Laser Velocimetry

1

NGUYEN Patrick

SUAREZ Carlos

Laser velocimetry measurements

Laser Project Tutors:

David Clark

Xavier Marie

2011/2012

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Introduction

Since dawn of human existence and throughout History, scientists have always been fascinated by the way how

fluids move whatever its nature, liquid or gas. In order to reach a continuously better and better

understanding of fluid behaviors, more and more complex flow measurement systems have been designed. All

these combined efforts allowed technical improvements concerning many various fields such as architectural

and mechanical design of bridges, hydrodynamics of boats and submarines, aerodynamics of automobiles and

aircrafts, development of pipelines webs, etc.

Fluids velocity measurements have always been a challenge. Even if for example, trajectory shapes of a liquid

flow can easily be observed introducing colorants in the liquid, accurately determining its velocity is not always

a simple task. This report first establishes a short history of existing velocimetry devices. Afterwards,

advantages of laser velocimetry over other techniques are highlighted as well as their applications for industry,

medicine and science. And finally, three different types of laser velocimetry are thoroughly described and their

general features compared.

I. Brief history of velocimetry

In 1732, the Pitot tube was invented by the French physicist Henry Pitot. 1

It is the first mechanism really able to

determine the velocity of a fluid. The Pitot tube is essentially a pressure sensor which measured value let us

deduce the velocity from Bernoulli’s laws. This device is constituted of two cylindrical concentric tubes: the

extern tube aperture has to be oriented parallel to the fluid flow measuring the environmental pressure

whereas the intern tube aperture faces the fluid flow, indicating the total pressure.

Some years later, in 1796, the Italian physicist Giovanni Venturi created the Venturi tube. This tube is a device

that creates a pressure drop by reducing cross section area of the flow path. It consists of a short straight pipe

between two conical sections where gauges are placed in order to measure the pressure difference between

the two parts of the tube having different section. And in the same way as the Pitot tube, it is possible to find

the fluid velocity from Bernoulli’s laws. Nevertheless, this kind of pressure sensors has several limitations such

as poor accuracy of measurements, disturbances due to the device introduction in the liquid flow and lack of

any turbulent phenomenon possibility.

In the 1830s, Michael Faraday stated his law of induction which would make the invention of the magnetic flow

meter possible. According to him, the voltage induced across any conductor as it moves through a magnetic

field is proportional to the velocity of that conductor. Although this device can be non-invasive, it has some

drawbacks such that the fluid on which the velocity is measured must be electrically conductive, for example,

water that contains ions, and an electrical insulating pipe surface is required.

Similarly, the first description of another physical phenomenon, the Doppler Effect, by the Austrian physicist

Christian Doppler in 1842 was an event which marked the beginning of development of a new kind of flow

sensors based on frequency shifts of an ultrasonic signal when it is reflected by moving gas bubbles or particles.

That frequency shift is proportional to the flow rate.

Another new device involving the Doppler Effect on coherent light waves is the Laser Doppler Velocimeter

(LDV) which was developed soon after the fabrication of the first He-Ne laser in 1962.

Yeh and Cummins developed in 1964, while working at Columbia University, the first prototype of a LDV and

they succeeded in getting accurate velocity measurements of water flowing through a pipe.2 In the late 1970s

several research groups began to experiment a new method of flow quantitative visualization called Particle

Image Velocimetry (PIV) where particles are illuminated by a laser light sheet and their locations filmed by a

3

camera. Finally, since its invention in 1990, a technique called Planar Doppler Velocimeter (PDV) has

undergone a rapid pace of development, since this method takes advantages of light scattering of particles or

molecules. 3

II. Why do we need laser velocimetry?

Laser velocimetry importance comes from the possibility to detect extremely tiny targets which exhibit

dimensions in the order of some micrometers and a great spatial and temporal resolution. As measurements

can be led without introducing any part of devices in the fluid and therefore avoids any external disturbance of

the measured flow velocity, laser velocimetry techniques are considered as non-invasive. Furthermore, some

laser velocimetry complex devices allows measurements of the three components of a fluid velocity at the

same time.

