal equilibrium and the temperature
was varied to obtain the best fit between the simulation and the
measurement.
Results and Discussion
High speed photography
A sample of the images taken with the high speed camera is
given in figure 3. The upper series of ten images show single
frames extracted at steps of 10 µs during the recording. The
second of these shows a small luminous region around the body
just after the arrival of the incident shock wave. At this early
stage, the flow is made up purely of the accelerator gas. A fur-
ther 10 µs later, a near maximum shock stand-off is established
before the flow settles to a near steady state at around 25 µs af-
ter arrival. The lower part of figure 3 shows the time history of
the shock stand-off over the duration of the test. Based on these
measurements, the steady test time seems to start about 30 µs
after the arrival of the incident shock. An approximately con-
stant shock stand-off distance (within the level of fluctuation)
is observed for about another 50 µs. Beyond this the stand-off
slowly decreases. This is consistent with static pressure mea-
surements (not shown) which show a pressure increase at about
80 µs after the arrival of the incident shock indicating a test time
of about 50 µs.
Holographic Interferometry
An interferogram of the flow over the cylinder was taken using
462
Periscope
Holographic
Plate
Periscope
Nd: YAG
Laser
Shutter
80/20 Beamsplitter
Adjustable
Micrometer
Mount
Acceleration Tube
Flow
Acceleration Tube
Flow
Spectrometer
+ camera
Beam
rotator
High-speed
camera
Test Section
Model
Test Section
Model
(a) High speed 
photography
and emission
(b) Holographic 
Interferometry
Figure 2: The optical arrangement for (a) high speed photography and emission spectroscopy, and (b) holographic interferometry.
0 50 100
0
2
4
Time (ms)
St
oc
k 
St
an
d-
of
f (
mm
)
-10 0 10 20 30
40 50 60 70 80
Figure 3: High speed imaging results. Upper - images with
indicated delays in microseconds. Lower - the time history of
the stand-off. Circles show image times.
0
2
4
6
Ph
as
e
Figure 4: Holographic interferometry results. The upper half of
the image shows a raw interferogram of flow over the cylinder.
The lower half shows the phase shift imparted on the light by
the flow obtained from processing the interferogram.
463
0123
0
0.01
0.02
0.03
Distance from body (mm)
D
en
sit
y 
(kg
/m
3 )
Figure 5: The density along the stagnation streamline obtained
from the holographic interferometry measurements.
holographic interferometry. The results of this measurement are
shown in figure 4. The upper half of the image shows the raw
interferogram. Due to some window damage and a poor laser
beam profile, the quality of the image is less than ideal. How-
ever, by using Fourier transform techniques, it is possible to
eliminate the noise and obtain the phase shift, φ, that the light
undergoes due to the flow. This is shown in the lower half of the
image. The phase shift is directly related to the density, ρ, via
the equation
ρ = ρ0 +
φ
2pi
λ
Gl (1)
where ρ0 is the free-stream density, λ is the wavelength of the
light used (here 532 nm), G is the Gladstone-Dale constant and
l is the path length that the light travels (here the width of the
cylinder, 100 mm). In a mixture of gases, the Gladstone-Dale
constant must be determined by combining the Gladstone-Dale
constants for individual species based on their relative den-
sities. Hence a knowledge of the species distribution is re-
quired to obtain an absolute density. In comparing the results
with numerical simulations it is thus best to calculate the phase
from the simulation (in which species concentrations are cal-
culated) rather than comparing the density. However, a den-
sity estimate can be made by assuming that, behind the shock
front, the gas is mostly a mixture of carbon monoxide and oxy-
gen atoms allowing the Gladstone-Dale coefficient to be deter-
mined. Equation (1) can then be evaluated to give the density as
ρ = 1.8×10−3 +3.7×10−3φ. A plot of the density along the
stagnation streamline obtained in this manner is shown in figure
5. A peak density ratio (density normalised by the free-stream
density) of about 14 is reached near the body.
Emission Spectroscopy
The results of the emission spectroscopy are shown in figure 6.
Figures 6(a) and 6(b) show the spectrum that was obtained from
the emission along the stagnation line in front of the cylinder. In
