ABSTRACT
Title of Thesis: DEVELOPMENT OF KRYPTON PLANAR LASER
INDUCED FLUORESCENCE METHODS FOR
THE MEASUREMENT OF HYPERSONIC
FLOW CONDITIONS
Chase Standage
Master of Science, 2021
UMD Department of Aerospace Engineering
Advised by: Dr. Kenneth Yu
UMD Department of Aerospace Engineering
Dr. Ashwani Gupta
UMD Department of Mechanical Engineering
Mr. Mike Smith
Arnold Engineering Development Complex
Conventional wind tunnel flow measurement techniques typically involve the
use of intrusive sensor systems, such as Pitot-probes and transducers which come in
contact with the flow. Intrusive methods become impractical for high Mach number
flows, as such methods can cause considerable disruption to the integrity of flow
measurements. Therefore, it is desirable to utilize non-intrusive methods in such
experiments, especially as hypersonic flow conditions are achieved. Schlieren and
shadowgraph imaging methods have been used successfully for decades as a method
of non-intrusive flow visualization. However, these methods become obsolete when
the path of light is obstructed, which is a common problem when analyzing concave
surfaces and complex geometries.
The goal of this project was to develop a scalable krypton planar laser induced
fluorescence flow visualization system for use on curved-surface geometries in sup-
port of the hypersonic Boundary Layer Transition (BOLT) program. The system
was designed to fit multiple wind-tunnel facilities, including the AEDC Tunnel 9
hypersonic test facility and UMD Ludwieg Tube.
In order to design and test the system, the AEDC Mach 3 Calibration wind
tunnel was utilized and Kr-PLIF measurements were taken about a 0.50? spherical
model and 2? BOLT model. A wide variety of equipment and methods were assessed
for their suitability of this project, including 3 cameras and 7 sheet combinations.
A beam from a single-diode Ti-Sapphire laser was amplified, modulated, and
shaped in order to create a thin laser-sheet of 0.25-1.0? width and 0.01?-0.025?
thickness, frequency of 1 kHz, and pulse width of 40 fs. The flow was seeded with
5% krypton, and tests were conducted at Mach 3.
The results were compared to Schlieren imaging tests conducted onsite in the
same Mach 3 wind tunnel. The Kr-PLIF method was moderately successful in
finding regions of relatively high flow density, such as boundary layers and leading
edges at an angle-of-attack. Additionally, Kr-PLIF was able to make measurements
about the curved region of the BOLT model, which was previously unobservable by
Schlieren imaging.
DEVELOPMENT OF KRYPTON PLANAR LASER INDUCED
FLUORESCENCE METHODS FOR THE MEASUREMENT OF
HYPERSONIC FLOW CONDITIONS
by
Chase Standage
Thesis submitted to the Faculty of the Graduate School of the
University of Maryland, College Park in partial fulfillment
of the requirements for the degree of
Master of Science
2021
Thesis Defence Committee:
Dr. Ashwani Gupta, Co-Chair/Co-Advisor
Dr. Kenneth Yu, Co-Chair/Co-Advisor
Dr. Christopher Cadou, Committee Member
Mike Smith, AEDC Tunnel 9 Advisor
? Copyright by
Chase Standage
2021
Acknowledgments
I owe my gratitude to all the people who have made this thesis possible and
because of whom my year of graduate education experience has been one that I will
take with me indefinitely the future.
First and foremost I?d like to thank my advisor, Dr. Ashwani Gupta for
reaching out to me with the invaluable opportunity to work in the challenging, yet
incredibly important field of hypersonics. He has been extremely helpful throughout
this whole process, and was always available. It has been humbling to work with
and learn from such an expert in the field.
I would also like to thank the entire team at the US Air Force AEDC Tunnel
9 facility, all of whom have selflessly lent their time and expertise into making this
project possible. Mr. Mike Smith has been invaluable to this research each step
of the way and, without him, this thesis would not be possible. Mr. Smith has
lent countless hours to assisting me, and provided me with the tools and training
necessary to accomplish meaningful work.
I would also like to acknowledge the role of Air Force and U.S. government
in providing the funding to make this project possible. Additionally, I owe my
gratitude for the U.S. Navy for allowing me to undergo graduate education at the
University of Maryland prior to reporting to flight school at NAS Pensacola.
ii
Finally, I owe my deepest thanks to my parents, who have always challenged
me to take the harder path, and who have supported me through every step of the
way in my education process.
iii
Table of Contents
Acknowledgements ii
Table of Contents iv
List of Figures vi
List of Abbreviations viii
List of Symbols ix
Chapter 1: Introduction 1
1.1 The Need for Hypersonic Testing and Evaluation . . . . . . . . . . . 1
1.2 BOLT Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Overview of Planar Laser Induced Fluorescence (PLIF) . . . . . . . . 6
1.4 Development of a PLIF System: Component Overview . . . . . . . . 8
1.4.1 Seeding Gas and Process . . . . . . . . . . . . . . . . . . . . . 9
1.4.2 Tunable High Power Laser . . . . . . . . . . . . . . . . . . . . 12
1.4.3 Sheet Forming Optics . . . . . . . . . . . . . . . . . . . . . . . 14
1.4.4 Image Capture . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.4.5 Post Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.5 Schlieren Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Chapter 2: Experimental Setup 16
2.1 Laser Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.1 Solstice Ace Ti-Sapphire Laser . . . . . . . . . . . . . . . . . . 16
2.1.2 Spectra Physics Topas Prime Optical-Parametric Amplifier
(OPA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.3 NirUVis Optical-Parametric Oscillator (OPO) . . . . . . . . . 17
2.1.4 Wavelength Generation . . . . . . . . . . . . . . . . . . . . . . 18
2.1.5 Laser Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.1 Mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.2 Sheet Forming Optics . . . . . . . . . . . . . . . . . . . . . . . 22
2.3 Krypton Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3.1 Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4 Mach 3 Calibration Tunnel . . . . . . . . . . . . . . . . . . . . . . . . 27
2.4.1 Seeding Method . . . . . . . . . . . . . . . . . . . . . . . . . . 29
iv
2.4.2 Pricing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.5 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.5.1 Stainless Steel Sphere . . . . . . . . . . . . . . . . . . . . . . . 30
2.5.2 BOLT Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.6 Imaging Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.6.1 Lens and Filter . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.6.2 Camera 1: Princeton Instruments PI-MAX 2 . . . . . . . . . . 35
2.6.3 Camera 2: PCO.Dimax HD & LaVision HighSpeed IRO . . . 36
2.6.4 Camera 3: PCO.Dicam C1 UHS . . . . . . . . . . . . . . . . . 38
2.7 Post-Processing Software . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.7.1 LaVision Davis . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.7.2 Density Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 41
Chapter 3: Results 43
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2 Schlieren Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2.1 Schlieren: Sphere . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2.2 Schlieren: BOLT, Side Profile . . . . . . . . . . . . . . . . . . 45
3.2.3 Schlieren: BOLT, Rotated 45o . . . . . . . . . . . . . . . . . . 46
3.3 Kr-PLIF: PI-MAX 2 Results . . . . . . . . . . . . . . . . . . . . . . . 47
3.3.1 Test: PI-Max 2 with 0.25? Focused Beam . . . . . . . . . . . 47
3.3.2 Test: PI-MAX 2 with 0.25? Focused Beam . . . . . . . . . . . 48
3.3.3 Test: PI-MAX 2 with 0.5? 1-D Focused Beam . . . . . . . . . 49
3.4 Kr-PLIF: PCO.Dimax Results . . . . . . . . . . . . . . . . . . . . . . 50
3.4.1 Test: PCO.Dimax, Single Image, No Averaging . . . . . . . . 50
3.4.2 Test: PCO.Dimax, Time Averaged for 250 pulses . . . . . . . 50
3.5 Kr-PLIF: PCO.Dicam C1 UHS Results . . . . . . . . . . . . . . . . . 52
3.5.1 Test: PCO.Dicam, Sphere, Time-Averaged for 200 pulses . . . 52
3.5.2 Test: PCO.Dicam, BOLT, Time-Averaged for 400 pulses . . . 54
3.5.3 Test: PCO.Dicam, BOLT rotated by 45o, Time-Averaged for
400 pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Chapter 4: Conclusions and Future Work 61
4.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Bibliography 64
v
List of Figures
1.1 BOLT Boundary Layer Transition Simulation . . . . . . . . . . . . . 4
1.2 Instrumented BOLT Wind Tunnel Model . . . . . . . . . . . . . . . . 4
1.3 BOLT Model onboard sounding rocket (top) . . . . . . . . . . . . . . 5
1.4 (a) CH2O-PLIF instantaneous images and (b) OH-PLIF instanta-
neous images [7] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Simplified PLIF System Setup. . . . . . . . . . . . . . . . . . . . . . 8
1.6 Two-photon emission spectra of krypton excited by a 1500V power
source at 50 kHz. NIST database results also shown [9]. . . . . . . . . 11
1.7 212.5 nm to 760/810 nm state-transition diagram for krypton gas. . . 12
1.8 Experimental two-photon emission spectra of krypton, as captured
by spectrometer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1 Spectra Physics TOPAS Prime OPA (Exterior) . . . . . . . . . . . . 17
2.2 Spectra Physics TOPAS Prime OPA (Interior) . . . . . . . . . . . . . 18
2.3 Example of the beam path as it travels from the first mirror, through
the sheet-forming optics, and into the tunnel. The labeled optics are
