# Sec 1 LDV Intro - TSI

Mechanics

Feb 22, 2014 (7 years and 5 months ago)

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Laser Doppler Velocimetry:

Introduction

TSI LDV/PDPA
Spring

Workshop & Training

Presented by Joseph Shakal Ph.D.

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Laser Doppler Velocimetry

Light Scattering Principles

Fringe Formation

Characteristics of Scattered Light

Doppler Signals

Properties of the Measurement Volume (Beam Waist)

System Optics

Conclusion

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Laser Doppler Velocimetry

i(t)

Signal is a
Time

Varying

Current

Photodetector (PMT)

Flow

Illuminating

Beams

Scattered Light

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LDV Hardware Components

Signal

Processor

FSA

Particles moving

with the fluid

Photo
-
detector

No Probe in the Flow

Small Measuring Volume

No Velocity Calibration

Large Dynamic Range

Desired Velocity Components

High Frequency Response

measured Directly

Transmitting
Optics

Receiving
Optics

Laser

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Fringe Description

d
f

d

u

d

f

x

f

D

f

2

sin

= Wavelength of incident light

= Frequency detected at PMT

f

D

Transmitting
Optics

Actual
Fringes

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Fringe Description

u
x

Focal Length =

f

u

d

f

x

2 sin
K

d

f

d

f

f

D

f

Focal Distance

Particle
crosses
a fringe

Pedestal

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Collection Optics Location

Backscatter

  

Forward scatter

  

Transceiver

Off
-
axis

Backscatter

Off
-
axis

Forward Scatter

Not Here

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Scattered Light Intensity Variation

0.5um PSL in Water
0.1
1
10
100
0
30
60
90
120
150
180
Angle from Forward (deg)
Scattered Intensity (AU)
0
20
40
60
80
100
120
0
30
60
90
120
Angle from Forward (deg)
Scattered Intensity (AU)
Log Scale

Linear Scale

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Typical Frequency vs. Velocity Curves

  
nm

 
14

 

14

0.001

0.1

10

1000

100000

1.0E+7

1.0 E
-
06

1.0 E
-
04

0.01

1.0

100

10,000

Velocity (m/sec)

Frequency, MHz

= 514.5 nm

=
14
0

=
0.14
0

Typ. Frequencies

Typ. Velocities

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Spectrum of Doppler Signal and
Filtering

Signal

-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-14.95
-1.5999999999999
11.75
After high pass
filter (HPF)

-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
After low pass

filter (LPF)

Frequency

Power

Pedestal

Doppler

HPF

LPF

Noise

-0.5
0
0.5
1
1.5
2
0.0037868912928165
0.93033565757253
0.063278488872161
Sum

Frequency

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Measurement Volume

0
0.2
0.4
0.6
0.8
1
1.2
-15
-2.0999999999999
10.8
1

1/e
2

Intensity

d
m

is the diameter of the measurement
volume, or in other words, the 1/e
2

waist diameter

d
m

d
m

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Measurement Volume
Dimensions

Beam Diameter

D
e
-
2

l
m

= d
e2

/ sin

S

z

x

y

z

x

1/e
2

Contour

d

m

Fringes

l

m

y

D

e

-
2

V

6 cos
2

sin


3

/

(

d
e2

Beams are in plane of page

d
e2

= 4
f

/

D
e
-
2

Focused Beam Dia.

d
e2

d
e2

= diameter here

d
m

= d
e2

/ cos

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Measurement Volume Parameters

f =
120mm

Example:

Measurement Volume Diameter

d
m

3m
m,

“small”

d
m

~ f /
4

since

~ 0.5
m
m

d
e2

=

f

4

e

D

2

d
m
~

d
e2
/
1

d
m

~ f

/
2

since D
e2

~ 2.5mm

Units:

D
m

will be in
m
m,

if

in

m
m, f
in

mm, D
e2

in

mm

since

is small

(from previous slide)

d
m

=
d
e2

/ cos

and

Diameter of Measuring Volume:

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Measurement Volume Parameters

Example :

TR
-
260 probe,

f
=
250 mm,
S

= 50 mm

Length of Measuring Volume

l
m

=
d
m

/
sin

Fringe Spacing

d
f

f

S

f

S

2 sin
K

~

~

0.5

l
m

=
10

d
m
= 620
m
m

and

d
f
=
2.5

m
m

e

D

-
2

f

S

tan

~ sin

~ (
S
/2) /
f

(from previous slide)

so
l
m

= 2
f

d
m

/
S
=

f d
m

/ 25

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Measuring Volume Parameters

Example

N
FR

d
m

d
f

4

S

e

D

2

Note:
N
FR

is
independent
of focal length (
f)
and beam expansion

N
FR

~ S / 2
if

S
is in mm, since D
e2

~ 2.5mm

for

S
= 50 mm
,
N
FR

=
25 (for = 2.6 mm)

e

D

2

4

f

/

D
e2

f

/ S

Number of Fringes

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System Parameters

Many of these parameters
are found in the FlowSizer
Run Setup

-
>
Optics

tab.