the two-dimensional image (figure 6b), the spatial dimension is
the vertical axis and flow is from top to bottom. The horizontal
axis shows the wavelength in the violet/blue region. The spec-
tral lines at around 390 nm and 415 nm result from transitions in
the CN molecule referred to as CN violet. The bands at 390 nm
are from no change in the vibrational quantum number (ie 0-0,
1-1, 2-2, ... bands) while the 415 nm bands have an increase in
quantum number of 1 (ie 0-1, 1-2, 2-3, ... bands). Other lines
result from impurities in the test gas such as iron.
The region between the shock wave and the body was divided
into about 15 horizontal slices effectively representing 15 mea-
surement points along the stagnation streamline. In each region,
400 420380
Wavelength (nm)
Shock
Body
Flow
In
te
ns
ity
 (a
.u.
)
Te
m
pe
ra
tu
re
 (K
)
0
10,000
15,000
Position
Shock Body5000
(a)
(b)
(c)
Figure 6: Emission spectroscopy results. (a) A single spectrum;
(b) Spatially and spectrally resolved image; (c) temperature dis-
tribution between the shock and the body.
the emission intensity was averaged along the spatial (vertical)
direction to obtain an emission spectrum, an example of which
is shown in figure 6(a). The LIFBASE program [3] was used
to fit spectra to the 390 nm band of the emission for each of
these regions using the temperature of the flow as the free vari-
able. In this process, it was assumed that thermal equilibrium
is established and that translational, rotational and vibrational
temperatures are all equal. This is not a good assumption di-
rectly behind the shock front where high levels of thermal non-
equilibrium are present, characterised by a significantly higher
translational/rotational temperature than the vibrational temper-
ature. The temperature distribution obtained is shown in figure
6(c). It shows an initial temperature behind the shock front of
just over 10,000 K. The temperature then slowly decreases most
likely due to an energy loss from radiative processes. There
follows a slight increase as the flow approaches the body and
kinetic energy is converted back to thermal energy.
Conclusions
Experiments have been performed to characterise the flow over
a blunt body in a hypersonic flow simulation capsule entry into
the Martian atmosphere. The flow establishment was studied
using high speed photography from which the test time could
be determined. Holographic interferometry allowed a measure-
ment of the flow density. The temperature along the stagnation
streamline was obtained using emission spectroscopy. Compar-
ison with numerical simulations are planned to allow a better
understanding of the flow.
Acknowledgements
The authors would like to thank the inputs of postgraduate
student Andrew Dann and from technical staff in the Schools
of Physical Sciences and Engineering at the University of
Queensland. Funding from the Queensland Government Smart
State scheme and the Australian Government Discovery Grant
scheme is also gratefully acknowledged.
References
[1] Bishop, A.I. “Spectrally Selective Holographic Interferom-
464
etry Techniques for Flow Diagnostics” PhD Thesis, The
University of Queensland, Brisbane, 2001.
[2] Eichmann, T.N., McIntyre, T.J., Bishop, A.I., Vakata,
S., Rubinsztein-Dunlop, H. “The influence of three-
dimensional effects on line-of-sight visualization measure-
ments”, Shock Waves Journal Vol. 16 (4-5), pp 299-307,
2007.
[3] Luque, J., Crosley, D.R. “LIFBASE: Database and spectral
simulation”, SRI International Report MP 99-009, 1999.
[4] McIntyre, T.J., Bishop, A.I., Wegener, M.J., Thomas, A.M.,
Rubinsztein-Dunlop, H. “Emission and holographic inter-
ferometry measurements in a superorbital expansion tube”,
AIAA Journal 36(5), 1049–1054, 1998.
[5] Neely, A.J., Morgan, R.G. “The Superorbital Expansion
Tube Concept, experiment and analysis”, Aero J 98,97–
105, 1994.
[6] Olejniczak, J., Wright, M. J., Prabhu, D., Takashima,
N., Zuby, V.E., Hollis, B.R. and Sutton, K. “An anal-
ysis of the radiative heating environment for aerocap-
ture of Titan, Paper 4953, Proceedings of the 39th
AIAA/ASME/SAE/ASEE Joint Propulsion Conference
and Exhibit, Huntsville, Alabama, July, 2003.
465


Radiation measurements in a simulated non-terrestrial atmosphere

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Radiation measurements in a simulated non-terrestrial atmosphere

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