The main applications of laser velocimetry measurements are:

Experimental verification of theoretical models in fluids: Laser velocimetry techniques give to scientists the

opportunity to validate and confirm the numerical algorithms resulting from Computational Fluid Dynamics

(CFD) which allow the analysis and resolution of problems that involve fluid flows. 4

Aerodynamics: In industry, many different aerodynamics experiments using laser velocimetry techniques are

implemented to optimize the design, fabrication and construction of automobiles, airplanes or helicopters, in

order to reduce air resistance for high performance, to decrease the energy consumption or even far to

increase the efficiency of heating systems in aircraft cabins.

Hydrodynamic experiments: In industry and laboratories, laser velocimetry techniques are part of a lot of

research experiments looking for design and manufacturing improvements. These kind of experiments involves

any mechanism immerged in a fluid such as sediment transport process, mixture characterization tests, design

of ship hull structures, simulation for swells of rivers, seas and canals and for pumps functioning.

Combustion process: In aerospace and automotive industry, it is necessary to study flow, temperature and

concentration insides engines in order to reduce pollutant emissions, improve fuel efficiency, optimize diesel

injection and decrease noise levels. Laser velocimetry are complementary techniques used to investigate how

the different swirl flames interact with each other. 5

Biomedical Research: Since the LDV is a non-invasive technique, it has been applied to measure continuous

circulation of blood flow on a microscopic level. The specific studies that have been performed all relate to

detecting blood flow abnormalities. 6

Study of blood in arteries, veins and capillaries, the detection of plasma

and red blood cells are required for the improvement of therapeutic medical procedures.

Flow visualization in wind tunnel: With the development of Planar Doppler Velocimetry, it is possible

nowadays to render the air movements visible in the wind tunnel. For this, submicronic particles are seeded in

the flow and illuminated by a laser light sheet such that these particles follow the stream lines of the vortex,

and scatter the laser light reaching the camera.7

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III. Laser velocimetry techniques

1. Seed particles and the Doppler effect

All three laser velocimetry techniques we are about to describe need seed particles to be performed and the

first two of them exploit the Doppler Effect induced by these seeded particles presence. Before being able to

measure a liquid flow, seed particles have to be injected in the liquid. These particles are usually spheres

composed with materials such as polystyrene, polyamide or hollow glass which size can ranges from 1 to 100

µm 8 so that they are small enough to be totally carried (no sedimentation effect) by the liquid flow but large

enough to scatter a detectable amount of light. Actually, what we measure is the particles velocities from

which the liquid flow can be deduced if they are properly-sized.

Fig. 1 Illustration of the Doppler shift detected from a seeded particle9

Figure 1 shows how the Doppler Effect gets involved. As a seeded particle crosses over a laser beam, it scatters

the incident light (frequency f) into all directions and consequently behaves itself as a secondary light source.

The scattered light can be caught by a photodetector. However, as the particle is moving as well as scattering

light at the same time, the detected light frequency will be slightly different from the original frequency. This

difference is called the Doppler shift (fD) and directly depends on the relative motion and distance between a

wave source and a detector. As an acoustic illustration we all have experienced, it explains why the pitch of the

sound seems to change when you are standing at a fixed point and a car passes quickly by you. As the car gets

closer and closer, the sound becomes shriller (high frequencies) and as it gets farther away, the sound becomes

deeper (low frequencies).

Going back to our seed particle, the Doppler shift can be expressed as:

2

cos sin2D

Vf

αβλ

=

This underlines the proportionality between the Doppler shift and the absolute value of the particle velocity (V)

as well as its dependence on the particle moving direction (β angle) and the photodetector location (α angle).

2. Laser Doppler Velocimetry

2.1 The heterodyne model

Because the Doppler shift fD is a very small value compared to the huge of the light frequency f, measuring it

accurately is nearly impossible. To get around this issue, the LDV (Laser Doppler Velocimetry) technique also

known as LDA (Laser Doppler Anemometry) introduces the using of the two laser beams of same intensity and

wavelength which crosses each other. When a particle passes through the crossing area, there will be two

different scattered light waves arriving on the photodetector which will therefore detect a light wave with a

frequency equal to f + fD1 and another with a frequency equal to f + fD2.

