1,136 research outputs found
Modelling waves and their impact on moored ships
No full textShips that are moored at a berth in coastal waters are subject to various external forcings, including the hydrodynamic loads that are induced by the local wave field. If the ship motions resulting from these wave-induced loads become too large, they may hamper safe operations (e.g., the loading of a container ship). Accurate predictions of the hydrodynamic loads are therefore desired to ensure safe operations of moored ships. In a coastal environment, the wave field is generally dominated by short waves. The majority of these waves originate from the open ocean, where they are generated by the wind. If the short waves are energetic at a berth, they may cause a significant response of a moored ship. In addition, nonlinear wave effects can excite significant ship motions, which may even occur during relatively calm wave conditions or in a region that is sheltered from energetic short waves. This significant response is primarily related to the presence of infragravity waves, which are excited through nonlinear interactions amongst pairs of short waves. An accurate description of this nonlinear wave field is therefore indispensable when predicting the hydrodynamic loads that act on a ship which is moored in coastal waters. The range of scales and physical processes involved in such studies make this a challenging problem to solve using numerical models. At present, the existing models that can predict the wave impact on a moored ship based on an offshore wave climate are restricted to relatively mild wave conditions. This thesis set out to develop a new modelling approach to advance our capabilities in solving this complex problem.The proposed model aims to be applicable at the scale of a realistic coastal or harbour region (say in the order of km), while accounting for the relevant physical processes. This includes the processes that govern the nonlinear wave evolution over a varying bottom topography (e.g., the nonlinear interactions that excite infragravity waves), and the interactions between the waves and a moored ship (e.g., the scattering of waves by a fixed floating body). The approach is based on the recently developed non-hydrostatic wave-flow model SWASH, which has been successfully applied to simulate a range of wave related processes. This work pursues the development of a new modelling approach through a further development and evaluation of the SWASH model in (i) simulating the nonlinear wave dynamics in a coastal region, and (ii) simulating the interactions between waves and a restrained ship. The first crucial step in this development is to determine if the model can resolve the nonlinear wave field in a coastal environment. Previous studies showed that models like SWASH can resolve the short-wave dynamics in coastal waters. However, they did not address if such models can resolve the dynamics of the infragravity-wave field. Furthermore, most of these studies focussed on laboratory applications due to computational limitations, whereas field scale applications of non-hydrostatic models have been rarely reported. With the ever increasing computational capabilities, such scales are now within the reach of the state-of-the-art computer systems. To advance the capability of the non-hydrostatic approach towards such realistic applications, this work presents a thorough evaluation of the SWASH model in resolving the nonlinear wave dynamics at the scale of a realistic coastal region. Given the importance of infragravity waves with respect to the wave-induced response of a moored ship, this work particularly determines if the model can resolve their nearshore evolution. The model was validated using both laboratory and field experiments, covering a range of wave conditions (varying from bichromatic waves to short-crested sea states). A comparison between model predictions and laboratory measurements showed that the model captures the frequency dependent cross-shore evolution of infragravity waves with a coarse vertical resolution (2 layers), including their steepening and eventual breaking close to the shoreline. These results demonstrate that the model can efficiently resolve the dominant processes that affect their nearshore evolution (e.g., nonlinear interactions, shoreline reflections, and dissipation), permitting applications at the scale of a realistic harbour or coastal region. To determine the capability of the model at such scales, SWASH was applied to study the infragravity wave dynamics at a field site near Egmond aan Zee (the Netherlands), which is characterised by a complex bottom topography. The model was used to reproduce a total of six sea states (including mild and storm conditions), which were measured as part of a two month field campaign. For all conditions, the predicted wave field gave a good representation of the natural conditions, supporting a further study into the infragravity wave dynamics. A unique feature of these predictions is their extensive spatial coverage, allowing analyses of the wave dynamics at scales not easily covered by in-situ measurement devices.Amongst others, this study showed that a significant portion (up to 50%) of the infragravity wave motion can be trapped at a nearshore bar. This shows the potential of the model to improve our understanding of such complex wave dynamics. The findings of the flume and field studies further show that the SWASH model provides a powerful tool to predict the nonlinear wave field at a coastal berth based on an offshore wave climate. To predict the impact of this wave field on a ship that is moored at such a berth, the next crucial step in the model development