(1) -50 mm cylindrical plano-concave lens, (2) 200 mm cylindrical
plano-convex lens, (3) 250 mm cylindrical plano-convex focusing lens. 21
2.4 [15] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.5 Average Laser Induced Damage Threshold vs Laser Pulse Duration
(Width). [14] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.6 Sheet Dimensions Diagram . . . . . . . . . . . . . . . . . . . . . . . . 23
2.7 0.5? x 0.025? Laser Sheet Example. . . . . . . . . . . . . . . . . . . . 24
2.8 Effect of Kr concentration on fluorescence . . . . . . . . . . . . . . . 27
2.9 Mach 3 Calibration Tunnel: Test Section Diagram . . . . . . . . . . . 28
2.10 Mach 3 Calibration Tunnel: Reynolds Number vs Orifice Diameter . . 28
2.11 Gas containment and seeding system. . . . . . . . . . . . . . . . . . . 30
2.12 0.5? Mounted Sphere Model. . . . . . . . . . . . . . . . . . . . . . . . 31
2.13 Nomenclature of Sphere Shock Structure . . . . . . . . . . . . . . . . 31
2.14 Instantaneous visualisation of the 3D supersonic flow past a sphere:
Re=300 and two different Mach numbers. [13] . . . . . . . . . . . . . 32
2.15 2? Mounted BOLT Model . . . . . . . . . . . . . . . . . . . . . . . . 33
2.16 Sphere Model (left) and BOLT Model (right) mounted inside tunnel
test section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.17 Princeton Instruments PI-MAX 2. . . . . . . . . . . . . . . . . . . . . 35
vi
2.18 PCO.Dimax HD Camera. . . . . . . . . . . . . . . . . . . . . . . . . 37
2.19 LaVision HighSpeed IRO. . . . . . . . . . . . . . . . . . . . . . . . . 37
2.20 HighSpeed IRO Photcathode Response. . . . . . . . . . . . . . . . . . 38
2.21 Mounted PCO.Dimax/IRO System. . . . . . . . . . . . . . . . . . . . 39
2.22 PCO.Dicam C1 UHS intensified CMOS camera . . . . . . . . . . . . 39
2.23 PCO.Dicam Intensifier Photocathode Quantum Efficiency . . . . . . . 40
3.1 Schlieren: 0.5? Sphere at Mach 3 . . . . . . . . . . . . . . . . . . . . 44
3.2 Schlieren: 2? BOLT Model at Mach 3 . . . . . . . . . . . . . . . . . . 45
3.3 Schlieren: 2? BOLT Model (rotated 45o) at Mach 3 . . . . . . . . . . 46
3.4 Test: PI-MAX 2 with 0.25? Focused Beam, Leading Edge. Static
(left), Mach 3 (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.5 Test: PI-Max 2 with 0.25? Focused Beam, Wake Area. Static (left),
Mach 3 (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.6 Test: PI-Max 2 with 0.5? 1-D Focused Beam, Wake Area. Static
(left), Mach 3 (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.7 Test: PCO.Dimax, Single Image, No Averaging . . . . . . . . . . . . 50
3.8 Test: PCO.Dimax, Series of Images from 5 Runs, Time Averaged for
250 pulses, 16k Resolution . . . . . . . . . . . . . . . . . . . . . . . . 51
3.9 Test: PCO.Dimax, Composite Image, Time Averaged for 250 pulses,
8k Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.10 Test: PCO.Dimax, Composite Image, Time Averaged for 250 pulses,
16k Resolution. Black White (left), False-Color (right). . . . . . . . . 52
3.11 Test: PCO.Dicam, Sphere, Composite Image Time-Averaged for 200
pulses. Relative intensity map (left), Sobel-filtered image (right). . . . 53
3.12 Schlieren/Kr-PLIF Overlay: Sphere . . . . . . . . . . . . . . . . . . . 54
3.13 Test: PCO.Dicam, BOLT, Composite Overlayed Image Time-Averaged
for 400 pulses. Relative intensity map (left), Sobel-filtered image (right). 55
3.14 Test: PCO.Dicam, BOLT, Raw Composite Image Time-Averaged
for 400 pulses. Relative Intensity Map (top) Compass Filter Edge
Detection (bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.15 Schlieren/Kr-PLIF Overlay: BOLT Model, Normal to FOV . . . . . . 56
3.16 Test: PCO.Dicam, BOLT rotated by 45o, Composite Overlayed Im-
age Time-Averaged for 400 pulses.Relative intensity map (left), Sobel-
filtered image (right). . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.17 Test: PCO.Dicam, BOLT rotated by 45o, Raw Composite Image
Time-Averaged for 400 pulses. Compass Filter Edge Detection. . . . . 59
3.18 Test: PCO.Dicam, BOLT rotated by 45o, Raw Composite Image
Time-Averaged for 400 pulses. 8k Resolution (top), 16k Resolution
(bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.19 Schlieren/Kr-PLIF Overlay: BOLT, Rotated 45o . . . . . . . . . . . . 60
vii
List of Abbreviations
AEDC Aerospace Engineering Development Center
APL Applied Physics Laboratory
BOLT BOundary Layer Transition
CARS Coherent Anti-Stokes Raman Scattering
CCD Charge-Coupled Device
CMOS Complementary Metal-Oxide-Semiconductor
FLEET Femtosecond Laser Electronic Excitation and Tagging
HTLT High-Temperature Ludwieg tube
ICCD Intensified Charge-Coupled Device
IR Infrared
IRO Intensified Relay Optics
JHU Johns Hopkins University
KTV Krypton Tagging Velocimetry
LIDT Laser-Induced Damage Threshold
Nd:YAG Neodymium-doped Yttrium-Aluminum-Garnet
NIST National Institute of Standards and Technology
OPA Optical-Parametric Amplifier
OPA Optical-Parametric Oscillator
PIV Particle Image Velocimetry
PLIF Planar Laser Induced Fluorescence
UMD University of Maryland
USAF United States Air Force
USN United States Navy
USNA United States Naval Academy
UV Ultraviolet
UVFS UV Fused Silica
viii
List of Symbols
? angle-of-attack
A21 Einstein coefficient for transition
c speed of light
C calibration coefficient
d collimation distance between optics
f focal length
h Planck constant
? wavelength
l sheet length
m magnification
M? freestream Mach number
n Kr number density
? density
?? freestream density
Re Reynolds number
?0 self-quenching cross section
S fluorescence signal
ix
Chapter 1: Introduction
1.1 The Need for Hypersonic Testing and Evaluation
Hypersonic systems represent a central part of the next generation of military
aviation technology and strategy. A country that can field such systems will have a
significant advantage in the next major conflict. In contrast to traditional ballistic
missiles, these systems may be highly capable of penetrating air defenses and allow-
ing for first-strike capability. Therefore, there is an intense interest by the United
States to research and develop hypersonic flight technology, vehicles, and testing
methods. As the major world powers invest into the development of hypersonic
flight vehicles and weapon systems, the need for accurate testing methods for the
hypersonic regime has become more important than ever. Numerical and analytic
models of the hypersonic regime can vary greatly in accuracy, and so physical tests
are one of the best ways to get fast and accurate information. However, real world
flight test of experimental vehicles can be extremely costly, with up to millions of
dollars potentially being invested for a single flight test. Therefore, the industry has
leaned heavily on the use of wind-tunnel testing in order to gather as much data as
possible in place of costly flight tests, and to assist in the validation of simulation
models.
1
The U.S. Air Force?s Arnold Engineering Development Complex (AEDC), co-
located in Tennessee and Maryland, has focused on the test and evaluation of hy-
personic flight vehicles for half a century. The AEDC Tunnel 9 facility, located in
White Oak, MD, is one of the nation?s premier hypersonic testing facilities, with
a wind tunnel capable of routinely producing flight conditions of up to Mach 18.
Apart from conducting tests for commercial and military clients, the Tunnel 9 facil-
ity has also been a pioneer in the design and development of unique testing methods
for hypersonic conditions. Many of these methods have gone on to be commonplace
in the larger hypersonic testing community.
The hypersonic flight regime is generally considered to begin above Mach 5,
where the dissociation of air begins and extremely high heat loads exist. Traditional
models governing supersonic flight begin to break down, leading to the rise of in-
creasingly unpredictable behavior as the Mach number is increased. For example,
certain phenomena like thin shock layers, high viscous interactions, and a thick en-
tropy layer lead to pronounced boundary layers that behave significantly different
from those in lower Mach number flight conditions. In order to reinforce models
that seek to accurately simulate such behavior, real world data to which the models
can be compared is needed.
In the hypersonic regime, it is more important than ever to utilize non-intrusive
methods of measurement, as the change in flow conditions due to intrusive probes
can entirely alter and ruin the results of a test. This has led to the development of
a wide variety of techniques that utilize light, particularly lasers, to make specific
measurements. These methods include Femtosecond Laser Electronic Excitation and
2
Tagging (FLEET) [1], Particle Image Velocimetry (PIV) [3], Coherent anti-Stokes
Raman scattering (CARS) [6], and Planar Laser Induced Fluorescence (PLIF). Ad-
ditionally, Schlieren and shadowgraph methods have been performed for decades
in order to visualize hypersonic shock/expansion waves, turbulence, and other phe-
nomena. The development of PLIF, as an aid to Schlieren/shadograph methods, is
the focus of this research.
1.2 BOLT Program
A secondary objective for this research into Kr-PLIF is to support the multi-
entity BOLT (BOundary Layer Transition) program. The BOLT program is a col-
laboration between the U.S. Air Force Research Laboratory?s Aerospace Systems
Directorate and AEDC, NASA Langley, the University of Minnesota, Purdue Uni-
versity, Texas AM University, CUBRC, Australia?s Defence Science and Technology
Group, the German Aerospace Center (DLR), VirtusAero, Johns Hopkins Applied
Physics Laboratory (APL), and the University of Maryland.
