IIT Madras. Sreenath K. Laser Doppler Anemometry ... IIT Madras. Sreenath K.
Laser Doppler Anemometry ..... of flow around a ship propeller in a cavitation tank
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Laser Doppler Anemometry
Sreenath K
IIT Madras
OUTLINE
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Flow measuring techniques
LDA principle
Components of LDA
Applications
Single Phase, Multi Phase measurements
Phase Doppler Anemometry
Quantification of fluid movement is pivotal in experimental and research studies
Bulk Flow measuring devices
Venturi, Rotameter, Flow nozzle, Orifice plate, Piston meter, Thermal mass flow meter
Electromagnetic, Ultrasonic, Coriolis flow meters, Optical flow meters Direct and Indirect measurement
Local or Bulk, Time-Averaged or Instantaneous values
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Convective heat transfer is correlated with velocity Limitations
Decomposition of fluid medium
Perturbations caused by measuring probes especially in regions of Thin boundary layer Recirculation regions, Flow separation
Deposition of solids
Particulate content
Fig 1: Hot wire anemometer, Reference (1)
Electrical conductivity 4
Laser Doppler Anemometry
Sreenath K
IIT Madras
Indirect measurement
Measured quantities are Thermodynamic state dependent
Proper knowledge of Fluid-property influence for good calibration
State parameter fluctuation e.g. Two phase flows, chemical reactions Blockage to flow in ducts with small dimensions Hostile environment. Flames etc FLOW DISTURBANCE
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Laser Doppler Anemometry
Sreenath K
IIT Madras
No flow disturbance
Almost direct measurement
Needs particles which can faithfully follow the flow Optical access
Takes advantage of particulate content to an extent High Spatial and temporal resolution
Additional instrumentation for size, concentration measurements
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Laser Doppler Anemometry
Sreenath K
IIT Madras
OPTICAL TECHNIQUES Image formation
Tracer Technique
Position Tracking
Photoelectric image tracing 7
Interference pattern formation
Photoelectric Interferometry
Photoelectric position tracking
Laser Doppler Anemometry
Sreenath K
IIT Madras
LASER DOPPLER ANEMOMETRY (LDA) Characteristics
Invented by Yeh and Cummins in 1964 No mechanical probe
No flow disturbance No pressure loss
Velocity, velocity changes, Instantaneous velocity and its correlations Rapid response to velocity changes High accuracy without calibration
Measures local, instantaneous velocity of tracer particles suspended 8
in the flow
Laser Doppler Anemometry
Sreenath K
IIT Madras
LDA Principle
Doppler shift
Laser light is sent through the flow region Scattered by seeded particles Scattered light is detected
Frequency of scattered light is Doppler shifted by an
Fig 2: Doppler Shift, Reference (1)
amount related to velocity of tracer particle
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Laser Doppler Anemometry
Sreenath K
IIT Madras
LDA Principle
Fig 3: LDA principle, Reference (2)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
LDA Principle
Doppler shift (~ 105 )is only a small fraction of incident frequency (~1015)
Estimating a small value as a difference of two big values - High uncertainty
Use two beams, find beat frequency.