is to account for the interactions between the waves and a restrained ship. For this purpose, a fixed floating body was schematised within SWASH. The model was validated by comparing model results with an analytical solution, a numerical solution, and two laboratory experiments that consider the wave impact on a restrained ship for a range of wave conditions (varying from a solitary wave to a short-crested wave field). These comparisons showed that the model captures the scattering of waves, and the hydrodynamic loads that act on the body. Remarkably, a coarse vertical resolution sufficed to resolve these dynamics. This shows the potential of the model in efficiently simulating the wave-ship interactions. The findings of this thesis demonstrate that, with the inclusion of a fixed floating body in SWASH, a novel modelling approach has been developed that can efficiently resolve the key dynamics that govern the nearshore evolution of waves and their interactions with a restrained ship. Although further work is required, for example, accounting for the motions of a moored ship, this demonstrates the approach has the potential to simulate the wave-induced response of a ship that is moored in coastal waters. This thesis thereby sets the stage to advance our modelling capabilities towards such realistic applications in a complex coastal environment
Simulating waves and their interactions with a restrained ship using a non-hydrostatic wave-flow model
No full textThis paper presents a numerical model to simulate the evolution of waves and their interactions with a restrained ship that is moored in coastal waters. The model aims to be applicable at the scale of a harbour or coastal region, while accounting for the key physical processes that determine the hydrodynamic loads on the ship. Its methodology is based on the non-hydrostatic wave-flow model SWASH, which provides an efficient tool to simulate the nonlinear dynamics that govern the nearshore wave field. In this work, we propose a new numerical algorithm that accounts for the presence of a non-moving floating body, to resolve the wave impact on a restrained ship. The model is validated through comparisons with an analytic solution, a numerical solution, and two laboratory campaigns. The results of the model-data comparison demonstrate that the model captures the scattering of waves by a restrained body. Furthermore, it gives a reasonable prediction of the hydrodynamic loads that act on a restrained container ship for a range of wave conditions. Importantly, the model captures these dynamics efficiently, which demonstrates that it retains this favourable property of the non-hydrostatic approach when a floating body is included. The findings of this study suggest that the model provides a promising new alternative to simulate the nonlinear evolution of waves and their impact on a restrained ship at the scale of a realistic harbour or coastal region.Environmental Fluid Mechanic
Numerical modelling of infragravity waves in coastal regions
No full textInfragravity waves are a common feature in the nearshore wave field and have a significant impact on numerous coastal processes. It is therefore important to accurately predict infragravity wave conditions at a given location. However, analytical relations do not exist with which to make such predictions and one has to rely on numerical models. In this study infragravity waves are simulated with a linear 1D surf beat model (IDSB) and a non-hydrostatic model (SWASH). The aim of this study is to perform a detailed numerical investigation into the nearshore behaviour of infragravity waves. Specific attention is paid to the generation, propagation and reflection of IG-waves.Environmental Fluid DynamicsHydraulic EngineeringCivil Engineering and Geoscience
Non-hydrostatic modelling of the wave-induced response of moored floating structures in coastal waters
No full textPredictions of the wave-induced response of floating structures that are moored in a harbour or coastal waters require an accurate description of the (nonlinear) evolution of waves over variable bottom topography, the interactions of the waves with the structure, and the dynamics of the mooring system. In this paper, we present a new advanced numerical model to simulate the wave-induced response of a floating structure that is moored in an arbitrary nearshore region. The model is based on the non-hydrostatic approach, and implemented in the open-source model SWASH, which provides an efficient numerical framework to simulate the nonlinear wave evolution over variable bottom topographies. The model is extended with a solution to the rigid body equations (governing the motions of the floating structure) that is tightly coupled to the hydrodynamic equations (governing the water motion). The model was validated for two test cases that consider different floating structures of increasing geometrical complexity: a cylindrical geometry that is representative of a wave-energy-converter, and a vessel with a more complex shaped hull. A range of wave conditions were considered, varying from monochromatic to short-crested sea states. Model predictions of the excitation forces, added mass, radiation damping, and the wave-induced response agreed well with benchmark solutions to the potential flow equations. Besides the response to the primary wave (sea-swell) components, the model was also able to capture the second-order difference-frequency forcing and response of the moored vessel. Importantly, the model captured the wave-induced response with a relatively coarse vertical resolution, allowing for applications at the scale of a realistic harbour or coastal region. The proposed model thereby provides a new tool to seamlessly simulate the nonlinear evolution of waves over complex bottom topography and the wave-induced response of a floating structure that is moored in coastal waters.Environmental Fluid Mechanic
