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| Titel |
Properties of small-scale interfacial turbulence from a novel thermography based approach |
| VerfasserIn |
Jana Schnieders, Christoph Garbe |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250082639
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| Zusammenfassung |
| Oceans cover nearly two thirds of the earth’s surface and exchange processes between the
Atmosphere and the Ocean are of fundamental environmental importance. At the
air-sea interface, complex interaction processes take place on a multitude of scales.
Turbulence plays a key role in the coupling of momentum, heat and mass transfer
[2].
Here we use high resolution infrared imagery to visualize near surface aqueous
turbulence. Thermographic data is analized from a range of laboratory facilities and
experimental conditions with wind speeds ranging from 1ms-1 to 7ms-1 and various
surface conditions.
The surface heat pattern is formed by distinct structures on two scales - small-scale short
lived structures termed fish scales and larger scale cold streaks that are consistent with the
footprints of Langmuir Circulations. There are two key characteristics of the observed surface
heat patterns: (1) The surface heat patterns show characteristic features of scales.
(2) The structure of these patterns change with increasing wind stress and surface
conditions.
We present a new image processing based approach to the analysis of the spacing of cold
streaks based on a machine learning approach [4, 1] to classify the thermal footprints of near
surface turbulence. Our random forest classifier is based on classical features in image
processing such as gray value gradients and edge detecting features. The result is a pixel-wise
classification of the surface heat pattern with a subsequent analysis of the streak spacing. This
approach has been presented in [3] and can be applied to a wide range of experimental
data.
In spite of entirely different boundary conditions, the spacing of turbulent cells near the
air-water interface seems to match the expected turbulent cell size for flow near a no-slip
wall. The analysis of the spacing of cold streaks shows consistent behavior in a
range of laboratory facilities when expressed as a function of water sided friction
velocity, u*. The scales systematically decrease until a point of saturation at u* = 0.7
cm/s. Results suggest a saturation in the tangential stress, anticipating that similar
behavior will be observed in the open ocean. A comparison with studies of small-scale
Langmuir circulations and Langmuir numbers shows that thermal footprints in infrared
images are consistent with Langmuir circulations and depend strongly on wind wave
conditions.
Our approach is not limited to laboratory measurments. In the near future, we will
deploy it on in-situ measurements and verify our findings in these more challenging
conditions.
References
[1]   L. Breimann. Random forests. Machine Learning, 45:5–32, 2001.
[2]   S. P. McKenna and W. R. McGillis. The role of free-surface turbulence and
surfactants in air-water gas transfer. Int. J. Heat Mass Transfer, 47:539–553,
2004.
[3]   J Schnieders, C. S. Garbe, W.L. Peirson, and C. J. Zappa. Analyzing
the footprints of near surface aqueous turbulence - an image processing based
approach. Journal of Geophysical Research-Oceans, 2013.
[4]   Christoph Sommer, Christoph Straehle, Ullrich Koethe, and Fred A.
Hamprecht. ilastik: Interactive learning and segmentation toolkit. In 8th IEEE
International Symposium on Biomedical Imaging (ISBI 2011), 2011.
[5]Â Â Â W.-T. Tsai, S.-M. Chen, and C.-H. Moeng. A numerical study on the evolution
and structure of a stress-driven free-surface turbulent shear flow. J. Fluid Mech.,
545:163–192, 2005. |
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