Waves reflect off the surfaces of the model and from sides of the tank. All tanks have resonant frequencies and often these lie within the working frequencies that are generated in the tank. A good beach will absorb much of the energy after is has passed the model but has little effect on cross-waves or models reflected from the model. This can be a major limitation on towing tanks where the productively of the whole facility is determined by the settling time after a run has been completed. Active absorbing wavemakers dramatically increase the performance of a tank by prolonging the time that an experiment can run without the build up of spurious waves and also by decreasing the settling time between runs. See the video below for more detail.
Traditional wavemakers work with a position feedback control system. This has the disadvantage that the swept volume of the paddle is dependent on the water level in front of the paddle. So the wave height generated is dependant on many factors including the size of an incoming wave or a poor quality beach.
During the first trials of the Duck wave energy converter Professor Stephen Salter found that wave height could vary by 30% which made it very hard to measure the absorption of the device. Early experiments were unstable because waves reflected back from the models and interacted with the wavemakers to create an uneven wave field. He overcame the problem by inventing a force feedback absorbing wavemaker that absorbed incoming waves by measuring the force on the front of the paddle and controlling the velocity.[1] Now the absorption control is calculated by a digital controller so absorption is totally predictable and can be optimised for specific experimental conditions.
Other researchers have implemented wave absorption using different techniques such as measuring the incoming wave with a wavegauge mounted to the front of the paddle. This signal is brought into the paddle controller and the motion is modified to absorb and damp out the unwanted wave.
References:
- Salter S.H., (1981). Absorbing wave-makers and wide tanks, Proceedings. Directional Wave Spectra Applications, Berkeley, Am.Soc.Civ.Eng, p185-202