Proceeding of ’99 Korea-Japan Joint Seminar
on Particle Image Velocimetry
pp15-31
Optical Diagnostics for Velocity, Temperature and
Species Measurements of the Flow Field
Toshio KOBAYASHI, Hui HU and Tetsuo SAGA
Institute of Industrial Science, University of Tokyo
Roppongi 7-22-1, Tokyo 106-8558, Japan
Abstract
The basic concept and principles of the optical techniques for the velocity, temperature and spice measurements of the
flow field, which include Particle Image Velocimetry (PIV), Thermos-Chromic Liquid Crystal (TCLC) and Planar Laser
Induced Florescence (PLIF) techniques, were briefly introduced in the present paper. The current development states of
these optical diagnostic techniques were overviewed and illustrated with examples picked up from recent published
journal and conference papers describing both the developments and applications of PIV, TCLC and PLIF techniques.
The highlight areas for the future development were also suggested in the present paper.
Keywords: Optical diagnostics, Particle Image Velocimetry (PIV) technique, Thermos-chromic Liquid Crystal
(TCLC) technique and Planar Laser Induced Florescence (PLIF) technique
Introduction
With the rapidly development of the model optical techniques and digital image processing techniques, optical
diagnostics is assuming an ever-expending role in the diagnostic probing of fluid mechanics by supplying the fluid
mechanics researcher with the capability for remote, non-intrusive, in-situ, spatially and temporally precise
measurements of important parameters of the flow field like velocity, temperature and species. This means that, optical
diagnostics provides the fluid mechanics researchers and engineers not only with the more accurate measurements but
also with completely new capabilities. Spatially precise, instantaneous measurement at high rates permits the flow field
to be frozen and tracking with high frequency response. Measurement at many locations simultaneously along a line,
over a plane or in a volume permits spatial correlation to be obtained providing new phenomenological insight into
fundamental behaviors of flow phenomena. The application of these new and exciting techniques is so promising that it
is expected to impact fluid mechanics science and technology in a very significant fashion.
Although various optical flow field diagnostic techniques had been developed, the context of present paper will just
focus on the techniques of Particle Imaging Velocimetry (PIV), Thermos-chromic liquid crystal (TCLC) and Planar
Laser Induced Fluorescence (PLIF). These techniques can be used to measure the velocity, temperature and spice of the
flow field instantaneously and globally. Compared with other traditional velocity, temperature and species measurement
techniques, PIV, TCLC and PLIF techniques have following characteristics:
1. Non-intrusive measurement techniques: In contrast to techniques for the measurement of flow velocities and
temperature employing probes as pressure tubes, hot wires, thermos-couple or other traditional methods, PIV, TCLC
and PLIF being optical techniques work non-intrusively. This allows the application of PIV, TCLC and PLIF techniques
even in high-speed flows with shocks or in boundary layers closed to the wall, where the flow may be disturbed by the
presence of probes.
2. Whole field measuring techniques: PIV, TCLC and PLIF are techniques, which allows to record image of large
parts of flow fields in a variety of applications in gaseous and liquid media and to extract the velocity, temperature and
spice information out of these images. For the velocity measurement, this feature is unique to the PIV techniques except
to Doppler Global Velocimetry (DGV), which is a technique particularly appropriate for high-speed air flows (the error
of DGV is almost bigger than 5m/s(Meyer, 1995), which made it is unacceptable for the most low speed case). For the
flow field temperature measurement, although infrared (IR) imagery technique can also get globe results, it suffers from
the poor spatial resolution and measurement accuracy. Other techniques for velocity, temperature and spice
measurement methods only allow the measurement of the flow at a single point. The instantaneous image capture and
high spatial resolution of PIV, TCLC and PLIF systems allow the detection of spatial structure of even in unsteady flow
fields.
Due to the above advantages, numerous researches had been reported about PIV, TCLC and PLIF techniques in the
past two decades. The applications of PIV, TCLC and PLIF almost touch upon all the fluids-related fields, range from
the fluid mechanics fundamental study of shear flow and turbulent transition to the engineering field of turbo- 15 -
machinery, automobile and aircraft designing. Many famous review papers and textbooks about PIV (Adrian 1991,
Grant, 1997, Wernet 1997, Reffel et al. 1998), TCLC (Dabiri et al. 1991) and LIF (Hesselink, 1988 and Eckebreth,
1995) are also available commercially.
