Transcript 投影片 1 - IYPT Archive

Report 10
Calm Surface
劉富蘭克林
Problem # 10
• Calm Surface
• When wind blows across a water
surface, waves can be observed. If
the water is covered by an oil layer,
the waves on the water surface will
diminish. Investigate the
phenomenon.
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Overview
• Introduction
– Observation
– Problem Analysis
• Experiment
– Experimental Setup
– Experiment
• Results and Discussion
• Conclusions & Summary
• References
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Introduction
• Observation
• Water surface waves can be categorized into
two types:
• Capillary wave regime (wavelength<1.73cm)
• Gravity wave regime (wavelength>1.73cm)


 c  2
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
g
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Introduction
• Wind on water surface:
1. Wind picks up on small disturbances on
water surface, giving the waves energy.
2. The water oscillates in a circular motion
3.The surface becomes a trochoid
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• Wind on oil over water surface:
• Gibbs elasticity: Change of surface tension to
change of area of surfactant layer.
E 2
d
d ln A
• different wave speeds in different materials-> phase
difference in water and oil layer-> shear gradient
• Oil applies a reaction force on wave -> Energy loss
Behroozi, Peter. Et. Al. The calming effect of oil on water. AJP, Vol. 75, Issue 5, pp. 407-414 (2007)
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• Calming effect failure:
• Large waves (gravity wave regime)
– Oil layer may be broken and lose calming effect
• Thick layer of oil on water surface:
– Wave height is much smaller than oil layer
thickness
– Waves form directly on oil surface
oil
water
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Experiment
• Parameters:
– Oil layer thickness
– Wind speed (small waves vs. large waves)
– Attenuation of point-source waves through a distance
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Experiment Setup
• Screen and
camera
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• Wide ripple tank (55x55 cm2)
• Light projector
• Crossflow fan with parallel vanes
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•
Second
ripple
Experiment Setup
tank with wave
• Micro-pipette
generator and
• Anemometer • Mineral oil
stroboscope
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Experiment Method
• The fan is turned on, and ripples start to form.
• The projector projects the image onto the
screen.
• A camera on fixed manual adjustments is used
to capture the images
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Experiment Method
1cm
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•Light bands indicate crests
•Dark bands indicate
troughs
•If the waves were larger,
the brightness of the
photos would be more
contrasted
•By analyzing the
brightness deviation of the
photo, we can compare the
“roughness” of the surfaces
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Experiment 1: Oil layer thickness
• Different amounts of oil are used added to the
water surface (total thickness=1cm):
• Pure water
• 1ml oil-> thickness: 4.7x10-6 m
• 5ml oil-> thickness: 1.6x10-5 m
• 20ml oil-> thickness: 6.6x10-5 m
• 1.5L of water+ 1.5L of oil-> thickness: 0.5 cm
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Experiment 1: Oil layer thickness
Wind speed: 7.7m/s
pure water
1ml
oil
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5ml oil
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oil
½20ml
water+oil
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Experiment 1: Oil layer thickness
• Video: Disturbances on water surface when
shear stress is applied
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1ml oil on wate
5ml oil on wate
20ml oil on wat
Pure water
1ml Oil on water 0.25
0.20
0.15
0.10
0.10
brightness
0.15
0.05
0.00
0.05
0.00
-0.05
-0.05
-0.10
-0.10
-0.15
-0.20
-0.15
-0.25
0
20
40
60
80
100
0
20
pixel
40
60
80
100
1ml pixels
oil on water
1/2 oil+water
0.20
0.15
0.10
0.05
brightness
brightness
0.20
0.00
-0.05
-0.10
-0.15
-0.20
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40
60
80
100
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Experiment 1: Results
0.09
Standard deviation of pixel brightness
(Roughness of water surface)
0.08
0.07
0.06
0.05
0.04
0.03
Standard deviation of
pixel brightness
0.02
0.01
0
Pure water
1ml oil on water
5ml oil on water
20ml oil on water
1/2 oil+water
• Oil resists the shear gradient applied by water
waves (Gibbs elasticity) -> Calming effect
• A thin layer of oil has the best calming effect
• If excess oil is used, ripples form directly on
the oil
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Experiment 2: Wind speed
0.20
0.20
0.15
0.15
0.10
0.10
0.05
0.05
brightness
brightness
Pure water
1ml Oil on water
0.00
-0.05
0.00
-0.05
-0.10
-0.10
-0.15
-0.15
-0.20
-0.20
-0.25
Pure water
1ml oil
-0.25
0
20
40
60
80
100
0
20
40
60
80
100
pixels
pixel
• Wind speed:7.7 m/s
• Wind speed:6.4 m/s
•Oil has better calming effect when wind speed is
lower, because at high wind speeds the oil layer
tends to break
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Experiment 3: Attenuation of waves
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Experiment 3: Attenuation of waves
0.20
• Oil on water also has
attenuation effect on
point-source waves
• The frequency
remains unchanged
0.15
brightness
0.10
0.05
0.00
-0.05
-0.10
pure
water
Oil on
water
water
oilPure
on water
-0.15
0
10
20
30
40
50
60
70
80
pixel
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Conclusions & Summary
•Waves travel at different speeds in
different media
•The Gibbs elasticity property of an oil
film makes it resist the shear gradient
applied by water waves
-> Calming effect
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Conclusions & Summary
•Oil layer thickness:
•A thin layer of oil has the best calming effect
•If excess oil is used, ripples form directly on the oil
Standard deviation of pixel brightness (Roughness of water surface)
0.1
0.08
0.06
0.04
0.02
•Wind speed:
Standard…
0
Pure water
1ml oil on water5ml oil on water20ml oil on water 1/2 oil+water
•Oil has better calming effect when wind speed is
lower, because at high wind speeds the oil layer tends
to break
•Attenuation of waves from a point source:
•While enhancing the decay of the waves, the
frequency remains unchanged
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References
• Behroozi, Peter. Et. Al. The calming effect of oil
on water. AJP, Vol. 75, Issue 5, pp. 407-414
(2007)
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23rd IYPT 2010
劉富蘭克林
2nd TYPT 2010
1st TYPT 2009
Thank you!
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