A new active absorption system and its performance to linear and non-linear waves

The present paper presents a new active absorption method for wavemakers in physical models based on digital filtering of signals from wave gauges in the nearfield. Such system is needed to maintain control of the waves when performing model tests where reflections from the models are significant. Similar systems have been described earlier, but in previous systems a causal IIR filter was applied while FIR filters were considered impossible to use. However, in the present paper it is shown that a causal FIR filter can indeed be fitted with similar or even better performance than the systems based on IIR filters. Furthermore, the existing fully linear absorption theory for gauges being flush mounted to the paddle is extended to gauges with a small gap to the paddle. Such gap can be used to compensate a large control delay in some existing setups. The performance of the new system on linear and non-linear regular waves has been tested in a new wave flume with wavemakers in both ends. Such flume is ideal for validating the theoretical performance curve and the results show excellent agreement between theory and measurements. Results for irregular waves confirm the broad banded absorption capabilities. Finally, a procedure for generating highly non-linear waves with simultaneous active absorption has been given. The results show that the superharmonics of such waves can also be well absorbed by the system.