(1)

When two waves of equal amplitude and nearly equal frequency are added, the resulting signal will have his

periodically amplitude rising and falling with

equal a half of the difference between the two

equal to |fD1 - fD2|/2. As this beat frequency is

Fig.

Considering a simple case as an example,

angle, a particle passing perpendicularly through the intersection area and a pho

bisector of the two beams. Using equation

would be given by:

1 2| | 2 2cos sin sin

2 4 4 2D Df f V Vθ θ θ

λ λ− = =

From that point, the particle velocity V is easily

2.2 The fringe model

A more common way to explain the

understand and closer from how light signals are really processed

Doppler Effect is less visible.

Fig. 2 (a) Scheme of a LDV device

A laser beam (most often produced by a He

the two beams are crossed over by a transmitter lens.

probe volume or measurement volume. As these two beams come from the same coherent light source an

interference fringes pattern appears at the probe volume location.

and dark fringes. The fringe spacing d

between the split beams:

2sin( / 2)fdλθ

= (3)

(a)

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waves of equal amplitude and nearly equal frequency are added, the resulting signal will have his

periodically amplitude rising and falling with a characteristic frequency called “beat frequency” and which is

of the difference between the two original frequencies. In our case, the beat frequency would be

As this beat frequency is much slower than f, it is also much easier to measure.

Fig. 2 A simple crossed beams geometry9

an example, figure 2 presents two incident beams crossing each other with a

angle, a particle passing perpendicularly through the intersection area and a photodetector located on the

ng equation (1) to express the two Doppler shifts fD1 and f

2 2cos sin sin

2 4 4 2

V Vθ θ θλ λ

= =

From that point, the particle velocity V is easily determined.

to explain the same phenomenon is to use the fringe model 10

which is

how light signals are really processed in devices. Nevertheless,

(a) Scheme of a LDV device11

. (b) Detail of the probe volume12

A laser beam (most often produced by a He-Ne laser source) is split into two beams of equal intensity. Then,

the two beams are crossed over by a transmitter lens. The intersection between the two beams is called the


interference fringes pattern appears at the probe volume location. This pattern consists of

and dark fringes. The fringe spacing df only depends on the wavelength of the laser source and the angle

(2)

(b)

waves of equal amplitude and nearly equal frequency are added, the resulting signal will have his

a characteristic frequency called “beat frequency” and which is

original frequencies. In our case, the beat frequency would be

than f, it is also much easier to measure.

incident beams crossing each other with a θ

todetector located on the

and fD2, the beat frequency

which is probably easier to

. Nevertheless, the link with the

12

Ne laser source) is split into two beams of equal intensity. Then,

ntersection between the two beams is called the


consists of alternating bright

only depends on the wavelength of the laser source and the angle

Fig. 4 (a) Image of a seeded particle about to cross

As a seeded particle crosses the fringe pattern, the intensity of the scattered light will fluctuate at the same

time of the fringes light intensity fluctuations. The scattered light intensity signal is converted by the

photodetector into a voltage signal

is transferred to a band-pass frequency filter in order to remove the Gaussian component and keep the

sinusoid. Actually the frequency of the final signal

corresponds to the beat frequency as seen for the previously described

equation (2). And as a consequence:

2

sin( / 2)Df

V Vf

dθ

λ= =

The particle velocity component which is perpendic

f DV d f=

This configuration doesn’t allow us to get

whether the particle crosses the probe volume from bottom to top or from top to bottom, the same scattered

light signal would be detected.

In more advanced devices, the directional information

incident beam. This will cause the fringe pattern to move permanently and allow disambiguation about the

particle motion direction.

More sophisticated devices are able to measure two and even all the three components of a particle velocity

vector. To achieve this performance

first one have to be included as shown below.

