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  • 1.
    Andersen, Jacob
    et al.
    Aalborg University; Denmark.
    Eskilsson, Claes
    Aalborg University, Denmark.
    Detached-Eddy Simulation of Normal Flow past Flat Plates: The Influence from Corner Curvature2023Ingår i: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 33, nr 4, s. 359-366Artikel i tidskrift (Refereegranskat)
  • 2.
    Bosi, Umberto
    et al.
    INRIA Bordeaux Sud-Ouest, France.
    Engsig-Karup, Allan P.
    DTU Technical University of Denmark, Denmark.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Ricchiuto, Mario
    INRIA Bordeaux Sud-Ouest, France.
    A spectral/hp element depth-integrated model for nonlinear wave–body interaction2019Ingår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 348, s. 222-249Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a depth-integrated Boussinesq model for the efficient simulation of nonlinear wave–body interaction. The model exploits a ‘unified’ Boussinesq framework, i.e. the fluid under the body is also treated with the depth-integrated approach. The unified Boussinesq approach was initially proposed by Jiang (2001) and recently analyzed by Lannes (2017). The choice of Boussinesq-type equations removes the vertical dimension of the problem, resulting in a wave–body model with adequate precision for weakly nonlinear and dispersive waves expressed in horizontal dimensions only. The framework involves the coupling of two different domains with different flow characteristics. Inside each domain, the continuous spectral/hp element method is used to solve the appropriate flow model since it allows to achieve high-order, possibly exponential, convergence for non-breaking waves. Flux-based conditions for the domain coupling are used, following the recipes provided by the discontinuous Galerkin framework. The main contribution of this work is the inclusion of floating surface-piercing bodies in the conventional depth-integrated Boussinesq framework and the use of a spectral/hp element method for high-order accurate numerical discretization in space. The model is verified using manufactured solutions and validated against published results for wave–body interaction. The model is shown to have excellent accuracy and is relevant for applications of waves interacting with wave energy devices.

  • 3.
    Bosi, Umberto
    et al.
    INRIA Bordeaux Sud-Ouest, France.
    Engsig-Karup, Allan P.
    DTU Technical University of Denmark, Denmark.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety.
    Ricchiuto, Mario
    INRIA Bordeaux Sud-Ouest, France.
    Solai, Elie
    Université de Toulon, France.
    A high-order spectral element unified Boussinesq model for floating point absorbers2018Ingår i: Coastal Engineering Proceedings, 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    Nonlinear wave-body problems are important in renewable energy, especially in case of wave energy converters operating in the near-shore region. In this paper we simulate nonlinear interaction between waves and truncated bodiesusing an efficient spectral/hp element depth-integrated unified Boussinesq model. The unified Boussinesq model treatsalso the fluid below the body in a depth-integrated approach. We illustrate the versatility of the model by predictingthe reflection and transmission of solitary waves passing truncated bodies. We also use the model to simulate themotion of a latched heaving box. In both cases the unified Boussinesq model show acceptable agreement with CFDresults – if applied within the underlying assumptions of dispersion and nonlinearity – but with a significant reductionin computational effort.

  • 4.
    Engsig-Karup, Allan
    et al.
    DTU Technical University of Denmark, Denmark.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety.
    Spectral element FNPF simulation of focused wave groups impacting a fixed FPSO-type body2019Ingår i: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 29, nr 2, s. 141-148Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A 3D fully nonlinear potential flow (FNPF) model based on an Eulerian formulation is presented. The model is discretizedusing high-order prismatic – possibly curvilinear – elements using a spectral element method (SEM) that has support foradaptive unstructured meshes. The paper presents details of the FNPF-SEM development, and a model is illustrated toexhibit exponential convergence for steep stream function waves to serve as validation. The model is then applied to the caseof focused waves impacting on a surface-piercing, fixed FPSO-like structure. Good agreement is found between numericaland experimental wave elevations and pressures.

  • 5.
    Engsig-Karup, Allan P.
    et al.
    DTU Technical University of Denmark, Denmark.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Spectral element FNPF simulations of focused wave groups impacting a fixed FPSO2018Ingår i: Proceedings of the Twenty-eighth (2018) International Ocean and Polar Engineering Conference, The International Society of Offshore and Polar Engineering , 2018, s. 1443-1450Konferensbidrag (Refereegranskat)
    Abstract [en]

    For  the assessment of experimental measurements of focused wave groups impacting a surface-piecing fixed structure, we present a new Fully Nonlinear Potential Flow (FNPF) model for simulation of unsteady water waves. The FNPF model is discretized in three  spatial dimensions (3D) using high-order prismatic - possibly curvilinear - elements using a  spectral  element  method (SEM) that has support for adaptive unstructured meshes. This  SEM-FNPF model is based on an Eulerian formulation and deviates from past works in that a  direct discretization of the Laplace problem is used making it straightforward to handle  accurately floating structural bodies of arbitrary shape. Our objectives are; i) present detail of a new SEM modelling developments and ii) to consider its application to address a wave-body interaction problem for nonlinear design waves and their interaction with a model-scale fixed Floating Production, Storage and Offloading vessel (FPSO).  We first reproduce  experimental measurements for focused design waves that represent a probably extreme  wave event for a sea state represented by a wave spectrum and seek to reproduce these measurements in a numerical wave tank. The validated input signal based on measurements is then generated in a NWT setup that includes the FPSO and differences in the signal caused by nonlinear diffraction is reported.

