Development of a
Doppler Global Velocimetry system in view of turbomachinery
applications
Carlo Bagnera
February 20, 2008
Contents
1 Introduction 1
1.1 Framework and motivation of thesis and work . . . . 1
1.2 The importance of measurements . . . . 2
1.3 The fascinating realm of laser technologies . . . . 3
1.4 Fluid dynamic measurements, a survey . . . . 5
1.5 Roadmap to the following chapters . . . . 6
2 Doppler Global Velocimetry 9 2.1 Doppler velocimetry . . . . 9
2.2 DGV at NASA Langley . . . . 12
2.3 DGV at DLR . . . . 15
2.4 DGV at ONERA . . . . 17
2.5 DGV at the University of Oxford . . . . 18
2.6 Conclusions . . . . 19
3 The laser 21 3.1 The physical working principle of a laser . . . . 21
3.1.1 The argon laser . . . . 22
3.1.2 The optical cavity . . . . 23
3.1.3 Theory of the single line operation . . . . 23
3.1.4 Definition of modes and free spectral range . . . . 24
3.1.5 The etalon . . . . 25
3.1.6 Variation of the cavity length . . . . 27
3.2 Modification of an existing laser . . . . 27
3.2.1 Coarse tuning the laser frequency . . . . 28
3.2.2 Fine tuning the laser frequency . . . . 31
3.3 Conclusions . . . . 33
4 The iodine cell 35 4.1 The choice of iodine and its characteristics . . . . 35
4.2 The design of the cells . . . . 41
4.2.1 The first cell . . . . 41
4.2.2 The second cell . . . . 43
4.3 Testing the cells . . . . 44
4.3.1 Quality of the optical surfaces . . . . 46
4.3.2 The set-up for the cell calibration . . . . 48
4.3.3 Calibration of the first cell . . . . 50
4.3.4 Calibration of the second cell . . . . 51
4.4 Cell calibration with an improved method . . . . 55
4.5 Conclusions . . . . 58
5 Laser frequency stability 61 5.1 The problem . . . . 61
5.2 The proposed solution . . . . 62
5.3 Real-time system . . . . 63
5.4 Natural laser (in)stability . . . . 64
5.5 Laser stabilization . . . . 66
5.6 Conclusions . . . . 68
6 DGV tests in fluid flows 69 6.1 Introduction . . . . 69
6.2 Tests in the low speed jet . . . . 70
6.2.1 Jet characterization with hot wire anemometry . . . . 70
6.2.2 DGV measurements, the set-up . . . . 70
6.2.3 DGV measurements, the results . . . . 73
6.3 Tests in the high speed jet . . . . 79
6.3.1 Jet characterization with hot wire anemometry . . . . 80
6.3.2 DGV measurements . . . . 81
6.4 Tests in a turbine cascade . . . . 85
6.4.1 The facility . . . . 86
6.4.2 The flow field conditions . . . . 89
6.4.3 Characterization with a 5-hole probe . . . . 91
6.4.4 DGV measurements, the receiving optics . . . . 93
6.4.5 DGV measurements, the set-up . . . . 99
6.4.6 DGV measurements, the results . . . 100
6.5 Conclusions . . . 105
7 Conclusions and prospects 109
List of Figures
1.1 Jean Michel Jarre - Hong Kong - May 23
rd, 1995 . . . . 4 2.1 The principle of Doppler velocimetry: determination of the mea-
sured velocity component . . . . 10 3.1 Part of the energy level diagram for argon ion transitions . . . . 22 3.2 Sketch of a laser resonant cavity composed of two mirrors and the
laser tube . . . . 23 3.3 Sketch of the mounting principle of a line selecting prism inside
the laser cavity . . . . 23 3.4 Sketch of the gain profile of the argon ion laser, with modes and
transmission peaks of the intracavity etalon . . . . 24 3.5 Sketch of the mounting principle of a tilting etalon inside the laser
cavity . . . . 25 3.6 Sketch of the mounted etalon assembly on the laser frame rods. A
demultiplication of 20:1 ensures a precise control on the vertical etalon tilting angle . . . . 29 3.7 Photo of the etalon mount . . . . 30 3.8 The back mirror assembly with the prism and the piezotranslator 32 3.9 Sketch of the laser with the intracavity etalon and the line select-
ing prism: a visual summary of the modifications introduced . . 33 4.1 Part of the iodine spectrum in the proximity of the argon ion line
at 514.5 nm, as published by Gerstenkorn and Luc [1] . . . . 36 4.2 Iodine transitions under argon ion gain profile at 514.5 nm . . . . 37 4.3 Vapour pressure of molecular iodine . . . . 38 4.4 Simulation of the absorption line for molecular iodine . . . . 39 4.5 Sketch and dimensions of the first iodine cell . . . . 41 4.6 Pictures of the first iodine cell. On the left, before closing the
Teflon casing, showing the heating wire coils, the appendix at the
bottom, and the bead of iodine in the appendix. The final cell on
the right, with the oil reservoir . . . . 42
4.7 The second iodine cell . . . . 43
4.8 The second iodine cell. On the left, before closing the Teflon casing, showing the wire coils and the smaller appendix, residual of the manufacturing method. On the right the final component 44 4.9 The check of the quality of the optical windows of the cell per-
formed by comparing two pictures of a pattern with and without the cell interposed . . . . 45 4.10 Vector fields representing the quality of the optical surfaces of the
iodine cell . . . . 47 4.11 Layout of the optical components and the iodine cells. A com-
puter acquires the data from the photodiodes and the thermocou- ples and controls the movements of the etalon . . . . 49 4.12 The set-up for the characterization of the iodine cells . . . . 49 4.13 Frequency scan with etalon: the red and the blue lines of the top
graph are the light intensities I and I
0respectively, measured by the photodiodes; the black line of the bottom graph is the ratio I/I
0. . . . 50 4.14 The laser mode hops are reflected by different absorption levels
of the iodine. The center of the plateaus is assumed to be repre- sentative of the iodine absorption line . . . . 51 4.15 Repeatability check: four tests are shown. The temperature of
the body is kept at 100
◦C and the temperature of the cold finger at 50
◦C . . . . 52 4.16 Comparison of the different absorption curves for different tem-
peratures of the cold finger. The temperature of the main body is kept constant at 100
◦C . . . . 52 4.17 Comparison of the absorption of the two cells. The side arm of
the first cell is kept at 50
◦C, while the temperature of the second cell is 100
◦C . . . . 53 4.18 Temperature variations of the second iodine cell: the shape of the
curve remains constant for temperature values higher than 90
◦C 54 4.19 Layout of the optical components and the iodine cells. A com-
puter acquires the data from the photodiodes and the thermo- couples and controls the movements of the piezotranslator in the back of the laser cavity . . . . 55 4.20 Changes of the laser cavity length: the back mirror is displaced
with a piezotranslator (PZT). The absorption of the iodine reveals the changes of the light frequency . . . . 56 4.21 Repetitions of cavity length changes with the etalon tilt angle set
at different positions . . . . 56
4.22 Absorption curve as a continuous line . . . . 59
4.23 Repeatability check of the continuous absorption line . . . . 59
List of Figures 5.1 One hour time history of iodine absorption, and temperature
monitor of the iodine cell body, of the side arm, and of the ambient 62 5.2 Time history of intensity ratio in a short term period and fre-