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Laser Doppler Anemometry
Sreenath K
IIT Madras
LDA Principle
Fig 3: LDA principle, Reference (3)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Optical System a)
Reference beam
c)
Dual Scattered Beam
b)
Dual beam
Fig 4: Reference (4)
Fig 5: Reference (4)
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Laser Doppler Anemometry
Fig 6: Reference (4)
Sreenath K
IIT Madras
Dual beam LDA
DOPPLER MODEL
Fig 7: Doppler model to explain dual beam LDA, Reference (3)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Dual beam LDA
FRINGE MODEL
Fig 8: Fringe model to explain dual beam LDA, Reference (2)
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Gaussian Intensity distribution
Particle moving little away from centre Laser Doppler Anemometry
Sreenath K
IIT Madras
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Particle size
2𝜋𝜋𝑟𝑟𝑝𝑝 ∆𝑥𝑥 2𝜋𝜋𝑟𝑟𝑐𝑐 𝐼𝐼𝑆𝑆 = 𝐼𝐼𝑆𝑆,𝐴𝐴 �1 + � � sin � � cos � �� 2𝜋𝜋𝑟𝑟𝑝𝑝 ∆𝑥𝑥 ∆𝑥𝑥
Very small particles – modulation depth is fully reflected (A) As particle diameter increases – signal shape similar to (B)
Particle of diameter EQUAL to one fringe spacing (C)
diameter larger than several fringe spacing
Good “visibility” have been obtained with particles of Incompleteness of fringe model Mie’s theory (scattering)
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Laser Doppler Anemometry
Fig 9: Effect of Particle Size on signal quality, Reference (5)
Sreenath K
IIT Madras
Particle concentration
Constructive or destructive superposition of signals can occur
Modulation Depth is likely to be small at large particle concentrations Poly-dispersed particle size distribution
Fig 10: Effect of Particle concentration on signal quality, Reference (5)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Directional ambiguity
Same velocity in opposite directions result in same frequency shift Shift frequency of one incident beam using bragg’s cell
Rotating diffraction grating
Fig 11: Direction determination by frequency shifting, Reference (2)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Particle generator
Transmitting Optics
Backward scattering
Fig 12: Components of a typical LDA, Reference (6)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Optics set-up
Ensure velocity information in signals obtained
Matching particle size to angle between two beams Effective measuring volume as seen by detector
Signal quality at the outer region of ellipsoid measuring volume is poor.
Fig 13: Influence of geometrical parameters, Reference (5)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Multidimensional measurement
One pair of beams per dimension
Different frequency for each pair for meaningful detection by filtering
Fig 14: Reference (6)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Signal Processing
Signal Characteristics
Frequency spectrum of signal from PMT
Modulation depth varies with particle size and concentration
Signal is not present at all times. Noise is always present
Selection on the basis of
Property to be measured
Precision
SNR
Particle concentration
Turbulence intensity etc.
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Fig 15: Various aspects of signal processing, Reference (5)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Fig 16: Various aspects of signal processing, Reference (5)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Phase Doppler Anemometry • Extension of LDA principle
• Simultaneously measures velocity and size as well as concentration, mass flux
• First introduced by Durst and Zare (1975)
• Commercial instrument in 1984 • No calibration required
• Very high accuracy and spatial resolution
• Preconditions: Optical access, Sphericity, homogeneity, known refractive indices, size and concentration
• Principle : Phase shift between scattered signals at different locations is proportional to diameter
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Laser Doppler Anemometry
Sreenath K
IIT Madras
X
Set up of PDA
Detector 1
Flow
• Optical parameters
Z
θ
• Beam intersection angle θ
ψ
• Scattering angle Φ
ϕ
• Elevation angle ψ
• Optical configuration • Collection lens • Mask
ψ
Y
Scattering plane
Detector 2 Fig 17: PDA optical setup, Reference (6)
• Vertical slit
Fig 18: PDA optical setup, Reference (7)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Scattering • Scattering of plane waves by spherical particles – Mie theory
• Scattering modes
• Phase shift between detectors
• Depends on θ, Φ, ψ, diameter, scattering mode, wavelength of laser
Fig 18,19: Scattering by a spherical particle, Reference (6)
• Phase – diameter correlation • Linear for each mode
• Intensity of scattered light • Scattering angle • Polarisation 28
Laser Doppler Anemometry
Sreenath K
IIT Madras
2π ambiguity • Phase difference is a modulo 2π function • Three detector setup increases measurable size range
Fig 20: Illustration of 2π ambiguity in PDA, Reference (6)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Dual PDA • Trajectory and slit errors eliminated • Optimised for spray with transparent particles • Reject non-spherical particles • Improved concentration and mass flux estimation
Fig 21: Dual PDA optical setup, Reference (6)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Dual PDA • Combination of conventional and planar PDA
Fig 22: Conventional PDA, Reference (6)
Fig 23: Planar PDA, Reference (6)
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Laser Doppler Anemometry
Fig 24: Dual PDA, Reference (6)
Sreenath K
IIT Madras
Tracer particles
• Scattering properties different from that of fluid • Small scattering particles penetrating the fringe system • Suitable density
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Laser Doppler Anemometry
Set up
• Particle concentration • Turbulence • Properties to be measured • Appropriate Signal Processing
Sreenath K
IIT Madras
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Characteristics
Two Phase
• Much more complex • Variety of flow patterns • Flow properties of each phase should be measured
• Bubbles/Suspended particles for secondary phase flow properties • Additional tracer elements to extract main flow characteristics • The scattering properties should be distinct.