In the following context, some basic principles of PIV, TCLC and PLIF techniques will be brief introduced. The
recently development of these techniques will also be reviewed by giving some examples of the velocity, temperature
and spice measurement results of the flow field by using these techniques.
1.
PIV Technique
The technical basement of the PIV to do velocity measurement is to measure the displacement of the tracer particles
seeded in the flow in the settled short time interval. A typical set-up of a PIV system always consists of several subsystems (Fig.1), which including particle seeding, flow field illumination, particle image acquisition and image
processing. In most PIV applications, tracer particles have to be added to the flow. These particles are illuminated in a
plane or in a volume of the flow at least twice within a short time interval. The light scattered by the particles is
recorded either on a single frame or a sequence of frames by either photographic film or CCD camera. It is assumed that
the trace particles move with the local flow velocity between the illuminations. With the settled time interval of the
illuminations and the displacement of the tracer particles between the two or more light illuminations determined
through PIV image processing, the velocity of the flow field is constructed.
1.1. Seeding
As the same as Laser Doppler Velocimetry (LDV), PIV technique measures the velocity of the fluid field indirectly by
means of the measuring the velocity of tracer particles within the flow. The kinematics of the local fluid field was
estimated by analyzing the motion of the tracer particles recorded on the photographic film or CCD camera. In order to
insure the tracer particles can follow to the flow motion accurately, the tracer particles should be chosen carefully to be
neutrally buoyant and to be efficiently scatter the illumination lights.
Since the obtained particle image intensity and the contrast of PIV images is directly proportional to the scattered
light power. It is often more effective and economical to increase the image intensity by properly choosing the
scattering tracer particles than by increasing the illuminating laser power. The light scattered by small particles is a
function of many factors, such as the ratio of the refractive index of the particles to that of surrounding medium, particle
size, particle shape, particle orientation and angle of observation. For spherical particle with diameters larger than the
illumination light wave length, the scattered light by the particle can be estimated by Mie's scattering theory (Van de
Hulst, 1957).
Figure 2 shows the normalized scattered intensity of different diameter glass particles in water according to the Mie
theory. From these figures, it can be seen that the scattered light intensity will be increasing as the particle diameter
increases. It should also be noted that the scattered light intensity is not blocked by the tracer particle but spread out in
all directions. Therefore, for a large number of particles inside the light sheet, massive scattering will appear. So, the
light intensity which is focused by the recording lens is not only due to direct illumination but also due to fraction light
which have been scattered by more that one particle. These imply that not only larger particles can be used to increase
the scattering efficiency but also the number density of tracer particles.
The scattering efficiency of the trace particle also strongly depends on the ratio of the refractive index of the particle
to that of the fluid. Such as, the refractive index of water is considerably larger than that of air. The scattering of
particles in air is at least one order of magnitude more powerful than particles of the same size in water (Raffel et al.
1998). So, the larger particles have to be used for water flow experiment, which can mostly be accepted since the
density matching of particles and fluid is usually better than that in air.
1.2 Illumination system
The illumination system of PIV is always composed of light source and optics. Lasers, such as Argon-ion laser and
Nd:YAG Laser, are widely used as light source in PIV systems due to their ability to emit monochromatic light with
high energy density which can easily be bundled into thin light sheet for illuminating and recording the tracer particles
without chromatic aberrations. The optical system is always consisted by a set of cylindrical lenses and mirrors to shape
the light source beam into a planar sheet to illuminate the flow field.
1.3 Particle Image acquisition
The widely used recording medium for PIV particle image acquisition is either photographic film or Charge Couple
Device (CCD) camera. The development of digital CCD camera has encouraged to the development of Digital Particle
Imaging Velocimetry (DPIV) (Willert et. al. 1991). Comparison with the photographic film based PIV techniques, DPIV
system has the many advantages. (1). It is fully digitized, so avoids expensive opt-mechanical devices and tedious
interrogation processes. (2). Various digital techniques can be implemented to handle noise and improve processing
speed. (3). It allows for the cross-correlation of two independent video frames and removes problems associated with
the self-convoluted images. (4). Conventional auto- or cross- correlation techniques combined with special framing
techniques can be used to measure higher velocities. While, the disadvantages of the DPIV in current stage are the low
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temporal resolution (defined by the video framing rate, always 30 Hz from 512 by 512 CCD camera and much lower for
the other high resolution camera) and the loss of the spatial resolution (the CCD camera used in current stage is almost
512by 5122, 1024 by 1024 or high to 2048 by 2048 pixel, which is still at least one or two order lower than that by
using photographic film.). These disadvantages are expected to be improved in the future with the development of the
computer technology.