quency analysis . . . . 64 5.3 Long time monitor of the natural laser frequency drift and mode
hops when no cavity control is activated . . . . 65 5.4 Changes of the laser cavity length monitored with both cells . . . 66 5.5 Long time acquisition, with PID . . . . 67 6.1 Hot wire characterization of the low speed jet. Velocity and tur-
bulence intensity profiles at Re = 40 000 . . . . 71 6.2 Hot wire characterization of the low speed jet. Map of iso-velocity
lines at Re = 40 000 . . . . 71 6.3 Layout of the optical set-up for measurements in the low speed jet 72 6.4 Photo of the table with the optical set-up and the low speed nozzle 72 6.5 DGV measurements in a low speed jet. The raw DGV measure-
ment signal is shown in black dots; a 1 s average is shown with green dots; the linear regression of the measurements in red . . . 74 6.6 Repeatability check of the measurements in a low speed jet . . . 74 6.7 Comparison on a velocity scale of the measured velocity fluctu-
ations (black dots) with the laser frequency fluctuations (blue dots). The green dots are the average over 1 s time windows . . . 75 6.8 The lack of recognizable patterns in the distribution of a veloc-
ity scatter plot indicates the absence of correlation between the fluctuation of the laser frequency with the fluctuation of velocity 76 6.9 Time history and frequency analysis of the intensity ratio of the
measurement signal . . . . 77 6.10 DGV measurements in a low speed jet low pass filtered at 50 Hz
(black dots) and 1 s average (green dots) . . . . 78 6.11 The high speed nozzle. Profile and photo . . . . 79 6.12 Sketches of the high speed nozzle with seeding entraining system
and settling chamber . . . . 79 6.13 Hot wire characterization of the high speed jet. Velocity and
turbulence intensity profiles at Re = 60 000 . . . . 80 6.14 Photo of the optical set-up and the high speed nozzle . . . . 81 6.15 DGV measurements in a high speed jet. Raw signals in black;
average over 1 s in red . . . . 82 6.16 Repeatability check of the measurements in a high speed jet . . . 82 6.17 Time varying measurements in a high speed jet. In red the raw
signal, in blue the running average with a 0.2 s window, and in
black the jet velocity as measured in the settling chamber . . . . 83
6.18 Definition of the geometrical parameters of a turbine cascade . . 85
6.19 The settling chamber of the C3 facility, with the mixing chamber
where the smoke generator is situated . . . . 87
6.20 The test section . . . . 87
6.21 The cascade in use, with the measurement positions 0.5 c
axup- stream and downstream the cascade . . . . 88
6.22 The measurement plane 0.5 c
axdownstream of the cascade, the direction of the 3 light sheets, and the direction of observation . 89 6.23 Pitch-wise velocity distribution upstream and downstream the cascade, to check the flow periodicity . . . . 90
6.24 Boundary layer velocity profile upstream the cascade . . . . 90
6.25 Pictures of the 5 hole probe: frontal and lateral views . . . . 91
6.26 Secondary velocity field obtained from measurements with the 5 hole probe: the mean velocity is subtracted from the velocity field, to highlight the secondary flow vortices. M
2,is= 0.6 . . . . 92
6.27 Losses distribution . . . . 94
6.28 Angle distributions . . . . 94
6.29 Sketch of the layout of the optical elements forming the receiving optics. The idea of the double view is represented by the camera field of view divided into the two regions: red and blue . . . . 96
6.30 Picture of the receiving optics . . . . 96
6.31 The double view recorded on the same sensor. The millimetric paper target serves as a reference for the dewarping coefficients and for the spatial calibration of the measurements . . . . 97
6.32 The dewarped left and right images of the double view . . . . 97
6.33 The double view of a flat white paper target, for the determination of the luminosity influence of one view on the other . . . . 98
6.34 Light intensity distribution along the red line of figure 6.33 for the left and right views recorded alone, and for the left and right views recorded together . . . . 98
6.35 Picture of the test section and the receiving optics . . . 100
6.36 Layout of the set-up for the first light sheet . . . 101
6.37 Layout of the set-up for the second light sheet . . . 101
6.38 Layout of the set-up for the third light sheet . . . 102
6.39 The set-up for the measurements with the third light sheet . . . 102
6.40 Secondary velocity flow field obtained from the average of 50 mea- surements per component . . . 104
6.41 Comparison of the intensity ratio across the iodine cell as seen by the photodiodes (reference) and by three points in a well illumi- nated region of an image . . . 106
6.42 Comparison of the intensity ratio across the iodine cell as seen by
the photodiodes (reference) and by six points in a region of an
image with weaker illumination . . . 106
Chapter 1 Introduction
This introductory chapter gives an overview of the measurement techniques to obtain fluid flow velocity information in wind-tunnel testing applications. On the basis of the literature on this subject and the current understanding of flow phenomena, especially in turbomachinery applications, the development of a non-intrusive measurement system will be proposed.
1.1 Framework and motivation of thesis and work
The work described in this thesis was done in the von Karman Institute (VKI) in a small suburb near Brussels. The VKI is a non-profit international educa- tional and scientific organisation, hosting three departments: aeronautics and aerospace, environmental and applied fluid dynamics, and turbomachinery &
propulsion. It provides post-graduate education in fluid dynamics (Research Master in Fluid Dynamics, doctoral program, stagiaire program and lecture series) and encourages “training in research through research”. A wide spec- trum of facilities has enabled research studies, basic and applied, to span the range from the low-speed regime of commercial aircraft to the hypersonic regime of re-entry vehicles; from internal flows as typically found in aero-engines and turbomachines, to the increasing concern of human interrelations with the en- vironment, such as large and small scale pollutant dispersal, forest fires, and micro-climates. The development of measurement techniques and instrumenta- tion in view of these studies plays a central role, and a number of techniques have been improved and developed in-house gathering a world-wide recognized expertise.
The present work concerns research on a new measurement technique for mea- suring velocity and velocity patterns in fluid dynamics, called Doppler Global Velocimetry (DGV). This technique was proposed and introduced in 1990, and a number of research institutes have since explored this technique. It was envis- aged that DGV had a number of advantages compared to existing techniques, like the popular Particle Image Velocimetry (PIV), but it turned out that DGV is more complex than expected and progress in development has slowed down.
