1. Report 10 Calm Surface
劉富蘭克林

2. 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.  
2019/1/18
Reporter: 劉富蘭克林

3. Overview Introduction Experiment Results and Discussion
Observation
Problem Analysis
Experiment
Experimental Setup
Results and Discussion
Conclusions & Summary
References
2019/1/18
Reporter: 劉富蘭克林

4. Introduction Observation
Water surface waves can be categorized into two types:
Capillary wave regime (wavelength<1.73cm)
Gravity wave regime (wavelength>1.73cm)
2019/1/18
Reporter: 劉富蘭克林

5. 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
2019/1/18
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6. Wind on oil over water surface:
Gibbs elasticity: Change of surface tension to change of area of surfactant layer.
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 (2007)
2019/1/18
Reporter: 劉富蘭克林

7. 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
2019/1/18
Reporter: 劉富蘭克林

8. Experiment Parameters: Oil layer thickness
Wind speed (small waves vs. large waves)
Attenuation of point-source waves through a distance
2019/1/18
Reporter: 劉富蘭克林

9. Experiment Setup Wide ripple tank (55x55 cm2) Light projector
Crossflow fan with parallel vanes
Screen and camera
2019/1/18
Reporter: 劉富蘭克林

10. Experiment Setup
Second ripple tank with wave generator and stroboscope
Micro-pipette
Mineral oil
Anemometer
2019/1/18
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11. 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
2019/1/18
Reporter: 劉富蘭克林

12. Experiment Method Light bands indicate crests
1cm
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
2019/1/18
Reporter: 劉富蘭克林

13. 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
2019/1/18
Reporter: 劉富蘭克林

14. Experiment 1: Oil layer thickness
Wind speed: 7.7m/s
½ water+oil
1ml oil
5ml oil
20ml oil
pure water
2019/1/18
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15. Experiment 1: Oil layer thickness
Video: Disturbances on water surface when shear stress is applied
2019/1/18
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16. 2019/1/18
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17. Experiment 1: Results
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
2019/1/18
Reporter: 劉富蘭克林

18. Experiment 2: Wind speed
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
2019/1/18
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19. Experiment 3: Attenuation of waves
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20. Experiment 3: Attenuation of waves
Oil on water also has attenuation effect on point-source waves
The frequency remains unchanged
Oil on water
Pure water
2019/1/18
Reporter: 劉富蘭克林

21. 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
2019/1/18
Reporter: 劉富蘭克林

22. Conclusions & Summary Oil layer thickness: Wind speed:
A thin layer of oil has the best calming effect
If excess oil is used, ripples form directly on the oil
Wind speed:
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
2019/1/18
Reporter: 劉富蘭克林

23. References
Behroozi, Peter. Et. Al. The calming effect of oil on water. AJP, Vol. 75, Issue 5, pp (2007)
2019/1/18
Reporter: 劉富蘭克林

24. Thank you! 23rd IYPT 2010 劉富蘭克林 2nd TYPT 2010 1st TYPT 2009 2019/1/18
Reporter: 劉富蘭克林