Accurate and fast generation of irregular short crested waves by using periodic boundaries in a mild-slope wave model

In this work, periodic lateral boundaries are developed in a time dependent mild-slope equation model, MILDwave, for the accurate generation of regular waves and irregular long and short crested waves in any direction. A single wave generation line inside the computational domain is combined with periodic lateral boundaries. This generation layout yields a homogeneous and thus accurate wave field in the whole domain in contrast to an L-shaped and an arc-shaped wave generation layout where wave diffraction patterns appear inside the computational domain as a result of the intersection of the two wave generation lines and the interaction with the lateral sponge layers. In addition, the performance of the periodic boundaries was evaluated for two different wave synthesis methods for short crested waves generation, a method proposed by Miles and a method proposed by Sand and Mynett. The results show that the MILDwave model with the addition of periodic boundaries and the Sand and Mynett method is capable of reproducing a homogeneous wave field as well as the target frequency spectrum and the target directional spectrum with a low computational cost. The overall performance of the developed model is validated with experimental results for the case of wave transformation over an elliptic shoal (Vincent and Briggs shoal experiment). The numerical results show very good agreement with the experimental data. The proposed generation layout using periodic lateral boundaries makes the mild-slope wave model, MILDwave, an essential tool to study coastal areas and wave energy converter (WEC) farms under realistic 3D wave conditions, due to its significantly small computational cost and its high numerical stability and robustness

An internal wave generation method for the non-hydrostatic model swash

Numerical wave propagation models are commonly used as engineering tools for the study of wave transformation in coastal areas. In order to simulate waves in the nearshore zone correctly, the generation and absorption of waves at the boundaries of the models need to be modelled accurately. In numerical models, incident waves are usually generated by prescribing their horizontal velocity component at the boundary of the computational domain over the vertical direction. Additionally, in order to absorb and to prevent re-reflections in front of the numerical wave generator, a weakly reflective wave generation boundary condition is applied in which the total velocity signal is a superposition of the incident velocity signal and a velocity signal of the reflected waves. However, this method is based on the assumption that the reflected waves are small amplitude shallow water waves propagating perpendicular to the boundary of the computational domain and hence this method is weakly reflective for directional and dispersive waves. Within the present study, an internal wave generation method combined with sponge layers is applied in the non-hydrostatic model SWASH, in order to more accurately generate waves and avoid re-reflections at the boundaries

Generation of homogeneous wave fields in phase resolving wave propagation models

One of the challenges that the engineering world has to face is the study of coastal environments, in order to assess their vulnerability due to the rising of the sea-level and the resulting increase of the wave heights. Towards this goal, numerical models constitute a valuable tool for coastal engineers. In recent years, phase-resolving wave models are used more and more often in order to get a realistic and accurate representation of the waves in the field and their transformation over time and space. In this dissertation, developments are considered in two phase-resolving models, the mild-slope wave model MILDwave and the non-hydrostatic wave model SWASH. The core aim is to improve the homogeneity of the generated wave fields in these wave models by enhancing their capability of accurately generating the target wave conditions and at the same time by minimising the disturbance of the generated wave field by unwanted wave diffraction and reflection patterns due to the imposed numerical boundaries. The conclusions of this research reveal that the new developments in MILDwave and SWASH can be successfully used to study long-existing engineering problems in a more accurate way.Een van de uitdagingen waarmee de ingenieurswereld geconfronteerd wordt, is de studie van kustomgevingen, om hun kwetsbaarheid veroorzaakt door het stijgende zeeniveau en de resulterende verhoging in golfhoogtes te beoordelen. Met het oog op dit doel vormen numerieke modellen een waardevol middel voor coastal engineers. In de laatste jaren worden fase-oplossende golfmodellen steeds meer gebruikt om een realistische en nauwkeurige voorstelling van de golven en hun transformatie in tijd en ruimte te verkrijgen. In deze thesis beschouwen we ontwikkelingen in twee fase-oplossende modellen, het milde-helling golfmodel MILDwave en het non-hydrostatische golfmodel SWASH. Het hoofddoel is het verbeteren van de homogeniteit van de opgewekte golfvelden in deze golfmodellen middels het opvoeren van hun vermogen om de beoogde golfcondities accuraat te genereren en terzelfdertijd het minimaliseren van de verstoring van het opgewekte golfveld door ongewenste golfdiffractie- en reflectiepatronen dankzij de opgelegde numerieke grenzen. Conclusies van dit onderzoek tonen aan dat de nieuwe methodes, ontwikkeld in MILDwave en SWASH, succesvol ingezet kunnen worden om lang bestaande ingenieursproblemen accurater te bestuderen

Wake Effect Assessment in Long- and Short-Crested Seas of Heaving-Point Absorber and Oscillating Wave Surge WEC Arrays

In the recent years, the potential impact of wave energy converter (WEC) arrays on the surrounding wave field has been studied using both phase-averaging and phase-resolving wave propagation models. Obtaining understanding of this impact is important because it may affect other users in the sea or on the coastline. However, in these models a parametrization of the WEC power absorption is often adopted. This may lead to an overestimation or underestimation of the overall WEC array power absorption, and thus to an unrealistic estimation of the potential WEC array impact. WEC array power absorption is a result of energy extraction from the incoming waves, and thus wave height decrease is generally observed downwave at large distances (the so-called “wake” or “far-field” effects). Moreover, the power absorption depends on the mutual interactions between the WECs of an array (the so-called “near field” effects). To deal with the limitations posed by wave propagation models, coupled models of recent years, which are nesting wave-structure interaction solvers into wave propagation models, have been used. Wave-structure interaction solvers can generally provide detailed hydrodynamic information around the WECs and a more realistic representation of wave power absorption. Coupled models have shown a lower WEC array impact in terms of wake effects compared to wave propagation models. However, all studies to date in which coupled models are employed have been performed using idealized long-crested waves. Ocean waves propagate with a certain directional spreading that affects the redistribution of wave energy in the lee of WEC arrays, and thus gaining insight wake effect for irregular short-crested sea states is crucial. In our research, a new methodology is introduced for the assessment of WEC array wake effects for realistic sea states. A coupled model is developed between the wave-structure interaction solver NEMOH and the wave propagation model MILDwave. A parametric study is performed showing a comparison between WEC array wake effects for regular, long-crested irregular, and short-crested irregular waves. For this investigation, a nine heaving-point absorber array is used for which the wave height reduction is found to be up to 8% lower at 1.0 km downwave the WEC array when changing from long-crested to short-crested irregular waves. Also, an oscillating wave surge WEC array is simulated and the overestimation of the wake effects in this case is up to 5%. These differences in wake effects between different wave types indicates the need to consider short-crested irregular waves to avoid overestimating the WEC array potential impacts. The MILDwave-NEMOH coupled model has proven to be a reliable numerical tool, with an efficient computational effort for simulating the wake effects of two different WEC arrays under the action of a range of different sea states