1.4. Image Processing
Many algorithms or methods had been proposed for PIV image processing. All of these methods or algorithms can be
conclude into three catalogues based on the particle image density.
When the tracer particles seeding density is low, the trajectory of the individual particles can be identified in the series
of images. The algorithms or method based on identification of the trajectory of the individual particles are always
called Particle Tracking Velocimetry (PTV) method. The most common used PTV method including 4T-PTV
(Kobayashi, et al.1989), Binary correlation method (Uemura et al. 1990), spring-model method (Okamoto, 1993) and
Super-Resolution Method (Keane et al.1995) et al..
As the seeding density increasing, the detection of individual particles becomes difficult. In this case an alternative
approach named correlation method can be used to tracking the patterns of particles instead of individual particle. By
using correlation method, The PIV recording images is divided into small sub-areas called "interrogation area", the local
displacement vector for the images of the tracer particles at the first and second illumination is determined for each
interrogation area by means of statistical methods. The correlation function is always used to judge the corresponding
pairs of the interrogation areas in the first and second illuminations.
The correlation methods may also be divided auto-correlation method and cross correlation method. When the two
exposure were recorded in one frame, auto-correlation method was always used. For the auto-correlation method, it will
always have the problem of direction ambiguity, which may be cured by using image shifting (Adrian et al. 1986) or
other techniques (Shen, 1997 and Marzoak, 1998). When the two exposures were recorded on two frames, the crosscorrelation method can be used. Since it did not have the problem of direction ambiguity, cross-correlation method is
the most popular method for PIV image processing in the current stage (Hu et al. 1998).
When the tracer particle concentration is so dense that individual particles are not longer distinguishable, the pattern
is formed through the variation in the speckle intensity variations on the image. It was always named as Laser Speckle
Velocimetry (LSV) for this case. The most common used method for LSV image processing is Young's fringe analysis
method (Grant et al. 1989).
1.5. The expanded application range of PIV technique
Since PIV technique can measure the velocity of whole flow field instantaneously without disturbing to reveal the
global structures of a complicated and/or unsteady flow field quantitatively, the application of PIV technique almost
touch upon all the fluids-related fields. The application range of the PIV technique has be expanded quite a lot in the
recent years. PIV technique hand been used to study either steady flow or unsteady flow, low speed flow (subsonic
flow) or high speed flow (supersonic flow); single phase flow or multi-phase flow; macros scale flow or micro scale
flow.
PIV technique had been used successfully to study many complex unsteady flow phenomena to reveal the evolution
of the vortex and turbulence structures in the flow field. Figure 3 shows the PIV measurement results in a self-induced
sloshing flow field (Hu et al 1999). The changes of the global flow pattern and vortices evolution during the selfinduced sloshing can be revealed very clearly from the PIV velocity vector field.
PIV technique has also been used to study many high-speed (supersonic) flow phenomena. Figure 4 shows the PIV
measurement result of a transonic flow field (the velocity range is from 280m/s to 520m/s) around a NACA 0012 airfoil
(Raffel et al., 1993). The structure of shock about the airfoil can be revealed very clearly in the velocity vector field.
PIV technique had also been used to research many multi-phase flow phenomena in recent years, which include gasliquid flow, gas-solid flow and liquid-solid flow. Since the PIV images can not only used to construct the velocity fields
of the multi-phase flow, but also can be used to get information about the geometry and concentration distributions of
the bubbles or solid particles in the multi-phase flow field through image processing. The interaction of the flow field
with the gas bubbles or solid particles can be revealed more easily and directly than other techniques. Figure 5 shows
the PIV measurement results of a liquid-gas two-phase flow. The effect of the gas bubbles rising on the water flow field
was analysis based on such kind of PIV results (Lindken et al. 1999).
In recent years, one of the most promising and challenging research topics is the study about MEMS (MicroElectronics Machine System). In order to study the flow behavior inside MEMS (the scale of the flow field is always in
the order of ｵ m), Micro PIV techniques have also been developed. Figure 6 shows the PIV measurement results of a
Hele-Shaw flow around 30ｵ m abscale (Santiago et al. 1998 ) and a supersonic flow in a micro-nozzle (Meihart et al.