The program seeks to gain a deeper understanding of hypersonic boundary
layer transition, and uses a novel test-model to do so. The project began in 2017
with computer simulation testing, as shown by Figure 1.1 and soon transitioned to
wind tunnel testing with a highly instrumented model, as shown by Figure 1.2. A
full-scale Mach 5-7 flight test of the BOLT Vehicle on-board a sounding rocket is
currently planned for the near future (Figure 1.3).
As the program is focused on boundary layer transition, testing methods that
3
can observe flow conditions close to the curved surface of the model are needed.
Conventionally, Schlieren imaging could be used to view just above the surface of
a flat model, but in the case of BOLT, the curved surface gradient is unable to be
examined by Schlieren or other conventional optical methods. Therefore, it could be
advantageous for the program to utilize PLIF as a method to make measurements
about this otherwise obscured region.
[2]
Figure 1.1: BOLT Boundary Layer Transition Simulation
Figure 1.2: Instrumented BOLT Wind Tunnel Model
4
Figure 1.3: BOLT Model onboard sounding rocket (top)
5
1.3 Overview of Planar Laser Induced Fluorescence (PLIF)
PLIF is an established method of measuring the relative density of a region
within a flow through the excitation of a gas by a thin laser sheet. PLIF involves
the excitation of a gas within the flow of interest and the measurement/recording
of photons subsequently emitted from the excited gas as it drops down to a lower
energy state. This measurement of emitted photons can be then correlated to the
relative density of the flow at the region of measurement. This region is defined by
the dimensions of the laser sheet by which the gas is excited. PLIF can also be used
to visualize shock and boundary layer formation and behavior, similar to Schlieren
imaging. However, PLIF has an advantage over Schlieren imaging in that it can be
used to analyze a the flow near a curved surface, which would otherwise obstruct
the pathway of light that is imaged via the Schlieren method. Therefore, for certain
models, PLIF can theoretically take data from regions which are otherwise obscured
to conventional optical methods.
PLIF requires that the flow being analyzed contains molecular particles that
have the capability to be excited by a laser, thereby resulting in fluorescence. This
means that energy state of the molecules are able to be excited to a higher energy
level than the ground state by the absorbance of light. Following this excitation,
the molecules are able to return to the original ground state through the release of
photons of longer wavelength of than the incident photons. In the case of excitation
by UV light, the corresponding photon release is typically in the visible light range,
and therefore observable by imaging equipment.
6
In the past, PLIF measurements have been accomplished in flames by the
excitation of combustion bi-products such as CH2O, OH, and NO by a Nd:YAG or
Ti-Sapphire laser and an OPO/OPA. An example of this is depicted by Figure 1.5.
The application of PLIF for high-speed wind tunnel applications is a rising field,
and has been examined in a very limited manner. [6]
Figure 1.4: (a) CH2O-PLIF instantaneous images and (b) OH-PLIF instantaneous
images [7]
The goal of this research was to develop hypersonic flow visualization methods
via planar laser induced fluorescence of krypton gas by a Ti-Sapphire femtosecond
laser. The krypton was chosen due to its relative inertness, which allows it to
be used in N2 based flows without disruption to the system. The development is
being conducted with the ultimate goal to use the system at the AEDC Tunnel 9
facility and the University of Maryland?s high-temperature Ludwieg tube (HTLT).
As stated earlier, a primary advantage of PLIF over Schlieren imaging is that it
allows measurements to be taken right down to the surface of curved geometries,
which would otherwise obscure the free passage of light needed to conduct Schlieren
7
imaging. The BOLT model is an example of one such geometry where Schlieren
images cannot be taken down to the surface. In the case of BOLT, the primary
focus of the experiment happens to be at the surface of the model (where boundary
layer transition occurs), so the development of Kr-PLIF in hypersonic conditions is
valuable as an experimental data supplement to the BOLT program.
1.4 Development of a PLIF System: Component Overview
PLIF systems principally require four main components:
1. Excitable Gas and Delivery System
2. High Power Laser
3. Sheet Forming Optics
4. Intensified Camera
A simplified PLIF system is shown by Figure 1.5. Each of the four main components
are described in the following sections.
Figure 1.5: Simplified PLIF System Setup.
8
1.4.1 Seeding Gas and Process
In order for measurements of PLIF to be accomplished, there must be some
molecular species in the flow that is able to be excited by a laser in order to induce
a release of visible photons (fluorescence). Fluorescence is the emission of light by
a substance following the absorbance of light or other electromagnetic radiation. In
most cases, the photons being absorbed by the substance are of a higher energy,
and therefore shorter wavelength, than the photons which are emitted. For this
research, the two-photon fluorescence of krypton is accomplished with an absorbed
light wavelength of 212.5 nm and a broadband of emitted wavelengths between 740
and 820 nm. The two most prominent of these emitted wavelengths are 760 nm and
810 nm, with 760 nm being the focus of this research. This is further discussed in
Section 1.4.1.1.
As stated before, PLIF has been extensively accomplished in the past using
compounds such as nitric-oxide (NO), formaldehyde (CH2O), and hydroxyl (OH)
radicals. The advantage performing PLIF with these gas particles is that they
are naturally occurring in their respective flows; nitric-oxide is a naturally present-
species in air-based hyper-velocity flows (particularly above Mach 6), and formalde-
hyde/hyrdroxyl radicals are commonly present in hydrocarbon-fueled combustion
systems. However, nitric-oxide, and many other hypersonic flow radicals, are highly
corrosive and potentially destructive for test systems. Optics, models, and other
system components have significantly degraded lifetimes when exposed to these de-
structive molecules. The degradation can be to the point where the experimental
9
setup only lasts for one tunnel run. While this can be acceptable under some cir-
cumstances, where the value of the data overshadows the cost of the experimental
setup, this is not often the case. This is one of the primary reasons for the use of
pure N2 in hyper-velocity flow systems. Therefore, it is desired to perform PLIF
using a seeded gas that gives measurable fluorescence without disrupting flow char-
acteristics or causing damage to the experimental setup. This is the advantage of
using krypton (Kr).
While Kr is a heavy gas, with an atomic weight of 83.798u and atomic num-
ber of 36, it does not have an appreciable effect on flow conditions or behavior
when introduced in low concentrations (<10% for Mach 3, <1% for Mach 18). Ad-
ditionally, it is one of the six naturally occurring noble gases, which means it is
relatively non-reactive under low-pressure conditions. [5] While xenon, and argon
are more naturally abundant, the fluorescence of krytpon is well characterized and
determined to be more energetic at its peak levels than the former noble gases.
1.4.1.1 Spectra Measurement
In order to decide upon an appropriate camera filter to be used for the imaging
of krypton PLIF, the emission spectra of fluoresced krypton was assessed. This was
compared with the spectra that was expected from literature.
According to literature, the most energetic two-photon releases from krypton
occur via excitement by light with a wavelength of 212.5 and 214.7 nm. Generally,
a stronger series of emissions occurs from an exciting by 212.5 nm. This excitation
10
results in the emission of photons among a broad spectrum range between 740 and
820 nm. However, the most prominent of these emissions occurs at 760 and 810 nm,
as shown by Figure 1.6. [9]
Figure 1.6: Two-photon emission spectra of krypton excited by a 1500V power
source at 50 kHz. NIST database results also shown [9].
The krypton state-transition diagram for the 212.5 nm excitation is shown by
Figure 1.7. 212.5 nm stimulates Kr from a resting state of 4p61s0 to an excited
state of 5p[3/2]2. The excited Kr then releases photons from 5s[3/2]1 and 5s[3/2]2
in order to return to the unexcited resting state. [19] These photon releases result in
measurable fluorescence. The number of photon releases, and therefore the magni-
tude of fluorescence intensity, is dependent on both the gas density and laser power.
The analytic relationship between the signal of fluorescence and Kr density is given
by 1.1:
A21
S = C n (1.1)
A21 + ?0nv
11
where S is the fluorescence signal, n is the Kr number density, A21 is the
Einstein coefficient for the transition, ?0 is the self-quenching cross-section, v is the
frequency of the released photon, and C is the calibration coefficient.
Figure 1.7: 212.5 nm to 760/810 nm state-transition diagram for krypton gas.
In order to verify the data from literature and the National Institute of Stan-
dards and Technology (NIST) database, an early analysis of the krypton emission
spectrum was made using a Acton Research Corporation SpectraPro-2500i spec-
trometer, enclosed krypton sample at 10 torr, and laser at approximately 212.5 nm.
The results from this evaluation are shown by Figure 1.8.
1.4.2 Tunable High Power Laser
In order to excite the krypton gas, a laser system is required that can produce
the desired wavelength as well as output enough power such that the krypton fluo-
rescence is measurable. Additionally, due to the high-speed nature of the flows that
this Kr-PLIF system is being designed to analyze, the laser must have a sufficiently
12
Figure 1.8: Experimental two-photon emission spectra of krypton, as captured by
spectrometer.
short pulse-width. As flow speeds are increased, it becomes increasingly important
to have a shorter pulse width, in order to prevent an image from blurring as an
excited molecule moves through the camera frame during the duration of the pulse
width. For example, at Mach 18, the flow velocity is about 2 mm/ms, which would
result in significant image blur for the long pulse width of a continuous wave laser.
Therefore, the requirements outlined for this system are that the laser must
produce an output at 212.5 nm with a short pulse width on the order of femtoseconds
(10?15s). In the past, PLIF setups have utilized nanosecond (10?9s) lasers, which
reduce the laser energy output requirements by a factor of about 100 [12]. However,
nanosecond lasers typically have a lower repetition rate, on the order of 10 Hz, which
does not provide a very large sample set of data for tests that last on the order of
just seconds. In contrast, the femtosecond laser being used has a repetition rate of 1
kHz, which provides a much larger sample size, especially for flows lasting less than
one second (as is often the case in shock tubes).