In view of the advantages of this technique it was decided to build an experi-
mental set-up for this technique in the VKI, to explore the problems and try to find improvements.
1.2 The importance of measurements
The continuous growth of energy needs in modern society, the increasing ex- changes and contacts of people throughout the world, and the presentday life standards that we’re all used to, demand a continuous progress in the develop- ment of technology able to produce machines that can satisfy the requirements.
The challenge includes many goals, among which we can name a continuously increasing power request, ever improved efficiencies, a reduction of manufactur- ing and maintenance costs, a smaller environmental impact, decrease of weights, longer lifespans. . . and so on.
The design of new or more advanced machines, devices, or components in any technological field becomes more and more a delicate and a fine-tuning activity, where the previous experiences and the understandings of the relevant physical phenomena assume a central role. The scientific world and more generally the modern world bases its evaluations, its judgments, and its action-taking pro- cesses on quantitative data that can be obtained with measurements or, since not long ago, computers and numerical codes. The recent trend of extensively using computer and numerical codes in the prediction and analysis of problems, strongly changed the scenario of tools available to designers and researchers.
The reason is of course that it is in general faster and cheaper to produce results to a specific physical problem with a numerical code rather than to build, instru- ment, and test a model. The core question becomes then the reliability and the accuracy of the results obtained by the numerical simulations. Measurements play a central role not only because they provide quantitative data, but also because they can be the needed support for the validation of numerical codes.
Measurements remain therefore the essential and invaluable base to extend the human phenomenological experience of nature.
Fluid dynamics is not secondary to the run and rush towards progress, on the
contrary, it is contributing more and more as it is a very active and evolving field,
where measurements and data obtained by instrumented models and tests in
wind tunnels strongly interact with the results produced by numerical codes. Not
only the codes for fluid dynamic computations are evolving fast and on a daily
basis, but also the measurement tools are in rapid evolution. The availability of
new inventions like lasers, is reflected in the development of the tools present in
all scientific laboratories and sets milestones in the progress and advancement
of measurement techniques. In fact, the scope of the present work is to discuss
the development of a laser application into a velocity measuring instrument for
fluid flows.
1.3 The fascinating realm of laser technologies
1.3 The fascinating realm of laser technologies
The laser was invented more than 50 years ago, in the early ’50s, independently by different researchers. It was first named MASER (Microwave Amplification by Stimulated Emission of Radiation), then at the beginning of the ’60s the first optical maser was set up and called LASER (Light Amplification by Stimulated Emission of Radiation). Since their invention, lasers still cause quite a stir, curiosity and interest for their many applications in different technological fields.
Even the less knowledgeable reader feels a special appeal to this interesting and continuously developing technology: this special appeal is probably due to the wide range of applications and to the continuous development of new lasers.
In these first 50 years of life, lasers have witnessed an impressive development:
the dimensions of the laser have been reduced to reach the micrometer scale (microlasers) or even nanometer scale (nanolasers), the pulse duration has been shortened down to only a few femtoseconds and durations of attoseconds are already in sight. In parallel, the peak power has been increased to extreme values of about 1 PW.
1The emitting wavelength of lasers has been reduced and reaches now the X-rays.
A complete list and a thorough discussion of laser applications would be long and not appropriate here, but a brief summary of the major applications and the latest developments will help us focusing the attention on the role played by the laser in the modern world.
The possibility of obtaining coherent light with wavelengths in the X-rays opens the way to a new remarkable approach to holography: visualization of tridimensional protein structures or even DNA structures becomes easily achiev- able and will certainly lead to further progress in biological studies. Microscopy and lithography with X-rays are other potential applications of this type of lasers (Dennis L. Matthews and Mordecai D. Rosen [2]). Such short wavelength lasers also introduce the possibility of reducing the dimensions of the devices to microlasers or nanolasers: the former are already an essential part in the field of optical communications and in the field of information processing (Jack L.
Jewell et al. [3]). As for the nanolasers, their dimensions are limited by half the wavelength of the emitting light. When all three dimensions are brought down to the lower limit, only one mode of oscillation is still possible in the laser and the activation threshold energy is also reduced to a minimum (Paul L. Gour- ley [4]). Approaching the lower limit dimension of a laser led to studies where only one atom at a time is present in the laser resonator: these experiments are conducted to study the fundamental phenomena of the quantum interactions, e.g. the transition from spontaneous emission to stimulated emission (Michael S. Feld and Kyungwon An [5]). The shortest event generated and controlled by mankind is probably the pulse of a laser that only lasts for a few femtoseconds.
11 PW = 1015W. Peta- (P) is the prefix for 1015.
Figure 1.1: Jean Michel Jarre - Hong Kong - May 23
rd, 1995
This offers the means for studying the behavior of nature at timescales never explored before. De Silvestri et al. [6] discuss the new challenges offered by this kind of lasers.
As far as biology is concerned, a relatively new application of lasers has been the combination of pulsed and continuous wave lasers with beams focused to micrometer dimensions for use as scissors and forceps (Michael W. Berns [7]).
Storing and transmitting data are becoming of great importance in the world.
Applications of laser technologies for reading and writing CDs are now long established, and further developments into DVDs and into the most recent HD- DVDs or Blue-Ray Discs are by now commercial reality. A further step could be expected from the development of holographic memories, as discussed by Demetri Psaltis and Fai Mok [8]. Flavio Fontana [9] explains the central role of the laser in communication systems: real highways of data are needed to sat- isfy long distance connections with very high transmission rates, suited for an intercontinental network. Optical systems composed of optical fibers, semicon- ductor lasers and associated devices are the solution to the everyday data traffic increase. Industrial applications of lasers have become by now daily life: high power lasers (easily up to some hundreds of Watt) are used for cutting, drilling, welding, and surface treatments (Marco Brandizzi et al. [10]).
Ophthalmology is probably the field which has benefited most from laser pro-
gresses. Many are the pathologies commonly treated with lasers (Rosario Bran-
cato and Francesco Carones [11]). Also oncology is benefitting from laser oper-
ations, since the controllable wavelength of light can interact with the tumoral
tissues according to different parameters. The advantages of lasers for medicinal
1.4 Fluid dynamic measurements, a survey applications lies also in the fact that it can easily be introduced into the body with small fiber optical probes (Pasquale Spinelli and Andrea Mancini [12]).
The realm of laser technologies is so fascinating that even artists and singers have used lasers for public performances. Suffice it to mention, as an example of an original application, Jean-Michel Jarre, who played a harp made of laser beams during a concert in Hong Kong on May 23rd, 1995 (figure 1.1).
