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Item type:Publication, Access status: with files , On the singularity and numerical instability of flow fields associated with the nonlinear Schrödinger equationThe nonlinear Schrödinger equation (NLS) is a well-known equation in the study of wave motion. In the context of nonlinear water waves, the NLS has been proven to accurately model deep-water waves with a narrow spectral bandwidth and moderate wave steepness. While many studies have focused on reconstructing free surface profiles using the NLS, the associated flow field within the fluid has received less attention. Recently, a paper was published in which the flow field associated with solutions of the NLS was constructed. Here, it has been noted that the associated modeled flow field can contain singularities below the water surface. This work analyzes the occurrence and location of such artificial singularities for different analytical NLS solutions. It is found that the flow field can generally become singular at any depth below the surface and may even appear periodically in water depth. Additionally, it is shown that the numerical computation of the flow field poses considerable challenges due to the instability of numerical schemes. These findings are crucial for the accurate analysis of flow fields corresponding to the NLS.Publicationtype: Journal Article53 13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A computational method for the fluid-structure interaction in nonlinear ocean waves using the nonlinear Schrödinger equationAn efficient computation of the fluid-structure interaction (FSI) between mechanical structures and ocean waves is an important topic for many applications in ocean engineering and naval architecture. For many of such applications, it is often sufficient to consider the FSI within the potential flow theory instead of using the Navier-Stokes equations. However, the numerical solution of the governing equations of the potential flow theory is still computationally expensive. This work contributes to the improvement of the currently available numerical schemes, which compute the FSI in nonlinear ocean waves. The approach used in this work is based on the nonlinear Schrödinger equation (NLS). Compared to the governing equations of potential flow theory, solutions of the NLS can be computed much more efficiently. This efficiency advantage is used to develop a method that allows a fast computation of the FSI in nonlinear water waves. The developed method is investigated with regard to its accuracy, application areas, and computational effort. Here, numerical results for the hydrodynamic forces acting on mechanical structures and the resulting motion of the structures are analyzed. It is found that the NLS offers an interesting possibility to efficiently investigate the behavior of structures excited by nonlinear ocean waves.Publicationtype: Journal Article80 30 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Bathymetry reconstruction from experimental data using PDE-constrained optimisation(2024-06-30); ; ; ; Knowledge of the bottom topography, also called bathymetry, of rivers, seas or the ocean is important for many areas of maritime science and civil engineering. While direct measurements are possible, they are time consuming, expensive and inaccurate. Therefore, many approaches have been proposed how to infer the bathymetry from measurements of surface waves. Mathematically, this is an inverse problem where an unknown system state needs to be reconstructed from observations with a suitable model for the flow as constraint. In many cases, the shallow water equations can be used to describe the flow. While theoretical studies of the efficacy of such a PDE-constrained optimisation approach for bathymetry reconstruction exist, there seem to be few publications that study its application to data obtained from real-world measurements. This paper shows that the approach can, at least qualitatively, reconstruct a Gaussian-shaped bathymetry in a wave flume from measurements of the free surface level at up to three points. Achieved normalised root mean square errors (NRMSE) are in line with other approaches.Publicationtype: Journal Article52 88 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Determination of particle paths and hydrodynamic forces of random wind forced nonlinear ocean waves(2024-05); ; ; The hydrodynamic forces of nonlinear deep water gravity waves acting on cylindrical offshore structures are studied. Thereby, the waves are excited by random wind and the corresponding effect on the particle paths and hydrodynamic forces is investigated. This is done for the Peregrine breather solution of the nonlinear Schrödinger equation, which is nowadays considered as a prototype of extreme waves in open seas. Using this theory, the loads on mechanical structures can be calculated efficiently. It is shown that the Peregrine breather can exist under strong and gusty wind conditions and the water particles experience a horizontal drift. This leads to a force with randomly increasing amplitude in time, whereby a mean wind velocity of 50 km/h results in an increase of about 3%. The increase of hydrodynamic forces caused by the wind should therefore be considered for the construction of mechanical structures operating in the ocean.Publicationtype: Journal Article104 20 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Hydrodynamic forces acting on mechanical systems in linear and nonlinear ocean wavesThe hydrodynamic forces acting on prescribed free‐floating mechanical systems are studied. Regular and irregular sea states are considered using Stokes wave theory of first and second order. It is analyzed how much the magnitude and temporal behavior of the forces can differ using nonlinear theories of different orders instead of the linear wave theory. The corresponding effects on the mechanical dynamics of the system are investigated. Therefore, the fluid–structure interaction and hydrodynamic forces are computed, solving the corresponding partial differential equations of the linear and nonlinear wave theory. In this way, the nonlinear interactions