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 20. Particle image displacement methods and others
2
Particle image displacement methods
- Optical, non- or minimally-intrusive, fluid flow measurement technique;
- Instantaneous flow measurements in two-dimensional (2D) area or
three-dimensional (3D) volume field of views;
- Basic procedure of particle image displacement methods
1. Flow visualization
- Flow field seeded with small tracer particles
- Particles usually illuminated by a laser light sheet
2. Image recording
- Particle images captured by an imaging system
- Saved in photographic film or digital image file
3. Image evaluation
- Young’s fringes method
- Particle image identification
- Correlation-based algorithm
3
Particle image displacement methods
Example:
4
Particle image displacement methods
Three groups of methods
Particle tracking velocimetry (PTV)
- flow seeded with tracer particles of very low concentration
- very low image number density in photo or video recordings
- single particle can be identified in image recording
- particle image tracking possible from frame to frame
- low information density in measurement plane
Laser speckle velocimetry (LSV)
- flow seeded with tracer particles of very high concentration
- very high image number density in photo or video recordings
- single particle can not be identified in image recording
- particle image tracking impossible from frame to frame
- high information density in measurement plane
Particle image velocimetry (PIV)
- flow seeded with tracer particles of high concentration
- high image number density in photo or video recordings
- single particle can be identified in image recording
- particle image tracking impossible from frame to frame
- high information density in measurement plane
5
Particle image displacement methods
Single frame image recordings
Single exposure
- Long exposure time
- Velocity determined by trajectory
- Direction ambiguity
- Low particle number density required
Double exposure
- Short exposure time
- Velocity determined by displacement
- Direction ambiguity
- Methods to avoid direction ambiguity:
a. color/intensity tagging
b. Image shifting techniques
Multi-exposure
- Short exposure time
- Velocity determined by displacement
- Direction ambiguity
- Used to increase particle image number
- Limited in steady flow
6
Particle image displacement methods
Multi frame image recordings
- velocity determined with particle image displacement between frames
- double/Multi exposure used to increase image number in steady flow
7
Particle image displacement methods
Frequently used evaluation methods
LID – low image density (PTV)
HID – high image density (PIV)
LS – laser speckle mode (LSV)
8
Particle image displacement methods
Data reduction
Image plane
Scale factor:
 = L/L’
Time interval:
t
Laser light sheet
Objective
Lens
Velocity: V=S/t=·S’/ t
S’
Image plane
Objective Lens
Laser light sheet
L’
L
S
9
Particle image displacement methods
Evaluation methods
Particle trajectory identification
Image recording
- single frame
- single long time exposure
- low image density
- film or digital recording
Evaluation
- read film recordings with
a microscope system
- identify particle trajectories
y
in digital recording
S   y 2  x 2
V 
S
t
x
10
Particle image displacement methods
Evaluation methods
Young’s fringes method
Image recording
- positive film
- single frame
- double/multiple exposed
- HID & LS mode
Young’s fringes system
laser
PC
2D traverse
system
frosted glass
CCD camera
- SM inversely proportional to SA
- fringes perpendicular to particle image displacement
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Particle image displacement methods
Evaluation methods
Particle image tracking
PIV recording
- Minimum 2 frames
- Single exposure
- LID mode
- Film or digital recording
Evaluation
- Identify particle images & determine position of each particle image center
- Pairing particles in two frames (many algorithms)
- Velocity determined by position difference of paired particles & t
x x
y  y1
t t
x 2 1,y 2
,t  2 1
2
2
2
Vx x, y, t   
x2  x1
t2  t1
Vy x, y, t   
y2  y1
t2  t1
y
t1
y
t2
(x2, y2)
(x1, y1)
o
xo
x
12
Particle image displacement methods
Evaluation methods
Correlation-based interrogation
(m, n)
Autocorrelation
-S
S
n
o
m
Cross-correlation
(m’,n’)
(m,n)
1.0
-10
M
0.0
15
N
 m, n     g1 i, j   g 2 i  m, j  n 
m
t
V y xm , ym   
10
5
0
m
i 1 j 1
Vx xm , ym   
0
-5
n
0.5
10
-10
-15
n
t
13
Particle image displacement methods
Standard 2D PIV
t=t
Lens
0
Measurement
volume
Laser
Light
sheet
Image #1
t=t
Fluid flow seeded with
small tracer particles
0
Lens system
& Camera
Single exposed recording
Exposure #1
Double exposed recording
14
Particle image displacement methods
Standard 2D PIV
t=t0+t
Lens
Measurement
volume
Laser
Light
sheet
Image #1
t=t
Fluid flow seeded with
small tracer particles
0
Image #2
t=t0+t
Single exposed recording
Lens system
& Camera
Exposure #1
Exposure #2
Double exposed recording
15
Particle image displacement methods
Micro-scale PIV (MPIV)
Micro Device
MCROFLUIDIC DEVICE
Flow out
Flow in
CCD CAMERA
Glass
cover
MICROSCOPE
Focal Plane
BEAM EXPANDER
Flood Illumination
Microscope
Beam
Expander
Nd:YAG LASER
Micro-Fluidics Lab
Purdue University
Epi-fluorescent
Prism / Filter Cube
Nd:YAG Laser
Micro-PIV image pair
l=532 nm
l = 610 nm
CCD Camera
(1280x1024 pixels)
16
Particle image displacement methods
Stereo PIV (SPIV)
- 3 velocity components in a plane
- Two cameras
- Translation systems (lateral displacement)
- Rotational systems (angular displacement)
Scheimpflug condition
17
Particle image displacement methods
Holographic PIV (HPIV)
- 3 velocity components in a 3 dimensional volume
- Complex and precise illumination
a. Hologram recording
b. Hologram reconstruction
18
Particle image displacement methods
Other image-based methods
– Defocusing PIV (Pereira et al. 2000)
• Allow images to become defocused
• Single camera/ color CCD, particle image tracking
– Multiple-sheet PIV (Raffel et al.,1995 )
• Multiple laser light sheet, single camera
– 3D scanning PIV (Brücker, 1997)
• Scanning a 3D volume with a laser beam
• Single high speed camera
– X-ray & Echo PIV
– Molecular Tagging Velocimetry
– Temperature measurement with particle Brownian motion
– More
19
Measurement of wind velocity
Cup anemometers
Propeller anemometers
Vane anemometers
Sonic anemometers
20
Homework
- Read textbook 11.4-11.5 on page 275 - 284
- Questions and Problems: 9 on page 287
- Due on 10/12
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