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Total System Parameters

All these
parameters and
many more are
found in the
PDPA LDV
performance

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Considerations in LDV

Optimize Optics and Seeding for:

Physical Limits of Experiment

Flow Media

Laser Power Required for Good

Signal
s

(SNR)

Required Data Density

Select Signal Processor Based on:

Frequency Range Required

(Maximum Flow Velocity)

Bandwidth (Dynamic Range)

Required Flow Information

Next we look at some applications

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Turbulence Characteristics of a Swirling Jet

Full turbulence statistics measured with a 3D LDV system.

See AIAA paper number 2008
-
761 for details.

Courtesy of Courtesy of Prof. J. Naughton and R. Semaan, Dept. of Mechanical Engineering, Univ. Wyoming.

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Turbulence Characteristics of a Swirling Jet

Axial normalized turbulent stress distribution (uu/U
2
o
) for a swirl number of 0.39,

Reynolds number of 100,000, and solid body type swirl.

See AIAA paper number 2008
-
761 for details.

Courtesy of Courtesy of Prof. J. Naughton and R. Semaan, Dept. of Mechanical Engineering, Univ. Wyoming.

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High Speed Flow

Gatetime 1 Histogram
0
0.467
0.933
1.400
0
1000
2000
3000
4000
Gate Time Ch. 1 (usec)
Gate Time Count Ch. 1
V
mean

= 595m/s

Freq
mean

= 118.8MHz

Valid Vel = 100%

Valid Dia = 91.7%

Gate Time
mean

= 110ns

Data Rate: Ch 1 = 55.8kHz, Ch 2 = 26kHz

Courtesy of Dr. Steven Lin, TaiTech Inc.

Velocity 1 Histogram
400.000
533.333
666.667
0
20
40
60
80
Velocity Ch. 1 (m/sec)
Velocity Count Ch. 1
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Analysis of a Fluttering Flow

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Aircraft Turbine Combustor

Fuel Rate = 0.75g/s

Eq. Ratio = 0.4

Tair = 380K

Twall = 540K

Courtesy of Jonathan Colby, Georgia Institute of Technology

Lean Low NOx Combustor
(GE CFM 56 Engine)

Cold Flow

Combustion

Courtesy of Jonathan Colby, Georgia Institute of Technology

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Phase Discriminated LDV

Wave Machine

Sand is Transported off the
Crests (Dispersed Phase)

Use a single probe, Ar ion wavelengths

NO dyes, NO wavelength filtering, NO expensive spherical particles
required

Uses ordinary seeding particles and ordinary sand

Tracers in the Water
(Continuous Phase)

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Phase Discriminated LDV

We do not expect the typical I
scatter

~ d
2

to hold for irregular particles

However, regardless of particle shape, surface texture, etc. larger particles
are expected to scatter more light than smaller particles

“Borrow” burst intensity measurement capability from PDPA*

Measured burst intensity histogram:

* US Patent 4986659

Sand

Tracers

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Phase Discriminated LDV

Compare intensity distribution for various measurement locations

Sand

Tracers

In the crest
region (both
sand and
tracers)

On the bed (sand only)

In the free stream
(tracers only)

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Phase Discriminated LDV

Tracers (Continuous Phase)

0.5Hz

1Hz

34cm/s

Sediment (Dispersed Phase)

0.5Hz

30cm/s

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Probes for Underwater LDV

Prism Attachments

Sealed Stainless Steel Probes

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Conclusions

Special properties of laser beams allow us to generate
fringe patterns

Particles are added to flow, their velocity is measured

Light is scattered in all directions, but not uniformly

Different lens focal lengths give different fringe spacings

Fringe crossing rate of particle generates Doppler
frequency

Velocity is determined directly from Doppler frequency

Multitude of applications