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Introduction to Laser Doppler Velocimetry
Ken Kiger
Burgers Program For Fluid DynamicsTurbulence School
College Park, Maryland, May 24-27
Laser Doppler Anemometry (LDA)• Single-point optical velocimetry method
Study of the flow between rotatingimpeller blades of a pump
3-D LDA Measurements ona 1:5 Mercedes-Benz
E-class model car in wind tunnel
Phase Doppler Anemometry (PDA)• Single point particle sizing/velocimetry method
Drop Size and Velocitymeasurements in an atomized
Stream of Moleten Metal
Droplet Size DistributionsMeasured in a Kerosene
Spray Produced by a Fuel Injector
Laser Doppler Anemometry
• LDA– A high resolution - single point technique for velocity measurements in
turbulent flows
– Basics• Seed flow with small tracer particles• Illuminate flow with one or more coherent, polarized laser beams to form a MV• Receive scattered light from particles passing through MV and interfere with additional light sources• Measurement of the resultant light intensity frequency is related to particle velocity
A Back Scatter LDA System for One Velocity Component Measurement (Dantec Dynamics)
LDA in a nutshell
• Benefits– Essentially non-intrusive– Hostile environments– Very accurate– No calibration– High data rates– Good spatial & temporal resolution
• Limitations– Expensive equipment– Flow must be seeded with particles if none naturally exist– Single point measurement technique– Can be difficult to collect data very near walls
• General wave propagation
Review of Wave Characteristics
tx
Atx 2cos,
x
A
x,t Re Aei kx t A = Amplitudek = wavenumberx = spatial coordinatet = time = angular frequency = phase
2
k
c
c
22
Electromagnetic waves: coherence• Light is emitted in “wavetrains”
– Short duration, t– Corresponding phase shift, (t); where may vary on scale
t>t
• Light is coherent when the phase remains constant for a sufficiently long time– Typical duration (tc) and equivalent propagation length (lc)
over which some sources remain coherent are:
– Interferometry is only practical with coherent light sources
E Eo exp i kx t t
Source nom (nm) lcWhite light 550 8 mMercury Arc 546 0.3 mmKr86 discharge lamp 606 0.3 mStabilized He-Ne laser 633 ≤ 400 m
Electromagnetic waves: irradiance• Instantaneous power density given by Poynting vector
– Units of Energy/(Area-Time)
• More useful: average over times longer than light freq.
Sc 2oEB
S coE2
I STco E 2
Tco2
EE co2
E02
tdtfT
fT
T
t
tT
2
2
1Frequency Range
6.10 x 1014
5.20 x 1014
3.80 x 1014
LDA: Doppler effect frequency shift• Overall Doppler shift due two separate changes
– The particle ‘sees’ a shift in incident light frequency due to particle motion
– Scattered light from particle to stationary detector is shifted due to particle motion
ˆb k
ek
u
ˆse
LDA: Doppler shift, effect I
• Frequency Observed by Particle– The first shift can itself be split into two effects
• (a) the number of wavefronts the particle passes in a time t, as though the waves were stationary…
ˆb k
ek
Δtbeu
Δtbeu
u
Number of wavefronts particle passes during t due to particle velocity:
LDA: Doppler shift, effect I
• Frequency Observed by Particle– The first shift can itself be split into two effects
• (b) the number of wavefronts passing a stationary particle position over the same duration, t…
k
ke bˆ
tc
tcNumber of wavefronts that pass a
stationary particle during t due to the wavefront velocity:
LDA: Doppler shift, effect I
• The net effect due to a moving observer w/ a stationary source is then the difference:
Δttc beu
Number of wavefronts that pass a moving particle during t due to combined velocity (same as using relative velocity in particle frame):
Net frequency observed by moving particle
cf
c
ct
f
b
b
p
eu
eu
ˆ1
ˆ1
s wavefrontof #
0
LDA: Doppler shift, effect II
• An additional shift happens when the light gets scattered by the particle and is observed by the detector– This is the case of a moving source and stationary detector (classic train
whistle problem)u
tseu
se
tcreceiver
lens
u
Distance a scattered wave front would travel during t in the direction of detector, if u were 0:
tc
p
s
p
ss f
c
Δtf
Δttc eueu ˆˆ
emitted wavesofnumber
by wave traveleddistancenet
Due to source motion, the distance is changed by an amount:
Therefore, the effective scattered wavelength is:
Δtseu
LDA: Doppler shift, I & II combined• Combining the two effects gives:
• For u << c, we can approximate
c
cf
c
f
c
cfcf
s
b
s
p
s
p
sobs eu
eu
eueu ˆ1
ˆ1
ˆ1
ˆ 0
bs
bs
ssb
sbobs
c
ff
cf
cccf
ccff
eeu
eeu
eueueu
eueu
ˆˆ
ˆˆ1
1
ˆˆ1
ˆ1
ˆ1
ˆ1
00
0
2
0
1
0
LDA: problem with single source/detector• Single beam frequency shift depends on:
– velocity magnitude– Velocity direction– observation angle
• Additionally, base frequency is quite high…– O[1014] Hz, making direct detection quite difficult
• Solution?– Optical heterodyne
• Use interference of two beams or two detectors to create a “beating” effect, like two slightly out of tune guitar strings, e.g.