  • 6.
    Engsig-Karup, Allan P
    et al.
    DTU Technical University of Denmark, Denmark.
    Monteserin, Carlos
    DTU Technical University of Denmark, Denmark.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Safety. RISE Research Institutes of Sweden, Säkerhet och transport, Elektrifiering och pålitlighet. Aalborg University, Denmark.
    A mixed Eulerian-Lagrangian spectral element method for nonlinear wave interaction with fixed structures2019Ingår i: Water Waves, ISSN 2523-367X, Vol. 1, s. 315-342Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a high-order nodal spectral element method for the two-dimensional simulationof nonlinearwaterwaves. The model is based on themixed Eulerian–Lagrangian(MEL) method. Wave interaction with fixed truncated structures is handled usingunstructured meshes consisting of high-order iso-parametric quadrilateral/triangularelements to represent the body surfaces as well as the free surface elevation. A numericaleigenvalue analysis highlights that using a thin top layer of quadrilateral elementscircumvents the general instability problem associated with the use of asymmetricmesh topology.We demonstrate howto obtain a robustMELscheme for highly nonlinearwavesusing an efficient combination of (i) global L2 projectionwithout quadratureerrors, (ii) mild modal filtering and (iii) a combination of local and global re-meshingtechniques. Numerical experiments for strongly nonlinear waves are presented. Theexperiments demonstrate that the spectral element model provides excellent accuracyin prediction of nonlinear and dispersive wave propagation. The model is also shownto accurately capture the interaction between solitary waves and fixed submerged andsurface-piercing bodies. The wave motion and the wave-induced loads compare wellto experimental and computational results from the literature.

  • 7.
    Eskilsson, Claes
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Elektrifiering och pålitlighet. Aalborg University, Denmark.
    Palm, Johannes
    Sigma Energy & Marine AB, Sweden.
    High-fidelity modelling of moored marine structures: multi-component simulations and fluid-mooring coupling2022Ingår i: Journal of Ocean Engineering and Marine Energy, ISSN 2198-6444, E-ISSN 2198-6452, Vol. 8, nr 4, s. 513-526Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-fidelity viscous computational fluid dynamics (CFD) models coupled to dynamic mooring models is becoming an established tool for marine wave-body-mooring (WBM) interaction problems. The CFD and the mooring solvers most often communicate by exchanging positions and mooring forces at the mooring fairleads. Mooring components such as submerged buoys and clump weights are usually not resolved in the CFD model, but are treated as Morison-type bodies. This paper presents two recent developments in high-fidelity WBM modelling: (i) a one-way fluid-mooring coupling that samples the CFD fluid kinematics to approximate drag and inertia forces in the mooring model; and (ii) support for inter-moored multibody simulations that can resolve fluid dynamics on a mooring component level. The developments are made in the high-order discontinuous Galerkin mooring solver MoodyCore, and in the two-phase incompressible Navier–Stokes finite volume solver OpenFOAM. The fluid-mooring coupling is verified with experimental tests of a mooring cable in steady current. It is also used to model the response of the slack-moored DeepCwind FOWT exposed to regular waves. Minor effects of fluid-mooring coupling were noted, as expected since this a mild wave case. The inter-mooring development is demonstrated on a point-absorbing WEC moored with a hybrid mooring system, fully resolved in CFD-MoodyCore. The WEC (including a quasi-linear PTO) and the submerged buoys are resolved in CFD, while the mooring dynamics include inter-mooring effects and the one-way sampling of the flow. The combined wave-body-mooring model is judged to be very complete and to cover most of the relevant effects for marine WBM problems. © 2022, The Author(s).

  • 8.
    Eskilsson, Claes
    et al.
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Palm, Johannes
    Chalmers University of Technology, Sweden.
    Simulations of floating wave energy devices using adaptive mesh refinement2019Ingår i: Advances in Renewable Energies Offshore / [ed] C. Guedes Soares, 2019, s. 431-438Konferensbidrag (Refereegranskat)
    Abstract [en]

    CFD simulations of floating wave energy converters are computationally very heavy. This paper deals with a straightforward attempt to cut down on the computational effort by using adaptive mesh refinement (AMR). We investigate the use of AMR for simulations involving floating bodies inside the open-source finite volume framework OpenFOAM. A simple error indicator based on the pressure jump over cell faces is used to drive the AMR. First the use of the error indicator is illustrated for propagation of a very steep stream function wave. Then the AMR technique is applied to two cases of floating bodies: (i) a floating box and (ii) a bottom reacting point-absorber. As expected the AMR significantly reduce the number of cells in the computational meshes and subsequently lower the  computational effort.

  • 9.
    Eskilsson, Claes
    et al.
    Aalborg University, Denmark.
    Palm, Johannes
    Chalmers University of Technology, Sweden.
    Bergdahl, Lars
    Chalmers University of Technology, Sweden.
    Simulations of Moored Wave Energy Converters Using OpenFOAM: Implementation and Applications2018Ingår i: Proceedings of 6th World Maritime Technology Conference, 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this paper we present and discuss the use of CFD for coupled mooring analysis of floating wave energy converters. We use the two-phase Navier-Stokes finite volume solver in OpenFOAM and a high-order finite element model for the cable dynamics. The implementation of the coupling is described in some detail and we show validation of the scheme against laboratory data. A comparison between RANS and Euler simulations isolates effects of viscosity and geometric scale for moored point-absorbers, and parametric pitch excitation is demonstrated. The inclusion of power take-off/control follow the same blueprint as the mooring restraint and we illustrate the use of phase control.

