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 16. Flow visualization with marker techniques
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Flow visualization
Marker techniques
- Foreign material added to the fluid flow
- Foreign energy (e.g. heat or electric
discharge) added to the fluid flow
Optical techniques
- Based on refraction index changes
- Compressible gas flow
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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
Trailing vortices behind an inclined delta-wing
as visualized by a tuft screen
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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
Oil-dot method - similar to oil-streak method, for surface with complex shapes
Oil-film method - works with interferometry to measure wall shear stress
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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
Example: Dye lines in water flow
Vortex flow behind a yawed cylinder visualized
with mixture of ink, milk and alcohol
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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 (pH<0.8), color changed to blue when pH>9.6
- electric current in water releases hydrogen ions at cathode and leaves OH ions to increase pH
vortex street behind a cylinder
Others: Tellurium wire technique
flow generated by a rotating cylinder
Electrolytic precipitation
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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
UV laser
Blue laser
Blue laser
- 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
Electric current causes electrolysis in water:
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
Consecutive rows of hydrogen bubbles indicating
Velocity profiles a flat plat boundary layer
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Flow visualization with marker techniques
Smoke, mists, and fogs
Common methods for generating smoke
Burning
vaporization and condensation
Smoke lines around an airfoil model
in a wind-tunnel
Aerosol generation
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Flow visualization with marker techniques
Solid markers, bubbles, and droplets
Particle images obtained with stroboscopic illumination
Tracer particles in water flow
Oil drops in laminar pipe flow
Propeller Turbulence Photographed
in Stroboscopic Light as
Water Passes the Torpedo
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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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