420
CHAPTER 7. LABORATORY WAVE GENERATION
wave energy is present in the laboratory seastate. Testing
that continues beyond the time it takes the re-reflected
free long wave to again reach the testing section can no
longer make claims of strictly reproducing natural bound
long wave energy. However, this situation is decidedly better than if no attempts had been made to suppress the
spurious long-wave energy.
A final important point needs to be made regarding the use of wave
board signals containing the spurious long wave correction. Common practice is to achieve different irregular significant wave heights in the flume
by using the same irregular wave board signal and increasing or decreasing
the amplifier gain. This will not work for the spurious long wave
correction! Increasing or decreasing the amplifier gain for a first-order
board signal is justified on the basis of linear superposition of sine wave
components. However, the spurious long wave correction is a second-order,
nonlinear correction, and adjusting the gain factor will produce a linearly
adjusted long wave correction that will not be appropriate for the adjusted
short-wave field. This severely constrains some flume calibration procedures.
Correct Reproduction of Higher Harmonics
In the section entitled Second-Order Stokes Waves, it was shown that
second-order Stokes waves are a combination of a first-order wave and a
second-order “bound” component having twice the frequency of the firstorder wave but travelling at the same celerity. It was also shown that attempts to generate this nonlinear wave using first-order wavemaker theory
result in an unwanted second harmonic.
Similarly, irregular waves in the ocean environment often exhibit nonlinear characteristics due to the presence of higher harmonics bound to
the primary waves. The second-order harmonics tend to steepen the wave
crests and flatten the wave troughs, giving the wave profile a distinctive
asymmetry about the horizontal axis.
hen first-order wavemaker theory is used to generate irregular waves,
the nonlinear aspects of the wave train will not be produced Tnis may
or mai not be important, depending on the narticutar problem being examined in the physica. mode). However m those tanorator} studies when
nonlinearities are considered imno^tant. the wave r»oarr signa. must
nroOnoe the higher harmonics without introducing spurious rret
'
-
- juiar wave tnenry ■
CHAPTER 7. LABORATORY WAVE GENERATION
wave energy is present in the laboratory seastate. Testing
that continues beyond the time it takes the re-reflected
free long wave to again reach the testing section can no
longer make claims of strictly reproducing natural bound
long wave energy. However, this situation is decidedly better than if no attempts had been made to suppress the
spurious long-wave energy.
A final important point needs to be made regarding the use of wave
board signals containing the spurious long wave correction. Common practice is to achieve different irregular significant wave heights in the flume
by using the same irregular wave board signal and increasing or decreasing
the amplifier gain. This will not work for the spurious long wave
correction! Increasing or decreasing the amplifier gain for a first-order
board signal is justified on the basis of linear superposition of sine wave
components. However, the spurious long wave correction is a second-order,
nonlinear correction, and adjusting the gain factor will produce a linearly
adjusted long wave correction that will not be appropriate for the adjusted
short-wave field. This severely constrains some flume calibration procedures.
Correct Reproduction of Higher Harmonics
In the section entitled Second-Order Stokes Waves, it was shown that
second-order Stokes waves are a combination of a first-order wave and a
second-order “bound” component having twice the frequency of the firstorder wave but travelling at the same celerity. It was also shown that attempts to generate this nonlinear wave using first-order wavemaker theory
result in an unwanted second harmonic.
Similarly, irregular waves in the ocean environment often exhibit nonlinear characteristics due to the presence of higher harmonics bound to
the primary waves. The second-order harmonics tend to steepen the wave
crests and flatten the wave troughs, giving the wave profile a distinctive
asymmetry about the horizontal axis.
hen first-order wavemaker theory is used to generate irregular waves,
the nonlinear aspects of the wave train will not be produced Tnis may
or mai not be important, depending on the narticutar problem being examined in the physica. mode). However m those tanorator} studies when
nonlinearities are considered imno^tant. the wave r»oarr signa. must
nroOnoe the higher harmonics without introducing spurious rret
'
-
- juiar wave tnenry ■