The pervasive aspect of lasers couldn’t let fluid dynamic aside, and its appli- cations set milestones also in this field.
1.4 Fluid dynamic measurements, a survey
Measurements of fluid properties are done with a large number of different mea- surement techniques. Researchers are interested in a variety of physical quanti- ties ranging from velocity, temperature, density, to concentration. Both quanti- tative and qualitative methods are available to measure these values. Qualitative techniques are for instance, smoke visualizations, oil flow visualizations or mea- surements with wool tufts, etc. These methods provide the researcher with a visual overview of the flow structure, and can therefore be very useful for struc- ture analysis, but in spite of the sometimes even good spatial resolution, they do not give quantitative information on the fluid properties with sufficient ac- curacy. As for quantitative measurements, quite a large choice is offered to the researcher, and categories of measuring techniques can be defined according to several approaches.
The traditional methods of measuring pressure and velocity of a flow is with probes: pressure probes and hot-wires are for sure the most wide spread sys- tems. These first two examples give us already the possibility to point out the first two categories of measuring techniques: pointwise (as opposed to field mea- surements) and intrusive. Although easily extensible to racks or arrays, pressure probes or hot-wires are able to determine quantities in one point only. So, in or- der to completely cover the whole flow domain, long wind tunnel running times are required, thus increasing the costs of the experiments. Additionally probes present an important blockage to the flow and introduce major disturbances both in the subsonic and in the supersonic ranges. In the subsonic range, the upstream flow is influenced by the downstream probe, and even if the values can be corrected via a calibration, the flow might be slightly changed by the presence of the probe. In the supersonic range, the introduction of the probe in the test section causes a blockage that might lead to choking effects and possibly to influence the massflow through the tunnel.
Optical measurement techniques are an alternative to probes: they avoid the
obstructions due to intrusion in the test section and belong therefore to the
non-intrusive category. They can still be pointwise, like the Laser Doppler Ve-
locimetry (LDV) or the Laser-Two-Focus (L2F), but they’re also appreciated
as field techniques, such as, for instance, Particle Image Velocimetry (PIV) or Backward Oriented Schlieren (BOS). Thin film measurements for heat transfer analysis or pressure tappings on models, although non-optical, are also non- intrusive techniques, but of course can only yield results that limit the study of the flow to the model surface. Surface measurements can also be performed optically: infrared thermography and liquid crystal techniques for temperature and heat-transfer measurements along with Pressure Sensitive Paints (PSP) or Temperature Sensitive Paints (TSP) are examples of such techniques. There are of course also optical measurements useful for the study of the bulk of the fluid flow: Schlieren, shadowgraph, interferometric techniques, Laser Induced Fluorescence (LIF), and BOS for density measurements; LDV, L2F, and PIV for retrieving velocity information.
The use of lasers for optical measurement techniques requires a bit of discus- sion. Not all the named techniques need all the properties of a laser (such as the monochromaticity of the emitted light, the coherence character of light, the particular polarization. . . just to name some), on the contrary, very often a simple white light can cope with all the requirements of the chosen measuring method. Let us concentrate, for instance, on the measurements of density: the light source used for the Schlieren does not require any of the special features of laser light, and, as a matter of fact, powerful bulbs or sparks are among the most common light sources for Schlieren imagery. If we consider interferomet- ric measurements, it is immediately apparent that the light must be at least monochromatic and the use of a laser is therefore very convenient. In a similar way, velocity measurements do not always require lasers: a light source output that can be shaped into a light sheet and that is powerful enough to record im- ages is perfectly suited for PIV applications, while a monochromatic light source is a must to perform LDV measurements, since it relies on the Doppler effects of the light scattered by the particles traveling with the flow.
A relatively new development of an optical velocimetry technique has been proposed with an American patent by Hiroshi Komine [13] and has raised the interest of a number of fluid dynamic laboratories across the scientific world. It is known as Doppler Global Velocimetry (DGV). James Meyers [14] from the NASA Langley research center even calls this new idea the “Next Generation”
velocimetry technique. Being indeed an interesting and quite intriguing topic, it was decided to investigate this technique at VKI. In the upcoming pages of the present work, the twists and turns of this appealing, powerful, and yet challenging and somewhat complicated measuring technique will be described.
1.5 Roadmap to the following chapters
In the following chapters first an overview of the development of DGV will be
given and the important work on DGV that has been done in the most important
1.5 Roadmap to the following chapters
research institutes willl be reviewed. As DGV is an optical technique, this is
followed by chapters on the requirements and the construction of the light source
to be used, i.e. a tunable single-mode laser, and of the device used to convert
optical frequency shifts in easy to measure intensity variations, i.e. a iodine cell,
and the calibration thereof. The interaction of the emitted laser light with the
iodine absorption sets the basis for the laser frequency stability control. After
that, the results of the experiments and the measurements on a low speed jet, a
high speed jet, and a turbine cascade will be given, followed by a discussion of
the results.
Chapter 2
Doppler Global Velocimetry
The principle underlying the DGV measuring method is presented in this chap- ter. Also an overview of published DGV systems and techniques is given.
2.1 Doppler velocimetry
The Doppler Global Velocimetry system is an instrument which performs the measurement of the Doppler shift of the light scattered by moving particles.
With reference to figure 2.1, let us consider a particle illuminated by laser light. The laser light has a frequency ν
0and propagates in direction ~l. The particle will scatter light in all directions. The light seen by an observer along the direction ~ o will have a frequency ν . When the particle moves with velocity V ~ , the frequency ν seen by the observer is different from ν
0because of the Doppler effect. The variation of frequency ∆ν = ν − ν
0is linked to the velocity V ~ of the particle, to the directions ~l and ~ o, and to the speed of light c by the formula:
∆ν = ν
0c
~ o − ~l
· V ~ (2.1)
This relationship represents the basic principle of Doppler velocimetry (see e.g.
G´ erard Degrez and Michel Riethmuller [15]).
Let us consider some numerical values, to have an idea of the order of magni- tude of the frequency shift that has to be measured. The light frequency ν
0is of the order of 10
14Hz, the speed of light c of the order of 10
8ms
−1and let us assume the value of the velocity V ~ equal to 100 ms
−1. For simplicity let us say that the direction ~ o − ~l
coincides with the direction of the velocity V ~ so that the dot product will give us a unitary value. Then, according to equation 2.1, the order of magnitude of ∆ν is of the order of 10
8Hz. This is a very small value compared to the initial light frequency ν
0. Only very accurate instruments with a very high resolution (at least 10
−9for a 10% accuracy) could be used for determining such a small value directly.
