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KRITI KAPOOR

INDIAN INSTITUTE OF TECHNOLOGY BOMBAY

TUTOR: PROF. FRANZ DURST

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Outline

Fluid flow measurements: pitot tubes, HWA, LDA

Introduction to LDA: Doppler and fringe model

Optical system set up

Electronic systems

Applications of LDA

Limitations of LDA

2LASER DOPPLER ANEMOMETRY

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Fluid Flow Measurements

Using Pitot Tubes

Uses the concept of

Bernoullis equation

Other techniques using

pressure difference:

Venturi tube, Rotameter,

Flow nozzles, Orifice plate

etc.

3LASER DOPPLER ANEMOMETRY

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Fluid Flow MeasurementsHot Wire Anemometry

Measures the convective heat transfer

occurring due to flowing fluid

Sensor having higher temperatureLimitations

Indirect measurement

Dirt is deposited resulting in insulation

Disturbs the flow

Fluid may decompose due to high temperature

LASER DOPPLER ANEMOMETRY 4

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Optical Techniques

Do not disturb the flow

Almost direct measurement

Need particles for measurements

For transparent fluids

Particles small enough to follow the flow

Measure displacement of a small particle in the fluid in

unit time


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Optical Techniques:

Examples

Particle Image Velocimetry PLIF


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Introduction to LDADOPPLER EFFECT

Change in frequency of a wave as perceived

by an observer moving relative to the source

of waves. Moving source

Moving observer = velocity of source,

= velocity of waves, and vo = velocity of observer.


A source of waves moving

to the left

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Introduction to LDADoppler Model Fringe Model


= A B =(1/) Ui (ki

mi)

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Dual Beam Anemometer

Explanation using beat frequency

Detected frequency being independent of the observingdirection is advantageous because a larger aperture can be

used.

LASER DOPPLER

ANEMOMETRY 10

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Drawback of Fringe Model

Intensity as square of electromagnetic energy integrated

over time

Only squared value detector can see intensity

Eyes and photodetector can see intensity

Particle cannot see the intensities and hence the fringes

LASER DOPPLER

ANEMOMETRY 11

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Direction of flow

Bragg cells

Diffraction grating


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Direction of flow

Laser-Doppler system based on rotating diffraction

grating


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Multidimensional

Measurement


Two Beam Anemometer

Using one pair of beam per measured flow direction

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Effect of size of particles Different modulation depths are obtained

with different sized particles

For very small particle, signal path A is

obtained.

For increased particle size, B is

obtained.

For particle size comparable to thedistance of the interference fringes,

modulation disappears as shown in C.


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Optical System Setup :Mathematical AnalysisTo ensure parallel

interference fringesand high modulationdepth

For the lens in front ofphoto detector

Quantity of effectivemeasuring volume can

be computed as

Effective diameter ofmeasuring volume

Number of fringeswithin the cross sectionarea


sin2

MNd

phph =

2sin

Nph=m

d

cos

1)(

5

1

1

D

fdin =

tan10

1

1

D

f

Nph =

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Electronic Systems for LDA

Measurement Signal comprising two

parts

Low Frequencydue to

Gauss Intensity

Distribution of Laser

High Frequency-Part of

Laser Doppler signal High Frequency portion is

of interest.

Output Signal


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MeasurementSpectrum Analyzer

VCO signal triggered by asaw tooth wave.

Mixer to mix VCO signaland Input signal.

IF filter to pass componentsaround stable frequency

Switching circuit for

squaring and smoothing ofoutput signal

Saw tooth wave is used fortriggering X axis.

Spectrum Analyzer


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MeasurementOffset Hetrodyne Tracker

Input Signal mixed with mixer.

Band pass filter to pass

components around center

frequency. Frequency discriminator to

generate signal proportional to

modification around center

frequency.

Integrator controls transient

behavior.

Offset Hetrodyne Tracker


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Measurement High Pass input signal in

form of bursts. Whenever input is greater

than threshold we get output.

Tracker changes the

frequency of precedingsignal measured at exit.

Thus output is in form ofsteps.

Integrate over severalfrequency changes to getlower noise signal.

Input and Output signal


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MeasurementPeriod Measurement System

Counter system has two level

detectors and one zer0 detector.

Pulse 1 Upper level and zeroposition

Pulse 2 Zero position only

Pulse 3 Lower level and zeroposition


Laser Doppler Counter

System

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Measurement Take only those zero

passages in which either 1

or 3 is set.

This avoids multiple zero

passages.

Use this to get number of

time pulse passes zero. Divide that by time taken

to get Doppler frequency.


Laser Doppler Counter System

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Applications of LDA

Multiphase flows

Turbulent flows

Through rectangular duct

Separated air flow

Supersonic flows

Flow near the wall


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Applications of LDATurbulent flow through rectangular duct


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Applications of LDATurbulent flow through rectangular duct


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Applications of LDATurbulent flow in form of separated air flows


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Applications of LDASupersonic flows


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Limitations of LDA

Particles do not always follow the fluid flow accurately.

Heating of fluid

Always needs particles

Applicable for transparent fluids only


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Conclusions Conventional techniques like HWA have limitations

LDA can be explained using

Doppler model

Fringe model Direction of flow and other components of velocity of

fluid can be measured using some modifications in setup.

Offset heterodyne tracker is better than spectrum analyzer.


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References Principles and Practice of Laser-Doppler Anemometry

by F. Durst, A. Melling and J. H. Whitelaw

http://web.mit.edu/fluids-

modules/www/exper_techniques/LDA.text.pdf

Chapter-21, Fluid flow measurements ,

http://web.mit.edu/fluids-modules/www/exper_techniques/LDA.text.pdfhttp://web.mit.edu/fluids-modules/www/exper_techniques/LDA.text.pdfhttp://web.mit.edu/fluids-modules/www/exper_techniques/LDA.text.pdfhttp://web.mit.edu/fluids-modules/www/exper_techniques/LDA.text.pdf

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