of the different wave components in irregular seas and the disturbances of the sea due to the presence of the mechanical system are captured.Publicationtype: Conference Paper65 76 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Investigation of the dynamics of a multibody wave energy converter excited by regular and irregular waves(Elsevier, 2022-12-01); ; ; ; The dynamics and the performance of a novel multibody wave energy converter is investigated, which is based on inclined single modules connected to a frame. The frame floats on the sea surface and the modules each move translationally along inclined guidance rods. Direct-drive linear generators or rotation based generators convert the relative translational motion between the frame and the modules into electrical power. This paper studies the conditions which influence the performance of the converters in regular and irregular waves. Different design layouts are investigated numerically, whereby the wave excitation is modeled by a random non-white Gaussian stochastic process.Publicationtype: Journal ArticleScopus© Citations 8 202 101 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Particle paths and hydrodynamic forces of random wind forced nonlinear ocean wavesThe hydrodynamic forces of nonlinear deep water gravity waves acting on cylindrical offshore structures are studied. Thereby, the waves are excited by random wind and the corresponding effect on the particle paths and hydrodynamic forces is investigated. This is done for the Peregrine breather solution of the nonlinear Schrödinger equation, which is nowadays considered as a prototype of extreme waves in open seas. Using this theory, the loads on mechanical structures can be calculated efficiently. It is shown that the Peregrine breather can exist under strong and gusty wind conditions and the water particles experience a horizontal drift. This leads to a force with randomly increasing amplitude in time, whereby a mean wind velocity of 50km/h results in an increase of about 3%. The increase of hydrodynamic forces caused by the wind should therefore be considered for the construction of mechanical structures operating in the ocean.Publicationtype: Conference Paper72 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Design and optimization of a wave energy converter for drifting sensor platforms in realistic ocean waves(Elsevier, 2022-09-01); ; ; ; One of the biggest challenges in converting wave energy is to enable the use of low frequency waves, since the highest waves in typical sea states have low frequencies, as can be seen from the corresponding wave spectra, such as the Pierson–Moskowitz or JONSWAP spectra. In this paper, we show that this challenge is indeed achievable for the operation of small autonomous drifting sensor platforms. We present the design and optimization of a compact wave energy converter that freely floats in random sea waves. An optimization of the dynamical behavior as well as the electromagnetic power take-off is conducted based on simulations and experiments. The platform has compact dimensions of 50 cm draft and 50 cm diameter, which leads to special requirements for size and appearance. To meet these requirements, a two-body self-reacting point absorber is designed and a flux switching permanent magnet linear machine is developed for the power take-off. The developed system is validated by experiments in a wave flume and the linear generator is analyzed on a test bench. A coupled model is used to simulate and optimize the corresponding mechanical system, which leads to an increased output power from below 10 mW for the simulated initial setup to a power output of more than 100 mW in the simulation. Simulations and experiments are performed for regular and random waves in order to provide realistic approximations of the total output power.Publicationtype: Journal ArticleScopus© Citations 13 175 813 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Dynamical analysis of a multibody wave energy converter excited by random waves(2022-07); ; ; The dynamics of a novel multibody wave energy converter based on inclined single modules connected to a frame are investigated, on which generators convert the corresponding relative motion into electrical power. Thereby, it is studied under which conditions the inclined individual modules perform the largest relative motions in regular and irregular waves. For this, different setups are analyzed in the presence of wave excitations, which is generated by a random non-white Gaussian stochastic process.Publicationtype: Conference Paper119 162 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Multivariate simulation of offshore weather time series: a comparison between markov chain, autoregressive, and long short-term memory models(Multidisciplinary Digital Publishing Institute (MDPI), 2022-06-16); ; ; ; In the estimation of future investments in the offshore wind industry, the operation and maintenance (O&M) phase plays an important role. In the simulation of the O&M figures, the weather conditions should contain information about the waves’ main characteristics and the wind speed. As these parameters are correlated, they were simulated by using a multivariate approach, and thus by generating vectors of measurements. Four different stochastic weather time series generators were investigated: Markov chains (MC) of first and second order, vector autoregressive (VAR) models, and long short-term memory (LSTM) neural networks. The models were trained on a 40-year data set with 1 h resolution. Thereafter, the models simulated 25-year time series, which were analysed based on several time series metrics and criteria. The MC (especially the one of second order) and the VAR model were shown to be the ones capturing the characteristics of the original time series the best. The novelty of this paper lies in the application of LSTM models and multivariate higher-order MCs to generate offshore weather time series, and to compare their simulations to the ones of VAR models. Final recommendations for improving these models are provided as conclusion of this paper.Publicationtype: Journal Article128 202