Measurements in Fluid Mechanics
058:180:001 (ME:5180:0001)
Time & Location: 2:30P - 3:20P MWF 218 MLH
Office Hours: 4:00P – 5:00P MWF 223B-5 HL
Instructor: Lichuan Gui
lichuan-gui@uiowa.edu
http://lcgui.net
Lecture 19. Laser Doppler velocimetry
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Laser Doppler velocimetry
Doppler effect
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Laser Doppler velocimetry
Doppler shift of light frequency
- monochromatic, coherent, linearly polarized, and collimated laser with wavelength  and frequency 
- transmitted through a fluid seeded with particles of velocity 𝑉
- particle adsorbs some of the light and reemits light following Mie-scattering laws
- photodetector receives the scattered light with frequency +D from view angle 
Doppler frequency:
𝑒𝑖 – unit direction vector of incident light
𝑒𝑠 – unit direction vector of scattered light
𝑉 – projection of velocity vector on the direction normal to the bisector of angle 
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Laser Doppler velocimetry
Dual-beam LDV
- laser beam split into two parallel beams
- focused at measuring volume with angle 
- small particles with velocity 𝑉 in measuring volume
- scattered lights collected with lens and superimposed on photodetector
- received light intensities:
𝐴1 sin2𝜋 𝜈 + 𝜈𝐷1 𝑡
𝐴2 sin2𝜋 𝜈 + 𝜈𝐷2 𝑡
- output voltage:
- Doppler frequency difference:
- photodetector output (Doppler signal) :
- flow velocity component:
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Laser Doppler velocimetry
The measuring volume
- Gaussian intensity distribution across laser beam
- beam diameter 𝑑𝑒 determined at 13.5% of maximum intensity
- focused beam diameter:
𝑓𝑇 - focal distance of the lens
- measuring volume with an ellipsoidal shape
Fringe model
- fringes in measuring volume with
fringe spacing
- fringe number in measuring volume
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Laser Doppler velocimetry
Scattered light intensity of moving particle
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Laser Doppler velocimetry
Directional ambiguity / Frequency shift
- Particles moving in either the forward or reverse direction
will produce identical signals and frequencies.
- With frequency shift (w. Bragg cell) in one beam relative to the other, the
interference fringes appear to move at the shift frequency.
- The fringe moving speed is subtracted from measured velocity.
- With frequency shifting, negative velocities can be distinguished.
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Laser Doppler velocimetry
LDV measuring system
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Laser Doppler velocimetry
Laser, characteristics and requirements
Monochrome
- light of a narrow frequency
Coherent
- spatial coherent
cross-correlation between two points has only 1 value at all times
- temporal coherent
perfectly correlated at one location
between any two different times
Linearly polarized
Low divergence (collimator)
Laser
Gaussian intensity distribution
Laser
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Laser Doppler velocimetry
System configurations
Forward scatter
and side scatter
(off-axis)
• Difficult to align,
• Vibration
sensitive
Flow
Detector
Backscatter
Receiving optics
with detector
Transmitting
optics
Transmitting and
receiving optics
• Easy to align
• User friendly
Bragg
cell
Laser
Flow
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Laser Doppler velocimetry
Multicomponent LDV systems
Two-component system
- two pairs of laser beams create two groups of
fringe planes perpendicular to each other to
measure two velocity components
- different wavelength for each pair of beams
e.g. Argon-ion laser spectral peaks at =514.5nm (G) and =488 nm (B)
- optical filters used to separate signals
Three-component system
- add a third pair of laser beams for 3D velocity measurement
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Homework
- Read textbook 11.2-11.3 on page 264 - 275
- Questions and Problems: 8 on page 286
- Due on 10/10
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