Fig. 3 Multi

Compared to other laser velocity measurement techniques, LDV is a way to obtain a fluid ve

local area as the probe volume is small.

can be reached provided that the injected seed particles concentration is adapted to

great amount of particles crosses the probe volume during short time duration, the signal processing is more

(4)

(5)

(a)

6

seeded particle about to cross the probe volume13

. (b) Voltage signal from the

photodetector. (c) Filtered signal14



to a voltage signal which is pseudo-sinusoid weighted by a Gaussian function.

pass frequency filter in order to remove the Gaussian component and keep the

sinusoid. Actually the frequency of the final signal called the Doppler frequency in this

corresponds to the beat frequency as seen for the previously described heterodyne

And as a consequence:

The particle velocity component which is perpendicular to the fringe pattern is deduced from

uration doesn’t allow us to get information about the motion direction of the particle becau

the probe volume from bottom to top or from top to bottom, the same scattered

ctional information can be obtained by slightly shifting the frequency of one

he fringe pattern to move permanently and allow disambiguation about the

able to measure two and even all the three components of a particle velocity

To achieve this performance, additional pairs of split laser beams with different wavelength from the

to be included as shown below.

Multi-components velocity measurement device14

Compared to other laser velocity measurement techniques, LDV is a way to obtain a fluid ve

local area as the probe volume is small. Devices handling is simple and a high spatial and temporal resolution

can be reached provided that the injected seed particles concentration is adapted to

f particles crosses the probe volume during short time duration, the signal processing is more

(4)

(5)

(b) (c) V

. (b) Voltage signal from the



weighted by a Gaussian function. Then, the signal

pass frequency filter in order to remove the Gaussian component and keep the

cy in this fringe model

heterodyne model and defined by

deduced from:

information about the motion direction of the particle because

the probe volume from bottom to top or from top to bottom, the same scattered

obtained by slightly shifting the frequency of one

he fringe pattern to move permanently and allow disambiguation about the

able to measure two and even all the three components of a particle velocity

pairs of split laser beams with different wavelength from the

Compared to other laser velocity measurement techniques, LDV is a way to obtain a fluid velocity at very small

igh spatial and temporal resolution

can be reached provided that the injected seed particles concentration is adapted to the fluid velocity (if a

f particles crosses the probe volume during short time duration, the signal processing is more

Time

difficult). And finally if larger areas of fluid flow have to be

global we describe thereafter have to be used.

3. Planar Doppler Velocimetry

Planar Doppler velocimetry (PDV) also called Doppler Global Velocimetry (

the Doppler Effect but in contrast to LDV, it allows instantaneous velocity measurement on much larger fluid

areas (up to 3m x 3m surfaces).15

Fig.

A laser source (most commonly a pulsed

a special light modulator illuminate

comparing the frequency of the light scattered by particles and the frequency of un

is proportional to the particle velocity component along the

0 01 0 .( ) | |D

f ff f f V R E V

c c= − = − =

r r r

Fig. 6 (a) Multi-components PDV device scheme.

As mentioned previously in the LDV description part,

as it is a negligible fraction of the total frequency of the scattered light. PDV uses another ingenious principle

to solve the problem. The key element of a PDV device is the absorption cell cont

absorption spectrum is depicted in figure 6

ray of this gas. A mix of scattered and un

which divides the incident beam into two beams of equal intensity. The first beam is transmitted to the camera

1 after passing through the absorption cell whereas the second one directly reaches the camera 2

supplies the reference image. The Doppler shif

the reference image and the image from the absorbed beam.

(a)

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difficult). And finally if larger areas of fluid flow have to be analyzed, other techniques such as the two more

global we describe thereafter have to be used.

Planar Doppler Velocimetry

Doppler velocimetry (PDV) also called Doppler Global Velocimetry (DGV) is another technique


Fig. 5 Basic principles of the PDV technique16

source (most commonly a pulsed Nd-YAG laser) which beam is transformed into a planar light sheet by

illuminate the liquid flow. The Doppler shift is measured by two

comparing the frequency of the light scattered by particles and the frequency of un-scattered light.

is proportional to the particle velocity component along the Rr

– Er

vector: 16

0 0.( ) | |R E

f ff f f V R E V

c c −= − = − = r r

components PDV device scheme. 17

(b) Transmission spectrum of the iodine absorption cell

As mentioned previously in the LDV description part, measuring the Doppler shift accurately is a real challenge


to solve the problem. The key element of a PDV device is the absorption cell containing iodine gas which light

sorption spectrum is depicted in figure 6. The laser light frequency has to be close from one of an absorption

A mix of scattered and un-scattered light is focused by an optical system on a beamsplitter

h divides the incident beam into two beams of equal intensity. The first beam is transmitted to the camera

1 after passing through the absorption cell whereas the second one directly reaches the camera 2

The Doppler shift is deduced from evaluation of the light intensity shift between

the reference image and the image from the absorbed beam.