1999), more details can be got from their papers.
1.6. Three Components and Three Dimensional PIV techniques
The "classical" PIV technique is just two dimensional method, which is only capable of recording the projection of
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velocity into the plane of the laser sheet, i.e the out-of-plane velocity component is lost while the in-plane components
are affected by an unrecoverable error due to the perspective transformation (Nisino 1999). For highly three-dimension
flows, this can lead to substantial measurement error of the local velocity vector. In order to get the out-of plane
velocity component, some three components PIV (3C-PIV) and three-dimensional PIV (3-D PIV) techniques were also
be proposed. They include stereoscopic PIV technique (Parsad, 1993), Dual-plane PIV technique (Raffel et al.1996), 3D PTV technique (Suzuki et al. 1999) and Holographic PIV (HPIV) (Barnhart et al. 1994, Zhang et al., 1997).
Stereoscopic PIV technique is a most straightforward, but not necessarily easily implemented, method for the three
component velocity measuring PIV technique. It used two cameras at different view axis to do PIV recording (Fig.7,
Saga, 1999). By doing the view reconciliation (Soffol et al., 1998), the corresponding image segments in the two views
are matched to get three component of the velocity vector in the laser sheet plane. Dual-plane PIV technique is a quasi
three component PIV technique, which is implemented by offsetting the light sheet a small amount between the
recordings to obtain the out of plane velocity of the flow field in the laser sheet plane. Based on the measuring change
of the respective correlation peak height from one recording to the next, the out-of-plane displacement component was
estimated (Raffel et al. 1998, Fig. 8). The Dual-plane PIV technique was has the problem of the poor accuracy for the
out of plane velocity in the current stage.
3D-PTV and HPIV are real three-dimensional measurement technique, which can get the three components of the
flow velocity in a volume simultaneously. Three cameras are always used to record the positions of the particle tracers
in the measurement volume from three different view directions for 3D-PTV technique (Fig. 9, Suzuki et al. 1999).
Through there-dimensional image reconstruction, the locations of the tracer particles in the measurement volume are
determined. By using particle-tracking method, the three dimensional movement displacements of the tracer particles
are calculated. Up to now, 3D-PTV method may suffer from the low spatial resolution (always less than 1,000
instantaneous velocity vectors can be got simultaneously by using common used CCD camera), which is expected to be
improved by using high resolution CCD-camera.
Holographic PIV (HPIV) uses holography technique to do PIV recording, (Fig. 10), which enables the measurement
of three components of velocity throughout a volume of flow to be obtained. Of the existing three-dimensional PIV
methods, HPIV is capable of the highest measurement precision and spatial resolution. However, HPIV is still at its
stage of development and not well suit for experiment, where set-up time, optical access and observation distance are
important factors (Okamoto, 1999). The further efforts should be needed to make the HPIV technique as a more
practical technique for fluid mechanics experiments in the coming years.
2. Micro-Capsulated Thermos-chromic Liquid Crystal (TCLC) Thermometry
Liquid crystal, which is thermos sensitive, can change its color with the changing of temperature. Since the first using
of liquid crystal in the field of fluid dynamics by Klein et al.(1968) in determining the location of laminar and turbulent
boundary layer transitions on a flat plant placed in a supersonic air stream, the development of the digital image
processing technology in the past two decades had enabled Thermos-Chromic Liquid Crystal (TCLC) to be used not
only a qualitative indicator to indicate the hot and cold region but also as a quantitative tool to visualize the temperature
of the flow field quantitatively.
The cholesteric liquid crystal is composed of layers of liquid crystal sheet (Fig. 11), and in each sheet the molecules
are aligned in a slightly different orientation with respect to their adjacent layer. When unpolarized light hits the liquid
crystal, it is split into two linear polarized components. Due to the anisotropy of the liquid crystal, the polarized plane of
the light will be rotated. When the optical wavelength is equal to the wavelength of the incident light, the circularly
polarized light in the same direction as the helix is reflected, while the circularly polarized light in the opposite direction
is transmitted through the liquid crystal.
As the temperature of the liquid crystal changes, the distance between the sheet of liquid crystal layers changes
tending to increase the pitch of the liquid crystal. And the angular direction of each layer of liquid crystals increases
with respect to its adjacent layers tending to decrease the pitch. Due to these effects, the color refection of the liquid
crystals is not only sensitive to the temperature but also to the viewing angle, shear stress and magnetic field as well.
