444
CHAPTER 7. LABORATORY WAVE GENERATION
Active Absorbers
Active wave absorbers are mechanical devices that can move in response
to a wave so that wave energy is extracted. One type of active absorber
resembles a wave board except it is placed at the far end of a wave tank
to absorb the progressive waves. As the wave reaches the absorber, the
absorber board moves so that it appears invisible to the wave, making it
seem like the wave passed on through the absorber (Dean and Dalrymple
1984). Efficient absorption depends on moving the absorber board in just
the right motion at just the right time so that the absorber velocity matches
the wave velocity.
First-order wave theory can be used to formulate the necessary absorber
board motion to absorb progressive regular waves (e.g., Dean and Dalrymple 1984). However, the absorber paddle velocities will not exactly match
those of the wave resulting in some reflection, and precise synchronization
of the wave and absorber phases may be difficult. Generally, some type of
measurement and active control feedback system will be required.
Milgram (1970) appears to have been the first to use a flap-type wave
board to absorb low-steepness waves in a short wave tank. He used a
capacitance-type wave gauge just in front of the flap to sense the fluctuating water level of the progressive waves and provide feedback to an active
control system driving the flap.
The second kind of active absorption takes place at the wave board
where reflected waves need to be absorbed at the same time waves are
being generated. Bullock and Murton (1989) pointed out that failure to
absorb reflected wave energy at the wave board (particularly when reflection
coefficients are high) results in re-reflection and a continued addition of
energy into the incident wave system until finally an energy balance is
obtained. For this reason, some tests on highly reflective structures, such
as breakwaters, are run in short bursts so that re-reflected wave energy does
not have sufficient time to contaminate the incident waves.
Salter (1985) discussed many of the aspects of absorbing wave makers,
and he advocated measuring the force on the wavemaker as the best means
of sensing the approach of a reflected wave. He stated this has the advantage
that a single transducer provides an average condition over the entire wave
board width, whereas several wave gauges would be needed to account
for any cross-tank variations. Another advantage of the for ce-measuring
system is that it can be combined with a velocity transducer to determine
the instantaneous rate at which energy is being given to the water (Salter
Bullock and Murton (1989) agree that force measurement is practical
for new low-inertia wave boards; but for retro-fitting existing high-inertia
CHAPTER 7. LABORATORY WAVE GENERATION
Active Absorbers
Active wave absorbers are mechanical devices that can move in response
to a wave so that wave energy is extracted. One type of active absorber
resembles a wave board except it is placed at the far end of a wave tank
to absorb the progressive waves. As the wave reaches the absorber, the
absorber board moves so that it appears invisible to the wave, making it
seem like the wave passed on through the absorber (Dean and Dalrymple
1984). Efficient absorption depends on moving the absorber board in just
the right motion at just the right time so that the absorber velocity matches
the wave velocity.
First-order wave theory can be used to formulate the necessary absorber
board motion to absorb progressive regular waves (e.g., Dean and Dalrymple 1984). However, the absorber paddle velocities will not exactly match
those of the wave resulting in some reflection, and precise synchronization
of the wave and absorber phases may be difficult. Generally, some type of
measurement and active control feedback system will be required.
Milgram (1970) appears to have been the first to use a flap-type wave
board to absorb low-steepness waves in a short wave tank. He used a
capacitance-type wave gauge just in front of the flap to sense the fluctuating water level of the progressive waves and provide feedback to an active
control system driving the flap.
The second kind of active absorption takes place at the wave board
where reflected waves need to be absorbed at the same time waves are
being generated. Bullock and Murton (1989) pointed out that failure to
absorb reflected wave energy at the wave board (particularly when reflection
coefficients are high) results in re-reflection and a continued addition of
energy into the incident wave system until finally an energy balance is
obtained. For this reason, some tests on highly reflective structures, such
as breakwaters, are run in short bursts so that re-reflected wave energy does
not have sufficient time to contaminate the incident waves.
Salter (1985) discussed many of the aspects of absorbing wave makers,
and he advocated measuring the force on the wavemaker as the best means
of sensing the approach of a reflected wave. He stated this has the advantage
that a single transducer provides an average condition over the entire wave
board width, whereas several wave gauges would be needed to account
for any cross-tank variations. Another advantage of the for ce-measuring
system is that it can be combined with a velocity transducer to determine
the instantaneous rate at which energy is being given to the water (Salter
Bullock and Murton (1989) agree that force measurement is practical
for new low-inertia wave boards; but for retro-fitting existing high-inertia