– Need to repeat for optical waves
bsobs c
fff eeu ˆˆ0
0
to 1111 cos rkEE
to 2222 cos rkEEP
cos 1t cos 2t 1
2cos 1 2 t cos 1 2 t
• Repeat, but allow for different frequencies…
Optical Heterodyne
I co2
E1 E2 E1 E2
I co2
Eo12 Eo2
2 2Eo1Eo2 cos k1 k2 r 1 2 t 1 2
1
2Io1 Io2 2 Io1Io2 cos k1 k2 r 1 2 t 1 2
ACIPEDI
E1 E01 exp i k1x 1t 1 E01 exp i1
E2 E02 exp i k2x 2t 2 E02 exp i2
I co2
Eo12 Eo2
2 2E01E02
exp i 1 2 exp i 1 2 2
I co2
Eo12 Eo2
2 E01 exp i1 E02 exp i2 E01 exp i1 E02 exp i2
I co2
Eo12 Eo2
2 2E01E02 cos 1 2
How do you get different scatter frequencies?
• For a single beam
– Frequency depends on directions of es and eb
• Three common methods have been used– Reference beam mode (single scatter and single beam)– Single-beam, dual scatter (two observation angles)– Dual beam (two incident beams, single observation
location)
bss c
fff eeu ˆˆ0
0
Dual beam method
Real MV formed by two beamsBeam crossing angle Scattering angle
‘Forward’ Scatter Configuration
Dual beam method (cont)
Note that ,1 ,2ˆ ˆ ˆ( ) 2sin( / 2) b b g e e x
so:
ˆ gMeasure the component of in the directionu x
1,2,0
2,2,0
02,
1,1,0
01,
ˆˆ
ˆˆ
ˆˆ
bb
bss
bss
c
ff
c
fff
c
fff
eeu
eeu
eeu
Dg f2sin2
xu
21212121
2sin4cos2
2
1
tIIIItI oooo gxurkk
Fringe Interference description
• Interference “fringes” seen as standing waves– Particles passing through fringes scatter light in regions of
constructive interference
– Adequate explanation for particles smaller than individual fringes
fs
xu
2sin2
Gaussian beam effects
-Power distribution in MV will be Gaussian shaped-In the MV, true plane waves occur only at the focal point-Even for a perfect particle trajectory the strength of the Doppler ‘burst’ will vary with position
A single laser beam profile
Figures from Albrecht et. al., 2003
Non-uniform beam effects
Centered Off Center
Particle Trajectory
DC
AC
DC+AC
- Off-center trajectory results in weakened signal visibility-Pedestal (DC part of signal) is removed by a high pass filter after photomultiplier Figures from Albrecht et. al., 2003
Multi-component dual beam
Three independent directions
xg
xb
Two – Component Probe LookingToward the Transmitter
Sign ambiguity…
• Change in sign of velocity has no effect on frequency
Ds f2sin2
xu
uxg> 0
uxg< 0
beam 2
beam 1
Xg
2121 2cos22 tfIII Doo rkk
Velocity Ambiguity
• Equal frequency beams– No difference with velocity direction… cannot detect
reversed flow
• Solution: Introduce a frequency shift into 1 of the two beams
beam 2
beam 1XgBragg Cell
fb = 5.8 e14
fb2 = fbragg + fb
fb1 = fb
,1 ,1 ,1
,2 ,2 ,2
,1 ,2 0
ˆ ˆ( )
ˆ ˆ( ) ( )
ˆ ˆ( )
bs b s b
bs b bragg s b
bD bragg b b bragg D
ff f
cf
f f fc
ff f f f
c
u e e
u e e
u e e
Hypothetical shift Without Bragg Cell
201 02 cos( 2 { } )D braggI E f f t
If fD < fbragg then u < 0
New Signal
Frequency shift: Fringe description
• Different frequency causes an apparent velocity in fringes– Effect result of interference of two traveling waves as
slightly different frequency
Directional ambiguity (cont)
nm, fbragg = 40 MHz and = 20°
Upper limit on positive velocity limited only by time response of detector
0( )
2sin( / 2)D bragg
xg
f fu
fbragg
uxg (m/s)
fD
s-1
Velocity bias sampling effects
• LDA samples the flow based on– Rate at which particles pass through the detection volume– Inherently a flux-weighted measurement– Simple number weighted means are biased for unsteady flows
and need to be corrected
• Consider:– Uniform seeding density (# particles/volume)– Flow moves at steady speed of 5 units/sec for 4 seconds (giving
20 samples) would measure:
– Flow that moves at 8 units/sec for 2 sec (giving 16 samples), then 2 units/sec for 2 second (giving 4 samples) would give
8.620
2*48*16
520
20*5
Laser Doppler AnemometryVelocity Measurement Bias
, ,n1 1x
1 1
U
N N n
xx i i x i ii ix N N
i ii i
U U UU
Mean Velocity nth moment
- The sampling rate of a volume of fluid containing particles increases with the velocity of that volume- Introduces a bias towards sampling higher velocity particles
Bias Compensation Formulas
Phase Doppler Anemometry
The overall phase differenceis proportional to particle diameter
The geometric factor, - Has closed form solution for p = 0 and 1 only - Absolute value increases with elevation angle relative to 0°)- Is independent of np for reflection
Multiple DetectorImplementation
Figures from Dantec
2niD
,,,np,ni