LASER DOPPLER
ANEMOMETRY
PRASHANT SATTURWAR
17
th
Dec. 2004
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