Abstract
A proper design of offshore and coastal structures requires further knowledge about extreme wave events. Such waves are highly nonlinear and may occur unexpectedly due to diverse reasons. One of these reasons is wave-wave interaction and the wave focusing technique represents one option to generate extreme wave events in the laboratory. The underlying mechanism is the superimposition and phasing of wave components at a predefined location. To date, most of the existing methods to propagate target wave profile backwards to the position of the wave generator apply linear wave theory. The problem is that the generated waves with different frequencies generate new components which do not satisfy the linear dispersion relation. As a result, small changes in the wave board control signal generally induce large and random shifts in the resulting focused wave. This means that iterations are necessary to get the required wave profile at the correct position in the flume. In this study, a Self Correcting Method (SCM) is applied to optimize the control signal of the wave maker in a Numerical Wave Tank (NWT). The nonlinearities are included in the control signal and accurate wave focusing is obtained irrespective of the prevailing seabed topography (horizontal or sloping) and type of structure (reflective or absorbing). The performance of the proposed SCM is numerically investigated for a wide variety of scenarios and validated by scale model tests in the Large Wave Flume (Groser Wellen Kanal, GWK), Hannover, Germany. The strengths and limitations of the proposed SCM are discussed, including the potential for further developments.
Highlights
In harsh weather, extreme events will lead to damages to ships, offshore and coastal structures
This section presents the validity of the Self Correcting Method (SCM) in order to generate predefined time series at the desired point for different cases including: (i) constant water depth, (ii) a fully reflective wall, (iii) wave reflection and uneven bottom, (iv) a submerged bar; and (v) experimental testing of wave focusing
Focused wave in constant water depth The aim of this section is to evaluate the performance of the SCM
Summary
Extreme events will lead to damages to ships, offshore and coastal structures. The design of reliable and economic coastal and offshore structures requires further knowledge of such episodic waves These are unpredictable waves that are out of the statistical logic Osborne et al, (2000). Extreme waves are considered like a strange phenomenon because they are highly asymmetric and nonlinear waves (high ratio between wave crest and wave trough) that can occur in a relative calm sea state. On the other hand, Funke and Mansard, (1988) developed a method of reversible dispersive technique to generate extreme waves (or episodic), that can deal with variable water depth. They used linear theory during the process of wave assemblage. To avoid the shifts at the focal point, xp, they empirically determined the value of xp in order to generate the focused wave at the desired position
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