436
CHAPTER 7. LABORATORY WAVE GENERATION
throughout the basin. In some cases an option is to use
a calibration procedure to incorporate the servo-control
system characteristics into the overall wave board transfer
function.
• Gaps beneath the wavemaker and between wave board
segments will reduce the wave height and cause nonuniformity in the directional wave field. Empirical correction
using measured wave data is the best method of compensating for this effect.
Dalrymple (1989) developed a directional wavemaker theory for a closed
basin which included wave diffraction, reflection, and shoaling. His model
was based on the mild slope equation, and a procedure was given that
utilized side wall reflections to produce uniform wave trains at specified
locations within the basin.
Isaacson (1989) developed a numerical model that used linear diffraction
theory and a point source representation of the wavemaker and basin walls
to describe the wave height variation within a directional basin. Isaacson
and Shaver (1991) compared the numerical model to physical model measurements for the case of uniform oblique waves and complete reflection by
the basin side walls. The comparison was reported to be “generally quite
favorable”.
7.7.4 Second-Order Directional Wave Generation
Generation of directional waves using a control signal developed from firstorder directional wavemaker theory does not adequately represent secondorder effects present in naturally-occurring wave fields. And just as occurs
for the two-dimensional case, unwanted second-order spurious waves will
be produced inadvertently by the wavemaker because it only satisfies the
wave board boundary conditions to first order. These waves may adversely
impact the experiment being conducted in the directional basin.
Sand (1982a) investigated the bound long waves that occur naturally
in a directional wave field, and he derived a means of estimating the amplitude of the bound wave. He determined that bound wave amplitudes
in a directionally-spread wave field were significantly smaller than those
produced by a unidirectional wave train. Also, the directional spreading
of the bound long waves was broader than the spreading of the irregular
short waves. This implies that correct reproduction of bound long waves
in physical models using directional irregular waves may not as important
as it is in the two-dimensional case.
CHAPTER 7. LABORATORY WAVE GENERATION
throughout the basin. In some cases an option is to use
a calibration procedure to incorporate the servo-control
system characteristics into the overall wave board transfer
function.
• Gaps beneath the wavemaker and between wave board
segments will reduce the wave height and cause nonuniformity in the directional wave field. Empirical correction
using measured wave data is the best method of compensating for this effect.
Dalrymple (1989) developed a directional wavemaker theory for a closed
basin which included wave diffraction, reflection, and shoaling. His model
was based on the mild slope equation, and a procedure was given that
utilized side wall reflections to produce uniform wave trains at specified
locations within the basin.
Isaacson (1989) developed a numerical model that used linear diffraction
theory and a point source representation of the wavemaker and basin walls
to describe the wave height variation within a directional basin. Isaacson
and Shaver (1991) compared the numerical model to physical model measurements for the case of uniform oblique waves and complete reflection by
the basin side walls. The comparison was reported to be “generally quite
favorable”.
7.7.4 Second-Order Directional Wave Generation
Generation of directional waves using a control signal developed from firstorder directional wavemaker theory does not adequately represent secondorder effects present in naturally-occurring wave fields. And just as occurs
for the two-dimensional case, unwanted second-order spurious waves will
be produced inadvertently by the wavemaker because it only satisfies the
wave board boundary conditions to first order. These waves may adversely
impact the experiment being conducted in the directional basin.
Sand (1982a) investigated the bound long waves that occur naturally
in a directional wave field, and he derived a means of estimating the amplitude of the bound wave. He determined that bound wave amplitudes
in a directionally-spread wave field were significantly smaller than those
produced by a unidirectional wave train. Also, the directional spreading
of the bound long waves was broader than the spreading of the irregular
short waves. This implies that correct reproduction of bound long waves
in physical models using directional irregular waves may not as important
as it is in the two-dimensional case.