Paths to 3D PIV

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Paths to 3D PIV
Holography
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Principle of HPIV
Displacement Velocity
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Advantage of holography

• True 3D imaging
• Instantaneous Volumetric
• High Information Capacity(106 - 109 Particles)
• Real-Time Recording but Off-line Data Transfer
  Processing

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How to get true 3D imaging?

• Phase Preservation
• OOexpi(f-wt)
• or OOsin(f-wt)
• How to record f?
• Any light sensitive media records intensity
• IO2 O2
• Need to encode phase f into some intensity
  modulation

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Encoding Phase
-- Use interference of coherent light!
E R O Reference wave Object wave where R
R expi(j-wt) , OOexpi(f-wt) Recorded
Intensity IRO2 R2 O2 2ROsin(f-j)
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Principle of Holography
O
I RO2 R2 O2 2ROsin(f-j)
R
I (RO)(RO) R2 O2 RORO
O
T R2 O2 RORO
Usually R exp(-iwt) T 1 O2 O O
O
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Experimental Demonstration
Reference beam, object beam Virtual, real
image Transmission or Reflection Hologram?
Setup Considerations Coherence length vs. path
length difference Exposure energy In the linear
range RO ratio
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Transmission or Reflection Hologram
Reflection hologram created by 2 plane waves
traveling towards opposite sides (Volume Hologram)
Transmission hologram created by 2 plane waves
traveling towards the same side
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Reflection Hologram
Bragg Condition 2dsinqml
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In-line (Gabor) Holography
Traditional for particle fields

• Simple geometry
• Low coherence energy requirement

• Speckle noise
• (limit seeding density seeding depth)
• Large depth of focus
• (practically only 2D vectors)

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Speckle Noise (in-line hologram)
Ok S ok Sk exp(ifk) Random Walk
Reconstruction field of an in-line hologram for
an ensemble of particles B S ok S ok Type-I
speckle -- interference between B and the
scattered waves ?Major Source of Speckle Type-II
speckle -- self-interference of the scattered
waves.
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Speckle noise decrease Signal-to-Noise Ratio
40 particles /mm3
6 particles /mm3
1 particle /mm3
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Off-Axis Holography as Solution

• Off-axis HPIV
• Higher SNR
• Higher Seeding Density
• Complex Geometry
• Higher Coherence Required

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IROV - In-line Recording Off-axis Viewing
Holography

• IROV Use side scattering
• Suppresses speckle noise
• Reduces image depth of focus

Making In-line based HPIV feasible
Meng Hussain (1995) Appl. Opt. 34, 1827
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IROV Experimental Setup
Recording
Reconstruction
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Use of High-Frequency Fringes on In-Line Holograms
Negligible influence of forward scattering Since
OL ltlt R, IL ltlt I sig
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IROV suppresses speckle noise
Reconstruction field of an in-line hologram for
an ensemble of particles B S ok S ok

• Completely avoids type-I speckle
• greatly reduces type-II speckle

Off-axis Viewing receives only S ok
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Improved SNR by IROV
In-line Viewed
IROV
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Reduction of Depth of Focus by IROV
In-line Fraunhofer diffraction
0 degree
20 degree
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Proof of Principle Experiment
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IROV Measurement of a Vortex Ring
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Post Processing
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IROV Data Processing Genetic Algorithm
Particle Pairing

• Low density requires intelligent pairing
• GA searches large solution space

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Genetic Algorithm Particle Pairing

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Why Genetic Algorithm?
Many possibilities to pair particles Need to
numerate and filter

• Conventional searching methods
• Computation intensive
• Difficult to incorporate intelligence
• Time consuming

• Genetic Algorithm
• Efficient in searching large space
• Built-in intelligence to follow fluid dynamics
• Fast and inherent parallel processing speed

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Two Approaches of HPIV Developed at LFD
Off-axis HPIV high-end
In-line (IROV) HPIV low-cost
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Digital In-line Holography
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Dual-Reference Off-Axis Technique

• High Seeding Density Allowed
• Small Depth of Focus
• Image Separation Removes
• Direction Ambiguity
• Complex Optical Geometry
• High Energy Laser Required
• High Coherence of Beam
• Needed

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Gemini Off-axis HPIV System
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Concise Cross Correlation(CCC) Algorithm

• Matching by particle groups
• Uses particle centroids only
• Group shifting for matching
• Decomposition of operation
• Low data volume / high compression rate
• High-speed processing

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System Test Flow- Excited Air Jet
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Phase-Locked Vortex
Side View
Top View
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Vorticity
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Vorticity Iso-surface
To be re-made
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HPIV Measurement of Tab Wake
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Vortab Flow HPIV Measurement Result

• Amount of Data 400,000 Vectors
• Mean Velocity 16.67 cm/sec.

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Vortab Flow Vorticity Iso-Surfaces
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Fundamental Challenges

• 3D Signal Decoding
• Complex Flow Mapping
• Large Data Quantity
• User-friendly?

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Holographic Flow Visualization a Tool for
Studying 3D Coherent Structures and Instabilities
Kansas State University, ISSI, Wright Laboratory,
WP/AFB
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Off-Axis HFV of Vortex Flame
(c)
(b)
(a)
Holographic Images of Three Vortex-Flame Systems
Photographed from Two Angles (a) or Using Two
Magnifications (b and c).
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IROV HFV of Turbulent Milk Drop
Holographic Images of A Milk Drop Undergoing Bag
Instability (a, b)
Holographic Images of A Turbulent Milk Drop (a)
and Its Downstream Breakdown (b, c)
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Naturally, HPIV is an ideal diagnostic tool for
studying particulate phase - 3D and dynamically