However, by encapsulating the liquid crystal and using them in the non-magnetic environment, the effect of the shear
and magnetic field can be eliminated.
The temperature response time of liquid crystal is also an important consideration since a slow response time would
result in erroneous temperature values. Since the liquid crystal is encapsulated, the response time of both the liquid
crystal and encapsulating shell should be take into account (Doh, 1995). By choosing the correctly sized neutrally
buoyant liquid crystals, the temperature field of the interested flow may be measured accurately and responding quickly.
To measure the temperature of the flow field by using Thermos-Chromic Liquid Crystal (TCLC), the calibration
process to get a quantitative relation of the color to temperature must be conducted before the experiment starting. For
color-to-temperature relation calibrating, the visualized images of the micro-capsulated liquid crystal particle at settled
temperatures were captured by color CCD camera in R, G and B color space. By using the acquired R, G and B values,
the other optical parameters such as Intensity, Saturation and Hue of the color can be computed.
Fig. 12 gives the typical measured optical characteristics of the micro-capsulated liquid crystals (R23CW3,
D=0.3mm), which including the acquired normalized R, G, B values and Hue in relation to the temperature. Since their
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relation to temperature is non-linearity, a neural network method (Ozawa et al., 1992) was suggested to construct the
color to temperature calibration profile with higher accuracy. By using the calibration profile or look-up tables of the
color-to-temperature relation, the temperature of the flow field can be calculated.
Figure 13 shows an example of temperature measurement results by using TCLC. By scanning the illumination light
sheet, three-dimensional temperature distributions of the flow field can also be constructed.
3. PLIF technique for spice (concentration) and temperature field measurement
Florescence process is a radioactive decay process that occurs by electronic transitions in molecules of fluorescence
material. After a fluorescent dye molecule is exposed to an electromagnetic field or exactitude by laser beam. Photons
entering the fluorescent dye molecule will cause displacements of electrons from one energy state to another. The
displacement of the electrons results in increased potential energy of the molecule from the ground state to the excited
state. When the electrons return to the ground state from excited state, fluorescent light will be radiated. Fig. 14 shows
an example of the spectrum of a fluorescent dye.
The intensity of the laser induced fluorescence measurement at any arbitrary point (x0,y0) along the excitation beam
can be expressed as:
H f ( x0 , y0 ) = I ( x0 , y0 ) AΦ εLC ( x0 , y0 )
(1)
where Hf ( x0,y0) is the measured fluorescence intensity at the point (x0, y0), A is the fraction of the fluorescence light
collected by camera. Φ is the quantum efficiency, L is the length of the sampling volume along the path of excitation
beam, ε is molar absorptivity, and C(x0,y0) is the molar concentration of the fluorescent dye. I(x0,y0) is the intensity of
excitation light beam at the point (x0,y0), which can also be expressed as:
I ( x0 , y 0 ) = I 0 e − εlc
(2)
Where I0 is intensity of the laser beam at the inlet of the measure flow field, l is the length of solution the excitation
laser beam traveled through before reaching the point (x0,y0), and c is the concentration of the fluorescent dye.
From the equation (2), it can be seen that, since the intensity of the excitation light I(x0,y0) is the function of the
position and the concentration distribution of the fluorescent dye along the excitation beam before reaching the
measurement point B. This means that concentration distribution of the fluorescent dye may attenuate the intensity of
the excitation beam. This attenuation effect will be increasing with the increasing of the concentration of the fluorescent
dye. Fig.15 shows the attenuation effect in the disodium fluorescein solution with different concentration (Hu et al.
1999). It can be seen that, in the low concentration condition, the attenuation effect is very small, which can always be
negligible.
3.1. The spice (concentration) measurement by using PLIF
From the equation (1), it can be seen that, if the temperature of the flourecent dye is kept in constant, the intensity of
induced fluorescent light will be change linear with the concentration of the fluorencent dye in the flow field for low
fluorescent concentration case with the attenuation effect being negligible. So, by measurement the intensity of the
fluorescent light, the concentration distribution of the fluorescent dye in the flow field can be calculated.
Before the measurement starting, the calibration procedure to get the quantitative relationship of the normalized
intensity of the florescent light to the concentration of the fluorescent dye should be conducted firstly. Figure 16 shows
a typical calibration profile of normalized intensity of the florescent light to the concentration of the fluorescent dye (Hu
et al. 1998).
