Improvement of Shallow-Water Basin Wave Generation Using ‘Anti-Waves’

Abstract

The demand for hydrodynamic research facilities to correctly model shallow-water effects associated with low-frequency (LF) waves increases. Modelling such shallow-water waves in a basin introduces extra challenges, on top of the issues such projects are associated with at full scale. Various unwanted LF wave contributions are introduced by the geometry of the basin and the wave generators. Measurements show significantly more LF wave energy than expected based on second-order bound wave theory, where certain basin modes can generally be identified. This paper describes a method to minimize the unwanted part of this LF wave energy, by creating an ‘anti-wave’ and choosing an appropriate location in the basin. A wave-splitting tool was used to separate the original wave field into bound and LF free wave components in incident and reflected directions. The resulting LF free wave travelling from the wave generator was used to define a deterministic ‘anti-wave’, which was designed to cancel this LF free wave. Some first tests of this theory in MARIN’s Offshore Basin showed that it is possible to reduce the amount of unwanted LF spurious wave energy in shallow water by generating an anti-wave at the wave generator. This is promising for further development, although phasing of the anti-wave is difficult for basin oscillation modes. A procedure to follow for future shallow-water basin projects was identified.

Linking Experimental and Numerical Wave Modelling

“…If the expected basin effects are known, through basin measurements and/or simulations, they can be mitigated using adjustments to the basin (beaches, adjusting a bathymetry, etc. ), active reflection compensation (ARC) or deterministic 'anti-waves' to cancel unwanted long wave reflections [34][35][36]. However, it can be imagined that significant structural modification of the experimental facility is not always possible.…”

“…Active reflection compensation (ARC) algorithms based on instantaneous wave measurements usually have a notch filter around the basin mode frequencies to prevent instability in the system. Attempts at reducing some of the unwanted low-frequency wave content in the basin with deterministic anti-waves were reasonably promising [34][35][36]. However, the results differed for different modes and wave conditions and such a procedure requires an extensive iterative measurement campaign prior to tests with a model.…”