TUHH Open Research (TORE)https://tore.tuhh.deTORE captures, stores, indexes, preserves, and distributes digital research material.Mon, 29 May 2023 22:47:50 GMT2023-05-29T22:47:50Z50131Study on the behavior of weakly nonlinear water waves in the presence of random wind forcinghttp://hdl.handle.net/11420/4268Title: Study on the behavior of weakly nonlinear water waves in the presence of random wind forcing
Authors: Dostal, Leo; Hollm, Marten; Kreuzer, Edwin
Abstract: Specific solutions of the nonlinear Schrödinger equation, such as the Peregrine breather, are considered to be prototypes of extreme or freak waves in the oceans. An important question is whether these solutions also exist in the presence of gusty wind. Using the method of multiple scales, a nonlinear Schrödinger equation is obtained for the case of wind-forced weakly nonlinear deep water waves. Thereby, the wind forcing is modeled as a stochastic process. This leads to a stochastic nonlinear Schrödinger equation, which is calculated for different wind regimes. For the case of wind forcing which is either random in time or random in space, it is shown that breather-type solutions such as the Peregrine breather occur even in strong gusty wind conditions.
Sat, 01 Feb 2020 00:00:00 GMThttp://hdl.handle.net/11420/42682020-02-01T00:00:00ZStudy of weakly nonlinear water waves subjected to stochastic wind excitationhttp://hdl.handle.net/11420/5416Title: Study of weakly nonlinear water waves subjected to stochastic wind excitation
Authors: Hollm, Marten; Dostal, Leo
Abstract: The behavior of deep-water gravity waves under the effect of wind and viscosity is studied. It is analyzed, how random wind affects the Peregrine breather, which is considered as a possible prototype of extreme waves in the oceans. Using the Euler equations of fluid dynamics and the method of multiple scales, the nonlinear Schrödinger equation and the modified nonlinear Schrödinger equation are obtained for the case of nonlinear deep water waves forced by random wind. For modeling the wind forcing of ocean waves, Miles’ theory is extended to include time varying random wind velocity processes, which leads to stochastic partial differential equations. For different regimes of stochastic wind it is shown that perturbed versions of the Peregrine solution occur also under strong gusty wind conditions and lead to extreme waves as well. Moreover, the improved modeling using the modified nonlinear Schrödinger equation does not considerably change this behavior.
Sun, 01 Sep 2019 00:00:00 GMThttp://hdl.handle.net/11420/54162019-09-01T00:00:00ZHydrodynamic Forces Acting on Cylindrical Piles Subjected to Wind-Forced Random Nonlinear Water Waveshttp://hdl.handle.net/11420/12050Title: Hydrodynamic Forces Acting on Cylindrical Piles Subjected to Wind-Forced Random Nonlinear Water Waves
Authors: Hollm, Marten; Dostal, Leo; Seifried, Robert
Abstract: A novel approach for the computation of hydrodynamic forces due to random nonlinear water waves acting on a cylindrical pile is presented. This approach is based on paths of fluid particles underneath of solutions to the nonlinear Schrödinger equation (NLS). This is computationally very efficient compared to the determination of the corresponding solutions of the Euler equations or even of the Navier–Stokes equation.In this chapter, specific solutions of the NLS, such as solitons, are considered in the presence of gusty wind. Using a spectral scheme for the numerical computation, the corresponding velocity potential is obtained. Depending on the corresponding wave envelope, the wave kinematics, the particle trajectories, and the forces acting on a submerged cylindrical pile are computed. Thereby, the equation of Morison is used for the computation of the hydrodynamic forces. With this, also effects of random wind on particle trajectories and forces are studied in detail. With the presented new approach it is possible to determine very efficiently the loads on structures that are exhibited to complicated random nonlinear ocean waves.
Sat, 01 Jan 2022 00:00:00 GMThttp://hdl.handle.net/11420/120502022-01-01T00:00:00ZStudy on the interaction of nonlinear water waves considering random seashttp://hdl.handle.net/11420/10921Title: Study on the interaction of nonlinear water waves considering random seas
Authors: Hollm, Marten; Dostal, Leo; Fischer, Hendrik; Seifried, Robert
Abstract: The nonlinear Schrödinger equation plays an important role in wave theory, nonlinear optics and Bose-Einstein condensation. Depending on the background, different analytical solutions have been obtained. One of these solutions is the soliton solution. In the real ocean sea, interactions of different water waves can be observed at the surface. Therefore the question arises, how such nonlinear waves interact. Of particular interest is the interaction, also called collision, of solitons and solitary waves.
Using a spectral scheme for the numerical computation of solutions of the nonlinear Schrödinger equation, the nonlinear wave interaction for the case of soliton collision is studied. Thereby, the influence of an initial random wave is studied, which is generated using a Pierson-Moskowitz spectrum.
Mon, 25 Jan 2021 00:00:00 GMThttp://hdl.handle.net/11420/109212021-01-25T00:00:00ZInvestigation of the dynamics of a multibody wave energy converter excited by regular and irregular waveshttp://hdl.handle.net/11420/13680Title: Investigation of the dynamics of a multibody wave energy converter excited by regular and irregular waves
Authors: Hollm, Marten; Dostal, Leo; Höhne, Joshua; Yurchenko, Daniil; Seifried, Robert
Abstract: 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.