13
The process used to produce the 212.5 nm laser beam is discussed in Section
2.1.4
1.4.3 Sheet Forming Optics
In order to acquire measurements about a whole region of interest, rather than
just a thin line, the laser beam must be formed into a sheet. This sheet must be of
sufficient width in one direction to interrogate the entire desired region of interest,
while also be of sufficient thinness to maximize the energy-density, and therefore
maximize the level of excitement from the krypton gas.
A variety of sheet forming methods and wide combination of optics were tested
in the development of the Kr-PLIF system. The specifics of each setup are discussed
in Section 2.2.2.
1.4.4 Image Capture
Due to the high velocity of the flow to be tested, a high-speed camera is de-
sired in order to capture the maximum amount of laser-pulses/krypton-fluorescence
cycles. Due to the anticipated weak signal of krypton fluorescence, the camera must
also be either internally-intensified or compatible with an external intensifier.
Additionally, the camera should be operated in a gate-mode, which allows each
pulse to be captured discreetly while minimizing noise from background sources. Due
to the weak signal of fluorescence, elimination of background noise is essential for the
analysis of signal-results, and is one of the key-considerations when post-processing
14
the PLIF data.
1.4.5 Post Processing
Following image capture and extraction, post-processing software can be used
to perform background reductions, smoothing functions, and calculations in order to
measure the relative density of the flow within the region of interest. The LaVision
Davis software is described in greater detail in Section 2.7.
1.5 Schlieren Imaging
In order to provide a reference baseline to which the Mach 3 Kr-PLIF results
could be compared, Schlieren imaging was also conducted about the sphere and
BOLT models. Schlieren is a visual process that is used to photograph the flow of
fluids of varying density. It is a highly valuable and non-invasive tool in visualizing
shock formation in supersonic conditions.
Schlieren utilizes a collimated light sources shining from behind a target object.
The collimated light beam is refracted by density gradients in the fluid, which can
be directly visualized by a camera. Typically, the light is focused with a converging
mirror/lens, and a knife edge is placed at the focal point of the light. This results
in lighter and darker shades which correspond to positive and negative fluid density
gradients in the direction normal to the knife edge. The knife edge of a Schlieren
system results in the first derivative of the density gradients to be observed by the
camera. [10]
15
Chapter 2: Experimental Setup
2.1 Laser Setup
There are three main components used to produce the laser beam at 212.5 nm:
the Ti-Sapphire laser, optical-parametric amplifier (OPA), and optical-parametric
oscillator (OPO).
2.1.1 Solstice Ace Ti-Sapphire Laser
The Solstice Ace Ti-Sapphire Laser is a tunable system that produces >7W
of power at 1 kHz within the range of 780?820 nm. The laser can produce a pulse-
width between 35-120 fs. First invented and used at the MIT Lincoln Laboratory in
1982, Ti-Sapphire lasers use a crystal sapphire Al2O3 lasing medium which is doped
with Ti3+ ions. One of the primary advantages of a Ti-Sapphire laser is its ability
to produce ultrashort pulses in the femtosecond range at very high repetition rates,
as opposed to the much lower repetition rates of nanosecond lasers, such as Nd:YAG
lasers commonly used in the past. While carrying less energy per pulse, this allows
femtosecond lasers to increase the data sampling rate by 1-2 orders of magnitude.
This is advantageous in extremely high speed flows, such as the Mach 7-18 range of
16
the Tunnel 9 facility, which may only last on the order of a couple seconds.
2.1.2 Spectra Physics Topas Prime Optical-Parametric Amplifier (OPA)
An OPA is a laser-light source which emits light of varying wavelengths fol-
lowing an optical parametric amplification process. The OPA takes in a pump beam
from the Ti-Sapphire laser and sum-frequency mixes it with an idler beam to gen-
erate a signal beam. The OPA then amplifies the resultant signal beam. The final
output of the OPA then is the amplified signal beam and the idler beam. These
beams are then used as inputs into the OPO.
The Spectra Physics Topas Prime OPA is a femtosecond capable amplifier that
can take up to 5 mJ of input energy (pump) from wavelengths between 770-830 nm.
The exterior and interior are shown by Figures 2.1 and 2.2 for reference.
Figure 2.1: Spectra Physics TOPAS Prime OPA (Exterior)
2.1.3 NirUVis Optical-Parametric Oscillator (OPO)
Similar to the OPA, the OPO takes in a pump beam to generate a new set
of wavelengths through a sum-frequency mixing process. However, it does this by
17
Figure 2.2: Spectra Physics TOPAS Prime OPA (Interior)
means of second-order nonlinear optical interaction within an optical cavity, which
contains a series of optical resonators and crystals. The primary output of the OPO
is a beam of the desired wavelength, 212.5 nm, as well as residual wavelengths (810,
576, 288 nm) from prior mixing and doubling processes.
The NirUVis is an add-on frequency mixing OPO for the TOPAS Prime OPA.
The advantage of the NirUVis system is its designed compatibility with the TOPAS
Prime OPA, as well as its ability to be automatically controlled and calibrated from
a master computer. It offers laser tuning from 189 nm to 20 ?m and has an estimated
conversion efficiency of > 25% at 212.5 nm.
2.1.4 Wavelength Generation
In order to generate the 212.5 nm beam, a series of sum-frequency mixing and
doubling processes occur with the OPA and OPO. First, the 810 nm ouput from
the Ti-Sapphire laser and the 2029 nm ouput from the OPA undergo sum-mixing
to produce 576 nm, as described by Equation 2.1.
18
1 1 1
= + (2.1)
?new ?1 ?2
Sum-mixing is a second order nonlinear process based on the annihilation of
two input photons with distinct angular frequencies, and the subsequent genera-
tion of an output photon that conserves the total incident angular frequency. This
process is governed by the conservation of energy. [18]
Next, the 576 nm beam is frequency-doubled through a doubling-crystal in
order to produce 288 nm, as described by Equation 2.2.
1
?new = ?old (2.2)
2
Frequency-doubling, or second-harmonic generation is a nonlinear optical two-
photon process where the photons, each with the same input frequency, interact with
a material and are combined, resulting in the generation of a new photon that carries
twice the energy of the initial photons. This is explained by Planck?s Equation (2.3):
hc
E = hf = (2.3)
?
where E is the photon energy, h is Planck?s constant, h is the frequency, h is
the speed of light, and ? is the wavelength.
Finally, the 288 nm beam undergoes sum-mixing by Equation 2.1 with a beam
of 810 nm from the Ti-Sapphire laser in order to produce a beam at approximately
212.5 nm. This 212.5 nm beam still carries residuals of 288, 576, and 810 nm which
19
could be seen by an unfiltered intensified camera. However, most of the residual
energy is dumped by the series of UV fused-silica mirrors/prisms.
2.1.5 Laser Tuning
The laser was initially tuned to the desired 212.5 nm wavelength by using an
Ocean Optics QE Pro High Performance Spectrometer. The spectrometer allows
a beam to be input via a fiber optic cable, and can measure wavelengths between
200-900 nm. However, this device has an accuracy band of +/- 0.5 nm, and so a
more accurate method was need to precisely tune to 212.5.
The laser was more precisely tuned to the desired wavelength by the following
method. A vacuum cell was pumped with 100% Kr gas to 1 atm. The cell had
windows appropriate for both the passage of the laser sheet, and the recording of
fluorescence with an intensified camera. The laser was tuned such that a maximum
signal of fluorescence was obtained from the krypton within the cell. This was done
by making 0.05 nm changes to the laser software settings, and adjusting for timing
delay changes with the final delay stage in the OPO.
2.2 Optics
2.2.1 Mirrors
The 0.25? diameter beam was directed towards the tunnel test-section by a
series of four mirrors. Each UV fused-silica mirror was one inch wide, and rated
for 213 nm with a reflectance of 95%. The first mirror results in a power loss of
20
about 0.3 mW, partially attributed to the transmission of residual infrared (IR)
wavelengths from the laser mixing process. While resulting in a power loss, this
transmission through the mirror removes the unwanted IR residuals from the UV
beam. Each successive mirror reflection thereafter resulted in a power loss of about
0.1 mW, for a total power loss due to mirrors of about 0.6 mW. The path of the
beam from the laser output to the tunnel test-section is shown by Figure 2.3.
Figure 2.3: Example of the beam path as it travels from the first mirror, through
the sheet-forming optics, and into the tunnel. The labeled optics are (1) -50 mm
cylindrical plano-concave lens, (2) 200 mm cylindrical plano-convex lens, (3) 250
mm cylindrical plano-convex focusing lens.
2.2.1.1 Alternate: Prism
Near the end of the research, it was discovered that the Yt-As coating on the
surface of the UV fused-silica mirrors was severely damaged through nano-cracking,
21
leading to a near total loss in laser power following the damaged mirror. It is
believed that the damage was due to the femtosecond pulse-width of the laser. As
the pulse width decreases for a laser, the power density increases, and the dielectric
coating on mirrors tends to become excited and displaced. This nano-cracking effect
is shown by Figure 2.4, and the relationship between laser pulse-width and mirror
damage threshold is shown by Figure 2.5. [14]
Figure 2.4: [15]
2.2.2 Sheet Forming Optics
A variety of laser sheet formations were evaluated. The creation of each specific
sheet profile is described in the following subsections. For this discussion, the sheets
are said to have width in the x-direction, and thickness in the y-direction, where
?x? and ?y? are shown by Figure 2.6.