  • 10.
    Eskilsson, Claes
    et al.
    Aalborg University, Denmark.
    Palm, Johannes
    Sigma Energy & Marine AB, Sweden.
    Johannesson, Pär
    RISE Research Institutes of Sweden, Material och produktion, Kemi och Tillämpad mekanik.
    Moura Paredes, Guilherme
    Universidade Lusófona do Porto, Portugal.
    Sensitivity analysis of extreme loads acting on a point-absorbing wave energy converter2022Ingår i: International Marine Energy Journal, ISSN 2631-5548, Vol. 5, nr 1, s. 91-101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    There are many uncertainties associated with the estimation of extreme loads acting on a wave energy converter (WEC). In this study we perform a sensitivity analysis of extreme loads acting on the Uppsala University (UU) WEC concept. The UU WEC consists of a bottom-mounted linear generator that is connected to a surface buoy with a taut mooring line. The maximum stroke length of the linear generator is enforced by end-stop springs. Initially, a Variation Mode and Effect Analysis (VMEA) was carried out in order to identify the largest input uncertainties. The system was then modelled in the time-domain solver WEC-SIM coupled to the dynamic mooring solver Moody. A sensitivity analysis was made by generating a surrogate model based on polynomial chaos expansions, which rapidly evaluates the maximum loads on the mooring line and the end-stops. The sensitivities are ranked using the Sobol index method. We investigated two sea states using equivalent regular waves (ERW) and irregular wave (IRW) trains. We found that the ERW approach significantly underestimate the maximum loads. Interestingly, the ERW predicted wave height and period as the most important parameters for the maximum mooring tension, whereas the tension in IRW was most sensitive to the drag coefficient of the surface buoy. The end-stop loads were most sensitive to the PTO damping coefficient.

  • 11.
    Eskilsson, Claes
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning. Aalborg University, Denmark.
    Palm, Johannes
    Sigma Energy & Marine AB, Sweden.
    Johannesson, Pär
    RISE Research Institutes of Sweden, Material och produktion, Tillämpad mekanik.
    Moura Paredes, Guilherme
    Universidade Lus´ofona do Porto, Portugal.
    Sensitivity Analysis of Extreme Loads Actingon a Point-Absorbing Wave Energy Converter2021Ingår i: Proceedings of the 14th European Wave and Tidal Energy Conference 5-9th Sept 2021, Plymouth, UK, 2021, s. 1992-1-1992-10Konferensbidrag (Refereegranskat)
    Abstract [en]

    There are many uncertainties associated with the estimation of extreme loads acting on a wave energy converter (WEC). In this study we perform a sensitivity analysis of extreme loads acting on the Uppsala University (UU) WEC concept. The UU WEC consists of a bottommounted linear generator which is connected to a surface buoy with a taut mooring line. The maximum stroke length of the linear generator is enforced by end-stops. Initially, a Variation Mode and Effect Analysis (VMEA) was carried out in order to identify the largest input uncertainties. The system was then modelled in the time-domain solver WECSIM coupled to the dynamic mooring solver Moody. A sensitivity analysis was made by generating a surrogate model based on polynomial chaos expansions, which rapidly evaluates the maximum loads on the mooring line and the endstops. The sensitivities are ranked using the Sobol index method. We investigated two sea states using equivalent regular waves (ERW) and irregular wave (IR) trains. We found that the ERW approach significantly underestimate the maximum loads. Interestingly, the ERW predicted wave height and period as the most important parameters for the maximum mooring tension, while the tension in IR waves was most sensitive to the drag coefficient of the surface buoy. The end-stop loads were most sensitive to the PTO damping coefficient.

  • 12.
    Eskilsson, Claes
    et al.
    Dep. of the Built Environment, Aalborg University.
    Pashami, Sepideh
    RISE Research Institutes of Sweden, Digitala system, Datavetenskap.
    Holst, Anders
    RISE Research Institutes of Sweden, Digitala system, Datavetenskap.
    Palm, Johannes
    Sigma Energy & Marine, Sweden.
    A hybrid linear potential flow - machine learning model for enhanced prediction of WEC performance2023Ingår i: Proceedings of the 15th European Wave and Tidal Energy Conference, 2023Konferensbidrag (Refereegranskat)
    Abstract [en]

    Linear potential flow (LPF) models remain the tools-of-the trade in marine and ocean engineering despite their well-known assumptions of small amplitude waves and motions. As of now, nonlinear simulation tools are still too computationally demanding to be used in the entire design loop, especially when it comes to the evaluation of numerous irregular sea states. In this paper we aim to enhance the performance of the LPF models by introducing a hybrid LPF-ML (machine learning) approach, based on identification of nonlinear force corrections. The corrections are defined as the difference in hydrodynamic force (vis- cous and pressure-based) between high-fidelity CFD and LPF models. Using prescribed chirp motions with different amplitudes, we train a long short-term memory (LSTM) network to predict the corrections. The LSTM network is then linked to the MoodyMarine LPF model to provide the nonlinear correction force at every time-step, based on the dynamic state of the body and the corresponding forces from the LPF model. The method is illustrated for the case of a heaving sphere in decay, regular and irregular waves – including passive control. The hybrid LPF-model is shown to give significant improvements compared to the baseline LPF model, even though the training is quite generic.

