Geoscience Reference
In-Depth Information
gauges. The technique deals with groups of waves, rather than an individual wave that is
breaking, and it was found
“advantageous to consider the momentum flux and energy flux loss from the wave packet as opposed
to considering individual waves”
(see also discussion in
Section 2.7
).
In
Section 2.7
and throughout the text, the results of
Rapp & Melville
(
1990
) are men-
tioned and discussed with regard to a number of relevant topics. This comprehensive and
thorough study represents a benchmark in terms of methodology of laboratory investiga-
tions of wave breaking and in terms of the physics of breaking which is caused by the linear
superposition of a number of waves. Some features of such breaking are general, but some
are distinctly different if compared with the physics of breaking that occurs, for example,
as a result of nonlinear evolution of wave groups (
Babanin
et al.
,
2010a
).
What needs to be additionally highlighted in this section is the technique of flow visu-
alisation of the breaking region devised by
Rapp & Melville
(
1990
) (in this regard, see
also
Janssen
(
1986b
)).
Rapp & Melville
(
1990
) performed photography on the propa-
gation and diffusion of dye initially floated on the surface. It was a precursor to mod-
ern flow-visualisation means for wave motion, such as PIV (e.g.
Melville
et al.
,
2002
;
Qiao & Duncan
,
2001
;
Grue & Jensen
,
2006
;
Oh
et al.
,
2008
;
Babanin & Haus
,
2009
),
but many results of
Rapp & Melville
(
1990
) on subsurface mixing due to a breaking
event, such as temporal and spatial measures of the mixing, their connection with wave
properties and momentum losses in the course of the breaking, remain state of the art
to date.
With respect to studying the early stages of wave breaking, dye-visualisation in fact
proved superior in some senses. The PIV techniques rely on projected laser sheets which
illuminate a plane in the wave flume seeded with small neutral-buoyancy particles. Sequen-
ces of images, illuminated at high rate, are taken by a sensitive camera and allow detection
of small displacements of the particles. Thus, two components of the water velocity can be
obtained with high frequency resolution (and modern PIV techniques in some flows can
obtain all three components).
The light, however, is scattered by air bubbles, which are many in the course of breaking
in progress, particularly in the case of plunging breakers. Thus, the measurements need
to be done outside of the aerated region.
Melville
et al.
(
2002
), for example, essentially
repeated the experimental setup of
Rapp & Melville
(
1990
) in order to investigate by PIV
details of the velocity and vorticity fields due to breaking. They, however, had to start their
measurements three wave periods after breaking onset, to allow for the large bubbles to sur-
face. This points out limitations in the PIV technique. First of all, the most intensive phase
of breaking is unavailable to investigation. Secondly, PIV instrumentation cannot attend to
the dynamics of flow with void fraction, and this is an important dynamics as a large pro-
portion of the breaking-wave energy is dissipated due to work against the bubble-buoyancy
forces (up to 50% in
Lammarre & Melville
(
1991
,
1992
) and at least 14% according to a
more recent study of
Blenkinsopp & Chaplin
(
2007
)).
Search WWH ::
Custom Search