386
CHAPTER 7. LABORATORY WAVE GENERATION
maximum forward position, followed by a slower retreat back to the maximum rearward position. Numerical implementation of the cnoidal wave
generation technique outlined above was given in detail by Goring (1979),
and the interested reader is referred to this source for further information.
Goring and Raichlen (1980) applied their method to generate laboratory
cnoidal wave trains with Ursell numbers varying between 10 < HL2/h3 <
1230. Comparison of wave profiles measured in the wave tank to theoretical
solutions of the Korteweg-de-Vries equation was reported to be “relatively
good with discrepancies apparent primarily in the region of the trough, especially for the cases of large Ursell numbers.”
7.5 Transient Wave Generation
Some studies in coastal engineering focus on the impacts due to a single
wave of a particular form or a group of waves having a specified pattern of
heights and periods. Example studies include capsizing of floating vessels
due to episodic waves on the verge of breaking, and stability of breakwater armor units exposed to a particular sequence of waves. These studies
require the capability to generate single waves or wave sequences having a
specified form at a specific location in the wave tank, this is not an easy
problem because most waves undergo dispersion as they propagate. The
following sections summarize progress that has been reported in the generation of transient waves.
7.5.1 Piston-Type Wavemaker Started From Rest
Before hydraulically-driven wavemakers became common, wave boards were
often driven by flywheel arrangements that established the stroke and period of the wave board motion. One problem encountered with these systems occurred at the initial startup and when the wavemaker was halted.
Rather than all waves being uniform, one of the first and one of the last
waves of the burst to arrive at a given location were considerably larger
than the average of the wave train. These large waves were considered a
transient response associated with starting and stopping the wavemaker,
and they were a concern to engineers conducting breakwater stability tests
because the larger wave may induce premature damage.
The general first-order solution for sea surface elevation resulting from
a wavemaker starting from rest was found by Kennard (1949) as
CHAPTER 7. LABORATORY WAVE GENERATION
maximum forward position, followed by a slower retreat back to the maximum rearward position. Numerical implementation of the cnoidal wave
generation technique outlined above was given in detail by Goring (1979),
and the interested reader is referred to this source for further information.
Goring and Raichlen (1980) applied their method to generate laboratory
cnoidal wave trains with Ursell numbers varying between 10 < HL2/h3 <
1230. Comparison of wave profiles measured in the wave tank to theoretical
solutions of the Korteweg-de-Vries equation was reported to be “relatively
good with discrepancies apparent primarily in the region of the trough, especially for the cases of large Ursell numbers.”
7.5 Transient Wave Generation
Some studies in coastal engineering focus on the impacts due to a single
wave of a particular form or a group of waves having a specified pattern of
heights and periods. Example studies include capsizing of floating vessels
due to episodic waves on the verge of breaking, and stability of breakwater armor units exposed to a particular sequence of waves. These studies
require the capability to generate single waves or wave sequences having a
specified form at a specific location in the wave tank, this is not an easy
problem because most waves undergo dispersion as they propagate. The
following sections summarize progress that has been reported in the generation of transient waves.
7.5.1 Piston-Type Wavemaker Started From Rest
Before hydraulically-driven wavemakers became common, wave boards were
often driven by flywheel arrangements that established the stroke and period of the wave board motion. One problem encountered with these systems occurred at the initial startup and when the wavemaker was halted.
Rather than all waves being uniform, one of the first and one of the last
waves of the burst to arrive at a given location were considerably larger
than the average of the wave train. These large waves were considered a
transient response associated with starting and stopping the wavemaker,
and they were a concern to engineers conducting breakwater stability tests
because the larger wave may induce premature damage.
The general first-order solution for sea surface elevation resulting from
a wavemaker starting from rest was found by Kennard (1949) as