  • 13.
    Eskilsson, Claes
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Maritima avdelningen. Aalborg University, Denmark.
    Pashami, Sepideh
    RISE Research Institutes of Sweden, Digitala system, Datavetenskap.
    Holst, Anders
    RISE Research Institutes of Sweden, Digitala system, Datavetenskap.
    Palm, Johannes
    Sigma Energy & Marine, Sweden.
    Estimation of nonlinear forces acting on floating bodies using machine learning2023Ingår i: Advances in the Analysis and Design of Marine Structures / [ed] J. W. Ringsberg, C. Guedes Soares, CRC Press, 2023, s. 63-72Kapitel i bok, del av antologi (Övrigt vetenskapligt)
    Abstract [en]

    Numerical models used in the design of floating bodies routinely rely on linear hydrodynamics. Extensions for hydrodynamic nonlinearities can be approximated using e.g. Morison type drag and nonlinear Froude-Krylov forces. This paper aims to improve the approximation of nonlinear forces acting on floating bodies by using machine learning (ML). Many ML models are general function approximators and therefore suitable for representing such nonlinear correction terms. A hierarchical modelling approach is used to build mappings between higher-fidelity simulations and the linear method. The ML corrections are built up for FNPF, Euler and RANS simulations. Results for decay tests of a sphere in model scale using recurrent neural networks (RNN) are presented. The RNN algorithm is shown to satisfactory predict the correction terms if the most nonlinear case is used as training data. No difference in the performance of the RNN model is seen for the different hydrodynamic models.

  • 14.
    Eskilsson, Claes
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Maritima avdelningen.
    Pashami, Sepideh
    RISE Research Institutes of Sweden, Digitala system, Datavetenskap.
    Holst, Anders
    RISE Research Institutes of Sweden, Digitala system, Datavetenskap.
    Palm, Johannes
    Sigma Energy & Marine, Sweden.
    Hierarchical Approaches to Train Recurrent Neural Networks for Wave-Body Interaction Problems2023Ingår i: The Proceedings of the 33rd International Ocean and Polar Engineering Conference, 2023, Vol. 33, artikel-id 307Konferensbidrag (Refereegranskat)
    Abstract [en]

    We present a hybrid linear potential flow - machine learning (LPF-ML) model for simulating weakly nonlinear wave-body interaction problems. In this paper we focus on using hierarchical modelling for generating training data to be used with recurrent neural networks (RNNs) in order to derive nonlinear correction forces. Three different approaches are investigated: (i) a baseline method where data from a Reynolds averaged Navier Stokes (RANS) model is directly linked to data from a LPF model to generate nonlinear corrections; (ii) an approach in which we start from high-fidelity RANS simulations and build the nonlinear corrections by stepping down in the fidelity hierarchy; and (iii) a method starting from low-fidelity, successively moving up the fidelity staircase. The three approaches are evaluated for the simple test case of a heaving sphere. The results show that the baseline model performs best, as expected for this simple test case. Stepping up in the fidelity hierarchy very easily introduce errors that propagate through the hierarchical modelling via the correction forces. The baseline method was found to accurately predict the motion of the heaving sphere. The hierarchical approaches struggled with the task, with the approach that steps down in fidelity performing somewhat better of the two.

  • 15.
    Eskilsson, Claes
    et al.
    Aalborg University, Denmark.
    Shiri, Alex
    RISE Research Institutes of Sweden, Säkerhet och transport, Maritima avdelningen.
    Katsidoniotaki, Eirini
    Uppsala University, Sweden.
    Solution verification of WECs: comparison of methods to estimate numerical uncertainties in the OES wave energy modelling task2023Ingår i: Proceedings of the 15th European Wave and Tidal Energy Conference, 2023, artikel-id 426Konferensbidrag (Refereegranskat)
    Abstract [en]

    High-fidelity models become more and more used in the wave energy sector. They offer a fully nonlinear simulation tool that in theory should encompass all linear and nonlinear forces acting on a wave energy converter (WEC). The focus on the studies using are usually dealing with validation. However, a validated model does not necessarily give reliable solutions. Solution verification is the methodology to estimate the numerical uncertainties related to a simulation. In this work we test four different approaches: the classical grid convergence index (GCI); a least-square version (LS-GCI), a simplified version of the least-square method (SLS-GCI) and the ITTC rec- ommended practice. The LS-GCI requires four or more solutions whereas the other three methods only need three solutions. We apply these methods to four different high- fidelity models for the case of a heaving sphere. We tested two parameters in the time-domain and two parameters in the frequency domain. It was found that the GCI and ITTC were hard to use on the frequency domain parameters as they require monotonic convergence which sometimes does not happen due to the differences in the solutions being very small. The SLS-GCI performed almost as well as the SL-GCI method and will be further investigated.

  • 16.
    Eskilsson, Claes
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Maritima avdelningen. Aalborg University, Denmark.
    Verao Fernandez, Gael
    Aalborg University, Denmark.
    Andersen, Jacob
    Aalborg University, Denmark.
    Palm, Johannes
    Sigma Energy & Marine, Sweden.
    Hydrodynamic simulations of a FOWT platform (1st FOWT comparative study) using Openfoam coupled to MoodyCore2023Ingår i: Proceedings of the 33rd International Ocean and Polar Engineering Conference, 2023, Vol. 33, artikel-id 068Konferensbidrag (Refereegranskat)
    Abstract [en]

    We numerically simulate the hydrodynamic response of a floating offshore wind turbine (FOWT) using CFD. The FOWT under consideration is a slack-moored 1:70 scale model of the UMaine VolturnUS-S semi-submersible platform. This set-up has been experimentally tested in the COAST Laboratory Ocean Basin at the University of Plymouth, UK. The test cases under consideration are (i) static equilibrium load cases, (ii) free decay tests and (iii) two focused wave cases with different wave steepness. The FOWT is modelled using a two-phase Navier-Stokes solver inside the OpenFOAM-v2006 framework. The catenary mooring is computed by dynamically solving the equations of motion for an elastic cable using the MoodyCore solver. The results of the static and decay tests are compared to the experimental values with only minor differences in motions and mooring forces. The focused wave cases are also shown to be in good agreement with measurements. The use of a one-way fluid-mooring coupling results in slightly higher mooring forces, but does not influence the motion response of the FOWT significantly.