Laser Doppler Anemometry
Sreenath K
IIT Madras
Fig 25: Two Phase flow patterns, Reference (6)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Fig 26: Setup for two phase flow measurement, Reference (7)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Fig 27: Setup for three phase flow measurement, Reference (8)
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Laser Doppler Anemometry
Sreenath K
IIT Madras
Pictures from Reference (6) 37
Laser Doppler Anemometry
Sreenath K
IIT Madras
1
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•Aircraft Fuel Injection
Laser Doppler Anemometry
Sreenath K
IIT Madras
2
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• Water flow measurement in a pump model
Laser Doppler Anemometry
Sreenath K
IIT Madras
3
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• Helicopter Rotor model testing
Laser Doppler Anemometry
Sreenath K
IIT Madras
4
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• Velocity profile measurement in a pipes
Laser Doppler Anemometry
Sreenath K
IIT Madras
5
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• Measurement of flow around a ship propeller in a cavitation tank
Laser Doppler Anemometry
Sreenath K
IIT Madras
6
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• Measurement of flow field around a 1:5 scale car model in a wind tunnel
Laser Doppler Anemometry
Sreenath K
IIT Madras
6
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• Measurement of air flow field around a ship model in a wind tunnel
Laser Doppler Anemometry
Sreenath K
IIT Madras
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•Nozzle Design
Laser Doppler Anemometry
Sreenath K
IIT Madras
T Y P I C A L A P P L I C AT I O N S Sprays and liquid atomization processes Water sprays Fuel injection Bubble dynamics Cavitation Multiphase mass transfer Aerodynamics Hydrodynamics Turbomachinery Combustion process Verification of turbulence models and CFD predictions 46
Laser Doppler Anemometry
Sreenath K
IIT Madras
Importance of undisturbed flow measurements in “fragile” flows Doppler effect applied in LDA
Dual beam LDA can be explained by Doppler model and fringe model Complete LDA system was analysed Principle of PDA was discussed
Physical conscience behind Multiphase Flow measurements were presented
We looked at some applications of this technology
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Laser Doppler Anemometry
Sreenath K
IIT Madras
1. 2. 3. 4. 5. 6. 7. 8.
http//www.efunda.com http://web.mit.edu/fluids-modules/www/exper_techniques/LDA.text.pdf Fluid Mechanics – An Introduction to the theory of fluid flows, F. Durst. Laser-doppler anemometry and Its application to flow investigations Related to the environment of vegetation, F. DURST and M. Zari~ “Principles and Practice of Laser-Doppler Anemometry” by F. Durst, A. Melling and J. H. Whitelaw http://www.dantecdynamics.com Measurement of velocities in gas-liquid two-phase flow using laser Doppler velocimetry ,P. F. Vassallo, T. A. Trabold, W. E. Moore, G. J. Kirouac Extended Phase-Doppler Anemometry for Measurements in Three-Phase Flows* By Heiko Braeske, Günter Brenn, Joachim Domnick, Franz Durst, Adrian Melling, and Maris Ziema
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Laser Doppler Anemometry
Sreenath K
IIT Madras
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Laser Doppler Anemometry
Sreenath K
IIT Madras