LASER DOPPLER

ANEMOMETRY

PRASHANT SATTURWAR

17

th

Dec. 2004

INTRODUCTION



Invented by Yeh and Cummins in 1964



Laser Doppler Anemometry (LDA) is a single
point optical measuring technique which
enables the velocity of the seeded particle
~0.5 - 5 microns (in air) or 1~20 microns (in
water) in a non-intrusive manner.



It determines the local velocity of the fluid
by analysing the Doppler i.e. equivalent
frequency of the laser light scattered by the
seeded particles with the flow.

Characteristics of LDA



No calibration required



Very high accuracy



Velocity range 0 to supersonic



Up to 3 velocity components



Measurement distance from centimeters to
meters



Non-intrusive measurements (optical technique)



Instantaneous and time averaged



Tracer particles are required



Can be used in flows of unknown direction



High spatial and temporal resolution

Principle

The basic configuration of an LDA consists of :



Continuous wave laser,



Transmitting optics, including a beam splitter
and a focusing lens,



Receiving optics, comprising a focusing lens,
an interference filtre and a photodetector,



A signal conditioner and a signal processor.

LDA PRINCIPLE

Velocity = distance/time

Flow with particles

d (known)

t (measured)

Signal

Time

Laser

Bragg

Cell

backscattered light

measuring volume

Detector

Processor

Types of LDA



Single-component dual-beam LDA system
in forward scatter mode



Single-component dual-beam LDA system
in back scatter mode



Single-component dual-beam LDA system
in side scatter mode



Two or three one-component systems



Reference beam systems (single laser
beam to illuminate particles in the flow)



Phase Doppler anemometer (PDAs) is an
extension of the laser Doppler
anemometer that usually uses two
receiving lenses and photodetectors

System configurations

Forward scatter
and side scatter
(off-axis)



Difficult to

align,



Vibration

sensitive

Backscatter



Easy to align



User friendly

Receiving optics

with

detector

Transmitting

optics

Flow

Rec

eiv

ing

op

tics

wit

h D

ete

cto

r

Flow

Laser

Bragg

cell

Detector

Transmitting

and receiving

optics

Two- and three-component

measurements



To measure two velocity components, two extra
beams can be added to the optics in a plane
perpendicular to the first beams.



All three velocity components can be measured
by two separate probes measuring two and one
components. Different wavelengths are
required to separate the measured components.
Three photo-detectors with appropriate
interference filtres are used to detect scattered
light of the three wavelengths.

LDA optics for measuring three

velocity components

 

                                                                              

Schematic of the three-dimensional

LDA system

 

 

                                                          

                  

Sch

Applications of LDA



Laminar and turbulent flows



Investigations on aerodynamics



Supersonic flows



High speed particle size measurements



Surface velocity and vibration measurement



Hot environments (flames, plasma etc.)



Velocity of particles



Biomedical application examples



Blood cell velocity



Zeta potential / Surface charge



Investigation of Vasomotion by laser Doppler

anemometry



Accurate measurement of prosthetic heart valves

(PHVs) flow field

Measurement of zeta

potential



Laser beams are aligned at the

stationary layer in the cell



At the crossing point of the beams,

Young's interference fringes of
known spacing are formed



Particles moving through the fringes

under the influence of the applied
electric field scatter light whose
intensity fluctuates with a frequency
that is related to the particles velocity



Photons detected are input to a digital

correlator



A frequency spectrum is produced from

which the mobility and hence zeta potential

are calculated

Disadvantages

Among the disadvantages of LDAs are

Expense (typically $40,000 for a simple
system)

The need for a transparent flow through
which the light beams can pass (Concentration
of seeded particle should be less)

They do not give continuous velocity signals.

Application examples



Measurements on wing / engine section of airbus



Research into wing geometries



measurements in transonic turbo machinery



Windshield velocity distribution measurements



Flowfield analysis in reation zones of wood stoves



Spherical couette flow experiment



Measurements around a ship model  



Measurement of mixing processes



Precipitation quality of stirred tank
precipitators



5-beam filberflow probe 3D measurements



Acoustic particle velocity measurements using LDA

 



Phase-triggered 3-D Laser Doppler Anemometry