(6)

(b)

analyzed, other techniques such as the two more

DGV) is another technique exploiting


YAG laser) which beam is transformed into a planar light sheet by

The Doppler shift is measured by two CCD cameras by

scattered light. This value

(b) Transmission spectrum of the iodine absorption cell17

measuring the Doppler shift accurately is a real challenge


aining iodine gas which light

close from one of an absorption

scattered light is focused by an optical system on a beamsplitter

h divides the incident beam into two beams of equal intensity. The first beam is transmitted to the camera

1 after passing through the absorption cell whereas the second one directly reaches the camera 2 which

t is deduced from evaluation of the light intensity shift between

In order to measure the spatial components of the veloc

on the device (each pairs determining velocity along one spatial component) or as seen on figure __ three

coplanar light sheets have to be generated through the fluid flow.

In short, PDV is attractive for measurement of very large scale flows (including gas flows), can reac

dimensional velocity resolution measurement about 5 m/s but devices are usually complex and expensive

(some devices need 6 CCD cameras)

4. Particle Image Velocimetry

A third and more recent laser velocimetry technique known as Particle Image Velocime

with the PDV as the two techniques are

LDV and PDV, the PIV technique doesn’t rely on coherence properties of laser sources.

Fig. 7 (a) Scheme of a PIV device.

In the same way as PDV, PIV requires intervention of

into a light sheet illuminating the fluid flow

an image of the illuminated seeded particles is taken by a CCD camera and a second one is taken as soon as the

second laser pulse occurs. Afterward

given area, locations of particles which appear on it for the first image are compared to particles locations of

the same area of the second image.

particles between the two images, two velocity components (belonging to the light sheet plan) of the particles

can be calculated.

Furthermore, estimation of the third velocity component is possibl

stereoscopic process (well known within

simple 2D images of same objects seen from

PIV is usually a simpler and cheaper mean to measure fluids velocities in large areas than PDV (though not as

large as what PDV is able to probe instantaneously) but in counterpart needs powerful computers to perform

fast images processing.

Conclusion

Nowadays, three competitive laser velocimetry techniques are mainly used for fluid velocity measurements in

both industry and laboratories. Each of them is characterized by specific advantages but also limitations

involved equipments and costs for imple

researchers to analyze situations and then to define which one is more suitable for their applications.

for some applications different techniques are of course usable, there will inevi

results more quickly and efficiently.

(a)

8

In order to measure the spatial components of the velocity, either three pairs of cameras have to be installed

pairs determining velocity along one spatial component) or as seen on figure __ three

sheets have to be generated through the fluid flow.

PDV is attractive for measurement of very large scale flows (including gas flows), can reac


).

Particle Image Velocimetry

A third and more recent laser velocimetry technique known as Particle Image Velocimetry (PIV) is competiting

with the PDV as the two techniques are suitable for velocity measurements in large areas. But conversely to

LDV and PDV, the PIV technique doesn’t rely on coherence properties of laser sources.

(a) Scheme of a PIV device.18

(b) Measurement of the third component of particles velocity by a

stereoscopic method8

PIV requires intervention of a laser beam (supplied by a Nd:YAG laser)

into a light sheet illuminating the fluid flow but this time the laser in only pulsed twice. During the first pulse,


pulse occurs. Afterward, a computer divides the two images into extremely small areas. For a

of particles which appear on it for the first image are compared to particles locations of

. Knowing the time between the two laser pulses and the moving distance of


Furthermore, estimation of the third velocity component is possible by using two cameras to implement

within the photography field as being able to give a 3D perception from two

D images of same objects seen from different angles).



owadays, three competitive laser velocimetry techniques are mainly used for fluid velocity measurements in

both industry and laboratories. Each of them is characterized by specific advantages but also limitations

involved equipments and costs for implementation are different. To conclude, it is up to engineers and

situations and then to define which one is more suitable for their applications.

for some applications different techniques are of course usable, there will inevitably be one allowing achieving

results more quickly and efficiently.

