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| Titel |
Experimental investigation of effect of surface gravity waves and spray on heat and momentum flux at strong wind conditions |
| VerfasserIn |
Yuliya Troitskaya, Daniil Sergeev, Maxim Vdovin, Alexander Kandaurov, Olga Ermakova, Vassily Kazakov |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105240
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| Publikation (Nr.) |
 EGU/EGU2015-4718.pdf |
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| Zusammenfassung |
| The most important characteristics that determine the interaction between atmosphere and
ocean are fluxes of momentum, heat and moisture. For their parameterization the
dimensionless exchange coefficients (the surface drag coefficient CD and the heat transfer
coefficient or the Stanton number CT) are used. Numerous field and laboratory experiments
show that CD increases with increasing wind speed at moderate and strong wind, and as
it was shows recently CD decreases at hurricane wind speed. Waves are known
to increase the sea surface resistance due to enhanced form drag, the sea spray is
considered as a possible mechanism of the “drag reduction” at hurricane conditions. The
dependence of heat transfer coefficient CD on the wind speed is not so certain
and the role of the mechanism associated with the wave disturbances in the mass
transfer is not completely understood. Observations and laboratory data show that this
dependence is weaker than for the CD, and there are differences in the character of
the dependence in different data sets. The purpose of this paper is investigation
of the effect of surface waves on the turbulent exchange of momentum and heat
within the laboratory experiment, when wind and wave parameters are maintained
and controlled. The effect of spray on turbulent exchange at strong winds is also
estimated.
A series of experiments to study the processes of turbulent exchange of momentum and
heat in a stably stratified temperature turbulent boundary layer air flow over waved water
surface were carried out at the Wind - wave stratified flume of IAP RAS, the peculiarity of
this experiment was the option to change the surface wave parameters regardless of the speed
of the wind flow in the channel. For this purpose a polyethylene net with the variable
depth (0.25 mm thick and a cell of 1.6 mm x 1.6mm) has been stretched along
the channel. The waves were absent when the net was located at the level of the
undisturbed water surface, and had maximum amplitude at the maximum depth of the net
(33cm). To create a stable temperature stratification of the wind, the air entering the
flume was heated to 30-40 oC. The water temperature was maintained about 15
degrees. The air flow velocity in the flume corresponded to the 10-m wind speed from
10 to 35 m/s. Turbulent fluxes of heat and momentum and roughness parameters
were retrieved from the velocity and temperature profiles measured at the distance
6.5 m from the inlet of the flume and subsequent data processing exploiting the
self-similarity of the temperature and velocity profiles. In a result surface drag and heat
exchange coefficients and roughness parameters were obtained. Wind wave spectra and
integral parameters (significant wave height, mean square slope) were retrieved from
measurements by 3-channel array wave gauge by coherent spectral data processing.
To estimate the amount of spray in the air flow, a spray marker was introduced
using the effect of a sharp decline in film anemometer readings in contact with a
droplet.
Dependences of the exchange coefficients on the wind speed, wave parameters and the
spray marker were obtained. It is shown that the exchange coefficients increase with the wind
speed and wave height. It was found, that the sharp increase of the drag and heat
exchange coefficients at wind speeds exceeded 25 m/s was accompanied by the
emergence and increasing concentration of the spray in the air flow over water. The
correlation coefficient between the drag coefficient and the spray marker was about
0.9.
Using high-speed video revealed the dominant mechanism for the generation of spray at
strong winds. It is shown that it is associated with the development of a special type of
instability of the air-water interface, which is known as "bag-breakup instability" in the
theory of fragmentation of liquids. The hypothesis is suggested, that the observed increase of
surface drag and heat exchange can be attributed to the development of this type of
instability.
This work was supported by the Russian Foundation of Basic Research (13-05-00865,
14-05-91767, 13-05-12093, 15-05-) and Alexander Kandaurov, Maxim Vdovin and Olga
Ermakova acknowledge partial support from Russian Science Foundation (Agreement No.
14-17-00667). |
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