TSI Incorporated Copyright© 2005 TSI Incorporated Laser Doppler Velocimetry: Introduction TSI...

TSI Incorporated

Copyright© 2005 TSI Incorporated

Laser Doppler Laser Doppler Velocimetry:Velocimetry:IntroductionIntroduction

TSI LDV/PDPA Spring Workshop & Training

Presented by Joseph Shakal Ph.D.

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Laser Doppler VelocimetryLaser 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 Laser Doppler VelocimetryVelocimetry

i(t)Signal is a Time Varying Current

Photodetector (PMT)

Flow

IlluminatingBeams

Scattered Light

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LDV Hardware ComponentsLDV Hardware ComponentsSignal

Processor

FSA

Particles movingwith the fluid

Photo-detector

No Probe in the Flow Small Measuring Volume

No Velocity Calibration Large Dynamic Range

Desired Velocity Components High Frequency Responsemeasured Directly

Transmitting Optics

Receiving Optics

Laser

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Fringe DescriptionFringe Description

df

d

u d

f

x fD f

2sin = Wavelength of incident light

= Frequency detected at PMTfD

Transmitting Optics

Actual Fringes

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Fringe DescriptionFringe Descriptionux

Focal Length = f

u

df

x

2 sin K

d fdff

D

f

Focal Distance

Particle crosses a fringe

Pedestal

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Collection Optics LocationCollection Optics Location

Backscatter

Receiver

Forward scatter

Transceiver

Off-axis Backscatter

Off-axisForward Scatter

ReceiverReceiver

Rec

eive

r

Not Here

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Scattered Light Intensity Scattered Light Intensity VariationVariation

Log Scale

Linear Scale

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Typical Frequency vs. Typical Frequency vs. Velocity CurvesVelocity 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)

Fre

qu

ency

, M

Hz

= 514.5 nm

= 140

= 0.140

Typ. Frequencies

Typ

. Vel

ociti

es

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Spectrum of Doppler Spectrum of Doppler Signal and FilteringSignal and Filtering

Signal After high pass filter (HPF)

After low pass filter (LPF)

Frequency

Pow

er

Pedestal

DopplerHPF LPFNoise

SumFrequency

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Measurement VolumeMeasurement Volume

0

0.2

0.4

0.6

0.8

1

1.2

-15 -2.0999999999999 10.8

1

1/e2

Inte

nsit

y

dm is the diameter of the measurement volume, or in other words, the 1/e2 waist diameter

dm

dm

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Measurement Volume Measurement Volume DimensionsDimensions

Beam DiameterDe-2

lm = de2 / sin

S

z

x

y

z

x1/e2 Contour

dm

Fringes

lm

yDe-2

V 6 cos2 sin 3 / (de2

Beams are in plane of page

de2 = 4 f / De-2

Focused Beam Dia.de2

de2 = diameter heredm = de2 / cos

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Measurement Volume Measurement Volume ParametersParameters

f = 120mmExample:

Measurement Volume Diameter dmm, → “small”

dm ~ f / 4 since ~ 0.5m

de2 =f4

e

D 2

dm ~ de2 / 1

dm ~ f / 2 since De2~ 2.5mm

Units: Dm will be in m, if in m, f in mm, De2 in mm

since is small

(from previous slide)dm = de2 / cos and

Diameter of Measuring Volume:

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Measurement Volume Measurement Volume ParametersParameters

Example : TR-260 probe, f = 250 mm, S = 50 mm

Length of Measuring Volume

lm = dm / sin

Fringe Spacing

df

f S

f

S

2 sin K

~ ~ 0.5

lm = 10 dm = 620 m and df = 2.5 m

eD -2

f

S

tan ~ sin ~ (S/2) / f

(from previous slide)

so lm = 2 f dm / S = f dm / 25

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Measuring Volume Measuring Volume ParametersParameters

Example

NFR

dm

df

4 S

e

D 2

Note: NFR is independent of focal length ( f) and beam expansion

NFR ~ S / 2 if S is in mm, since De2 ~ 2.5mm

for S = 50 mm, NFR = 25 (for = 2.6 mm) e

D 2

4 f/ De2

f / S

Number of Fringes

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System ParametersSystem Parameters

Many of these parameters are found in the FlowSizer Run Setup -> Optics tab.

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Total System ParametersTotal System Parameters

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

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Considerations in LDVConsiderations in LDV• Optimize Optics and Seeding for:

– Physical Limits of Experiment– Flow Media– Laser Power Required for Good Signals (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

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Turbulence Characteristics of a Turbulence Characteristics of a Swirling JetSwirling 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 Turbulence Characteristics of a Swirling JetSwirling Jet

Axial normalized turbulent stress distribution (uu/U2o) 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 FlowHigh Speed FlowGatetime 1 Histogram

0 0.467 0.933 1.4000

1000

2000

3000

4000

Gate Time Ch. 1 (usec)G

ate

Tim

e C

ou

nt

Ch

. 1

Vmean = 595m/sFreqmean = 118.8MHzValid Vel = 100%Valid Dia = 91.7%

Gate Timemean = 110nsData Rate: Ch 1 = 55.8kHz, Ch 2 = 26kHz

Courtesy of Dr. Steven Lin, TaiTech Inc.

Velocity 1 Histogram

400.000 533.333 666.6670

20

40

60

80

Velocity Ch. 1 (m/sec)

Vel

oci

ty C

ou

nt

Ch

. 1

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Analysis of a Fluttering Analysis of a Fluttering FlowFlow

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Aircraft Turbine Aircraft Turbine CombustorCombustor

Fuel Rate = 0.75g/sEq. Ratio = 0.4Tair = 380KTwall = 540K

Courtesy of Jonathan Colby, Georgia Institute of Technology

Lean Low NOx Combustor (GE CFM 56 Engine)

Cold Flow

CombustionCourtesy of Jonathan Colby, Georgia Institute of Technology

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Phase Discriminated LDVPhase Discriminated LDV

Wave Machine

Sand is Transported off the Crests (Dispersed Phase)

Use a single probe, Ar ion wavelengthsNO 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 LDVPhase Discriminated LDVWe do not expect the typical Iscatter ~ d2 to hold for irregular particlesHowever, 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

SandTracers

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Compare intensity distribution for various measurement locations

SandTracersIn the crest region (both sand and tracers)

On the bed (sand only)In the free stream (tracers only)

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Tracers (Continuous Phase)

0.5Hz1Hz 34cm/s

Sediment (Dispersed Phase)

0.5Hz30cm/s

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Probes for Underwater LDVProbes for Underwater LDV

Prism AttachmentsSealed Stainless Steel Probes

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ConclusionsConclusions

• 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

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