Geoscience Reference
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If the model is constructed so as to allow multiple modes, another numerical feature
still distinguishes the model from nature. The background noise, the source of the neces-
sary modes dictated by
M
I
, cannot be completely reduced to zero in nature. In a digitised
medium it can, however, be made very small. For example, in the CS numeric scheme, the
11th-order decimal accuracy is employed. Such accuracy is essential for precise simula-
tions, but since it is the only source of noise in the system, it can obviously slow down
the development of the initial modes. As is sometimes done in numerical simulations
(e.g.
Dold & Peregrine
,
1986
;
Banner & Tian
,
1998
;
Song & Banner
,
2002
), the modes
can be deliberately introduced as initial conditions. Such an approach was not, however,
employed in the simulation described in
Section 4.1
, since in this scenario
M
I
of the sys-
tem is pre-defined rather than formed naturally, and wave development to breaking may
be altered. Other implications of numerical modelling, essential for the present discussion,
have already been considered in
Sections 2.1
and
4.1
above.
The laboratory experiment described in
Babanin
et al.
(
2007a
,
2009a
,
2010a
) was con-
ducted at the ASIST wind-wave facility at RSMAS, University of Miami (http://peas.
rsmas.miami.edu/groups/asist). The tank is of stainless-steel construction with a working
section of 15m
1m. Its programmable fan is capable of generating centreline
wind speeds in the range 0 to 30m
×
1m
×
s. Immediately downstream of the fan, extensive
flow-straightening devices are installed to condition the air flow and introduce appropri-
ately scaled turbulence. Further values of wind speed used here will be those of
U
10
, i.e.
extrapolated to 10m height.
The ASIST facility includes a fully programmable wave maker able to produce both
monochromatic waves and waves with a predefined spectral form. These waves are dissi-
pated at the opposite end of the facility by a minimum-reflection beach. The ASIST beach
design had been the subject of a special research project. A gently sloping (10 degrees)
grid of 2
/
.
5 cm-diameter acrylic rods is used. A perforated acrylic plate is placed beneath
the rods to split wave orbital velocities into multiple turbulent jets to increase viscous dis-
sipation. The energy of the reflected component is approximately 5-10% of the incident
energy depending on the initial wavelength.
In the experiment described, monochromatic deep-water two-dimensional wave trains
were generated by the wave paddle. The water depth was held at 0
4m, thus providing
deep-water conditions for the wave frequencies involved. With a tank length of 13
.
.
24m,
surface elevations were recorded at 4
56m from the pad-
dle. For each record, the initial monochromatic steepness (IMS) was varied in such a way
that the waves would consistently break just after one of the wave probes. In this way,
the dimensional distance to breaking (and therefore the breaking probability), wave train
properties immediately prior to breaking and detailed properties of the incipient breaker
could be measured. Note that this breaker is the result of natural nonlinear wave evolu-
tion, rather than being an outcome of an imposed modulation, being forced or simulated
by means of, for example, coalescing linear wave packets. The fact that breaking could
be predicted and controlled by manipulating IMS only is a strong corroboration of the
numerical model.
.
55m
,
10
.
53m
,
11
.
59m and 12
.
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