  • 17.
    Huffmeier, Johannes
    et al.
    RISE Research Institutes of Sweden. Minesto AB, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning.
    Co-Location of Wave and Wind Power: Results from Screening 226 Locations Worldwide2021Ingår i: Proceedings of the 14th European Wave and Tidal Energy Conference 5-9th Sept 2021, Plymouth, UK, 2021, s. 2307-1-2307-9Konferensbidrag (Refereegranskat)
    Abstract [en]

    The levelized costs of energy (LCoE) of wavepower is still not fully competitive with other sourcesof renewable energy. However, wave energy is partlyin a different phase than other renewable energy typesand could thus contribute to a better predictability andsmoothed power output. This work focuses on co-locationof wave and wind power by investigating the intermittencyof wind and waves power based on measured historicaldata from several hundreds of locations worldwide. Employingwind power curves and wave power matrices, thesites are evaluated based on several different metrics. Theresults indicate that there are several spots where wavepower has a much lower intermittency than wind powerproviding reliable energy supply. Best sites for co-locationin terms of energy yield was found in North-WesternEurope. However, both wind and wave production have thesame seasonal variability in these sites. Only a handful ofsites found in California showed the possibility of seasonalpower smoothing using the combination of wind and wave.

  • 18.
    Johannes, Palm
    et al.
    Sigma Energy & Marine, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Maritima avdelningen.
    Verification and validation of MoodyMarine: A free simulation tool for modelling moored MRE devices2023Ingår i: Proceedings of the 15th European Wave and Tidal Energy Conference, 2023, Vol. 15, artikel-id 317Konferensbidrag (Refereegranskat)
    Abstract [en]

    This work presents the verification and validation of the freely available simulation tool MoodyMarine, developed to help meet some of the demands for early stage development of MRE devices. MoodyMarine extends the previously released mooring module MoodyCore (Discontinuous Galerkin Finite Elements) with linear radiation-diffraction bodies, integrated pre-processing workflows and a graphical user interface. It is a C++ implementation of finite element mooring dynamics and Cummins equations for floating bodies with weak nonlinear corrections. A newly developed nonlinear Froude-Krylov implementation is verified in the paper, and MoodyMarine is compared to CFD simulations for two complex structures: a slack-moored floating offshore wind turbine and a self-reacting point-absorber with hybrid mooring. 

  • 19.
    Katsidoniotaki, Eirini
    et al.
    Uppsala University, Sweden; Centre of Natural Hazards and Disaster Science, Sweden.
    Shahroozi, Zahra
    Uppsala University, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Elektrifiering och pålitlighet. Aalborg University, Denmark.
    Palm, Johannes
    Sigma Energy & Marine AB, Sweden.
    Engström, Jens
    Uppsala University, Sweden.
    Göteman, Malin
    Uppsala University, Sweden; Centre of Natural Hazards and Disaster Science, Sweden.
    Validation of a CFD model for wave energy system dynamics in extreme waves2023Ingår i: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 268, artikel-id 113320Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The design of wave energy converters should rely on numerical models that are able to estimate accurately the dynamics and loads in extreme wave conditions. A high-fidelity CFD model of a 1:30 scale point-absorber is developed and validated on experimental data. This work constitutes beyond the state-of-the-art validation study as the system is subjected to 50-year return period waves. Additionally, a new methodology that addresses the well-known challenge in CFD codes of mesh deformation is successfully applied and validated. The CFD model is evaluated in different conditions: wave-only, free decay, and wave–structure interaction. The results show that the extreme waves and the experimental setup of the wave energy converter are simulated within an accuracy of 2%. The developed high-fidelity model is able to capture the motion of the system and the force in the mooring line under extreme waves with satisfactory accuracy. The deviation between the numerical and corresponding experimental RAOs is lower than 7% for waves with smaller steepness. In higher waves, the deviation increases up to 10% due to the inevitable wave reflections and complex dynamics. The pitch motion presents a larger deviation, however, the pitch is of secondary importance for a point-absorber wave energy converter. © 2022 The Author(s)

  • 20.
    Koukounas, Dimitrios
    et al.
    Chalmers University of Technology, Sweden.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Engsig-Karup, Allan P.
    DTU Technical University of Denmark, Denmark.
    Numerical simulations of Peregrine breathers using a spectral element model2018Ingår i: Proceedings of the 37th International Conference on Ocean, Offshore and Arctic EngineeringOMAE 2018June 17-22, 2018, Madrid, Spain, 2018, Vol. 11A, artikel-id OMAE2018-77648Konferensbidrag (Refereegranskat)
    Abstract [en]

    Breather solutions to the nonlinear Schrödinger equation have been put forward as a possible prototype for rouge waves and have been studied both experimentally and numerically. In the present study, we perform high resolution simulations of the evolution of Peregrine breathers in finite depth using a fully non- linear potential flow spectral element model. The spectral ele- ment model can accurately handle very steep waves as illustrated by modelling solitary waves up to limiting steepness. The an- alytic breather solution is introduced through relaxation zones. The numerical solution obtained by the spectral element model is shown to compare in large to the analytic solution as well as to CFD simulations of a Peregrine breather in finite depth pre- sented in literature. We present simulations of breathers over variable bathymetry and 3D simulations of a breather impinging on a mono-pile.