An example of an x = 0.50? x y = 0.025? collimated sheet is shown by Figure
22
Figure 2.5: Average Laser Induced Damage Threshold vs Laser Pulse Duration
(Width). [14]
Figure 2.6: Sheet Dimensions Diagram
2.7.
The expansion ratio produced by a pair of cylindrical plano-concave and cylin-
drical plano-convex lenses is given by Equation 2.4,
( )
fconvex
m = (2.4)
fconcave
Where m is magnification, fconvex is the absolute value of the focal length of the
cylindrical plano-convex lens, and fconcave is the absolute value of the focal length
23
Figure 2.7: 0.5? x 0.025? Laser Sheet Example.
of the cylindrical plano-concave lens.
The distance required between optics in order to produce a collimated sheet
is conventionally given by Equation 2.5,
d = fconcave + fconvex (2.5)
However, Equation 2.5 is appropriate only if the wavelength of light being
transmitted is equivalent to the manufacturer specified optic rating. In this case,
the optic rating for the cylindrical lenses is given as 580 nm. The corrected distance
can be approximated by Equation 2.6,
?
dcorrected = (fconcave + fconvex) (2.6)
?spec
where ? is the laser wavelength (212.5 nm) and ? is the optic rating (580
nm). However, this only applies to the assumption of a thin lens. Because of
24
the relatively thick nature of the plano-convex lens, Gullstrand?s Equation must be
utilized instead:
fconcavefconvex
dGullstrand =
f + f ? t
(2.7)
concave convex n
where dGullstrand is the corrected distance between lenses, t is the thickness of
the lenses, and n is the refractive index of the lenses. This gives a close approxi-
mation of the separation needed between the lenses for collimation, and the exact
collimation of the sheet can be further tuned by hand.
Using this process, a variety of sheet combinations between 0.25? and 1.0? in
length, 0.025? and 0.25? in thickness, and collimated or focused were formed and
tested. In all, 10 different sheet combinations were evaluated in preliminary testing.
2.3 Krypton Gas
2.3.1 Concentration
Kr/N2 mixtures can be purchased in any concentration. 100% Kr gas was
used for initial laser fluorescence testing, but it is not practical to seed the tunnel
directly with 100% Kr, as the exact total concentration of Krypton amongst other
tunnel gases is then just a rough estimate based on the approximated volume of Kr
added to an approximate volume of total gas.
Therefore, it was desired to purchase a known Kr/N2 mixture. Due to the
relatively heavy atomic weight of Kr (83.798 u) compared to N2 (14.01 u), it is
25
desired to minimize the concentration of Krypton in order to not appreciably alter
the density and behavior of the flow being studied from what would be encountered
in a 100% N2 flow.
In order to determine a suitable Kr/N2 concentration, which is as minimal as
possible but still high enough to produce a suitable fluorescence signal, a vacuum
cell was filled with 100% Kr and then pumped down to fractions of 1 atm in order
to simulate lower concentrations of Kr. For example, to simulate a 5% Kr, 95%N2
mixture, the cell was filled with 100% Kr at 1 atm, and then pumped down to 0.05
atm. The limitation of this method is that there is still trace amounts of oxygen in
this cell, particularly at lower pressures where more oxygen can leak in, which can
lead to quenching.
As shown by Figure 2.8, the signal of fluorescence was very strong at 100% Kr,
with a 10:1 average signal-to-noise ratio. As the concentration was brought down to
5%, the signal was still suitable, with a 4:1 average signal-to-noise ratio. However,
as the concentration was decreased to 3%, the signal-to-noise ratio decreased to an
average of 1.5:1, which was not acceptable for Kr PLIF. It was also noted that the
fluorescence signal decreased slightly (by about 5%/inch, as viewed from top to bot-
tom) as the laser sheet passed through the Kr gas. This is due to the absorbance
of the laser power by the Kr molecules as they are excited to a higher energy level.
While not significant on a small scale, this could lead to large reductions in fluo-
rescence if the laser sheet has to pass through a large region of Krypton gas before
reaching the region of interest, which may be encountered in larger test sections.
26
Figure 2.8: Effect of Kr concentration on fluorescence
2.4 Mach 3 Calibration Tunnel
The Mach 3 AEDC Calibration Tunnel is an indraft supersonic wind tunnel,
driven by a large vacuum tank with a converging-diverging nozzle attached to it.
To start the tunnel, a valve is cycled downstream of the nozzle throat. The regular
effective reservoir is the ambient laboratory air and so the freestream conditions are
fixed at about 1 atm.
The test section, shown by Figure 2.9 has three separate stations which each
have four locations to insert windows, aluminum blocks, valves, etc.. This allows
more than one experiment to be setup at a time using the calibration tunnel, as was
the case during this project. The test section has a square cross section of about
2.5? in each dimension, which allows for a model with a frontal area of about 1-in2 to
be used without unstarting the flow. The exact maximum frontal area is dependent
on the geometrical nature of the model.
For each run, the tunnel was pumped down to 20 Torr. The valve was opened
27
Figure 2.9: Mach 3 Calibration Tunnel: Test Section Diagram
until a cutoff point of 50 Torr. This fully drained the reservoir bag without sucking
the bag into the test-section inlet, and allowed for about 5 seconds of flow in each
tunnel run. Additionally, the Reynolds number could be manually adjusted by
changing the orifice plate diameter. This relationship is shown by Figure 2.10.
Figure 2.10: Mach 3 Calibration Tunnel: Reynolds Number vs Orifice Diameter
For the tests conducted in this research, a 19.1 mm (0.75?) opening was used,
resulting in Re? ? 2.30 x 106 (1/m) and M? ? 2.77. This also results in ?? ?
28
0.030 kg/m3, or 0.025 atm.
The initial and final 500 ms of each run are dominated by unsteadiness and
shock reflections as the flow and shocks develop within the test section and about
the test article. The development of supersonic flow and shock structures within the
tunnel is known as starting. Conversely, the reverse process is known as unstarting.
Unstarts commonly occur when there is a rapid change in mass flow rate through a
supersonic duct. This change in mass flow rate is experienced during the initial and
final phases of each tunnel run. Therefore, data was mainly analyzed from portions
of each test after the initial second and before the final second of flow. Through
Schlieren imaging, steady supersonic flow could be observed during the 2-3 seconds
between these extremes.
2.4.1 Seeding Method
An 8-ft long, 1-ft diameter cylindrical PVC pipe was attached to the inlet end
of the tunnel test-section, before the nozzle. On the free end of the pipe, an 85 gallon,
7mm thick, drum-shaped plastic bag was attached to store gas for each tunnel run.
This allows for a maximum gas capacity of about 36 ft3. In order to prevent the
bag from being ingested into the inlet pipe, a plastic frame was constructed to give
rigidity to the bag section. Gas was directly injected into the test section before
each run through a line connected to a pressurized tank of 95%N2/5%Kr mixture.
The setup is shown by Figure 2.11.
The system took approximately three minutes to fully fill to 1 atm with the
29
gas mixture before each tunnel run. The 36 ft3 of gas allows for a total run time of
about 5 seconds.
Figure 2.11: Gas containment and seeding system.
2.4.2 Pricing
The 5% Kr, 95% N2 mixture was purchased in pressurized 256 ft
3 bottles for
a price of $800. Each bottle provided enough gas for about 15 runs in the Mach 3
calibration tunnel. This results in an average cost of about $55/run.
2.5 Models
2.5.1 Stainless Steel Sphere
A 0.5? diameter stainless-steel sphere was mounted to a sting and positioned
in the center of the tunnel test section. The sphere was chosen for initial testing
due to the abundance of prior shock data provided via Schlieren imaging to which
the results of the Kr-PLIF could be compared.
Examples of the shock formation at Mach 1.2 about a spherical object are given
30
Figure 2.12: 0.5? Mounted Sphere Model.
by Figures 2.13. As the Mach number is increased towards the test conditions of
Mach 3, the angle of the shock, expansion, and recompression waves are expected to
deflect and narrow down towards the sphere body. This is depicted by the simulation
in Figure 2.14
Figure 2.13: Nomenclature of Sphere Shock Structure
31
Figure 2.14: Instantaneous visualisation of the 3D supersonic flow past a sphere:
Re=300 and two different Mach numbers. [13]
32
2.5.2 BOLT Model
A 2? scale-model of the BOLT test article was mounted on a sting in the
tunnel test section for later tests. The model was 3-D printed out of plastic resin,
and was durable enough to withstand the laser for the extent of the tests. Various
aspects of the model are shown by Figure 2.15.
Figure 2.15: 2? Mounted BOLT Model
Figure 2.16: Sphere Model (left) and BOLT Model (right) mounted inside tunnel
test section.
33
2.6 Imaging Equipment
Three different cameras were used in the development of the PLIF system, each
with their own benefits and drawbacks. The first camera tested was the Princeton
Instruments PI-MAX 2, which was suitable for identifying sheet dimensions, but
not suitable for Kr-PLIF tunnel tests. The next camera tested was the PCO.Dimax
HD, which was suitable for making Kr-PLIF measurements at 1000 fps. Finally, the
PCO.Dicam was tested, which offered the highest signal-to-noise ratio.
In each case, an intensifier was necessary in order to increase the signal of
the Kr fluorescence, which is weak by nature. The PI-Max 2 and PCO.Dicam
were equipped with an internal intensifier, whereas the PCO.Dimax HD utilized an
externally mounted LaVision HighSpeed IRO intensifier.
