The program RWSYN, an acronym for Random Wave Synthesis, then is
used to synthesize a pseudo-random wave train on the basis of a specified target
spectrum obtained using PARSPEC. Each wave train consists of a time series
record of wave elevation. Several input parameters require defining, including
the model scale factor (normally = 1.0) and the duration of the wave train.
Typically, the duration of the wave train is normally set equal to 200 Tp, the
peak period of the wave spectrum. The length of the wave record is equal to the
recycling period, the interval at which the control system automatically cycles
back to the beginning of a time series when the MTS control software reaches the
end of the time series. The wave train time series can be generated using one of
three different methods, although the Random Phase Method is normally used.
In this technique, the phases of the Fourier components are chosen at random, but
the amplitudes are set deterministically according to the target spectrum. The
resulting wave records thus have spectra that exactly match the target spectrum.
Next, the program RWREP5 is used to compute the wave machine drive
signals to drive the four wave generators. These drive signals define the
commanded position of the actuators (i.e., the position of the wave boards) of the
wave machines, as a function of time. Several input parameters must be defined,
including the water depth at the wave generators, the lower and upper cutoff
frequency, and a wave amplification factor, WAF (usually WAF = 1.0). If the
calibration process was carried out correctly, the measured wave height should be
within 10 percent of the required wave height (with WAF = 1.0). After the
waves have been generated and measured, the wave amplification factor can be
adjusted to either increase or decrease the generated wave height.
Synchronization of wave generators
The program RWREP5 also contains a subroutine to synchronize the four
wave generators to create unidirectional long-crested waves with a total wave
front 30.5 m in length. Four wave propagation distances are entered into the
program RWREP5, one for each wave generator. The propagation distances are
calculated based on the length of each wave board (L = 7.64 m) and the geometry
of the layout of the wave boards. The propagation distance is the distance that a
wave must propagate to be in phase with waves generated by the adjacent wave
generator (travelling in the shoreward direction). This can be illustrated most
effectively by referring to Figure 1 and by taking note of the layout and
numbering of the wave generators. Waves generated from Wave Board No. 1 are
closest to shore for all angles of incidence. If the boards are positioned at a
10-deg angle of incidence, relative to shore normal, Wave Board No. 2 is
1.347 m further offshore than wave board No. 1. Wave Board No. 3 is 2.694 m
(2 1.347 m) further offshore than Wave Board No. 1, and Wave Board No. 4 is
4.041 m (3 1.347 m) further offshore than Wave Board No. 1. In this case, the
propagation distance is 1.347 m. The drive signal for Wave Generator No. 1 is
considered to be the master drive signal. Therefore, the drive signals for Wave
Generators No. 2, 3, and 4 are a function of the master drive signal, and have the
appropriate increase in phase at all wave frequencies to create unidirectional
long-crested waves. Figure 4, shown in Chapter 2, shows the four wave
generators creating unidirectional long-crested waves. This photograph was
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Chapter 7
Wave Generation and Current Recirculation
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