Methods to work around the small value of frequency shift and that retrieve
the speed of the particle, have been proposed and used since the very beginning
of Doppler velocimetry. The first application of laser Doppler velocimetry in
Figure 2.1: The principle of Doppler velocimetry: determination of the mea- sured velocity component
fluid dynamics was performed already in 1964 by Yeh and Cummins [16]. They measure the velocity in a pipe flow of water: their system comprises a HeNe laser beam split into two by a beam splitter. One beam acts as a reference and the other illuminates the particles in the flow. The light scattered by the par- ticles is focused and recombined with the reference beam on a photomultiplier:
such a configuration behaves as an optical heterodyne detection set-up of the frequency difference between the two light beams. The photomultiplier records the frequency difference as a beat signal which has a frequency proportional to the particle velocity.
The configuration chosen restricts the light collection to the direction of the reference beam. Moreover, since the beat signal is a function of the scatter- ing angle, small solid angles are required to collect the light in order to avoid broadening of the frequency spectrum.
A similar, yet different approach to measure the scattered light frequency shift
is followed in 1970 by Jackson and Paul [17] who use a Fabry-P´ erot interferome-
ter for velocity investigations at supersonic speed. The output of an argon laser
is split into two, one beam focused into the region to be investigated, the other
beam, which serves as a zero velocity reference, is focused at the scattering vol-
ume. The light scattered from the particles is collected by a lens and directed
into a confocal piezo-electrically scanned Fabry-P´ erot interferometer. Scanning
the Fabry-P´ erot interferometer with a sawtooth ramp and detecting the trans-
2.1 Doppler velocimetry mitted light with a photomultiplier gives the frequency spectrum of the signal which is finally linked to velocity.
Yet another way is followed by Smeets and George [18] in 1981, who choose a phase stabilized Michelson interferometer as a high resolution spectrometer to yield a signal proportional to the Doppler shifted frequency. The higher complication of the optical set-up is compensated by a reported high sensitivity for small velocities and a micro-second response time. Demonstration of flow measurements in a shock tube and in a supersonic jet confirm the fast response of the instrument to velocity variations.
An alternative method to resolve the small frequency shift of light is the crossed beam system, also called the fringe method. This technique lets two beams of equal intensity crossing. Particles transiting the region where the two laser beams cross, scatter light at a frequency that is independent of the direction of observation. The name fringe method derives from the heuristic explanation that the two laser beams form a pattern of interference fringes in the beam crossing region. Systems with this configuration are probably the most wide spread in fluid dynamic laboratories, and are known as Laser Doppler Velocimetry (LDV). A conventional LDV system with two crossing laser beams measures only one component of velocity and only in one point of the flow. In order to extend the measurements to two or three components, additional laser beams (usually of different colors) are needed from a different orientation: for the second component, a system can be realized with a single emitting optical device that comprises two sets of two beams, each in mutually perpendicular planes; for the third component, a supplementary emitting device is required which must be focused at the same measurement point. As for covering the whole region of interest in the flow with measurements, a patient and lengthy process must be followed, displacing the measurement point throughout the test section. Details on the theory, use, and operation of LDV systems can be found, e.g. in the lecture of Boutier [19].
It is not before 1990 with an idea patented by Hiroshi Komine [13] that
Doppler velocimetry could rely on a further alternative method to resolve the
light frequency shift. The new proposal of Komine claims an atomic or molec-
ular vapour as a filter able to convert the Doppler shifted frequency content
of scattered light into intensity variations: it is much easier to measure the in-
tensity of light than its frequency. Since the light intensity going through the
vapour cloud can be subject to variations because of external factors, part of
the incoming light ray is split off and its intensity I
0monitored: the ratio I/I
0of the light intensity which actually crosses the vapour cloud I and the moni-
tored incident light I
0gives an indication of the energy absorbed by the vapour
cloud. The capability of filtering a light signal through a frequency-to-intensity
converter is certainly not limited to a single ray, but a wider beam can simul-
taneously be analyzed. Measurements can therefore be performed in a whole
plane if photomultipliers are substituted with cameras. Still, only one compo-
nent of the velocity vector can be retrieved: it is intrinsic in the basic principle of Doppler velocimetry, expressed by the relationship 2.1. The extension to two or three components can be achieved by changing the direction of the vector
~ o −~l
, which in turn can be achieved by changing one or both of the directions of the observer ~ o or the light source ~l. For the three components of velocities no different colors are required, no different beams have to be focused and crossed on the same measurement point: one light sheet can simply be observed from three different directions and the global velocity field can be measured. The enthusiasm for this simplicity led Hiroshi Komine et al. [20] to refer to the novel technique as Doppler Global Velocimetry (DGV). Other research groups working on the same technique named the same principle with different acronyms, which through the years has summed-up to a quite confusing cluster of terminology:
Miles [21] refers to it as Filtered Rayleigh Scattering (FRS), McKenzie [22] calls it Planar Doppler Velocimetry (PDV), Elliott is not even coherent with himself as he uses first the term Filtered Rayleigh/Mie Scattering (FRMS) [23] then the term Filtered Planar Velocimetry (FPV) [24], Smith [25] names it Absorption Filter Planar Doppler Velocimetry (AFPDV).
The “new way to look at velocity” (James Meyers [26]) promises a potential real-time analysis of the images, thanks to the easy data processing required if compared with the more resource-eating PIV, and the choice between instan- taneous or averaged measurements, thanks to the use of pulsed or continuous wave lasers respectively.
The introductory statements of the DGV technique sound very appealing:
it can be a field technique, three components of the velocity can be acquired simultaneously, the processing requirements are less demanding than PIV (the counterpart of field velocity measurement techniques), instantaneous as well as averaged measurements can be performed. Due to all the claimed advantages, many research centers devoted time, effort and energy to the development of this technique. As can be expected, nothing is ever so easy, and before reaching a satisfactory level of robustness, many issues have to be faced and overcome.
In the following sections, a summary of the studies done by the most important research centers working on DGV is presented. The preference in the discussion is given to the research centers working mainly with a continuous wave argon ion laser, since the present work addresses the development of a DGV system with this type of laser. A more general overview of research centers studying DGV is reported in table 2.1 at the end of this chapter.
2.2 DGV at NASA Langley
After the initial steps at the Northrop Research and Technology Center by Hi-
roshi Komine [13, 20], an immediate reaction came from the NASA Langley
Research Center that took over the idea and started working on the topic under
2.2 DGV at NASA Langley the leading personality of James Meyers.
In 1991 Meyers and Komine [26] discuss the new measurement concept, and show their first application. They use a continuous wave argon ion laser oper- ating in single-mode at 514.5 nm as a light source, and they frequency tune the emission by means of a tilting etalon mounted inside the laser cavity. The edge of an absorption line in molecular iodine serves as the frequency discriminator.