2.6.1 Lens and Filter
For all of the cameras, a Nikon f-mount macro lens was used with a 760 (+/-
10) nm filter. The filter has a transmission of 60%. The filter was necessary in order
to reduce the noise associated with room light and residual laser reflections from the
Ti-Sapphire?s 810 nm beam. Even in a gated mode, the noise from these sources
can be quite high relative to the signal emitted by the krypton fluorescence due to
the intensifier gain.
34
2.6.2 Camera 1: Princeton Instruments PI-MAX 2
The PI-MAX 2 intensified camera system, shown by Figure 2.17 was the first
one used, and was utilized during much of the sheet-forming and laser-tuning tests
of this project. This camera was used previously for krypton tagging velocimetry
(KTV) [17] at the 760 nm wavelength of interest here.
The PI-MAX 2 is a 16-bit 1024x1024 pixel imaging system with an internal
CCD and 18 mm Gen III Extended Blue intensifier. For the Kr-PLIF experiments,
the gain was set at 255 (maximum), and a 25 ms gate allowed approximately 25
fluorescence cycles to be captured per frame.
Figure 2.17: Princeton Instruments PI-MAX 2.
The PI-MAX 2 camera head houses both the internal CCD and intensifier,
and is cooled by an attached fan and liquid coolant circulation system. The camera
had also has an F-mount to which the Nikon macro lens was mounted. An exter-
nal programmable timing generator and power supply is connected via wire to the
camera. A high-speed serial link allows data to be transferred directly to the host
35
computer during recording windows.
The camera is able to be operated in shutter or gate mode. For the Kr-PLIF
experiment, the camera was set to gate mode and externally triggered by the 1 kHz
laser signal.
The camera was not sensitive enough to gather an appreciable signal from
single pulses, so the 25 pulse-per-frame capture was necessary. Further boosts in
signal were made by averaging 10 frames (250 pulses, 250 ms) together. A major
issue with the PI-MAX 2 at these settings was that the 4 Hz framerate only allowed
5-6 steady state PLIF images to be captured in each tunnel run. This would not be
acceptable in a tunnel with shorter run times, such as the UMD Ludwieg Tube.
2.6.3 Camera 2: PCO.Dimax HD & LaVision HighSpeed IRO
The PCO.Dimax HD camera, shown by Figure 2.18 utilizes a Complementary
Metal Oxide Semiconductor (CMOS) active pixel sensor in order to convert light
into electrical signals. The sensor is very similar in function to a CCD. The camera
is capable of capturing images at up to 2128 fps at full HD resolution (1920 x 1080
pixel). It has 12-bit dynamic range, and images are stored on-board in internal
memory (36 GB maximum) as 12-bit .tif files.
In the Kr-PLIF setup, with the camera/intensifier externally gated, the oper-
ational frame-rate was identical to the pulse frequency of the laser (1 kHz = 1,000
fps).
The LaVision HighSpeed IRO, shown by Figure 2.19, is an intensified relay
36
Figure 2.18: PCO.Dimax HD Camera.
optic system equipped with gated high precision shutter control. It is specifically
designed to be used in series with common CMOS cameras, and provides a high
sensitivity peak within the visible range. The S25 photo-cathode response is near
its peak at the 760 nm wavelength emitted by the krypton fluorescence, as shown
by Figure 2.20.
Figure 2.19: LaVision HighSpeed IRO.
Gating times can be brought down to as short as 100 ns, which serves to
minimize the noise obtained while measuring the extremely brief (20-60 ns) periods
of krypton fluorescence during each laser pulse. With a max repetition rate of 2
MHz, the intensifier is well suited for triggering on the 1 kHz repetition of the laser.
37
Figure 2.20: HighSpeed IRO Photcathode Response.
During the tests, the PCO.Dimax/IRO was operated with a 135 ns gate and 90 ns
delay. The gain was set at 90%.
The Nikon macro lens was mounted directly to the F-mount of the LaVision
HighSpeed IRO. The IRO was then mounted directly to the PCO.Dimax HD. This
effectively created a single rigid body system, which was mounted to a series of
rails, which allow this large system to be easily mobile between multiple different
experiments. The rail system also allowed for adjustments to the camera height and
viewing angle. This system is shown by Figure 2.21
2.6.4 Camera 3: PCO.Dicam C1 UHS
The final camera tested was the PCO.Dicam C1 UHS intensified 16 bit sCMOS
camera, shown by Figure 2.22. The camera has a resolution of 1504 x 1504 pixels,
38
Figure 2.21: Mounted PCO.Dimax/IRO System.
and can capture images at 143 fps at full 2.3 MP resolution. Additional repetition
rate above 143 fps can be achieved by lowering the resolution to view a particular
region of interest. The camera has the lowest readout noise of any gated intensified
camera system on the market, and can gate as short as 2.5 ns.
Figure 2.22: PCO.Dicam C1 UHS intensified CMOS camera
The internal intensifier is directly coupled to the 16 bit sCMOS sensor via an
efficient tandem lens. The intensifier can utilize one of three photocathodes, each
with a peak efficiency at a different wavelength, as shown by Figure 2.23. For the
39
760 nm application of Kr-PLIF, the GaAs photocathode was utilized.
Figure 2.23: PCO.Dicam Intensifier Photocathode Quantum Efficiency
An immediate advantage of the PCO.Dicam over the PCO.Dimax/LaVision
IRO is its high mobility. The PCO.Dicam is about half the length and width of
the latter system, and is much more easily mounted/manipulated. This can be
particularly important if the camera is to be rotated in order to image a particular
surface region on a model.
2.7 Post-Processing Software
2.7.1 LaVision Davis
The LaVision Davis imaging software was used in order to process the .tif
images gathered from the tunnel runs by each camera. The Davis package is a com-
plete software solution for intelligent (laser) imaging for fluid dynamics, combustion,
spray applications as well as material strain and deformation imaging.
Of particular interest to this project, the Davis software can produce calibra-
40
tions to de-warp images, which allowed images to be taken of the Kr-PLIF setup
in the test section from non-right angles. This is fundamental to being able to an-
alyze the Kr-PLIF data on curved surfaces, which was a major motivation for this
research.
The Davis software also allows for the intuitive construction of masking func-
tions. In most cases, the PLIF data was time-averaged and underwent Gaussian
smoothing. This was necessary in order to construct a more accurate representation
of shock qualities. Additionally a mask was generated for each test setup in order
to subtract background noise from the .tif images, which served to maximize the
signal-to-noise ratio of the fluorescence signal. In most cases, image background
subtraction increased the signal-noise-ratio by about 5-10x.
Processing of an image set for each tunnel run took anywhere between 2-15
minutes, depending on the masking functions utilized. This represents a relatively
quick solution for extracting usable data following a successful test.
2.7.2 Density Evaluation
The main goal of PLIF is to translate a fluorescence signal into a quantifiable
density measurement. This relation is again described by Equation 2.8:
A21
S = C n (2.8)
A21 + ?0nv
where S is the fluorescence signal, n is the Kr number density, A21 is the
Einstein coefficient for the transition, ?0 is the self-quenching cross-section, v is
41
frequency of the emitted photon, and C is the calibration coefficient.
The density can also be found by Equation 2.11,
S
? = ?0 (2.9)
S?0
where ? is the density at a point in the flow, ?0 is the ambient density in
static air, S is the signal of fluorescence at some point in the flow, and and S0 is the
average signal of fluorescence in static conditions.
Alternatively, a numeric approach can be taken for processing of .bmp images
of x-rows and j-columns by the following modification to Equation 2.11:
S(x, j)
?(x, j) = ?0 (2.10)
S?0
? 1
S?0 = S0(x, j) (2.11)
N
where N is the number of pixel elements contained within the bounds of (x,j).
42
Chapter 3: Results
3.1 Overview
Each of the three cameras were evaluated with wind-tunnel tests at approxi-
mately Mach 3. During the tests, the first goal was to be able to visualize the shock
with Kr-PLIF. Once the shock could be visualized, the next goal was to evaluate
relative densities change within the flow as a function of the krypton fluorescence
signal.
Additionally, Schlieren imaging was conducted about the models in order to
establish a baseline for visualization of shock formation at Mach 3 to which the
Kr-PLIF results could be referenced.
3.2 Schlieren Imaging
3.2.1 Schlieren: Sphere
40 images were first taken about the 0.5? stainless steel sphere at 100 Hz, and
then time-averaged to form a single image across 0.4 seconds of flow. The resultant
shock and expansion waves about the sphere are shown in Figure 3.1.
The primary shock had a standoff distance of approximately 0.04 in (1 mm).
43
The expansion wave began at approximately the midpoint of the sphere body. Both
the shock and expansion wave had an angle of about 36o relative to the x-plane of
the sphere. Minor shock reflections could be seen throughout the tunnel run and
are visible in the image.
Figure 3.1: Schlieren: 0.5? Sphere at Mach 3
44
3.2.2 Schlieren: BOLT, Side Profile
Next, 50 images were taken about the 2? BOLT model at 100 Hz, and then
time-averaged to form a single image across 0.5 seconds of flow. The orientation of
the model was normal to the field of view (no bank/rotation), and was mounted at
approximately 3o AoA.
The resultant shock about the BOLT model is shown by Figure 3.2. The
shock had no measurable standoff distance from the leading edge of the model. The
upper-surface shock had an angle of about 27o relative to the x-plane of the model.
The lower-surface shock had an angle of about 25o relative to the x-plane of the
model. This slight difference is due to the angle-of-attack of the model, which was
expected to also create a greater density buildup on the lower-surface of the model,
as will be evaluated with Kr-PLIF in the sections to follow.
