Measurements in Fluid Mechanics 058:180 (ME:5180) Time

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Measurements in Fluid Mechanics058180
(ME5180)Time Location 230P - 320P MWF 3315
SCOffice Hours 400P 500P MWF 223B-5 HL

• Instructor Lichuan Gui
• lichuan-gui_at_uiowa.edu
• Phone [phone] (Lab), 319-400-5985 (Cell)
  http//lcgui.net

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Lecture 16. Flow visualization with marker
techniques
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Flow visualization
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Flow visualization
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Flow visualization
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Flow visualization with marker techniques
Tufts
- short pieces of yarn, string, or other flexible
material
- fastened one end on surface or thin support in
flow
- used in both gaseous and liquid flows
- indicate flow direction, separation region,
flow instability
- performance affected by density, stiffness, and
length
- tufts off surface
tuft screen - tufts mounted on mesh of thin wires
tuft wand - tufts mounted on a thin rod
pin tufts - tufts attached to thin needles
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Flow visualization with marker techniques
Surface marking methods
- visualize flow direction and flow patterns near
solid surfaces
Oil-streak method
- applicable to both gas and liquid flows
- surface coated with a paint consisting of a
pigment suspended in mineral oil
- pigment coagulates and is deposited on surface
in form of small lumps forming short streaklines
- commonly used pigment TiO2, china clay, lamp
black, copier toner, and fluorescent chrysene
- commonly used oil kerosene, diesel oil, and
light oil
- special coatings used to visualize pressure and
temperature distributions
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Flow visualization with marker techniques
Dyes
- visualize liquid flow with injecting dye
solution or producing colored region in fluid
Dye injection
- dye solutions injected into liquid stream
through wall orifices or tubes in midstream
- dyed stream should be injected at local speed
of the flow
- dye solution should match the density the fluid
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Flow visualization with marker techniques
Dyes
- visualize liquid flow with injecting dye
solution or producing colored region in fluid
Electrochemical methods
- chemical reaction triggered by electric current
Thymol blue (pH indicator) technique
- orange-yellow in acidic environment (pHlt0.8),
color changed to blue when pHgt9.6
- electric current in water releases hydrogen
ions at cathode and leaves OH ions to increase pH

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Flow visualization with marker techniques
Dyes
- visualize liquid flow with injecting dye
solution or producing colored region in fluid
Photolytic methods
- working fluid contains photochromic indicator
- temporarily capable of absorbing photons in
red-green range after illuminated by ultraviolet
light
- visible dark blue region appears when
illuminated by white light
- photochromic substances can be dissolved in
alcohol, kerosene, and other solvents, but not in
water
- photochromic substances return to original
state in a few seconds
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Flow visualization with marker techniques
Hydrogen bubbles
Thin metallic wire of 10-100 ?m used as cathode,
much thick carbon rod as anode
Hydrogen bubbles release to water flow from the
thin cathode
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Flow visualization with marker techniques
Smoke, mists, and fogs
Common methods for generating smoke
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Flow visualization with marker techniques
Solid markers, bubbles, and droplets
Particle images obtained with stroboscopic
illumination
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Homework
- Read textbook 10.1-10.2 on page 221 - 231

• Questions and Problems 2 on page 245

- Due on 10/03
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Learn to write a Matlab program

• Reconstructing an image after a shifting

Shift of an image S (Ix, Jy) I,J integer
numbers x and y decimal numbers and 0?xlt10?ylt
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Other interpolation methods available
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Try to write a Matlab program

• To cut a 6464-pixel image sample from a
  12801024-pixel image at i400, j200

6464-pixel image sample
http//lcgui.net/ui-lecture2012/hw/00/A001_1.BMP

• Reconstruct the 6464-pixel image with a shift of
  S ( 1.6, 2.3 )

i column number
j line number