Thu, 01 Dec 2022 00:00:00 GMThttp://hdl.handle.net/11420/136802022-12-01T00:00:00ZLocalized stationary seismic waves predicted using a nonlinear gradient elasticity modelhttp://hdl.handle.net/11420/10967Title: Localized stationary seismic waves predicted using a nonlinear gradient elasticity model
Authors: Dostal, Leo; Hollm, Marten; Metrikine, Andrei; Tsouvalas, Apostolos; Dalen, Karel N. van
Sat, 01 Jan 2022 00:00:00 GMThttp://hdl.handle.net/11420/109672022-01-01T00:00:00ZPerformance increase of wave energy harvesting of a guided point absorberhttp://hdl.handle.net/11420/11805Title: Performance increase of wave energy harvesting of a guided point absorber
Authors: Hollm, Marten; Dostal, Leo; Yurchenko, Daniil; Seifried, Robert
Abstract: The dynamics of a novel wave energy converter based on a guided inclined point absorber are investigated. Thereby, it is studied through simulations and experiments whether different inclination angles of the guided point absorber lead to larger motion amplitudes and velocities in regular and irregular waves, from which energy can be harvested. For that, different simulations and experimental setups are analyzed in the presence of wave forcing. In the case of irregular waves a random non-white Gaussian stochastic process based on a sea spectrum is used. It is shown that the inclination angle has a significant influence on the energy harvesting output. Based on this insight, a simple control strategy is introduced in order to further increase the energy harvesting output.
Wed, 02 Mar 2022 00:00:00 GMThttp://hdl.handle.net/11420/118052022-03-02T00:00:00ZDesign and optimization of a wave energy converter for drifting sensor platforms in realistic ocean waveshttp://hdl.handle.net/11420/12846Title: Design and optimization of a wave energy converter for drifting sensor platforms in realistic ocean waves
Authors: Harms, Julius; Hollm, Marten; Dostal, Leo; Kern, Thorsten Alexander; Seifried, Robert
Abstract: 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.
Thu, 01 Sep 2022 00:00:00 GMThttp://hdl.handle.net/11420/128462022-09-01T00:00:00ZMultivariate simulation of offshore weather time series: a comparison between markov chain, autoregressive, and long short-term memory modelshttp://hdl.handle.net/11420/12929Title: Multivariate simulation of offshore weather time series: a comparison between markov chain, autoregressive, and long short-term memory models
Authors: Eberle, Sebastian; Cevasco, Debora; Schwarzkopf, Marie-Antoinette; Hollm, Marten; Seifried, Robert
Abstract: 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.
Thu, 16 Jun 2022 00:00:00 GMThttp://hdl.handle.net/11420/129292022-06-16T00:00:00ZA nonlinear gradient elasticity model for the prediction of seismic waveshttp://hdl.handle.net/11420/13744Title: A nonlinear gradient elasticity model for the prediction of seismic waves
Authors: Dostal, Leo; Hollm, Marten; Metrikine, Andrei; Fărăgău, Andrei; Dalen, Karel N. van
Abstract: We present a novel equation of motion for a nonlinear gradient elasticity model. Thereby, higher-order gradient terms are introduced to capture the effect of small-scale soil heterogeneity/micro-structure. Using a newly established finite difference scheme, corresponding solutions including stationary waves are determined. In comparison with a commonly used model for nonlinear seismic waves, which has leading derivatives of second order, the solutions of the novel equations are much smoother. This allows much more accurate numerical computations as well as more realistic predictions of the seismic waves.
Fri, 01 Jul 2022 00:00:00 GMThttp://hdl.handle.net/11420/137442022-07-01T00:00:00ZDynamical analysis of a multibody wave energy converter excited by random waveshttp://hdl.handle.net/11420/13742Title: Dynamical analysis of a multibody wave energy converter excited by random waves
Authors: Hollm, Marten; Dostal, Leo; Höhne, Joshua; Seifried, Robert
Abstract: 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.
Fri, 01 Jul 2022 00:00:00 GMThttp://hdl.handle.net/11420/137422022-07-01T00:00:00ZParticle paths and hydrodynamic forces of random wind forced nonlinear ocean waveshttp://hdl.handle.net/11420/14232Title: Particle paths and hydrodynamic forces of random wind forced nonlinear ocean waves
Authors: Hollm, Marten; Dostal, Leo; Seifried, Robert
Abstract: 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 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.
Thu, 01 Sep 2022 00:00:00 GMThttp://hdl.handle.net/11420/142322022-09-01T00:00:00ZFirst passage times for nonlinear ship dynamics using Gaussian random fields and effective waveshttp://hdl.handle.net/11420/15334Title: First passage times for nonlinear ship dynamics using Gaussian random fields and effective waves
Authors: Dostal, Leo; Hollm, Marten; Maki, Atsuo
Abstract: It is important to know the mean time until critical roll motion occurs in various operating and sea conditions, in order to determine and ensure the safety of ship designs and operating ships. Since typical ocean waves are irregular, the forcing and roll response of the ship is considered to be a stochastic process of colored noise type. However, the simulation of the corresponding first passage times is very time consuming. Therefore, an approach for the determination of mean first passage times of critical roll motion of ships is proposed in this paper which needs much less computation time. This approach is based on explicit formulas for the roll energy of the ship. These formulas are used to determine the mean first passage times based on integral expressions, which were previously obtained. The resulting integral expressions can be computed very fast using standard quadrature formulas. Moreover, the underlying model for ship dynamics is extended by introducing a new effective wave for short-crested sea states. This is an extension to the improved Grim's effective wave concept.
Tue, 01 Aug 2023 00:00:00 GMThttp://hdl.handle.net/11420/153342023-08-01T00:00:00Z