7.7. DIRECTIONAL (3-D) WAVE GENERATION
435
variation between adjacent wave board segments, and they also developed
a different numerical technique to simulate gaps between adjacent wavemaker segments. Their simulations agreed with measurements reported by
Madsen (1974) for the case of only two segments. Harkins, et al. (1990)
concluded that even small variations in segment stroke, as well as gaps
between segments, can create large spatial variations in the wave field.
Sand and Mynett (1987) discussed other laboratory effects that prevent
uniformity of the directional wave field characteristics over a large area of
the basin. These included the following:
• Diffraction occurs at both ends of a finite-length directional wavemaker as generated oblique waves are diffracted
into the quiescent regions of the basin. This reduces the
wave height and decreases the region of the basin experiencing uniform wave conditions. The diffracted waves may
be reflected back into the wave field by solid boundaries.
Often solid vertical boundaries called “wave guides” are installed extending into the basin from the ends of the wavemaker and orientated perpendicular to the oblique wave
crests. The wave guides prevent diffraction from occurring
over their length, but they can create reflection problems.
Another option is to let diffraction occur, but compensate
by generating a larger wave height so the required height
will occur where it is needed in the model.
• Reflection in varying amounts will occur from the boundaries of the basin, and the reflected waves will contaminate
the wave field. The boundaries should be lined with effective wave absorbers to minimize this effect as much as
possible. If the model being tested has highly reflective
surfaces, an evaluation should be made to determine how
much of the reflective wave energy will be re-reflected by
the wave board or wave guides.
• Local Disturbances, created by the wave board not exactly matching the wave’s velocity distribution over the
depth, distort the wave shape immediately in front of the
wavemaker. However, these disturbances decay exponentially with distance from the wavemaker, and their effect
is minimal at two wavelengths from the wave board.
• Servo-Control System characteristics must be sufficient
for precise control of individual wave board segments. As
shown by Harkins, et al. (1990) small variations in segment stroke can lead to significant variations in wave form
435
variation between adjacent wave board segments, and they also developed
a different numerical technique to simulate gaps between adjacent wavemaker segments. Their simulations agreed with measurements reported by
Madsen (1974) for the case of only two segments. Harkins, et al. (1990)
concluded that even small variations in segment stroke, as well as gaps
between segments, can create large spatial variations in the wave field.
Sand and Mynett (1987) discussed other laboratory effects that prevent
uniformity of the directional wave field characteristics over a large area of
the basin. These included the following:
• Diffraction occurs at both ends of a finite-length directional wavemaker as generated oblique waves are diffracted
into the quiescent regions of the basin. This reduces the
wave height and decreases the region of the basin experiencing uniform wave conditions. The diffracted waves may
be reflected back into the wave field by solid boundaries.
Often solid vertical boundaries called “wave guides” are installed extending into the basin from the ends of the wavemaker and orientated perpendicular to the oblique wave
crests. The wave guides prevent diffraction from occurring
over their length, but they can create reflection problems.
Another option is to let diffraction occur, but compensate
by generating a larger wave height so the required height
will occur where it is needed in the model.
• Reflection in varying amounts will occur from the boundaries of the basin, and the reflected waves will contaminate
the wave field. The boundaries should be lined with effective wave absorbers to minimize this effect as much as
possible. If the model being tested has highly reflective
surfaces, an evaluation should be made to determine how
much of the reflective wave energy will be re-reflected by
the wave board or wave guides.
• Local Disturbances, created by the wave board not exactly matching the wave’s velocity distribution over the
depth, distort the wave shape immediately in front of the
wavemaker. However, these disturbances decay exponentially with distance from the wavemaker, and their effect
is minimal at two wavelengths from the wave board.
• Servo-Control System characteristics must be sufficient
for precise control of individual wave board segments. As
shown by Harkins, et al. (1990) small variations in segment stroke can lead to significant variations in wave form