Figure 3.2: Schlieren: 2? BOLT Model at Mach 3
45
3.2.3 Schlieren: BOLT, Rotated 45o
Finally, 50 images were again taken about the 2? BOLT model at 100 Hz,
and then time-averaged to form a single image across 0.5 seconds of flow. The
orientation of the model was rotated to 45o of bank, and the model was again
mounted at approximately 3o AoA.
The resultant shock about the BOLT model is shown by Figure 3.3. As before,
the shock had no measurable standoff distance from the leading edge of the model.
The upper-surface shock had an angle of about 26o relative to the x-plane of the
model. The lower-surface shock had an angle of about 24o relative to the x-plane of
the model.
As expected, it is impossible to gain any information about the shock down to
the curved surface of the BOLT model using Schlieren, even with the model rotated,
as the surface is entirely obscured by the through-and-through FOV requirements
of Schlieren and shadowgraph methods. This test, however, did provide a reference
to which the Kr-PLIF test in Section 3.5.3 could be compared.
Figure 3.3: Schlieren: 2? BOLT Model (rotated 45o) at Mach 3
46
3.3 Kr-PLIF: PI-MAX 2 Results
The PI-MAX 2 was the first camera tested, and a wide variety of sheet com-
binations were evaluated, including:
? 1.0? Wide Sheet, 1-D Focused
? 1.0? Wide Sheet, 2-D Focused
? 0.5? Wide Sheet, 1-D Focused
? 0.5? Wide Sheet, 2-D Focused
? 0.5? Wide Sheet, Collimated
The most notable results of the tests are shown in the following sections. Overall,
results were not consistent nor promising when using the PI-MAX 2. In order
to reliably see the fluorescence, the images had to be added and averaged during
capture, and in doing so, the camera could only export about 3-5 usable images for
each run (approximately a 1 Hz frame rate). In the following examples, 25 images
were captured and averaged per second in order to produce the following signals.
3.3.1 Test: PI-Max 2 with 0.25? Focused Beam
An attempt was first made to visualize the bow shock about the leading edge of
the ball bearing. The 0.25? laser sheet was focused at the leading edge of the sphere,
and the beam was allowed to slightly impinge the sphere geometry. Therefore, the
primary region of interest is the upper half of the sphere, prior to impingement. The
47
Figure 3.4: Test: PI-MAX 2 with 0.25? Focused Beam, Leading Edge. Static (left),
Mach 3 (right)
PI-MAX 2 was not sensitive enough, even with averaging, to consistently capture
the fluorescence signal in this setup. A visualization of the bow shock was not
readily discernible.
3.3.2 Test: PI-MAX 2 with 0.25? Focused Beam
Figure 3.5: Test: PI-Max 2 with 0.25? Focused Beam, Wake Area. Static (left),
Mach 3 (right)
The same 0.25? focused beam was then tested in the aft-portion of the sphere
in an attempt to visualize the flow separation following the mid-body expansion
wave (referred to in Figure 2.13). This effort was reasonably successful, as there
48
was a consistent point at which the signal of the krypton fluorescence was disturbed.
However, the quality of this signal was too deficient to be able to characterize this
disturbance in an exact manner.
3.3.3 Test: PI-MAX 2 with 0.5? 1-D Focused Beam
Figure 3.6: Test: PI-Max 2 with 0.5? 1-D Focused Beam, Wake Area. Static (left),
Mach 3 (right)
A 0.5? 1-D focused sheet was tested order to visualize a wider region of interest
along the length of the sphere body. The 250 mm cylindrical plano-convex focusing
lens resulted in a usable fluoresced region of interest that was about 0.3? x 0.3?. In
this test, the shock location was able to be roughly visualized, and a difference in
density was observable above and below the shock. However, the observed signal was
not strong or consistent enough in order to accurately assess the values of density
in this region. This was the primary issue with the PI-MAX 2 across all tests.
49
3.4 Kr-PLIF: PCO.Dimax Results
3.4.1 Test: PCO.Dimax, Single Image, No Averaging
The PCO.Dimax 2 and LaVision IRO were tested next. Initially, a 0.25?
x 0.025? collimated sheet was used. The camera gate was set to 25 ms, which
allowed 25 pulses to be captured per image. The noise background was subtracted
to maximize the signal-to-noise ratio. Some slight bow shock activity could be
observed, as shown by Figure 3.7, and there were definable variations in fluorescence
signal across the shock.
Figure 3.7: Test: PCO.Dimax, Single Image, No Averaging
3.4.2 Test: PCO.Dimax, Time Averaged for 250 pulses
In order to improve upon the fluorescence signal fro the previous test, a 0.25?
sheet was focused down in the y-dimension. Additionally, the 25-pulse images were
50
time-averaged across 10 images, producing time-averaged images that contained the
signal representation of 250 pulses. Following the success of the first image about
the leading-edge/bow shock, 4 more images were taken along the sphere geometry.
All 5 images were then summed together in order to form a single, composite image
about the entire test article, as shown by Figures 3.8, 3.9, and 3.10.
Figure 3.8: Test: PCO.Dimax, Series of Images from 5 Runs, Time Averaged for
250 pulses, 16k Resolution
From Figure 3.9, a shock and expansion zone can clearly be identified across the
sphere. The fluorescence signal is significant enough in order to obtain a consistent
depiction of the flow density in the fluoresced regions. Additionally, flow activity
can be seen within the boundary region close to the surface of the sphere. However,
the resolution of the PCO.Dimax is too low, even with a macro lens, to get a clear
view of motions within the boundary layer region.
51
Figure 3.9: Test: PCO.Dimax, Composite Image, Time Averaged for 250 pulses, 8k
Resolution
Figure 3.10: Test: PCO.Dimax, Composite Image, Time Averaged for 250 pulses,
16k Resolution. Black White (left), False-Color (right).
3.5 Kr-PLIF: PCO.Dicam C1 UHS Results
3.5.1 Test: PCO.Dicam, Sphere, Time-Averaged for 200 pulses
The PCO.Dicam was initially tested against the sphere model with a gate of
100 ns, framerate of 200 Hz at 900 x 900 pixel resolution, no software delay, and
maximum gain. The PCO.Dicam captured one laser pulse/fluorescence cycle per
52
gate, which resulted in much lower noise that the 25 ms, 25 pulse gate utilized by
the PCO.Dicam. Unlike the PCO.Dicam, the PCO.Dimax was capable of seeing
adequate signal from this single pulse/gate scenario.
In post-processing, the background was removed and the signals were mag-
nified for visibility. Five images, time-averaged across 200 individual gates, were
combined to form a composite image, as shown by Figure 3.11.
Figure 3.11: Test: PCO.Dicam, Sphere, Composite Image Time-Averaged for 200
pulses. Relative intensity map (left), Sobel-filtered image (right).
A region of high fluorescence intensity can be seen alongside the bottom of
the sphere, which grows in thickness along the length of the sphere. At the edge
of this region is the location of the expected primary shock (across a shock, there
is a sharp rise in density). However, when overlayed with the Schlieren image for
the sphere, as shown by Figure 3.12, it appeared that this region of high density
does not actually delineate the shock line. One possible explanation then for this
exceptionally high relative density and Kr fluorescence, is that this region is actually
the boundary layer of the sphere, which is expected to grow as a function of distance
across the sphere surface.
53
Figure 3.12: Schlieren/Kr-PLIF Overlay: Sphere
3.5.2 Test: PCO.Dicam, BOLT, Time-Averaged for 400 pulses
Following the success of visualizing density variation about the sphere, the
BOLT model was tested next. The BOLT test was averaged for 400 images at
200 Hz (2 seconds), during the duration of steady flow in the Mach 3 tunnel. The
background noise intensity was about 95 counts, and the average fluorescence signal
intensity in each test was about 100-175 counts above average. This resulted in
average pre-filtered intensity counts of about 195-270 counts. The results are shown
by Figure 3.13 and 3.18.
A region of relative high density was observed on the underside of the leading
edge. This region extends for approximately 5 mm along the BOLT model, and
has a maximum thickness of about 1.5 mm (extending out from the surface). This
54
Figure 3.13: Test: PCO.Dicam, BOLT, Composite Overlayed Image Time-Averaged
for 400 pulses. Relative intensity map (left), Sobel-filtered image (right).
Figure 3.14: Test: PCO.Dicam, BOLT, Raw Composite Image Time-Averaged for
400 pulses. Relative Intensity Map (top) Compass Filter Edge Detection (bottom).
55
region of high density was likely a result of the 3o AoA of the model.
There is also a region of moderate fluorescence which extends for the entire
length of the laser sheet-composite along the body of the model. Because of the
linear nature and relatively defined edge of this region of moderate fluorescence, it
was first believed to be an indicated of the weak shock about the model. Figure
3.15 displays an overview of the Schlieren and Kr-PLIF tests in order to investigate
this further.
Figure 3.15: Schlieren/Kr-PLIF Overlay: BOLT Model, Normal to FOV
There was an approximately 4-6o angular distance between the edge of the
region of moderate Kr fluorescence and the location of the shock observed in the
Schlieren test. One possible explanation for this could be that the seeding of the
gas with 5% Kr resulted in a slightly greater flow density and specific heats ratio,
from what was experienced in the Schlieren test. This would cause a change in shock
angle (by about 2o), and could partially explain the difference in shock geometries as
directly observed in the Schlieren test and inferred by the Kr-PLIF test. However,
the signal of the Kr-PLIF was still too weak to say with absolute certainty that this
explanation is accurate.
56
3.5.3 Test: PCO.Dicam, BOLT rotated by 45o, Time-Averaged for
400 pulses
In this test, the BOLT model was rotated to 45o of bank, such that the camera
had a view of the laser sheet all the way down to the curved surface, which was
obscured in the previous test and in Schlieren imaging. The results are shown by
Figures 3.16. Prior to filtering, the fluorescence signal had an average intensity of
about 100-200 counts above background in each laser pulse. With a background of
about 95 counts, this resulted in fluorescence intensities between 200 and 300 counts.
