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Publications (10 of 31) Show all publications
Eskilsson, C., Pashami, S., Holst, A. & Palm, J. (2023). A hybrid linear potential flow - machine learning model for enhanced prediction of WEC performance. In: Proceedings of the 15th European Wave and Tidal Energy Conference: . Paper presented at The 15th European Wave and Tidal Energy Conference.
Open this publication in new window or tab >>A hybrid linear potential flow - machine learning model for enhanced prediction of WEC performance
2023 (English)In: Proceedings of the 15th European Wave and Tidal Energy Conference, 2023Conference paper, Published paper (Refereed)
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.

Keywords
Linear potential flow, machine learning, recurrent neural network, floating bodies, wave energy
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-72107 (URN)10.36688/ewtec-2023-321 (DOI)
Conference
The 15th European Wave and Tidal Energy Conference
Funder
Swedish Energy Agency, 50196-1
Available from: 2024-03-02 Created: 2024-03-02 Last updated: 2024-03-08Bibliographically approved
Andersen, J. & Eskilsson, C. (2023). Detached-Eddy Simulation of Normal Flow past Flat Plates: The Influence from Corner Curvature. International Journal of Offshore and Polar Engineering, 33(4), 359-366
Open this publication in new window or tab >>Detached-Eddy Simulation of Normal Flow past Flat Plates: The Influence from Corner Curvature
2023 (English)In: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 33, no 4, p. 359-366Article in journal (Refereed) Published
Place, publisher, year, edition, pages
International Society of Offshore and Polar Engineers, 2023
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-72115 (URN)10.17736/ijope.2023.jc912 (DOI)
Available from: 2024-03-02 Created: 2024-03-02 Last updated: 2024-03-08Bibliographically approved
Eskilsson, C., Pashami, S., Holst, A. & Palm, J. (2023). Estimation of nonlinear forces acting on floating bodies using machine learning. In: J. W. Ringsberg, C. Guedes Soares (Ed.), Advances in the Analysis and Design of Marine Structures: (pp. 63-72). CRC Press
Open this publication in new window or tab >>Estimation of nonlinear forces acting on floating bodies using machine learning
2023 (English)In: Advances in the Analysis and Design of Marine Structures / [ed] J. W. Ringsberg, C. Guedes Soares, CRC Press, 2023, p. 63-72Chapter in book (Other academic)
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.

Place, publisher, year, edition, pages
CRC Press, 2023
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-72114 (URN)10.1201/9781003399759 (DOI)9781003399759 (ISBN)
Funder
Swedish Energy Agency, 50196-1
Available from: 2024-03-02 Created: 2024-03-02 Last updated: 2024-03-08Bibliographically approved
Eskilsson, C., Pashami, S., Holst, A. & Palm, J. (2023). Hierarchical Approaches to Train Recurrent Neural Networks for Wave-Body Interaction Problems. In: The Proceedings of the 33rd International Ocean and Polar Engineering Conference: . Paper presented at The 33rd International Ocean and Polar Engineering Conference. , 33, Article ID 307.
Open this publication in new window or tab >>Hierarchical Approaches to Train Recurrent Neural Networks for Wave-Body Interaction Problems
2023 (English)In: The Proceedings of the 33rd International Ocean and Polar Engineering Conference, 2023, Vol. 33, article id 307Conference paper, Published paper (Refereed)
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.

Keywords
Wave-body interaction; hierarchical modelling; linear potential flow; hybrid modeling; machine learning; recurrent neural net- work.
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-72110 (URN)
Conference
The 33rd International Ocean and Polar Engineering Conference
Funder
Swedish Energy Agency, 50196-1
Available from: 2024-03-02 Created: 2024-03-02 Last updated: 2024-03-08Bibliographically approved
Eskilsson, C., Verao Fernandez, G., Andersen, J. & Palm, J. (2023). Hydrodynamic simulations of a FOWT platform (1st FOWT comparative study) using Openfoam coupled to MoodyCore. In: Proceedings of the 33rd International Ocean and Polar Engineering Conference: . Paper presented at The 33rd International Ocean and Polar Engineering Conference. , 33, Article ID 068.
Open this publication in new window or tab >>Hydrodynamic simulations of a FOWT platform (1st FOWT comparative study) using Openfoam coupled to MoodyCore
2023 (English)In: Proceedings of the 33rd International Ocean and Polar Engineering Conference, 2023, Vol. 33, article id 068Conference paper, Published paper (Refereed)
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.

National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-72111 (URN)
Conference
The 33rd International Ocean and Polar Engineering Conference
Funder
Swedish Energy Agency, 44423-2
Available from: 2024-03-02 Created: 2024-03-02 Last updated: 2024-03-08Bibliographically approved
Eskilsson, C., Shiri, A. & Katsidoniotaki, E. (2023). Solution verification of WECs: comparison of methods to estimate numerical uncertainties in the OES wave energy modelling task. In: Proceedings of the 15th European Wave and Tidal Energy Conference: . Paper presented at The 15th European Wave and Tidal Energy Conference. , Article ID 426.
Open this publication in new window or tab >>Solution verification of WECs: comparison of methods to estimate numerical uncertainties in the OES wave energy modelling task
2023 (English)In: Proceedings of the 15th European Wave and Tidal Energy Conference, 2023, article id 426Conference paper, Published paper (Refereed)
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.