Two cameras form the receiving optics: the factors influencing the amount of collected scattered light, such as particle distribution and density, are minimized by viewing the same scene with the two cameras. The first camera reads the light intensity directly from the particles, while the other looks through the io- dine line filter. If good alignment of the cameras is obtained, a normalization of the images gives the absolute attenuation through the absorption line filter.
The presented results show the potential of the system with a one component prototype: demonstration of velocity measurements are presented for a spinning wheel, for a jet at 100 ms
−1, and for a delta wing.
Difficulties are not late in showing up. In 1992, Jimmy Usry et al. [27] point out the importance of the precision of the alignment of the cameras in the set-up of Meyers and process the images both with an analog and a digital method. Meyers [14] elaborates further on the difficulties of camera alignment and adds other important factors that play a role in the correct overlap of the images: lens aberrations, imperfections of beam-splitters, mirrors, and iodine cell windows. A numerical algorithm that can cope with all the deformations of the images seems the solution: the basis for a custom image warping software is discussed. Image warping, pixel sensitivity, and laser output drift are the topic of a paper by Lee et al. [28]. Background illumination images recorded under the same condition as during the tests but with no seeding particles are subtracted from each image, then the ratio of the images can be computed. Correction for unequal pixel sensitivities is performed. Low-pass filtering is applied to the images to remove some of the camera noise. More delicate aspects are underlined by Meyers [29] in 1994. Careful examination of the intensity variations in the images showed saturated pixels in the center of the light sheet and very dim regions near the edges. The cause was traced back to the simple expansion of the laser beam through lenses, which open the natural Gaussian distribution of the beam, and produce high intensity at the center and lower intensity on the sides. The solution presented is the use of a high-speed galvanometer scanner that sweeps the beam into a sheet of uniform intensity. Additional attention is given to the software that has to correct for the alternate line interlacing acquisition system of the cameras.
In 1995, Meyers [30] reviews the work done at NASA up to that moment in an
article where yet additional problems are discussed. Minor misalignment of the
images are corrected by cross-correlating the images and detecting with precision
the displacement for an improved overlap. With a reference to the work carried
out at the German DLR (discussed in a following section), a discussion on the
laser frequency drift and its impact on the measurement precision is presented, arriving at the conclusion that a stable laser or monitoring the laser frequency is an essential standpoint for further progress. A number of applications is reported, sweeping from jet flows, to an oblique shock on a flat plate, and to the flow behind a delta wing at several different angles of attack. In spite of a lack of comparison with more traditional measurement techniques, an accuracy of the measurements is claimed in the range of ±2 ms
−1. Supplementary details of the parameters influencing the measurements accuracy are given by Meyers in 1996 [31], where large temperature fluctuations of the iodine cell are recognized to be the main source of errors.
In order to make a step forward and to improve the technology(!), the switch from an argon ion laser to an Nd:YAG laser is considered by Meyers [32] in 1998. The introduction of the pulsed laser brings along unexpected surprises:
although laser speckle is to be expected as a more important effect from nar- rower line-width lasers, such as argon ion (reported linewidth 10 MHz) when compared with the Nd:YAG (reported linewidth 80 MHz), it revealed to be a much greater problem because of the short pulses. The non-satisfactory results obtained with this work set a stop to the rush towards continuous innovations and define a starting point for a more accurate characterization of measure- ment error sources. In 2001, Meyers [33] analyzes step by step all the elements that make up the straightforward, yet difficult to utilize measurement technique.
Recommendations are given for an accurate temperature control of the iodine vapour cell, that should be stable within 0.1
◦C. A significant improvement is the vapour-limited iodine cell as described by Elliott et al. [34] (Ohio State Uni- versity, United States). Given the key role of the shape of the absorption line of iodine, an accurate determination is pursued for the first time using the rotating wheel as a calibration tool. Recommendations are given also for the other essen- tial element among the DGV components: the laser. Ambient temperature and pressure variations as well as structural vibrations are indicated as parameters to be monitored if a stable laser operation is desired. Final warnings on the effects of the aperture of the receiving optic lens and on the possible secondary scatter from high concentrations of seeding particles are addressed.
More recent research (2006) is directed at obtaining temporal information from a reduced system based on DGV that only looks at one measurement point (Cavone et al. [35]), and at a more flexible system inspired on the work of David Nobes et al. [36] (Cranfield university, United Kingdom) that uses fiber optics to collect three different views onto only one camera sensor (Meyers et al. [37]).
To be noted in the latter work, the use of a dual-pass Bragg cell to shift the
laser frequency. Normally, DGV systems are operated with the laser frequency
adjusted to the midpoint along the side of the absorption line, while the laser
frequency is tuned to the bottom and only the Doppler shifted light frequency
is seen on the side of the absorption line. The advantage is a wider range of
measurable frequencies covered by the iodine absorption line, and therefore the
2.3 DGV at DLR
bigger range of velocity that can be measured.
2.3 DGV at DLR
Research on DGV at the Deutsches Zentrum f¨ ur Luft- und Raumfahrt (DLR) started very soon after Komine’s publications [13, 20]. Ingo Roehle and Richard Schodl [38] publish in 1994 the details of their system with the application to a subsonic jet. The stability of the laser frequency, as well as a reproducible and stable calibration of the iodine absorption line are treated as essential features for a reliable and accurate measurement system. The stability control of the laser is obtained with feedback loops that monitor the laser light by means of a spectrum analyzer and through the iodine cell. The outcome of the loops are input values to the temperature control of the intracavity etalon oven and to the displacement stage of the backward mirror of the laser cavity. The long- term stability achieved is ±1 MHz. The calibration of the iodine is obtained by using the frequency scale given by the hyperfine structure of iodine. Their set-up includes a pinhole placed at the focal point of the front lens and an optical chopper disc in front of the laser beam. The photodetector signals are amplified synchronously with the laser pulse: this method should reduce the noise from daylight influences. A comparison of the measurements with the established L2F (Laser-Two-Focus) technique shows good agreement.
The strong bases of the optical light source and the absorption line filter mo- tivate the DLR to move forward and extend the technique to three-dimensional velocimetry. The discussion whether one or multiple observations are best for three dimensional measurements brings Ingo Roehle [39] to adopt the former solution, with only one observation point and three directions of the laser sheet.
The laser sheet is generated in a sheet forming box by a wobbling prism that sweeps the beam and does not introduce luminosity gradients, a problem en- countered in the work of NASA Langley. Fiber optic cables bring the light from the laser output to the sheet forming box. The acquisition time is reduced to a minimum of less than 30 s approaching the much praised “real-time” capabilities as described in all the papers by James Meyers. Measurements in a swirl nozzle and behind the wake region of a car model are presented.
