Transcript LDA principles and applications

Laser Doppler Velocimetry
Introduction to principles and applications
Dr. Arnold A. Fontaine
ARL / Bioengineering
Office: Water Tunnel Building
Ph: 3-1765
email: [email protected]
Some Info compliments of Dantec Inc.
Characteristics of LDA
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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
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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.
Direction of motion
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Direction of motion
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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.
Incident beams
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Incident beams
LDA - Optical principle
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.
Frequency to velocity conversion
Ux
 K2
 /2
D  D1  D2
fD 
2U x
U
  
 U  (k1  k2 )
 sin  / 2
K1
U x  Cf D

C
2 sin  / 2
LDA - Fringe model
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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
Velocity = distance/time
Flow with particles
Signal
Processor
d (known)
t (measured)
Detector
Time
Bragg
Cell
Laser
measuring volume
backscattered light
LDA - Fringe model
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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
Basic modules:
• Beam splitter
• Achromatic lens
BS
Laser
Lens
Options:
• Frequency shift
(Bragg cell)
Bragg
cell
– low velocities
– flow direction
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
D E
Beam expanders
– reduce
measurement
volume
– increase power
density
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D
DL
F
Measurement volume
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The transmitting
system generates the
measurement volume
Transmitting
system

DL
Dimensions/diameters
x, y and z are given
by the 1/e2 intensity
points
Y
F
The measurement
volume has a Gaussian
intensity distribution in
all 3 dimensions
The measurement
volume is an ellipsoid
Z
1
0
X
1/e 2
Intensity
distribution
z
x
y
X
Z
Measurement
volume
Y
Measurement volume
Length:
z 
Width:
4F 
y 
 E DL
4F
 
 E D L sin 
 2
x 

2sin  
 2

 
 E DL cos 
 2
Z
No. of fringes:
 
8 F tan 
 2
Nf 
 E DL
4F
z
Fringe
separation:

f 
Height:
f
x
X
Laser, characteristics and
requirements
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Monochrome
Coherent
Laser
Linearly polarised
Low divergence
(collimator)
L-Diode
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Gaussian intensity
distribution
Laser
collimator
Principle of LDA, differential beam
technique
Flow
Laser
HeNe
Ar-Ion
Nd:Yag
Diode
PC
Transmitting
optics
Beamsplitter
(Freq. Shift)
Achrom. Lens
Receiving optics
with detector
Gas
Liquid
Particle
Achrom. Lens
Spatial Filter
Photomultiplier
Photodiode
Signal
processing
Signal
conditioner
Spectrum analyser
Correlator
Counter, Tracker
Amplifier
Filter
Signal characteristics
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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.
Directional ambiguity / Frequency shift
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Particles moving in either the forward or reverse direction will
produce identical signals and frequencies.
f
fmax
fshift
fmin
umin
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u
umax
umin
umax
shift
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.
Frequency shift / Bragg cell
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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
fs40 MHz
Piezoelectric
transducer
fL
wave front

Absorber
fL + fS
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
Detector
Transmitting and
receiving optics
Bragg
cell
Laser
Flow
Seeding: scattered light intensity
90
90
120
60
150
120
30
180
0
210
330
240
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300
90
120
60
150
30
180
0
210
330
240
300
270
270
dp0.2
dp1.0
60
150
30
180
0
210
330
240
300
270
dp10
Polar plot of scattered light intensity versus scattering angle
The intensity is shown on a logarithmic scale
Seeding: ability to follow flow
Particle Frequency Response
d
 U p U f
U p  18 2
dt
dp p /  f
12
Following Lumley (1976):
1
a F R  74 ( for  1% error )
a  particle time const .
F  characteristics fluid frequency.
R  Reynolds number based o  n integral length and velocity.
1
u
l
1
u
l
 

p


a1  36  2   1


f
d
2
d  particle diameter.
  fluid kinematic viscosity.
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p or f
 particle or fluid density.
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.
DIGITAL SIGNAL PROCESSORS
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:
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High pass filters to remove low frequency components such as the
signal pedestal and low frequency noise.
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Low pass filters to limit high frequency noise components.
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High speed A/D converter.
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Burst detection algorithms to help identify burst from background
signal. Digital signal analyzer to estimate the frequency.
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
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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.