AAE 520 Experimental Aerodynamics
Laser Doppler Anemometry
Introduction to principles and applications
adapted from
DANTEC literature
by John Sullivan,
Purdue AAE.
Edited by S.P.
Schneider, Purdue
AAE.
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Why Measure?
" Almost all industrial flows are turbulent.
" Almost all naturally occurring flows on earth, in oceans,
and atmosphere are turbulent.
Dui "Ä ij "p
Á =+ Áfi -
Dt "X "X
j j
Turbulent motion is 3-D, vortical, and diffusive
governing Navier-Stokes equations are very hard
(or impossible) to solve.
Measurements are easier (easy?)
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
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 (no calibration
required)
" Very high accuracy
" Very high spatial resolution due to small measurement
volume
" Tracer particles are required
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
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
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Velocity = distance/time
Flow with particles
Signal
Processor
d (known)
t (measured)
Detector
Time
measuring volume
Bragg
Laser
Cell
backscattered light
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
LDA - Fringe Model
" The fringe model
assumes as a way
of visualization
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
f=frequency
frequency equal to
the velocity of the
v=particle velocity
particle divided by
the fringe spacing.
¸=angle between laser beams
=wavelength of laser light
See Adrian paper in Goldstein
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Principle of LDA, differential beam
technique
Flow
Transmitting Receiving Optics
Laser
Optics with Detector
HeNe Beamsplitter Gas Achrom. Lens
Ar-Ion (Freq. Shift) Liquid Spatial Filter
Nd:Yag Achrom. Lens Particle Photomultiplier
Diode Photodiode
Signal
Signal
PC
conditioner
Processing
Spectrum analyzer Amplifier
Correlator Filter
Counter, Tracker
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Laser, Characteristics and
Requirements
" Monochrome
Laser
" Coherent
" Linearly polarized
" Low divergence
(collimator)
L-Diode collimator
Laser
" Gaussian intensity
distribution
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Transmitting Optics
Basic modules:
" Beam splitter
BS
" Achromatic lens
Laser
Lens
Options:
" Frequency shift (Bragg
Bragg
cell)
Cell
 low velocities
D × E
 flow direction
× •
" Beam expanders
D
Ń
 reduce measurement
volume
 increase power density
DL F
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement Volume
Transmitting
System
" The transmitting system
Z
¸
generates the
DL
measurement volume
F Y
" The measurement
volume has a Gaussian X
Intensity
1
intensity distribution in
Distribution
all 3 dimensions
1/e 2
0
´z
" The measurement
volume is an ellipsoid
´x
Z
" Dimensions/diameters ´x,
Measurement
´y and ´z are given by the
Volume
1/e2 intensity points
´y
X
Y
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement Volume
Width: Height:
Length:
4F
4 F 
4F
´x =
´ =
´y =
z
¸
¸
Ä„EDL
Ä„EDLcosëÅ‚ öÅ‚
ìÅ‚
Ä„ EDL sinëÅ‚ öÅ‚
ìÅ‚ ÷Å‚
íÅ‚2÷Å‚
Å‚Å‚
íÅ‚
2Å‚Å‚
´z
Fringe
Separation:

´ =
f
Ń
¸
Z
2sinëÅ‚ öÅ‚
ìÅ‚ ÷Å‚
íÅ‚ Å‚Å‚
2
No. of Fringes:
¸
8FtanëÅ‚ öÅ‚
ìÅ‚
´x
íÅ‚2÷Å‚
Å‚Å‚
´f
Nf =
X
Ä„ EDL
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Receiving Systems
" Receiving Optics
 Receiving optics
Lenses
 Multimode fibre
acting as spatial
filtre
Multimode
fibre
Photomultiplier
 Interference filtre
" Detector
 Photomultiplier
 Photodiode
Interference
filtre
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
System Configurations
Forward scatter
and side scatter
Receiving Optics
Transmitting
(off-axis)
with Detector
Optics
" Difficult to align,
" vibration Flow
sensitive
Detector
Transmitting and
Backscatter
Receiving Optics
" Easy to align
" User friendly
Bragg
Laser
Cell
Flow
Purdue University - School of Aeronautics and Astronautics
R
e
c
e
i
w
v
i
i
n
t
h
g
D
O
e
p
t
t
e
i
c
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o
r
AAE 520 Experimental Aerodynamics
Backscatter Configuration
Interference
filtres
Single mode polarisation
preserving fibres
Multimode
PM
fibre
PM
Bragg Colour
Laser
Cell splitter
Colour
Fibre manipulators
splitter
Flow
Multimode
fibre
Back scattered light Single mode
fibres
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Directional Ambiguity / Frequency
Shift
" Particles moving in either the forward or reverse direction will
produce identical signals and frequencies.
f
f
max
f
shift
f
min
u
u
u
shift
max
min
u u
min max no shift
" 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.
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Frequency Shift / Bragg Cell
" Acousto-optical Modulator
fs = 40 MHz
" Bragg cell requires a signal
Piezoelectric
generator (typically: 40 MHz)
Transducer
fL
" Frequency of laser light is
wave front
increased by the shift
fL+ fS
frequency
ÕÅ‚
" Beam correction by means of
additional prisms
Absorber
Purdue University - School of Aeronautics and Astronautics
r
e
s
a
L
AAE 520 Experimental Aerodynamics
3-D LDA Applications
" Measurements of boundary layer separation in wind
tunnels
" Turbulent mixing and flame investigations in combustors
" Studies of boundary layer-wake interactions and
instabilities in turbines
" Investigations of flow structure, heat transfer, and
instabilities in heat exchangers
" Studies of convection and forced cooling in nuclear
reactor models
" Measurements around ship models in towing tanks
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Seeding: ability to follow flow
ParticleFrequencyResponse
U - U
d ½
p f
U = -18
p
2
dt Áp / Á
dp
f
Diameter (µm)
Particle Fluid
f = 1 kHz f = 10 kHz
Silicone oil atmospheric air 2.6 0.8
TiO2 atmospheric air 1.3 0.4
MgO methane-air flame 2.6 0.8
(1800 K)
TiO2 oxygen plasma 3.2 0.8
(2800 K)
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Seeding: scattered light intensity
90
90 90
120 60 120 60
120 60
150 30
150 30
150 30
180 0 180 0 180 0
210 330 210 330
210 330
240 300 240 300
240 300
270 270
270
dp 10
dp 1.0
E" 
E" 
dp 0.2
E" 
" Polar plot of scattered light intensity versus scattering angle
" The intensity is shown on a logarithmic scale
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Signal Characteristics
" Sources of noise in the LDA signal:
 Photodetection 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 maximize the SNR.
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement of air flow around a
helicopter rotor model in a wind tunnel
Photo courtesy of University of Bristol, UK
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement of flow field around a
1:5 scale car model in a wind tunnel
Photo courtesy of Mercedes-Benz, Germany
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement of wake flow around a
ship model in a towing tank
Photo courtesy of Marin, the Netherlands
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement of air flow field around
a ship model in a wind tunnel
Photo courtesy of University of Bristol, UK
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Wake flow field behind hangar
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement of flow around a ship
propeller in a cavitation tank
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Measurement of flow in a valve model
Photo courtesy of Westsächsische Hochschule Zwickau, Germany
Purdue University - School of Aeronautics and Astronautics
AAE 520 Experimental Aerodynamics
Comparison of EFD and CFD results
Purdue University - School of Aeronautics and Astronautics