In comparison to the previous test, where the BOLT model was not rotated, there
was significantly more reflection that had to be taken into account during post-
processing. This is due to the angle of the model (which reflected light towards the
camera), as well as the material (the white plastic-resin used to 3D-print the model
is not light-absorbent).
There was a slight growth in the fluoresced region that can be observed. The
region of highest intensity grows from 0 mm to about 2 mm across the span of
about 20 mm. This may be due to the shock about the BOLT model, which results
in a region of higher density between the shock and the model surface. Another
possible explanation for this is that it is an indication of boundary layer growth
along the surface of the model. However, this is unlikely given the thickness of the
layer relative to its position near the leading edge of the model. This is something
that can be investigated further as more computational simulations and boundary
layer specific tests are performed by the BOLT program. The edge of this zone can
57
Figure 3.16: Test: PCO.Dicam, BOLT rotated by 45o, Composite Overlayed Image
Time-Averaged for 400 pulses.Relative intensity map (left), Sobel-filtered image
(right).
58
Figure 3.17: Test: PCO.Dicam, BOLT rotated by 45o, Raw Composite Image Time-
Averaged for 400 pulses. Compass Filter Edge Detection.
Figure 3.18: Test: PCO.Dicam, BOLT rotated by 45o, Raw Composite Image Time-
Averaged for 400 pulses. 8k Resolution (top), 16k Resolution (bottom).
be more clearly depicted with a compass filter, as shown by Figure 3.17.
This is a result that cannot be visualized with Schlieran imaging due to the
geometric constraints of the curved body surface, which was a primary motivation
for the development of Kr-PLIF. Figure 3.19 displays the region of fluorescence
observed, relative to what was observable by the Schlieren image for the rotated
BOLT model.
The fluoresced region, particularly the region of relatively high fluorescence,
lies almost entirely out of the observable region for Schlieren imaging. While there
59
Figure 3.19: Schlieren/Kr-PLIF Overlay: BOLT, Rotated 45o
was speculation as to the the cause of the fluorescence variation in this region, Kr-
PLIF was successful in providing information about a region that was previously
unobservable by most other non-intrusive flow visualization methods.
60
Chapter 4: Conclusions and Future Work
4.1 Conclusions
Three cameras and several laser sheet sizes were evaluating for Kr-PLIF using
5% Kr/95% N2. Measurements were taken about a 0.5? metal sphere, and a 2? model
of the BOLT test article. It was found that the best Kr-PLIF results came from
using the PCO.Dicam C1 UHS intensified camera with a 760 +/- 10 nm filter and
a 0.225? long, focused sheet. The focused sheet provided a 0.225? x 0.3? region of
usable fluorescence. Signal intensities varied between an average of 200-300 counts,
above a background of an average of 95 counts. Density variations across a shock
were able to be visualized, and regions of relatively high density were identified.
The primary shock itself was also able to be partially visualized. Composite images
were formed from 4-5 tunnel runs for each test article condition and compared to
Schlieren images about the same test articles. The Kr-PLIF was able to visualize a
density variation and possible shock formation on the concave surface of the BOLT
model which was inaccessible to Schlieren imaging. However, more signal is needed
to clearly delineate the shock and obtain numerical results for flow density.
61
4.2 Future Work
There is much work to be done towards improving the Kr-PLIF method if it is
to be a viable form of visualizing shocks and density fluctuations about curved sur-
faces in hypersonic conditions in the future. For the near term, the most impactful
improvement can be found in the increasing of laser power. The fluorescence signal
scales exponentially with laser power, so as femtosecond laser power is increased, the
quality of results should increase accordingly. Another improvement can be made by
increasing the concentration of kyrpton seeded to the flow. However, this is limited
by the needs of the experiment at hand, as the original flow behavior changes as
more krypton is added, and hypersonic flows may be limited to 1-5% Kr. However,
this can be overcome in certain applications through the use of localized seeding of
high concentration krypton gas from the target model itself. This is one possibility
for the BOLT program moving forward in the UMD Ludwieg Tube. Lastly, with the
proper optics, thinner sheets can be made, which can allow for greater power den-
sities and therefore a greater fluorescence signal or larger max-allowable sheet size.
Based on the results of these tests, the PCO.Dicam C1 UHS series camera is the
most suitable for Kr-PLIF due to its very high signal-to-noise ratio and moderate
framerate.
With the necessary improvements made in order to increase signal, Kr-PLIF
may become a possible near-future tool to supplement Schlieren imaging for complex
geometries in high speed flows. It is unlikely, however, for Kr-PLIF to fully replace
Schlieren imaging in cases where Schlieren can be used to observe the entire region
62
of interest. A major limitation in all cases moving forward is the scale of PLIF
observations. Typical sheet sizes are on the scale of fractions of an inch, and so full
coverage of a large scale model is not currently realistic. However, the small sheet
sizes of PLIF may be reasonably used in the near future to visualize features like
specific points of boundary layer transition, or behavior within corners/joints.
63
Bibliography
[1] L.E. Dogariu, A. Dogariu, R. Miles, M.S. Smith, E.C. Marineau Femtosecond
Laser Electronic Excitation Tagging Velocimetry in a Large-Scale Hypersonic
Facility (AIAA, Volume 57, Number 11. November 2019).
[2] B. Wheaton, D. Berridge, T. Wolf, R. Stevens Boundary Layer Transition
(BOLT) Flight Experiment Overview (Fluid Dynamics Conference. 2018).
[3] I. Grant, Particle Image Velocimetry: A Review (Proceesings of the Insitution
of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science.
Volume 211, Issue 1, January 1997).
[4] P.M. Danehy, S. O?Byrne, A.D. Cutler, C.G. Rodriguez. Coherent Anti-Stokes
Raman Scattering (CARS) as a Probe for Supersonic Hydrogen-Fuel/Air Mix-
ing (JANNAF APS/CS/PSHS/MSS Joint Meeting, Colorado Springs, CO, De-
cember 1-5, 2003).
[5] P.Z. Ejgierd, P.M. Lata. Krypton Oxides Under Pressure (Scientific Reports.
Article 18938. 2016).
[6] P.M. Danehy, J.A. Wilkes, D.W. Alderfer, S.B. Jones, A.W. Robbins. Planar
laser-induced fluorescence (PLIF) investigation of hypersonic flowfields in a
Mach 10 wind tunnel (invited) (Heat, Flow and Combustion Diagnostics. NASA
Langley Research Center, Hampton, VA. 2012).
[7] L. Yang, W. Weng, Y. Zhu, Y. He, Z. Wang, Z. Li. Investigation of Dilution
Effect on CH4/Air Premixed Turbulent Flame Using OH and CH2O Planar
Laser-Induced Fluorescence (Engineering Fluid Dynamics, 12(2), 325. 2020).
[8] J. Brooks. Development and Application of Mach 10 PIV in a Large Scale Wind
Tunnel (University of Maryland, College Park, MD. 2018).
64
[9] N.L.D. Bachir, A. Belasri, P.H. Guillot, B. Caillier. Radiative Emissions in
Visible?IR of Krypton Excilamp: Experimental and Theoretical Interpretations
(Plasma Chemistry and Plasma Processing. 2019).
[10] A. Mazumdar. Principles and Techniques of Schlieren Imaging Systems
(Columbia University. 2013).
[11] S.W. Grib, P.S. Hsu, H.U. Stauffer, C.D. Carter, S.Roy. Comparison of fem-
tosecond and nanosecond two-photon-absorption laser-induced fluorescence of
krypton. (Applied Optics Vol. 58, Issue 27, pp. 7621-7627. 2019).
[12] S.W. Grib, P.S.Hsu, N. Jiang, J.J. Felver, S.A. Schumaker, C.D. Carter, S.
Roy. 100 kHz krypton planar laser-induced fluorescence imaging (Optics Letters
45(14). 2020).
[13] J. Latt, C. Coreixas, J. Beny, A. Parmigiani. Efficient supersonic flow simu-
lations using lattice Boltzmann methods based on numerical equilibria (Royal
Society. Mathematical, Physical, and Engineering Sciences. 2020).
[14] P. Velpula, D. Kramer, B. Rus Femtosecond Laser-Induced Damage Charac-
terization of Multilayer Dielectric Coatings (Institute of Physics of the Czeck
Academy of Sciences. 2020).
[15] Y. Liao, Y. Cheng Femtosecond Laser 3D Fabrication in Porous Glass for
Micro- and Nanofluidic Applications (Shanghai Insitute of Optics and Fine
Mechanics. 2014).
[16] P.N. Butcher and D.N. Cotter, The Elements of Nonlinear Optics (Cambridge
University Press, Cambridge, UK, 1990).
[17] D. Zahradka, N.J. Parziale, M.S. Smith, E.C. Marineau. Krypton Tagging Ve-
locimetry (KTV) in Supersonic Turbulent Boundary Layers (AIAA SciTech
Forum. San Diego. 2016).
[18] H.Y. Shen, W.X. Lin, R.R. Zeng. Second-harmonic generation and sum-
frequency mixing of double-wavelength Nd:YALO3 laser to 413.7-nm violet co-
herent radiation in LiIO3 crystal (Journal of Applied Physics. Volume 72, Issue
9. 1998).
[19] D. Shekhtman, N. Parziale, M.A. Mustafa. Excitation Line Optimization for
Krypton Tagging Velocimetry and Planar Laser-Induced Fluorescence in 200-
220 nm Range (AIAA 2021-1300).
65