  • 21.
    Kramer, Morten
    et al.
    Aalborg University, Denmark; Floating Power Plant, Denmark.
    Andersen, Jacob
    Aalborg University, Denmark.
    Thomas, Sarah
    Floating Power Plant, Denmark.
    Buus Bendixen, Flemming
    Sintex, Denmark.
    Bingham, Harry
    DTU Technical University of Denmark, Denmark.
    Read, Robert
    DTU Technical University of Denmark, Denmark.
    Holk, Nikolaj
    Aalborg University, Denmark.
    Ransley, Edward
    University of Plymouth, UK.
    Brown, Scott
    University of Plymouth, UK.
    Yu, Y-H
    National Renewable Energy Laboratory, USA.
    Tran, Thanh Toan
    National Renewable Energy Laboratory, USA.
    Davidson, Josh
    Budapest University of Technology and Economics, Hungary.
    Horvath, Csaba
    Budapest University of Technology and Economics, Hungary.
    Janson, Carl-Erik
    Chalmers University of Technology, Sweden.
    Nielsen, Kim
    Ramboll Group A/S, Denmark; Aalborg University, Denmark.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning. Aalborg University, Denmark.
    Highly accurate experimental heave decay tests with a floating sphere: A public benchmark dataset for model validation of fluid–structure interaction2021Ingår i: Energies, E-ISSN 1996-1073, Vol. 14, nr 2, artikel-id 269Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Highly accurate and precise heave decay tests on a sphere with a diameter of 300 mm were completed in a meticulously designed test setup in the wave basin in the Ocean and Coastal Engineering Laboratory at Aalborg University, Denmark. The tests were dedicated to providing a rigorous benchmark dataset for numerical model validation. The sphere was ballasted to half submergence, thereby floating with the waterline at the equator when at rest in calm water. Heave decay tests were conducted, wherein the sphere was held stationary and dropped from three drop heights: a small drop height, which can be considered a linear case, a moderately nonlinear case, and a highly nonlinear case with a drop height from a position where the whole sphere was initially above the water. The precision of the heave decay time series was calculated from random and systematic standard uncertainties. At a 95% confidence level, uncertainties were found to be very low—on average only about 0.3% of the respective drop heights. Physical parameters of the test setup and associated uncertainties were quantified. A test case was formulated that closely represents the physical tests, enabling the reader to do his/her own numerical tests. The paper includes a comparison of the physical test results to the results from several independent numerical models based on linear potential flow, fully nonlinear potential flow, and the Reynolds-averaged Navier–Stokes (RANS) equations. A high correlation between physical and numerical test results is shown. The physical test results are very suitable for numerical model validation and are public as a benchmark dataset. © 2021 by the authors.

  • 22.
    Monteserin, Carlos
    et al.
    DTU Technical University of Denmark, Denmark.
    Engsig-Karup, Allan
    DTU Technical University of Denmark, Denmark.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Nonlinear wave-body interaction using a mixed-eulerian-lagrangian spectral element model2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    We present recent progress on the development of a newfully nonlinear potential flow (FNPF) model for estimation ofnonlinear wave-body interactions based on a stabilised unstructuredspectral element method (SEM). We introduce new proofof-concepts for forced nonlinear wave-body interaction in twospatial dimensions to establish the methodology in the SEM settingutilising dynamically adapted unstructured meshes. The numericalmethod behind the proposed methodology is describedin some detail and numerical experiments on the forced motionof (i) surface piercing and (ii) submerged bodies are presented.

  • 23.
    Palm, Johannes
    et al.
    Chalmers University of Technology, Sweden.
    Bergdahl, Lars
    Chalmers University of Technology, Sweden.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Parametric excitation of moored wave energy converters using viscous and non-viscous CFD simulations2019Ingår i: Advances in Renewable Energies Offshore / [ed] C. Guedes Soares, Taylor & Francis Group, 2019, s. 455-462Konferensbidrag (Refereegranskat)
    Abstract [en]

    The paper discusses the use of CFD simulations to analyse the parametric excitation of moored, full scale wave energy converters in six degrees of freedom. We present results of VOF- RANS and VOF-Euler simulations in Open FOAM® for two body shapes: (i) a truncated cylinder; and (ii) a cylinder with a smooth hemispherical bottom. Flow characteristics show large differences in smoothness of flow between the hull shapes, where the smoother shape results in a larger heave response. However the increased amplitude makes it unstable and parametric pitch excitation occurs with amplitudes up to 30°. The responses in surge, heave and pitch (including the transition to parametric motion) are found to be insensitive to the viscous effects. This is notable as the converters are working in resonance. The effect of viscous damping was visible in the roll motion, where the RANS simulations showed a smaller roll. However, the roll motion was found to be triggered not by wave-body interaction with the incident wave, but by reflections from the side walls. This highlights the importance of controlling the reflections in numerical wave tanks for simulations with WEC motion in six degrees of freedom.

  • 24.
    Palm, Johannes
    et al.
    Sigma Energy and Marine, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Elektrifiering och pålitlighet.
    Facilitating Large-Amplitude Motions of Wave Energy Converters in OpenFOAM by a modified Mesh Morphing Approach2022Ingår i: International Marine Energy Journal, Vol. 5, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    <p>High-fidelity simulations using computational fluid dynamics (CFD) for wave-body interaction are becoming increasingly common and important for wave energy converter (WEC) design. The open source finite volume toolbox OpenFOAM is one of the most frequently used platforms for wave energy. There are currently two ways to account for moving bodies in OpenFOAM: (i) mesh morphing, where the mesh deforms around the body; and (ii) an overset mesh method where a separate body mesh moves on top of a background mesh. Mesh morphing is computationally efficient but may introduce highly deformed cells for combinations of large translational and rotational motions. The overset method allows for arbitrarily large body motions and retains the quality of the mesh. However, it comes with a substantial increase in computational cost and possible loss of energy conservation due to the interpolation. In this paper we present a straightforward extension of the spherical linear interpolation (SLERP) based mesh morphing algorithm that increase the stability range of the method. The mesh deformation is allowed to be interpolated independently for different modes of motion, which facilitates tailored mesh motion simulations. The paper details the implementation of the method and evaluates its performance with computational examples of a cylinder with a moonpool. The examples show that the modified mesh morphing approach handles large motions well and provides a cost effective alternative to overset mesh for survival conditions.