The reliable and user-friendly system of DLR becomes a powerful tool for measurements in flows with industrial interest: thanks to the flexible light sheet delivery apparatus, it is used to check the velocity inhomogeneities and mass flows at the intake of an aircraft (Roehle et al. [40]). The detailed description of the system, the image processing steps and a summary of applications are documented in a lecture by Roehle [41], who also defended his PhD [42] in the same year (1999).
The maturity of their system is demonstrated by the fan of papers presented
at the biannual symposium on laser techniques for fluid mechanics in Lisbon,
Portugal in 2000. Chris Willert et al. [43] demonstrate an original use of DGV for phase-averaging velocity data of an engine exhaust flow: the emission of the continuous wave argon ion laser is modulated with a Bragg cell in order to phase-synchronously expose the camera. Fischer et al. [44] explain the working principle of a narrow band, frequency stabilized, tunable, long pulse Nd:YAG laser that has been developed through a collaboration between the DLR and the Laser Center Hannover (LZH). The higher light intensities obtained with such a laser allow for measuring the velocity field in kerosene flames. F¨ orster et al. [45] combine L2F with DGV for three component pointwise measurement applied to a compressor flow: a classic two component, back scatter L2F system measures the components of the velocity in a plane perpendicular to the optical axis, while, thanks to a background on DGV experience, a Doppler shift analysis of the scattered light is added to the system, thus allowing the measurement of the third component of the velocity without further optical access needed.
The work on phase-averaged DGV measurements leads Roehle [46] to inves- tigate further the accuracy of DGV. Interesting findings of his work are the non linearity of some cameras, the influence of polarization effects as a function of the iodine cell transmission, the detrimental effects of multi-scattering, reflec- tions, and scattered light reflected from surfaces. Worth to note are also the remarks on measurement accuracy: when three light sheets are observed with one camera, the determination of the in-plane components is independent of the meticulous determination of the observation direction and of the exact laser frequency; only the out-of-plane component accuracy depends on the accuracy of the determination of those parameters.
The application of DGV to industrially interesting flows continues with a to- mographic analysis of the steady state, isothermal velocity field of the in-cylinder flow of a piston engine cylinder (Willert et al. [47]). Time averaged velocity data is measured with an almost automatic system, that reduces significantly the run- ning time of the experiments and the data processing time.
The successful applications of DGV to a variety of combustor flows is the topic of a summary paper by Richard Schodl et al. [48]: besides the (usual) description of the by now known systems, the stress lies on the celerity of obtaining non- intrusive, field measurements which makes it a tool “pre-destined for rapid flow analysis and feedback into the design process”.
Measurements in a cryogenic wind tunnel appear as a challenge more for
the troublesome accessibility to the test section than for the underlying DGV
technique (Willert et al. [49]). Problems are faced with regard to the seeding
particles, then solved by the introduction of nitrogen and water vapour into the
tunnel: the cryogenic atmosphere immediately forms tiny ice crystals, whose
dimensions are not known. The novelty with respect to their previous works is
introduced by the use of fiber optics (first used by David Nobes et al. [36]) that
collect the light scattered by the particles and guide it to the camera sensor,
which brings along problems of luminosity variations that need to be corrected.
2.4 DGV at ONERA A further (and minor) improvement in the technique is the replacement of the square prism with an octagonal prism for the creation of the light sheet: the smaller angles between the prism surface and the incoming laser beam keep the losses to a minimum. A conclusive note brings back the simplicity of PIV when compared with DGV, and the advise to stick to PIV whenever possible.
Contrary to the expectations and the hopes of the early works, DGV still struggles its way through fluid dynamic laboratories without encountering en- thusiastic approvals. The reasons can lie in the higher level of complexity when compared with PIV and which makes it less appealing for researchers. More- over, the results that can be obtained by DGV are similar to those achievable by PIV, if the time for acquisition is not critical. The wide, consolidated, and affirmed base of PIV users adds negatively to the growth of DGV as an everyday instrument. A need is felt to question and face these issues with other teams: a workshop organized at the laboratories of ONERA
1between DLR and ONERA offers the ground for a comparative study of the systems of the two institutes (Willert et al. [50]).
2.4 DGV at ONERA
In the early years of DGV research, the quest for originality and the attempts to explore different paths, led ONERA to look into different lasers and different absorption vapours, while still faithful to the Doppler principle and to Komine’s idea of absorption line filtering. In 1996, Leporcq and Le Roy [51] describe a DGV system based on a single mode tunable dye laser which generates a narrow linewidth. The advantage consists in the wide tuning range, covering wavelengths from 500 to 700 nm, which allows for selecting absorption lines with specified characteristics of well-resolved shape and linear side profile. With this wide range of laser tuning capabilities, the selection of the absorption line is not limited any more to the fortuitous correspondence of argon ion emission and iodine absorption, but is now open to any combination. In particular, the use of bromine seems promising: it is a topic of discussion in a following paper (1997) by Leporcq et al. [52].
The innovative trial of a dye laser and a bromine cell is left aside, and studies continue in 2000 with a more traditional argon ion laser and a usual iodine cell (Barricau et al. [53]). The laser is provided with the newest commercial frequency stabilization and jitter control for optimal stability. The iodine cell, manufactured by the Bureau International des Poids et Mesures, comprises a side arm where the temperature is kept at a minimum and the solid iodine is supposed to condensate. A diffractive lens yields a solution to the concerns of a non uniformly distributed laser sheet intensity: the diffractive LASIRIS lens
1Office National d’ ´Etudes et de Recherches A´erospatiales
yields a flat intensity profile. The light sheet forming box of DLR already creates a flat intensity profile, however, a parasite Doppler effect is introduced to the measurements that has to be taken into account when processing the results.
In spite of the frequency stabilization and the jitter control, a perfect laser stabilization can not be reached and in the long term the frequency still drifts.
In 2001, Barricau et al. [54] present the creative DEFI system (the acronym DEFI stands for Dispositif d’Etalonnage Fr´ equence/Intensit´ e), which provides an on-line calibration on every measurement image acquired. The system relies on an acousto-optic device able to shift the light frequency by known steps F . About 10% of the laser power is split from the main beam and passed through this acousto-optic device, which splits further the rays into five, each frequency shifted by a difference F from the preceding. The five beams are then driven with optical fibers to a lateral strip of the camera sensor. In this way, five points on the absorption line of iodine are recorded along with the measurement image.
Since the DEFI system multiplies the outgoing beams and shifts the laser frequency to four values different from the laser output frequency, it can be advantageously used also for calibrating the iodine absorption lines: for every laser frequency, five points on the absorption line are tested, thus speeding up the calibration process or delivering more results when sweeping the argon ion gain profile (Christine Lempereur et al. [55]).
