Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 400

the bubble andgas exchange through the interface becomes a continuouslydistributed feature
rather than sporadic and intermittent.
Johnson ( 1986 ), Hwang et al. ( 1991 ), Thorpe et al. ( 1992 ) and Bortkovskii ( 1997 )
pointed out another environmental dependence for bubble formation, entrainment and
collapse - on the water temperature. This apparently happens through temperature depen-
dence of the air-water fluid properties such as cavitation stability, surface tension, air-to-
water diffusivity and water viscosity. The latter, for example, can change more than two
times under realistic temperature variations of the water (7.77) .
The onset of the acoustic methods of bubble detection and measurements ( Thorpe , 1982 )
signalled, in a way, a new era in this kind of investigation. The theory and practice of these
methods was discussed in Section 3.5 , and here we will only outline applications relevant
to the present section.
Such methods do have their limitations when dealing with individual bubble events,
under high void-fraction concentrations, because of the peculiarities of the acoustic-wave
propagation through the two-phase medium with essentially different densities and phase
speeds in the air and the water. Overall, however, the capabilities of the active acoustic sens-
ing by sonars and even of passive listening to the bubble-produced noise by hydrophones
are much greater by comparison to the photographing of the bubbles in the hostile and non-
stationary environment, particularly as far as the underwater investigations are concerned.
These methods were reviewed and discussed in Section 3.5 with respect to detection
and quantifying the wave-breaking events as such. In short, they either make bubbles ring
by exciting them artificially by means of active acoustic radiation emitted by sonars, or
utilise the acoustic signatures of bubbles naturally produced when the bubbles are created,
coalesce or collapse.
As an example of a modern application of acoustic methods to researching the bubble-
cloud structure, we refer to the recent study by Gemmrich & Farmer ( 2004 ). They used
free-floating underwater acoustic resonators to determine the size-distribution of microbub-
bles with radii of 20-200 μm, at a sampling rate of 2 2 Hz. As mentioned above, the method

is not suited for high levels of void fraction, but for 10 − 8

.

10 − 4 it produced data
hardly possible to achieve otherwise. Note that these magnitudes of air fraction are much
lower than those in (9.47) ,but Gemmrich & Farmer ( 2004 ) measured this fraction some 10
times deeper than Bezzabotnov et al. ( 1986 ) and dealt with microbubbles which would be
impossible to detect by the photographic method of the latter study in principle.
In particular, Gemmrich & Farmer ( 2004 ) analysed in detail the underwater signatures of
a breaking wave with period T

≤ N V ≤

5 s, recorded at the depth of approximately 1m below the
surface.Thesearenoticeablebothinthevelocityfield(turbulence)andvoidfraction(bubbles).
Turbulence due to the breaking causes the standard deviation of fluctuations of the verti-
cal component of the orbital velocity to rise six times. The peak of this rise almost coincides
with the passage of the crest of the breaker above (slightly lags) and the overall duration of
this response lasts for a quarter of the wave period. The volumetric turbulence-dissipation
rate

=

dis increases by more than three orders of magnitude. It is interesting that the gener-
ation of the turbulence precedes the arrival of the whitecapping crest, and 0

. 7 s before this