  • 25.
    Palm, Johannes
    et al.
    Sigma Energy and Marine AB, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning. Aalborg University, Denmark.
    Facilitating Large-Amplitude Motions of WaveEnergy Converters in OpenFOAM by a Modified Mesh Morphing Approach2021Ingår i: Proceedings of the 14th European Wave and Tidal Energy Conference 5-9th Sept 2021, Plymouth, UK, 2021, s. 2107-1-2107-8Konferensbidrag (Refereegranskat)
    Abstract [en]

    High-fidelity simulations using computational fluid dynamics (CFD) for wave-body interaction are becoming increasingly common and important for wave energy converter (WEC) design. The open source finite volume toolbox OpenFOAM® is one of the most frequently used platforms for wave energy. There are currently two ways to account for moving bodies in OpenFOAM: (i) mesh morphing, where mesh deforms around the body; and (ii) an overset mesh method where a separate body mesh moves on top of a background mesh. Mesh morphing is computationally efficient but may introduce highly deformed cells for combinations of large translational and rotational motion. The overset method allows for arbitrarily large body motions (in certain conditions) and retains the quality of the mesh. However, it comes with a substantial increase in computational cost and possible loss of energy conservation due to the interpolation. In this paper we present a straightforward extension of the spherical linear interpolation (SLERP) based mesh morphing algorithm that increase the stability range of the method. The mesh deformation is allowed to be interpolated independently for different modes of motion, which facilitates tailored mesh motion simulations. The paper details the implementation of the method and evaluates its performance with computational examples of a cylinder with a moonpool. The examples show that the modified mesh morphing approach handles large motions well and provides a cost effective alternative to overset mesh for survival conditions.

  • 26.
    Palm, Johannes
    et al.
    Chalmers University of Technology, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning. Aalborg University, Denmark.
    Influence of bending stiffness on snap loads in marine cables: A study using a high-order discontinuous galerkin method2020Ingår i: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 8, nr 10, artikel-id 795Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Marine cables are primarily designed to support axial loads. The effect of bending stiffness on the cable response is therefore often neglected in numerical analysis. However, in low-tension applications such as umbilical modelling of ROVs or during slack events, the bending forces may affect the slack regime dynamics of the cable. In this paper, we present the implementation of bending stiffness as a rotation-free, nested local Discontinuous Galerkin (DG) method into an existing Lax–Friedrichs-type solver for cable dynamics based on an hp-adaptive DG method. Numerical verification shows exponential convergence of order P and P + 1 for odd and even polynomial orders, respectively. Validation of a swinging cable shows good comparison with experimental data, and the importance of bending stiffness is demonstrated. Snap load events in a deep water tether are compared with field-test data. The bending forces affect the low-tension response for shorter lengths of tether (200–500 m), which results in an increasing snap load magnitude for increasing bending stiffness. It is shown that the nested LDG method works well for computing bending effects in marine cables. © 2020 by the authors. 

  • 27.
    Palm, Johannes
    et al.
    Sigma Marine and Energy, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning. Aalborg University, Denmark.
    On End-Stops and Snap Loads for Taut-Moored Wave Energy Converters2021Ingår i: Proceedings of the 14th European Wave and Tidal Energy Conference 5-9th Sept 2021, Plymouth, UK, 2021, s. 1984-1-1984-10Konferensbidrag (Refereegranskat)
    Abstract [en]

    The power output from many wave energy converters (WECs) is limited by a finite stroke length in the power take-off (PTO) mechanism. As the PTO approaches its maximum stroke length, an end-stop system needs to be engaged to avoid damage to the machinery. Still the on-set of the end-stop is a nonlinear trigger force, a stiff point in the system. In this respect it is similar to how snap loads in the mooring cables affect the system after a period of cable slack. This paper presents a detailed study into the dynamics of end-stop events and snap loads for a WEC. The WEC is a bottom-mounted linear generator connected to a surface buoy via a steel wire. By comparing a linear spring model with three dynamic mooring line models we conclude that large differences are observed in the low-tension and slack regions of the cable during moderate wave loads, while minor differences are seen in the estimated peak tension. By further varying end-stop parameters we observe that the peak tension in the line changes mildly with the axial stiffness for moderate wave heights. The peak tension is surprisingly unaffected by the introduction of a critical damping level to the end-stop system, despite the significant increase in end-stop force which causes the translator to come to a sudden stop. We discuss how the connection between maximum line force and end-stop parameters is highly dependent on the buoy position in the wave at the instant of end-stop onset.

