Review of multidirectional active wave absorption methods

Active wave absorption in a physical wave flume or basin requires a wavemak er, a hydrodynamic feedback, and an absorption control system. The feedback provides the information on the waves to be absorbed, and the control syst em ensures that the paddle of a wavemaker moves accordingly. Different ways of viewing the principle of active absorption are discussed, and basic con cepts an examined fur the simple case of shallow water waves, starting with the flume case and subsequently including wave obliqueness. A brief review is given of existing 2D systems for active absorption in wave flumes. Quas i-SD systems for multidirectional waves constituted by an array of independ ent flume systems are discussed, and fully 3D systems, including a coupling between neighboring feedback signals, are reviewed. The diverse appearance of these systems is mainly due to different ideas of accounting for wave o bliqueness. An attempt is made to unify the different approaches using the notion of a 2D filter giving the relation between the paddle control and th e feedback, regarding each of the two signals as one function of time and s pace (along the wavemaker), instead of as an array of 1D functions of time. So far, active absorption relies on linear systems. Suggestions are made f or the direction of future research, including the prospects for nonlinear extensions.