Laser Doppler Velocimetry Introduction to principles and applications Some Info compliments of...
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Transcript of Laser Doppler Velocimetry Introduction to principles and applications Some Info compliments of...
Laser Doppler VelocimetryIntroduction to principles and applications
Some Info compliments of Dantec Inc.
Dr. Arnold A. FontaineARL / Bioengineering
Office: Water Tunnel BuildingPh: 3-1765
email: [email protected]
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 (one calibration required)
• Very high accuracy
• Very high spatial resolution due to small measurement volume
• Tracer particles are required
Applications of LDA
• 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.
LDA - Optical principle
• 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.
• 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.
Photodetector
Inci
dent
bea
ms
Dire
ctio
n of
mot
ion
Inci
dent
bea
ms
Photodetector
Dire
ctio
n of
mot
ion
Frequency to velocity conversion
UxU
K2 / 2 K1
D D D U k k 1 2 1 2
( )
fU
Dx
22 sin /
U Cfx D
C 2 2sin /
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
Flow with particles
d (known)
Velocity = distance/time
t (measured)
Signal
Time
LaserBraggCell backscattered light
measuring volume
Detector
Processor
LDA - Fringe model
• 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
Laser
Braggcell
BS
F
D E
D
DL
Lens
Basic modules:
• Beam splitter• Achromatic lens
Options:
• Frequency shift (Bragg cell)
– low velocities– flow direction
• Beam expanders– reduce
measurement volume
– increase power density
Measurement volume
• The transmitting system generates the measurement volume
• The measurement volume has a Gaussian intensity distribution in all 3 dimensions
• The measurement volume is an ellipsoid
• Dimensions/diameters x, y and z are given by the 1/e2 intensity points
F
DL
Y
Z
X
Transmittingsystem
Measurementvolume
Intensitydistribution
0 1/e 2
1
z
x
y X
Z
Y
Measurement volume
Length:
Width: Height:
No. of fringes:
z
x
X
Z
f
Fringe separation:
f
22
sin
Laser, characteristics and requirements
• Monochrome
• Coherent
• Linearly polarised
• Low divergence (collimator)
• Gaussian intensity distribution
Laser
L-Diode collimator
Laser
Principle of LDA, differential beam technique
Laser
Signalprocessing
Transmittingoptics
Receiving opticswith detector
Signalconditioner
Flow
HeNeAr-IonNd:YagDiode
Beamsplitter(Freq. Shift)Achrom. Lens
GasLiquidParticle
Achrom. LensSpatial FilterPhotomultiplierPhotodiode
Spectrum analyserCorrelatorCounter, Tracker
AmplifierFilter
PC
Signal characteristics
• 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.
• Particles moving in either the forward or reverse direction will produce identical signals and frequencies.
Directional ambiguity / Frequency shift
fmax
fshift
fmin
f
u
uminumax
umin umax
• 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.
no shift shift
Frequency shift / Bragg cell
• 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
Piezoelectrictransducer
fs40 MHz
Absorber
wave front
Laser
fL
fL + fS
System configurations
Forward scatterand side scatter(off-axis)
• Difficult to align,• Vibration sensitive
Backscatter
• Easy to align• User friendly
Receiving opticswith detector
Transmittingoptics
Flow Receiving optics
with Detector
FlowLaser
Braggcell
Detector Transmitting and receiving optics
Seeding: scattered light intensity
180 0
90
270
210
150
240
120
300
60
330
30
180 0
330210
240 300270
150
12090
60
30
180 0
210
150
240
120
270
9060
300
30
330
• Polar plot of scattered light intensity versus scattering angle
• The intensity is shown on a logarithmic scale
Seeding: ability to follow flow
fp
fp
pp
UU
dU
/18
2
dt
d
ResponseFrequencyParticle
Following Lumley (1976):1
1 2
1
1
2
1
741%
362 1
a F R
aFR
a d
u l
u
l
p
f
fo r error
partic le tim e const
characteristics flu id frequency
d partic le d iam eter
flu id kinem atic
partic le or flu id density
( )
.
.
.
.
R eyno lds n um ber based o n in tegra l leng th an d velocity .
v iscosity .
p or f
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.
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:
• High pass filters to remove low frequency components such as the signal pedestal and low frequency noise.
• Low pass filters to limit high frequency noise components.
• High speed A/D converter.
• Burst detection algorithms to help identify burst from background signal. Digital signal analyzer to estimate the frequency.
DIGITAL SIGNAL PROCESSORS
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
• 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.