  • 28.
    Rashid, A
    et al.
    Ocean Harvesting Technologies AB, Sweden.
    Sidenmark, M
    Ocean Harvesting Technologies AB, Sweden.
    Eskilsson, Claes
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning.
    Wallentin, M
    Ocean Harvesting Technologies AB, Sweden.
    IWEC – Model Validation and CostOptimization of Infinity WEC Wave EnergyConverter2021Ingår i: Proceedings of the 14th European Wave and Tidal Energy Conference 5-9th Sept 2021, Plymouth, UK, 2021, s. 2192-1-2192-10Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper presents the methods developed and key findings of the IWEC project performed by Ocean Harvesting Technologies AB (OHT). It aimed to reduce the levelized cost of energy (LCoE) of OHT’s wave energy converter InfinityWEC, by analysing how different key parameters impact cost and annual output using a model of a 100-MW array installation. Component-level cost functions were developed and mapped to key parameters and constraints of the system. A large number of system configurations were then evaluated with a numerically efficient 3 degree-of-freedom (DoF) nonlinear radiationdiffraction model in WEC-Sim along with OHT’s sea statetuned polynomial reactive control (PRC). The most promising configurations were identified and investigated in more detail. The configuration with the best LCoE were finally identified and analysed further, including estimation of the effect of changing the PRC to model predictive control, which resulted in 17-34% higher annual output and 12-23% lower LCoE. The final LCoE was found to be 93 – 162 EUR / MWh at 100 MW installed capacity. An important finding from the study is that using simplified metrics such as CAPEX/ton was found to be irrelevant. Numerical wave tank testing, high-fidelity computational fluid dynamics (CFD), were used to tune the viscous drag of the 3 DoF WEC-Sim model. Applying verification and validation (V&V) techniques the CFD simulations showed a relatively large numerical uncertainty, but the average power and the motion responses were found to be sufficiently accurate.

  • 29.
    Sriram, V.
    et al.
    Indian Institute of Technology Madras, India.
    Xu, G.
    RISE Research Institutes of Sweden, Säkerhet och transport, Säkerhetsforskning.
    Li, G
    Ningbo University, China.
    A comparative study on the nonlinear interaction between a focusing wave and cylinder using state-of-the-art solvers: Part A2021Ingår i: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 31, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents ISOPE’s 2020 comparative study on the interaction between focused waves and a fixed cylinder. The paper discusses the qualitative and quantitative comparisons between 20 different numerical solvers from various universities across the world for a fixed cylinder. The moving cylinder cases are reported in a companion paper as part B (Agarwal, Saincher, et al., 2021). The numerical solvers presented in this paper are the recent state of the art in the field, mostly developed in-house by various academic institutes. The majority of the participants used hybrid modeling (i.e., a combination of potential flow and Navier–Stokes solvers). The qualitative comparisons based on the wave probe and pressure probe time histories and spectral components between laminar, turbulent, and potential flow solvers are presented in this paper. Furthermore, the quantitative error analyses based on the overall relative error in peak and phase shifts in the wave probe and pressure probe of all the 20 different solvers are reported. The quantitative errors with respect to different spectral component energy levels (i.e., in primary, sub-, and superharmonic regions) capturing capability are reported. Thus, the paper discusses the maximum, minimum, and median relative errors present in recent solvers as regards application to industrial problems rather than attempting to find the best solver. Furthermore, recommendations are drawn based on the analysis. 

  • 30.
    Wendt, F.
    et al.
    National Renewable Energy Laboratory, US.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Yasutaka, I.
    Saga University, Japan.
    Ocean energy systemswave energy modelling task: Modelling, verification and validation ofwave energy converters2019Ingår i: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 7, nr 11, artikel-id 379Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The International Energy Agency Technology Collaboration Programme for Ocean Energy Systems (OES) initiated the OES Wave Energy Conversion Modelling Task, which focused on the verification and validation of numerical models for simulating wave energy converters (WECs). The long-term goal is to assess the accuracy of and establish confidence in the use of numerical models used in design as well as power performance assessment of WECs. To establish this confidence, the authors used different existing computational modelling tools to simulate given tasks to identify uncertainties related to simulation methodologies: (i) linear potential flow methods; (ii) weakly nonlinear Froude-Krylov methods; and (iii) fully nonlinear methods (fully nonlinear potential flow and Navier-Stokes models). This article summarizes the code-to-code task and code-to-experiment task that have been performed so far in this project, with a focus on investigating the impact of different levels of nonlinearities in the numerical models. Two different WECs were studied and simulated. The first was a heaving semi-submerged sphere, where free-decay tests and both regular and irregular wave cases were investigated in a code-to-code comparison. The second case was a heaving float corresponding to a physical model tested in a wave tank. We considered radiation, diffraction, and regular wave cases and compared quantities, such as the WEC motion, power output and hydrodynamic loading.

  • 31.
    Xu, Hui
    et al.
    Imperial College London, UK.
    Cantwell, Chris D.
    Imperial College London, UK.
    Monteserin, Carlos
    DTU Technical University of Denmark, Denmark.
    Eskilsson, Claes
    RISE - Research Institutes of Sweden, Säkerhet och transport, Safety. Aalborg University, Denmark.
    Engsig-Karup, Allan P.
    DTU Technical University of Denmark, Denmark.
    Sherwin, Spencer J.
    Imperial College London, UK.
    Spectral/hp element methods: Recent developments, applications, and perspectives2018Ingår i: Journal of Hydrodynamics, ISSN 1001-6058, E-ISSN 1000-4874, Vol. 30, nr 1, s. 1-22Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The spectral/hp element method combines the geometric flexibility of the classical h-type finite element technique with the desirable numerical properties of spectral methods, employing high-degree piecewise polynomial basis functions on coarse finite element-type meshes. The spatial approximation is based upon orthogonal polynomials, such as Legendre or Chebychev polynomials, modified to accommodate a C0 - continuous expansion. Computationally and theoretically, by increasing the polynomial order p , high-precision solutions and fast convergence can be obtained and, in particular, under certain regularity assumptions an exponential reduction in approximation error between numerical and exact solutions can be achieved. This method has now been applied in many simulation studies of both fundamental and practical engineering flows. This paper briefly describes the formulation of the spectral/hp element method and provides an overview of its application to computational fluid dynamics. In particular, it focuses on the use of the spectral/hp element method in transitional flows and ocean engineering. Finally, some of the major challenges to be overcome in order´to use the spectral/hp element method in more complex science and engineering applications are discussed

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