Keywords
CFD, numerical uncertainty, solution verification, wave energy converter
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-72109 (URN)10.36688/ewtec-2023-426 (DOI)
Conference
The 15th European Wave and Tidal Energy Conference
Funder
Swedish Energy Agency, 44423-2
Available from: 2024-03-02 Created: 2024-03-02 Last updated: 2024-03-08Bibliographically approved
Katsidoniotaki, E., Shahroozi, Z., Eskilsson, C., Palm, J., Engström, J. & Göteman, M. (2023). Validation of a CFD model for wave energy system dynamics in extreme waves. Ocean Engineering, 268, Article ID 113320.
Open this publication in new window or tab >>Validation of a CFD model for wave energy system dynamics in extreme waves
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2023 (English)In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 268, article id 113320Article in journal (Refereed) Published
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)

Place, publisher, year, edition, pages
Elsevier Ltd, 2023
Keywords
CFD, Extreme waves, OpenFOAM, Point-absorber, Validation model, Wave energy, Mooring, Mooring cables, CFD-model, Energy systems, Point absorber, System Dynamics, Wave conditions, Wave energy converters, Wave energy conversion
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-62361 (URN)10.1016/j.oceaneng.2022.113320 (DOI)2-s2.0-85144021092 (Scopus ID)
Note

Funding details: 47264-1; Funding details: Centrum för naturkatastrofslära, Uppsala Universitet, CNDS; Funding details: Vetenskapsrådet, VR, 2015-04657; Funding details: Alexander S. Onassis Public Benefit Foundation; Funding text 1: The research in this paper was supported by the Centre of Natural Hazards and Disaster Science , Sweden, the Swedish Research Council (VR, grant number 2015-04657) , the Swedish Energy Authority (project number 47264-1 ). This scientific paper was also supported by the Onassis Foundation , scholarship ID: F ZP 021-1/2019-2020 . The CFD simulations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the HPC cluster: Tetralith at the National Supercomputer Centre, Linköping University. All authors have read and agreed to the published version of the manuscript.

Available from: 2023-01-23 Created: 2023-01-23 Last updated: 2023-05-16Bibliographically approved
Johannes, P. & Eskilsson, C. (2023). Verification and validation of MoodyMarine: A free simulation tool for modelling moored MRE devices. In: Proceedings of the 15th European Wave and Tidal Energy Conference: . Paper presented at The 15th European Wave and Tidal Energy Conference. , 15, Article ID 317.
Open this publication in new window or tab >>Verification and validation of MoodyMarine: A free simulation tool for modelling moored MRE devices
2023 (English)In: Proceedings of the 15th European Wave and Tidal Energy Conference, 2023, Vol. 15, article id 317Conference paper, Published paper (Refereed)
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. 

Keywords
mooring dynamics, hydrodynamic modelling, free evaluation tool
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-72108 (URN)10.36688/ewtec-2023-317 (DOI)
Conference
The 15th European Wave and Tidal Energy Conference
Funder
Swedish Energy Agency, 50196-1
Available from: 2024-03-02 Created: 2024-03-02 Last updated: 2024-03-08Bibliographically approved
Palm, J. & Eskilsson, C. (2022). Facilitating Large-Amplitude Motions of Wave Energy Converters in OpenFOAM by a modified Mesh Morphing Approach. Paper presented at EWTEC 2021 Special issue papers (Part 3). International Marine Energy Journal, 5(3)
Open this publication in new window or tab >>Facilitating Large-Amplitude Motions of Wave Energy Converters in OpenFOAM by a modified Mesh Morphing Approach
2022 (English)In: International Marine Energy Journal, Vol. 5, no 3Article in journal (Refereed) Published
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.

National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-62526 (URN)10.36688/imej.5.257-264 (DOI)
Conference
EWTEC 2021 Special issue papers (Part 3)
Note

This work was supported by the Swedish Energy Agency underproject numbers 40428-1 and 47264-1

Available from: 2023-01-23 Created: 2023-01-23 Last updated: 2023-05-16Bibliographically approved
Eskilsson, C. & Palm, J. (2022). High-fidelity modelling of moored marine structures: multi-component simulations and fluid-mooring coupling. Journal of Ocean Engineering and Marine Energy, 8(4), 513-526
Open this publication in new window or tab >>High-fidelity modelling of moored marine structures: multi-component simulations and fluid-mooring coupling
2022 (English)In: Journal of Ocean Engineering and Marine Energy, ISSN 2198-6444, E-ISSN 2198-6452, Vol. 8, no 4, p. 513-526Article in journal (Refereed) Published
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).

Place, publisher, year, edition, pages
Springer Science and Business Media Deutschland GmbH, 2022
Keywords
Cable dynamics, CFD, MoodyCore, Mooring systems, OpenFOAM, Submerged buoys, Arctic engineering, Buoyancy, Buoys, Drag, Galerkin methods, Mooring, Mooring cables, Navier Stokes equations, Cable dynamic, Computational fluid dynamics modeling, High-fidelity, High-fidelity modeling, Mooring system, Multicomponent fluid, Multicomponent simulation, Submerged buoy, Computational fluid dynamics
National Category
Marine Engineering
Identifiers
urn:nbn:se:ri:diva-61202 (URN)10.1007/s40722-022-00263-w (DOI)2-s2.0-85139647369 (Scopus ID)
Note

Funding details: National Science Council, NSC; Funding details: Vetenskapsrådet, VR, 2018-05973; Funding details: Energimyndigheten, 50196-1; Funding text 1: This work was supported by the Swedish Energy Agency through Grant no 50196-1. Computational resources were provided by the Danish e-infrastructure Cooperation (DeiC) National HPC (g.a. DeiC-AAU-N5-202200002) and the Swedish National Infrastructure for Computing (SNIC) at NSC partially funded by the Swedish Research Council through Grant Agreement No. 2018-05973.

Available from: 2022-12-06 Created: 2022-12-06 Last updated: 2023-05-16Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-6934-634x

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