(b)

ity, either three pairs of cameras have to be installed

pairs determining velocity along one spatial component) or as seen on figure __ three

PDV is attractive for measurement of very large scale flows (including gas flows), can reaches a 3-


try (PIV) is competiting

for velocity measurements in large areas. But conversely to

Measurement of the third component of particles velocity by a

(supplied by a Nd:YAG laser) is transformed

but this time the laser in only pulsed twice. During the first pulse,


, a computer divides the two images into extremely small areas. For a

of particles which appear on it for the first image are compared to particles locations of

me between the two laser pulses and the moving distance of


two cameras to implement

the photography field as being able to give a 3D perception from two



owadays, three competitive laser velocimetry techniques are mainly used for fluid velocity measurements in

both industry and laboratories. Each of them is characterized by specific advantages but also limitations and

conclude, it is up to engineers and

situations and then to define which one is more suitable for their applications. Though

tably be one allowing achieving

9

References

[1] Henri Pitot, « Description d'une machine pour mesurer la vitesse des eaux courantes et le sillage des

vaisseaux », dans Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de

physique tirés des registres de cette Académie, 1732, p. 363-376

[2] Yeh Y, Cummins H.Z., “ Localized fluid flow measurements with an He-Ne Laser Spectrometer” Appied

Physics Letters, 15 May 1964

[3] Roehle I et al, “Recent developments and aplications of quantitative laser light sheet measuring techniques

in turbomachinery components”, German Aerospace Centre (DLR), 1999.

[4] García Vizcaino, D. “Sistema laser de medida de velocidad por efecto doppler de bajo coste para

aplicaciones industriales e hidrodinámicas”. Universitat Politècnica de Catalunya. Ph. D Dissertation 2005.

[5] Analysis of Hydrogen Enriched Flames by Laser Diagnostics”.

Authors: Andrea Olivani, Fabio Cozzi, Aldo Coghe

13th Intl Symp on Application of Laser Techniques to Fluid Mechanics

Lisbon, Portugal, 26-29 June, 2006

[6] Morf Susanne et al, “Microcirculation abnormalities in patients with fibromyalgia measured by capillary

microscopy and laser fluxmetry”, Arthritis Research Therapy, 2005, Volume 7(2), p. 209-216.

[7] http://www.onera.fr/photos-en/mesexp/doppler-global-velocimetry.php

[8] Particle Image Velocimetry measurement principles http://www.dantecdynamics.com/Default.aspx?ID=820

[9] Laser Doppler Anemometry [LDA] http://web.mit.edu/fluids-modules/www/exper_techniques/LDA.text.pdf

[10] Experiment 4 - LASER DOPPLER ANEMOMETRY W. J. Devenport Last Modified December 21st, 2006

http://www.dept.aoe.vt.edu/~devenpor/aoe3054/manual/expt4/index.html

[11] Laser Doppler Velocimetry/Phase Doppler Interferometry

http://www.lavision.de/en/techniques/ldv_pdi.php

[12] Laser Doppler Velocimetry http://www.erc.wisc.edu/ldv.php

[13] Vélocimétrie laser à franges http://www.onera.fr/conferences/mesures-aerodynamique/14-velocimetrie-

laser-franges.php

[14] How an LDV/LDA works http://measurementsci.com/about_LDV-LDA.html

[15] Doppler Global Velocimetry (DGV) / Planar Doppler Velocimetry (PDV) http://www.holomap.com/dgv.htm

[16] DGV, Doppler Global Velocimetry or the photography of velocity by laser http://www.onera.fr/photos-

en/mesexp/doppler-global-velocimetry.php

[17] Reduction of the measurement uncertainty in Doppler Global Velocimetry

http://www.ptb.de/en/org/1/nachrichten1/2010/fundamentals/dgv.htm

[18] Particle Image Velocimetry (PIV) http://www.dlr.de/as/en/desktopdefault.aspx/tabid-183/251_read-

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Laser Velocimetry