Sec 1 LDV Intro - TSI

rangebeaverΜηχανική

22 Φεβ 2014 (πριν από 3 χρόνια και 5 μήνες)

85 εμφανίσεις

TSI Incorporated

Copyright© 2005 TSI Incorporated

Laser Doppler Velocimetry:

Introduction


TSI LDV/PDPA
Spring

Workshop & Training


Presented by Joseph Shakal Ph.D.

TSI Incorporated

Copyright© 2005 TSI Incorporated

Laser Doppler Velocimetry


Light Scattering Principles


Fringe Formation


Characteristics of Scattered Light


Doppler Signals


Properties of the Measurement Volume (Beam Waist)


System Optics


Conclusion

TSI Incorporated

Copyright© 2005 TSI Incorporated

Laser Doppler Velocimetry

i(t)

Signal is a
Time

Varying

Current

Photodetector (PMT)

Flow

Illuminating

Beams

Scattered Light

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

Collection Optics Location

Backscatter

  

Receiver



Forward scatter

  

Transceiver

Off
-
axis

Backscatter

Off
-
axis

Forward Scatter

Not Here

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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


TSI Incorporated

Copyright© 2005 TSI Incorporated

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:

TSI Incorporated

Copyright© 2005 TSI Incorporated

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


TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

System Parameters

Many of these parameters
are found in the FlowSizer
Run Setup

-
>
Optics

tab.

TSI Incorporated

Copyright© 2005 TSI Incorporated

Total System Parameters

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

Considerations in LDV


Optimize Optics and Seeding for:


Physical Limits of Experiment


Flow Media


Laser Power Required for Good

Signal
s

(SNR)


Adequate Spatial Resolution


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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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.


TSI Incorporated

Copyright© 2005 TSI Incorporated

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.


TSI Incorporated

Copyright© 2005 TSI Incorporated

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
TSI Incorporated

Copyright© 2005 TSI Incorporated

Analysis of a Fluttering Flow

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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)

TSI Incorporated

Copyright© 2005 TSI Incorporated

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

TSI Incorporated

Copyright© 2005 TSI Incorporated

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)

TSI Incorporated

Copyright© 2005 TSI Incorporated

Phase Discriminated LDV

Tracers (Continuous Phase)

0.5Hz

1Hz

34cm/s

Sediment (Dispersed Phase)

0.5Hz

30cm/s

TSI Incorporated

Copyright© 2005 TSI Incorporated

Probes for Underwater LDV

Prism Attachments

Sealed Stainless Steel Probes

TSI Incorporated

Copyright© 2005 TSI Incorporated

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