By using the above calibration profiles, the concentration distribution of the fluorescent dye in the flow field can be
measured. Figure 17 shows some examples of the concentration measurement results in a lobed jet mixing flow by
using PLIF technique (Hu et al. 1998).
3.2. The temperature measurement by using PLIF
For some fluorescent dyes, the molar absorptivity is temperature dependent. Therefore, equation (2) can be rewritten
as follows:
H f ( x0 , y 0 ) = I ( x0 , y0 ) AΦ ε (T ) LC ( x0 , y 0 )
(3)
If the concentration of the fluorescent dye is now kept in constant, the temperature of the flow field can be calculated
by measuring the intensity of the fluorescent light.
To do the temperature measurement by using PLIF, the calibration procedure to get the quantitative relationship of
the normalized intensity of the florescent light to the temperature should also be conducted firstly before the
measurement beginning. By using the calibration profile of the normalized intensity of the florescent light to the
temperature (Figure 18, Sakakibara et al. 1993), the temperature distribution of the flow field can be constructed.
3.3 Dual emission laser induced fluorescence technique
The PLIF temperature measurement method described in the above section just used a single dye as fluorescent dye.
In order to insure the measurement result has a good accuracy, a set of the calibrations have to be done to eliminate the
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errors due to the non-uniform distribution of light sheet, the laser light reflection and the attenuation effect.
One way to by pass these issues is to use a dual emission PLIF technique which normalize the fluorescent light
intensity of temperature dependant dye with a temperature independent dye. According to the equation (3), when the
laser light passed through a homogeneous fluorescent solution containing two dyes (both with a constant concentration
through out the solution), the fluorescence ratio at any point can be expressed as:
H 1 f ( x, y )
H 2 f ( x, y )
∝
ε1 (T )c1Φ1
ε 2c2 Φ 2
(4)
Where H1f (x,y) is the fluorescence intensity of the temperature dependent dye and H2 f (x,y) is the fluorescence
intensity of a temperature independent dye. From equation (4), it is evidence that the fluorescence ratio is only a
function of a few physical properties of the dyes, not the excitation intensity. Assuming that both fluorescence are
present in constant concentration everywhere in the flow field, these physical properties can be normalized through a
calibration ratio of fluorescence ratio versus temperature. That is the ratio of the physical properties of the two
fluorescent dyes is a constant and does not influence the change in the ratios with temperature. Note that the
fluorescence intensity of the temperature dependent dye contains the temperature information, while the temperature
fluorescence intensity of independent fluorescent dye contains the excitation intensity information at every point in the
laser sheet. If the florescence intensity from each dye is measured simultaneously, the fluorescence ratios will be
independent of laser light alignment, distribution, and intensity.
Figure 19 (Coppeta et al. 1998) shows the comparison of the two (flourescein and rhodamine B) and single dye
(flourescein) system's ability to predict temperature. It shows that the change in single dye absorption with temperature
causes the error to increase in the direction of light sheet propagation while the dual system is immune to the changes in
absorption.
An example of the temperature measurement result by using dual emission laser induced fluorescence technique were
shown on the Figure 20 (Sakakibara et al. 1999). By scanning the illumination laser sheet, the three-dimensional isosurface of the temperature in the flow field was shown in the figure.
4. Simultaneous measurement of the velocity and temperature/spice fields
As mentioned above, PIV technique can be used to do vector field (velocity) measurement of the flow field, while
TCLC and PLIF techniques can be used to measure the scalar (temperature and spice) distributions in the flow field. In
many engineering applications like combustion phenomena, the relationship between the vector transfer processing and
scalar transfer processing may also be very important and interesting. So, the methods to combine PIV technique with
TCLC or PLIF techniques to measure the velocity and temperature (or spice) distributions of the flow field
simultaneously have also been suggested in the recent years.
A method named as Particle Image Thermometry and Velocimety (PITV) was suggested by Doh, et al. (1995) with
the combination of PIV technique with TCLC technique. PITV method uses micro-capsulated thermos-chromic liquid
crystal particle as tracer particles seeded in the flow flied. The spatial position information of the micro-capsulated
liquid crystal particles was used to construct the velocity field by using PIV technique, while the temperature
distributions of the flow field was measured by the color-to-temperature relationship of the micro-capsulated liquid
crystal particle. The PITV technique had been successfully used to conduct the temperature and velocity measurements
in a vertical buoyant jet flow (Doh et al.1995), a typical measurement result were shown on Figure 21.
