Chemistry Reference
In-Depth Information
mal sublayer. From the heat transfer velocity, the transfer velocity of an
arbitrary gas is estimated using the so-called “Schmidt number scaling”
(Jähne and Haußecker 1998):
n
§
·
k
Sc
¨
©
¸
¹
g
h
(1.2)
k
Sc
h
g
with the Schmidt numbers
Sc
h
(heat) and
Sc
g
(gas) and the Schmidt number
exponent n (2/3 for a smooth surface, 1/2 for a wavy). The large difference
in the Schmidt numbers of heat (
Sc
= 7) and gas (Sc = 600 for carbon diox-
ide) might cast doubt whether the extrapolation is valid. However, detailed
laboratory measurements have shown that the extrapolation is correct
within a relative error of 10% provided that the uncertainty in the diffusion
coefficient of the gas is less than 5% and the Schmidt number exponent is
known with an absolute error less than 0.02 (Jähne et al. 1987).
3 Experimental Setup
The laboratory measurements were carried out in the small circular wind-
wave facility (Fig. 3), located in the Institute of Environmental Physics,
University of Heidelberg. This facility is gas tight and equipped with a
ventilation system that controls the flush rate of the air space independent-
ly of the wind speed. The outer diameter of the flume is 4 m and the chan-
nel width is 30 cm. The flume is loaded with 872 litres of deionised water
which corresponds to a water depth of 30 cm. Wind is generated up to 12
m/s by a rotating paddle wheel under the ceiling. A detailed description of
the facility can be found by Schmundt et al. (1995). The water surface is
observed via a mirror by the infrared camera system (see Fig. 1.3) and the
imaged footprint at the water surface has a size of 20 by 20 cm. The infra-
red camera (Amber Radiance, Raytheon, Goleta, CA) has a 256 x 256 fo-
cal plane array and is sensitive in the wavelength regime from 3 to 5 ȝm
with a noise equivalent temperature difference of 26 mK.
The field measurements were carried out in coastal and offshore waters
south of Cape Cod, New England in July 1997 in the framework of the
NSF-sponsored Coastal Ocean Processes study (CoOP97) conducted on-
board the RV Oceanus. The main part of the setup combined an infrared
camera, an infrared carbon dioxide laser and a temperature calibration de-
vice (Fig. 4).
