Laser Doppler Velocimetry

Laser Doppler VelocimetryIntroduction to principles and applications

Some Info compliments of Dantec Inc.

Dr. Arnold A. FontaineARL / Bioengineering

Office: Water Tunnel BuildingPh: 3-1765

email: aaf1@psu.edu

Characteristics of LDA

• Invented by Yeh and Cummins in 1964

• Velocity measurements in Fluid Dynamics (gas, liquid)

• Up to 3 velocity components

• Non-intrusive measurements (optical technique)

• Absolute measurement technique (one calibration required)

• Very high accuracy

• Very high spatial resolution due to small measurement volume

• Tracer particles are required

Applications of LDA

• Laminar and turbulent flows

• Investigations on aerodynamics

• Supersonic flows

• Turbines, automotive etc.

• Liquid flows

• Surface velocity and vibration measurement

• Hot environments (flames, plasma etc.)

• Velocity of particles

• ...etc., etc., etc.

LDA - Optical principle

• When a particle passes through the intersection volume formed by the two coherent laser beams, the scattered light, received by a detector, has components from both beams.

• The components interfere on the surface of the detector.

• Due to changes in the difference between the optical path lengths of the two components, this interference produces pulsating light intensity, as the particle moves through the measurement volume.

Photodetector

Inci

dent

bea

ms

Dire

ctio

n of

mot

ion

Inci

dent

bea

ms

Photodetector

Dire

ctio

n of

mot

ion

Frequency to velocity conversion

UxU

K2 / 2 K1

D D D U k k 1 2 1 2

( )

fU

Dx

22 sin /

U Cfx D

C 2 2sin /

LDA - Fringe model

• Focused laser beams intersect and form the measurement volume

• Plane wave fronts: beam waist in the plane of intersection

• Interference in the plane of intersection

• Pattern of bright and dark stripes/planes

Flow with particles

d (known)

Velocity = distance/time

t (measured)

Signal

Time

LaserBraggCell backscattered light

measuring volume

Detector

Processor

LDA - Fringe model

• The fringe model assumes as a way of visualisation that the two intersecting beams form a fringe pattern of high and low intensity.

• When the particle traverses this fringe pattern, the scattered light fluctuates in intensity with a frequency equal to the velocity of the particle divided by the fringe spacing.

Transmitting optics

Laser

Braggcell

BS

F

D E

D

DL

Lens

Basic modules:

• Beam splitter• Achromatic lens

Options:

• Frequency shift (Bragg cell)

– low velocities– flow direction

• Beam expanders– reduce

measurement volume

– increase power density

Measurement volume

• The transmitting system generates the measurement volume

• The measurement volume has a Gaussian intensity distribution in all 3 dimensions

• The measurement volume is an ellipsoid

• Dimensions/diameters x, y and z are given by the 1/e2 intensity points

F

DL

Y

Z

X

Transmittingsystem

Measurementvolume

Intensitydistribution

0 1/e 2

1

z

x

y X

Z

Y

Measurement volume

Length:

Width: Height:

No. of fringes:

z

x

X

Z

f

Fringe separation:

f

22

sin

Laser, characteristics and requirements

• Monochrome

• Coherent

• Linearly polarised

• Low divergence (collimator)

• Gaussian intensity distribution

Laser

L-Diode collimator

Laser

Principle of LDA, differential beam technique

Laser

Signalprocessing

Transmittingoptics

Receiving opticswith detector

Signalconditioner

Flow

HeNeAr-IonNd:YagDiode

Beamsplitter(Freq. Shift)Achrom. Lens

GasLiquidParticle

Achrom. LensSpatial FilterPhotomultiplierPhotodiode

Spectrum analyserCorrelatorCounter, Tracker

AmplifierFilter

PC

Signal characteristics

• Sources of noise in the LDA signal:

- Photo detection shot noise.- Secondary electronic noise, thermal noise from

preamplifier circuit- Higher order laser modes (optical noise).- Light scattered from outside the measurement volume, dirt,

scratched windows, ambient light, multiple particles, etc.- Unwanted reflections (windows, lenses, mirrors, etc).

• Goal: Select laser power, seeding, optical parameters, etc. to maximise the SNR.

• Particles moving in either the forward or reverse direction will produce identical signals and frequencies.

Directional ambiguity / Frequency shift

fmax

fshift

fmin

f

u

uminumax

umin umax

• With frequency shift in one beam relative to the other, the interference fringes appear to move at the shift frequency.

• With frequency shifting, negative velocities can be distinguished.

no shift shift

Frequency shift / Bragg cell

• Acousto-optical modulator

• Bragg cell requires a signal generator (typically: 40 MHz)

• Frequency of laser light is increased by the shift frequency

• Beam correction by means of additional prisms

Piezoelectrictransducer

fs40 MHz

Absorber

wave front

Laser

fL

fL + fS

System configurations

Forward scatterand side scatter(off-axis)

• Difficult to align,• Vibration sensitive

Backscatter

• Easy to align• User friendly

Receiving opticswith detector

Transmittingoptics

Flow Receiving optics

with Detector

FlowLaser

Braggcell

Detector Transmitting and receiving optics

Seeding: scattered light intensity

180 0

90

270

210

150

240

120

300

60

330

30

180 0

330210

240 300270

150

12090

60

30

180 0

210

150

240

120

270

9060

300

30

330

• Polar plot of scattered light intensity versus scattering angle

• The intensity is shown on a logarithmic scale

Seeding: ability to follow flow

fp

fp

pp

UU

dU

/18

2

dt

d

ResponseFrequencyParticle

Following Lumley (1976):1

1 2

1

1

2

1

741%

362 1

a F R

aFR

a d

u l

u

l

p

f

fo r error

partic le tim e const

characteristics flu id frequency

d partic le d iam eter

flu id kinem atic

partic le or flu id density

( )

.

.

.

.

R eyno lds n um ber based o n in tegra l leng th an d velocity .

v iscosity .

p or f

Typically: Particle should be small and near neutrally buoyant.d < 50 microns

DATA PROCESSING

What is a Signal Processor?

System Requirements: Accurate discrimination of burst.High dynamic range.Large frequency range.High data rate capacity.Can discriminate signal in low SNR.

Processor Types: Spectrum Analyzers.Photon Correlators.Trackers.Counters.Covariance processor.Digital Signal processor.

Digitally sample the burst with a high frequency, accurate A-D and then perform a variety of signal processing techniques to determine the Doppler frequency.

These are the state of the art in processing.

These processors combine:

• High pass filters to remove low frequency components such as the signal pedestal and low frequency noise.

• Low pass filters to limit high frequency noise components.

• High speed A/D converter.

• Burst detection algorithms to help identify burst from background signal. Digital signal analyzer to estimate the frequency.

DIGITAL SIGNAL PROCESSORS

LDV SIGNAL BIAS

What is Bias?

Examples?

Types of bias in LDV signals:1) Velocity bias.2) Fringe bias.3) Gradient bias.

Probe volume alignment for 3D velocity measurements

• To measure three velocity components requires careful alignment.

• The simplest method is by using a fine pinhole with an opening just large enough that the focused beam can pass through.

• Fine adjustment can be made using a power meter behind the pinhole maximising the power of light passing through the pinhole for each beam.

REFERENCES

1. “The laser Doppler technique,” L.E. Drain, J Wiley and Sons Publishers, 1980.

2. “Report of the Special Panel on Statistical Particle Bias in Laser Anemometry,” R.V. Edwards, J. Fluids Engineering, Vol 109, pp89-93, 1987.