The ideals with the combination of PIV and PLIF to resolve the velocity and temperature (or spice) distributions in
the flow field have also been suggested (Grissino et al. 1999, O’Hern et al. 1999). The flow field was seeded with small
spherical tracer particles embedded with fluorescent dyes. The laser-induced fluorescent light and the laser light
scattered by tracer particles are separated by using optical filters and detected by camera separately. The information
from the scattering Laser light was used to construct the velocity distribution of the flow field by using PIV image
processing. While, the signals from the laser induced fluorescent light are used to calculate the scalar distributions
(either temperature or spice concentration) in the flow field. A typical results of the velocity and temperature
simultaneous measurements from a PIV/LIF system was shown on Figure 22.
5. Conclusions and highlighting areas of future development
Recent advances of model optical techniques and digital image processing techniques are leading PIV, TCLC and
PLIF techniques to pay ever-expending roles in the research fields of fluid mechanics and thermodynamics. Evolution
of these techniques had been the principal activities of a number of laboratories over the last twenty years. These
techniques are now well established with its limitations and strengths being understood and well documented as
indicated in the many referenced papers in this paper and elsewhere (Adrian, 1996)
Although much progress had been made in improving the processing speed of PIV, TCLC and PLIF techniques by
using ingenious software and complex hardware, the sustained development in this field should still be need in the
coming years. The processing speed problem will become more serious for the stereogrammetry and Holographic
version of these techniques.
Besides the need to develop more powerful hardware for the techniques of PIV, TCLC and PLIF, considerable efforts
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are also still need for the image processing algorithms development in the future to increase the accuracy and resolution
(both spatial and temporal resolution) capability of the measurement results by using these techniques. Such as, two
major problems can be identified with current image processing techniques related to PIV are (1). Limitation on spatial
resolution of the estimated displacement field. (2). Limitation on the dynamical range of displacements (i.e., the
difference between the largest and smallest displacement) that can be accurate measured. Although some progress have
been made in recent years (Keane et al, 1995, Kumar et al. 998, Hart, 1999 and Hu et al 1999), sustained efforts should
still be made in future.
The relationship between the development of experimental techniques and Computational Fluid Dynamics (CFD)
simulation methods is being one of the most popular topics in the recent days. With the fast development of the modern
experimental techniques, in particular, the appearance of the field measurement techniques, like PIV TCLC and PLIF
techniques, it has become possible to reveal the flow field turbulent structures instantaneous and globally by using
experimental tools. The measurement results of these techniques will allow not only for simultaneous valid comparison
between CFD simulation results and experiment data and more importantly will enable the development of robust and
reliable turbulence models for CFD simulation (Moser et al., 1998). With the accumulating of a sufficient body of
turbulence data got by optical diagnostics like PIV, TCLC and PLIF techniques, the prospects for developing reliable
turbulence models are indeed bright.
The need to understand the complex flow phenomena are reflected in many sponsors of the researches of PIV, TCLC
and PLIF techniques with the interests coming from civil engineering companies, automobile and aircraft
manufacturers, energy producers and defense agencies. The necessary and importance of the flow field optical
diagnostic techniques like PIV, TCLC and PLIF have been well recognized. It is not doubtful that these optical
diagnostics will pay more and more important roles in the fluid-related researches with the continuos development of
these techniques in the future.
Reference
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Flow field with
tracer particles
Illumination
system
display
Synchronizer
camera
computer
Figure 1. A typical PIV system
a. d=1ｵ m
b. d=10ｵ m
c. d=30ｵ m
Figure 2. The light scattering by glass particle in water (Raffel et al. 1998)
-30.00 -24.00 -18.00 -12.00 -6.00
0.00
6.00
12.00 18.00 24.00 30.00
150
Spanwise Vorticity ( Z-direction )
Y mm
200
Spanwise Vorticity ( Z-direction )
Y mm
200
-30.00 -24.00 -18.00 -12.00 -6.00
0.00
6.00
12.00 18.00 24.00 30.00
150
Re =6,700
Re =6,700
Uin = 0.33 m/s
Uin = 0.33 m/s
50
50
0
0
-50
-50
0
50
100
150
U out
100
U out
100
X mm
200
250
300
-50
-50
a. t=t0
0
50
100
150
X mm
200
b. t=t0+1.0/7.5s
Figure 3. PIV measurement result for a self-induced sloshing flow field (Hu et al. 1999)
- 24 -
250
300
Figure 4. PIV measurement result for a transonic flow field above a NACA 0012 airfoil (Raffel et al.1993)
7
Figure 5. PIV measurement result of a multi-phase flow (Lindken et al. 1999)
a. Hele-Shaw flow around 30ｵ m obscale (Santiago et al. 1998) b flow in a supersonic micro-nozzle (Meinhart et al. 1999).
