Multi-element Wavemakers

Multi-element Wave makers
Geometry
The CSIR has added to its
research and development capabilities by acquiring multi-element
dynamic absorption wave generators used in the 2D flumes and 3D
wave basins. These wave makers
comprise rack and pinion paddles
(made by HR Wallingford).
The 2D flumes have a single
paddle (0.75m wide) with wave
absorption capabilities.
For the 3D basins, the wave
generators are made up of narrow
paddle multi-element wave makers.
Each paddle is 50 cm wide, and
each module has a width of 4 m. A
total of six modules are available
for model tests making up 24 m.
The wave paddles are mounted on
a linear slide assembly that is
supported by a common framework. All the bearing and precision
components are situated above
water level. Long-crested and
short-crested waves can be
generated with this wave maker
system. The angle of incidence of
the waves can be varied depending
on the frequency distribution of the
wave condition. The maximum
wave height generated in 80 cm
water depth is approximately 47 cm
between frequencies of 0.4 Hz and
0.6 Hz. Both irregular and regular
waves can be generated. The
wave makers are equipped with a
dynamic wave absorption feature.
Software
The
wavemaker
program
generates
regular
(sinusoidal)
waves and random waves using
two methods. These are digitally
filtering white noise in real time and
summation of sine waves in both
real time and using an offline
playback method. The option to
generate user defined spectral
shapes is also available.
Waveforms can be long or shortcrested and the mean direction of
wave propagation can be set at
any angle to the face of the wave
machine. Wave fields from two or
more directions can also be
defined.
The random waves produced by
wavemaker conform to one of two
standard
spectral
shapes,
JONSWAP
and
Pierson
Moskowitz. Many other less often
used spectral definitions can be
input using the user-defined option
as a series of spectral densities
separated by a constant frequency
increment.
Set-down
The second order effect of setdown beneath wave groups is
important in shallow water harbour
studies where resonances and
ranging of moored vessels can be
a problem. The same is true for
some coastal studies where longperiod disturbances may be of
interest. As they move into shallow
water groups of waves of high and
low amplitude produce changes in
the mean water level which
induces a long period disturbance
not dissimilar to a real long wave
but different in that it is tied to the
wave groups. Set-down derives its
name from the fact that a
depression in the mean water
surface occurs beneath groups of
large waves. This wave-like
perturbation of the water surface
may induce a response in the
system being studied and must be
accurately represented in physical
models.
The software includes the option to
modify the motion of the pistontype wave machines in real time to
ensure that the set-down propagates realistically without contamination by other unwanted
second order effects.
Directional spreading functions
include cos2θ, cos6θ and the
Mitsuyasu type among others.
Contact: Kishan Tulsi
Email: ktulsi@csir.co.za Ph: +27(0)21 888 2455
Website: www.csir.co.za
Contact: Kishan Tulsi
Email: ktulsi@csir.co.za Ph: +27(0)21 888 2455
Website: www.csir.co.za