Non-hydrostatic modelling of infragravity waves using SWASH
No full textThis paper presents numerical modelling of the nearshore transformation of infragravity waves induced by bichromatic wave groups over a horizontal and a sloping bottom. The non-hydrostatic model SWASH is assessed by comparing model predictions with analytical solutions over a horizontal bottom and with detailed laboratory observations for a sloping bottom. Good agreement between model predictions and data is found throughout the domain for bound infragravity waves. Furthermore the model predicts greater outgoing free infragravity wave-heights for steeper slope regimes which is consistent with the measurements. The model however tends to overestimate the magnitude of the outgoing infragravity waves.Hydraulic EngineeringCivil Engineering and Geoscience
Cross-shore transformation of bound and free infragravity waves off the Dutch coast
No full textInfragravity (IG) waves are key drivers for coastal erosion and thus need to be properly included in process-based modelling of coastal hazards. Uncertainties remain regarding the offshore boundary conditions for these long waves. Typically, only bound IG waves are included at the boundary, which means that the possible contribution of free IG waves, such as those radiated from distant coastlines, is neglected. Recent studies however suggest that incoming free IG waves could be significant, particularly in semi-enclosed basins such as the North Sea where they could contribute to coastal hazards (e.g., Reniers et al., 2021, Rijnsdorp et al. 2021). The objective of this work is to improve the understanding of the incoming IG wave field along the Dutch coast. We will quantify how bound and free IG waves develop in intermediate water depths and assess in which conditions (onshore directed) free IG waves become significant.Environmental Fluid MechanicsCoastal Engineerin
New exact coherent states in channel flow
No full textThree spatially periodic travelling wave exact coherent states are presented for channel flow. Two of the flows, which are asymmetric with respect to the channel centreplane, are derived by homotopy from solutions for channel flow subject to a spanwise rotation investigated by [1]. The third flow satisfies a half-turn rotational symmetry about a point on the channel centreplane, and turns out to be the flow from which one of the asymmetric flows bifurcates in a symmetry breaking bifurcation. One of the asymmetric flows is found to substantially reduce the value of the lowest Reynolds number at which exact solutions are known to exist down to 665
Free Infragravity Waves in the North Sea
No full textInfragravity waves are low-frequency surface waves that can impact a variety of nearshore and oceanic processes. Recent measurements in the North Sea showed that significant bursts of infragravity energy occurred during storm events. Using a spectral wave model, we show that a substantial part of this energy was radiated from distant shorelines where it was generated by the incident sea-swell waves. These radiated infragravity waves can cross the North Sea basin and reach distant shorelines. The origin of the infragravity wave energy varied between the different storms, and particularly depends on where largest sea-swell waves made landfall. Along the coastlines of the North Sea, shoreward directed infragravity waves that originate from a remote source were non-negligible during storm events. This suggests that radiated infragravity waves can potentially contribute to coastal dynamics and hazards away from their region of generation.Environmental Fluid Mechanic
Reconstruction of Directional Spectra of Infragravity Waves
No full textUnderstanding directional spectra of infragravity (IG) waves composed of free and bound components is required due to their impacts on various coastal processes (e.g., coastal inundation and morphological change). However, conventional reconstruction methods of directional spectra relying on linear wave theory are not applicable to IG waves in intermediate water depths (20–30 m) due to the presence of bound waves. Herein, a novel method is proposed to reconstruct directional spectra of IG waves in intermediate depth based on weakly nonlinear wave theory. This method corrects cross-spectra among observed wave signals by taking account of the nonlinearity of bound waves in order to reconstruct directional spectra of free IG waves. Numerical experiments using synthetic data representing various directional distributions show that the proposed method reconstructs free IG wave directional spectra more accurately than the conventional method. The method is subsequently applied to observations of severe sea-states at two field sites. At these sites, free IG waves are not isotropic and have clear peak directions. Numerical modeling of the wave fields shows that these peak directions correspond to the reflection of IG waves from the shore and/or coastal structures. Additionally, the validity of the underlying weakly nonlinear wave theory of the present method is assessed by a newly proposed method employing bispectral analysis. The bound wave response generally agrees with the theory at the field sites but deviates slightly for energetic sea states. The applicability of the present method on a sloping bottom is further discussed by an analytical solution.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Environmental Fluid Mechanic
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