Naar Hybride Numeriek-Fysische Modellering van Multidirectionele Golven en Stromingen

This PhD research aims to advance hybrid numerical-physical modelling methodologies for complex multidirectional wave and current conditions, through the adaptation of the High-Order Spectral - Numerical Wave Tank (HOS-NWT) to the specific setup of the Coastal & Ocean Basin (COB) in Ostend, Belgium. The COB features limited-span segmented wavemakers along two adjacent sides in an L-shaped layout, and the ability to generate currents via flow chambers beneath the basin. This configuration enables realistic testing of coastal and offshore structures, but also introduces significant modelling challenges. To address these, the HOS-NWT—a well-established nonlinear potential flow solver—was extended and validated against experimental data from the COB. A central challenge in replicating COB conditions numerically is the limited wavemaker extent, which gives rise to diffraction effects and transverse wave components. These induce spatial inhomogeneities in the wave field that must be accurately reproduced. The HOS-NWT was therefore modified to account for these boundary effects and validated using regular wave data from multiple experimental campaigns. The results confirm that the adapted model reproduces the observed wave field deformations with high fidelity, supporting its use in pre-test simulations, wavemaker calibration, and measurement interpretation. To replicate the COB's L-shaped wavemaker layout, the model was further enhanced to allow wave input from two adjacent boundaries. This extension is essential for developing a numerical tool that faithfully reflects the COB, as the dual wavemaker configuration is one of its defining features. A custom dualboundary forcing scheme was implemented and validated against experiments involving simultaneous operation of both wavemakers. The methodology proved robust and effective for simulating two-sided wave generation. While the model captured key aspects of the resulting interference patterns, challenges remain—particularly in reproducing the initiation of diffraction effects in the dual-wavemaker setup. Nonetheless, the implementation provides a solid foundation for simulating two-sided wave generation in the hybrid context. Wave breaking plays a dominant role in energy dissipation under steep wave conditions and remains a key challenge for potential flow solvers like the HOS-NWT, particularly in multidirectional sea states. This work focuses on steepness induced breaking in constant-depth environments and explores two strategies to represent its effects. First, a visual detection method was developed to identify physical breaking events from overhead video footage. This approach could support the development and validation of multidirectional breaking onset criteria for local dissipation models. Second, a global dissipation mechanism was implemented in the HOS-NWT via spectral low-pass filtering to approximate the statistical effects of breaking-induced energy loss without explicitly resolving individual breaking events. The filtering improved qualitative agreement with physical measurements across all cases, particularly in the high-frequency tail of the spectra. However, in steep and narrow-spread conditions, the dissipation remained insufficient. Filter performance also varied with distance from the wavemaker. While the global dissipation method provides a solid and practical first step for incorporating multidirectional breaking effects in HOS-NWT, further development of deterministic local dissipation models remains desirable. To represent wave-current interaction, the HOS-NWT was extended to include a uniform background current by modifying the free surface boundary conditions. This implementation was validated against prototype experiments in the COB, involving six regular wave cases with and without a 0.3 m/s following current. The model accurately reproduced the theoretically expected modulation of wave heights and wave lengths. While good agreement with experimental measurements was found for some cases, others showed an overestimation of wave height, likely due to the absence of turbulence effects in the potential flow formulation. Nonetheless, these results confirm the feasibility of the chosen approach and establish a foundation for hybrid modelling of wave-current interaction in the COB. In conclusion, this research extends and validates the HOS-NWT to adress the specific characteristics of the COB, enabling accurate simulation of diffraction effects, two-sided wave generation, breaking processes, and wave-current interaction under laboratory conditions. The resulting numerical model supports efficient test preparation by enabling virtual pre-calibration of wavemaker motions and helping to identify optimal test locations for specific conditions, thereby reducing the need for iterative physical adjustments. Its full spatial resolution also allows detailed analysis of local flow dynamics that are difficult to capture through physical measurements alone. As such, the model provides a valuable tool for experimental design and interpretation, and it can support the development of a fully integrated hybrid modelling framework for coastal and offshore engineering.