Figure 6. PIV measurement result of micro flow fields
- 25 -
Laser
Sheet
d
x1
x1
d
x1
d
x1
1
2
α1
x3
d
x3
α2
V
x2
β
β
1
x
x
d
x2
d
x
1
1
Camera 2
Camera 1
2
V
dx
d
x3
22
x2
x3
a. Optical arrangement
Y
20
0
-10
-20
-30
-20
-30
-10
Y mm
10
Z
X
30
W m/s
21.0896
20.0605
19.0314
18.0023
16.9732
15.9441
14.9149
13.8858
12.8567
11.8276
10.7985
9.7694
8.7403
7.7112
6.6821
5.6530
4.6238
3.5947
2.5656
1.5365
0.5074
20
10
0
-10
-20
-30
-20
-30
-10
0
Xm
m
Y mm
Z
30
Y
0
10
Xm
m
20
30
b. instantanous results
10
20
30
c. time average results
Figure 7. Optical arrangement and measurement results of a Stereo PIV system (Saga, 1999)
a. optical arrangement
b. measurement results
Figure 8. Optical arrangement and measurement result of Dual-plan PIV system (Rafel et al. 1998)
- 26 -
X
W m/s
21.0896
20.0605
19.0314
18.0023
16.9732
15.9441
14.9149
13.8858
12.8567
11.8276
10.7985
9.7694
8.7403
7.7112
6.6821
5.6530
4.6238
3.5947
2.5656
1.5365
0.5074
b. system arrangement
b. measurement results
Figure 9. System arrangement and measurement result of 3D-PTV system (Suzuki, 1998)
a. recording system
b. reconstruction system
c. 3D velocity distribution
Figure 10. Optical arrangement and measurement result of a Holograph PIV system (Barnhart et al. 1993)
- 27 -
Figure 11. The cholestric liquid crystal structure (Dabiri et al. 1991)
a. Normalized R, G, and B color values vs. temperature
b.Hue value vs. temperature
Figure 12. The optical characteristics of thermal-chromic liquid crystal (Doh, 1995)
Figure 13. Temperature measurement results by using thermos-chromic liquid crystal thermometry (Kimura et al. 1997)
- 28 -
Figure 14. The Spectrum of fluorescence dye Rohdamine B (Sakakibara, 1995 )
1.2
H(X)/H(0)
1
0.8
0.6
c0:1g;1250l
0.4
laser power 2.5w
c0:1g:100l
0.2
0
0
50
100
150
200
250
L length (m m )
Figure 15. Attenuation effect of the laser light (HU et al. 1999)
(I-Ib)/(I0-Ib)
1
0 .8
L a s e r p o w e r = h ig h
L a s e r p o w er= m iddle
L a s e r p o w e r = lo w
T h e o r y V a lu e
0 .6
0 .4
0 .2
C /C 0 , C 0 = 0 . 3 m g /l
0
0
0 .2
0 .4
0.6
0.8
1
Figure 16. The calibration profile for the concentration measurement by using LIF technique (Hu et al. 1998)
- 29 -
Figure 17. Spice concentration measurement result in a jet mixing flow by using PLIF (Hu et al. 1998)
Figure 18. The calibration profile for the temperature measurement by using PLIF technique (Sakakibara et al. 1997)
Figure 19. A comparison of single and dual florescence dye system for temperature measurement (Coppeta et al. 1998)
- 30 -
Figure 20. A typical temperature measurement result by using dual emission LIF technique (Sakakibara et al. 1999)
Figure 21. A typical measurement result of a PITV system (Doh et al. 1995 )
Figure 22. A typical measurement result of a PIV-PLIF system (Grissino et al. 1999)
(velocity vector from PIV and temperature distribution (background) form PLIF )
- 31 -