2.5 DGV at the University of Oxford
At the university of Oxford, the work on DGV is motivated by the appealing potential measurements of transonic turbo-machinery flows in their Isentropic Light Piston Tunnel. Roger Ainsworth et al. [56] start in 1994 with an extensive discussion on the elements needed in a DGV system. The discussion covers both their initial experiments and a compendium of the published works. The original point in the work is the proposed imaging system with only one camera, which reduces the electrical complexity of synchronizing two cameras and assures the simultaneous image capture. The drawback is a decreased spatial resolution, since the two images (signal and reference) are placed side by side on the same sensor. The system is tested on a rotating disc, and measurements in a free jet are presented by Steven Thorpe et al. [57], along with the discussion of the error sources. Manners et al. [58] devote many efforts to a careful analysis of the best method to process the images: dewarping algorithms and interpolation methods are the center of their studies.
The work with an argon ion laser at Oxford is concluded by Ainsworth [59]
with a review of the works of other laboratories and a detailed examination
of the relevant parameters in the measurement chain of a DGV system. The
experience built up by the team takes a turn and the research continues with
a pulsed Nd:YAG laser (Steven Thorpe and Roger Ainsworth [60]). Measure-
2.6 Conclusions ments are performed in a low speed air-brush plume and in a high transient flow emerging from an open-ended shock-tube, but more interesting is subsequently the application to the time-varying high-speed flow of a new type of transdermal delivery device (Thorpe et al. [61]): for once, the particle velocity is the relevant parameter to be measured and not the velocity of the fluid flow which carries the particles.
2The initial use of fiber optics by David Nobes [36] caught also the attention of the Oxford team, who started the development of a three component DGV system: faithful to a reduction of synchronization difficulties (and costs) they image the three double views (a signal and a reference image per component) on one camera sensor (Graham Hawkes et al. [63]).
2.6 Conclusions
In this chapter the principles behind DGV are outlined: information of flow ve- locity can be obtained by illuminating the flow with a suitably chosen line of an argon ion laser (at a wavelength of 514.5 nm, in the green part of the spectrum) and measuring the optical frequency shift (Doppler shift) of the light scattered by particles travelling with the flow. The main innovation over previous Doppler velocimetry techniques is the use of a cell containing iodine vapour, whose ab- sorption is strongly dependent on the on the optical frequency and matches the emission line of the argon ion laser. Measuring the scattered light intensity through the cell provides a quantitative determination of the optical frequency shift. By observing the flow velocity from different directions, or by repeating the measurements with different direction of the laser illumination, three com- ponent velocity measurements can be obtained. In addition, an overview of the research activity in the main centers is given. Since, by now, DGV has not yet become a commercial reality, and in view of its possibility and advantages, it may provide an important and valuable tool for the ongoing work on fluid dy- namics at the von Karman Institute. The decision to start the development of such a technique is at the basis for the work of this thesis, and the goal of the research is to contribute to the growing knowledge on the topic.
2The interested reader can satisfy his/her curiosity with further details on the study of the needle-free drug delivery device that can be found in the article of Quinlan et al. [62]
Team System Applications References Northrop
Research and Tech- nology Center, USA
·Doppler Global Velocimetry (DGV)
·Ar+ (Coherent Innova90-5), 1W
· Nd:YAG (Spectra Physics DCR-11) with seed laser (Lightwave Electronics 120-03), 15ns
·Image size = 50×100mm
·3C, 6CCD(simultaneously)
Near sonic velocity in a small wind tunnel
[13, 20]
NASA, Langley Research Center
·Doppler Global Velocimetry (DGV)
·Ar+
·Nd:YAG 10Hz,10ns, Laser band width = 120MHz
·Image size = 100×200mm
·1C, 2 CCD
·Velocity calibration system with spinning wheel
·600-grift sandpaper for reference monitoring
Wind tunnel testing (Delta wing ˜67m/s)
[26, 33, 32, 31, 30, 29, 27, 14, 64]
· Absorption Filter Planar Doppler Velocimetry (AF- PDV)
·Nd:YAG (Quanta-Ray YG-590) 30Hz, 15ns, Laser band width = 100MHz, 100mJ
·1C, 1 CCD with image splitter
Overexpanded supersonic jet (˜600m/s, 30Hz)
[65, 25, 66]
NASA, Ames Research Center
·Planar Doppler Velocimetry(PDV)
·Ar+ , 6W
·Nd:YAG, Laser band width = 140MHz
·Image size = 76×76mm
·Development of uncertainly model
·Rotating wheel (2m/s˜)
· low-speed turbulent jet (˜60m/s)
·Large Wind tunnel testing (˜60m/s)
[67, 68, 69]
DLR ·Ar+ , 1-2W, PID controller, Frq.jitter=1˜4MHz
·Optical Fiber for 3C
·Bragg cell for periodic flow
· DASA-Engine Inlet (40˜130m/s)
· Piston Engine Cylinder (cold)
[46, 70, 41, 40, 71]
Long pulsed Nd:YAG, 1.6mJ ·Kerosine burner
·Downstream of the turbo- charger
[48, 44]
·Ar+ , 2W
·3C-Doppler-L2F
Transonic Centrifugal Com- pressor (30˜75m/s)
[45]
ONERA, Fr.
·Ar+ (Spectra Physics 2060/65 with Jitter Lock) 2W
· 2C DGV with DEFI (Dispositid d’Etalonnage Fre- quence/Intensite)
Wind tunnel testing (Mach 0.74)
[54, 72]
Ohio Univ.
USA
·Planar Doppler Velocimetry (PDV)
·Nd:YAG (Spectra Physics GCR-4 ) 9ns, 10Hz, 660mJ
·Real Time PDV (RT-PDV)
·Wind tunnel testing (Mach 0.51-0.86)
· Supersonic Micro Flow (Mach 2)
[73, 34, 24]
Princeton Univ.
USA
·Filtered Rayleigh Scattering (FRS)
·Nd:YAG, cw, 50mW
·Meas. Value (Velocity, Temp., Press.)
Free jet (Mach 2) [74, 75, 76, 77, 78, 79]
Cranfield Univ. UK
·3C Planar Doppler Velocimetry (PDV)
·Nd:YAG (Spectra Physics GCR 190-30), 300mJ
·Fiber Image Bundle (1-I2cell, 2-CCD for 3C velocity meas.)
·Frequency-switching technique by AOM (acousto-optic modulator) (1-I2cell, 1-CCD)
[36]
Univ. of Oxford, UK
·Ar+ (0.5W) for rotating disc
·Nd:YAG, Laser band width = 90MHz, 10Hz
·1-CCD for 1C, capturing both reference and signal im- ages (Sony XC77CE)
Jet (˜450m/s) [61, 60,
59, 58, 56]
