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Imaging Techniques for Flow and Motion Measurement
Lecture 4
Tracer Particle & Illumination
Lichuan Gui
University of Mississippi
2011
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Tracer Particle
Fluid mechanical properties
Spherical particle in still fluid
Gravitational induced velocity (Ug)
STOKES drag law for low Re
U g d p2
p
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g
dp – particle diameter
Ug
p – particle density
– fluid density
– dynamic viscosity
g – gravity acceleration
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Tracer Particle
Fluid mechanical properties
Velocity lag of spherical particle in
continuously accelerating fluid Us
Accelerating flow
U
Estimated Us for low Re:
Us U p U d
2
p
p
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Up
a
Us
dp – particle diameter
p – particle density
– fluid density
– dynamic viscosity
a – fluid acceleration
Up – particle velocity
U
Up
Decelerating flow
U – fluid velocity
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Tracer Particle
Thermo fluid properties
Brownian motion
-
Considerable for sub-micro particles in slow flows
Quantified by the mean square distance of diffusion <s2>
Diffusion coefficient D first derived by Einstein (1905)
s 2 2 D t
D
T
3d p
dp – particle diameter
– dynamic viscosity
– Boltzman’s constant
T – absolute temperature
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Tracer Particle
Light scattering behavior
Factors influencing the scattered light power
-
Light source power
-
Ratio of refractive index of particles to that of surrounding
medium
-
Particle size
-
Particle shape and orientation
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Polarization and observation angle
-
Others
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Tracer Particle
Light scattering behavior
MIE’s scattering (dp>) for spherical particles
Back scattering
Side scattering
Forward scattering
Light scattering by a 1 m oil particle in air with 532 nm laser
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Tracer Particle
Light scattering behavior
MIE’s scattering of different particle sizes
1 m glass particle in water
10 m glass particle in water
30 m glass particle in water
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Tracer Particle
Light scattering behavior
Rayleigh scattering (dp</10)
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Tracer Particle
Particles used in liquid flows
(Melling
1997, Meas.
Imaging
techniques
for fluidSci.
flowTechnol.)
measurements
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Tracer Particle
Particles used in liquid flows
-
No special seeding system necessary in small and
circulating systems
-
Seeding system required for open channel or big tank
-
Example: seeding system in a 3x3x100 m3 water tank
Seeding system
CONTAINER FOR
PARTICLES
RAKE
15m GLASS
SPHERES
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Tracer Particle
Particles used in gas flows
Gas flows more difficult to seed than liquid flows
- Small liquid droplets will either evaporate (bad) or coat flow apparatus (bad)
- Particles must be smaller because of the greater density difference
(Melling 1997, Meas. Sci. Technol.)
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Tracer Particle
Particles used in gas flows
Gas flows seeders
- Oil droplets generator
Seeding oil generator
Sketch of a laskin nozzle
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Tracer Particle
Particles used in gas flows
Gas flows seeders
- Solid powder seeder
Seeding system in a supersonic mixing layer facility
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Illumination
Light sources usually used in PIV systems
Helium-Neon lasers
–
Continuous wave laser
–
Extremely monochromatic with wave
length of =632.8 nm
–
High temporal coherence (typical
coherence length of 1030 cm)
–
Spatially coherent
–
Unidirectional, parallel to the body of
the laser
–
Beam of Gaussian intensity
distribution
–
Cheap but not very powerful
–
Traditionally used for evaluation of
PIV images
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Illumination
Light sources usually used in PIV systems
Argon-ion lasers
–
Gas laser
–
Continuous wave
–
Multiple wavelengths with very
narrow bandwidths
–
two dominant wavelengths, 514nm
and 488nm, make up about 67% of
the total beam output power
–
Single line operation possible by
inserting prisms, diffraction gratings
and other optical devices to "filter
out" the unwanted wavelengths
–
Powerful enough to illuminate
particles in PIV tests
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Illumination
Light sources usually used in PIV systems
Copper-vapor lasers (Cu lasers)
– High pulse speed, can be considered either CW or individual pulses
for PIV particle illumination
– Wavelength within the yellow and green spectrum
– High average power (Typically 130 W)
– Properties of a commercial Cu laser
Wavelength:
Average power:
Pulse energy:
Pulse duration:
Peak power:
Pulse frequency:
Beam diameter:
Beam divergence:
510.6 nm and 578.2 nm
50 W
10 mJ
15 ns – 60 ns
<300 kW
5 kHz – 15 kHz
40 mm
0.6·10-3 rad
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Illumination
Light sources usually used in PIV systems
Nd:YAG laser
– Most popular solid-state laser for PIV
– Available wavelengths: 1064, 532, 355, 266 nm etc.
– Short laser pulses (~5 ns)
– Slow repeat rate (10-15 Hz)
– Operated in triggered mode with quality switch (Q-switch)
– Dual-cavity configuration enables short time interval between laser
pulses
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Illumination
Light sources usually used in PIV systems
•
White light
•
–
For flood illumination in MPIV systems
–
Example:
- Xe (xenon) flash lamps have high rep rates (kHz) and short pulse durations
- Overhead projector lamps using CCD camera or spinning wheel to gate
LED’s (Light Emitting Diodes)
–
Available in various colors
–
Short pulses possible
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Illumination
Light sheet optics
With three cylindrical lenses (one with negative focal length)
-
Diverging lens used first to avoid focal lines
Limited light sheet height
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Illumination
Light sheet optics
With two spherical lenses and one cylindrical lens
-
More versatile illumination system
Focal line exists but has a relatively large extension
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Illumination
Light sheet optics
With three cylindrical lenses
-
Independently change of light sheet height and thickness
Smaller light sheet than beam diameter possible
Not suitable for high power lasers
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Homework
Practice with EDPIV software
1. Read EDPIV help manual to know details in “Imaging scale/time interval”
window
2. Determine imaging scale of target image “target.bmp”.
EDPIV software and help manual are available at http://www.edpiv.com/
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