Claes Eskilsson

Claes Eskilsson
Aalborg University · Department of Civil Engineering

PhD, Docent

About

98
Publications
45,890
Reads
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1,578
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Introduction
Claes Eskilsson works at the Department of the Built Environment, Aalborg University (Denmark) and at the Renewable Energy unit, RISE (Research Institutes of Sweden). Claes does research in numerical models - focusing on high-order finite element methods - for Ocean Engineering, Naval Engineering and Civil Engineering. Applications are predominately for marine renewables.
Additional affiliations
July 2017 - present
RISE Research Institutes of Sweden
Position
  • Senior Researcher
June 2017 - present
Aalborg University
Position
  • Professor (Associate)
April 2009 - May 2017
Chalmers University of Technology
Position
  • Senior Researcher
Education
April 1999 - February 2005
Chalmers University of Technology
Field of study
  • Engineering Hydraulics

Publications

Publications (98)
Conference Paper
The levelized costs of energy (LCoE) of wave power is still not fully competitive with other sources of renewable energy. However, wave energy is partly in a different phase than other renewable energy types and could thus contribute to a better predictability and smoothed power output. This work focus on co-location of wave and wind power by inves...
Conference Paper
Full-text available
The power output from many wave energy converters (WECs) is limited by a finite stroke length in the power takeoff (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 onset of the end-stop is a nonlinear trigger force, a stiff point in the system. In th...
Conference Paper
Full-text available
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 i...
Conference Paper
Full-text available
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 which is connected to a surface buoy with a taut mooring li...
Conference Paper
Full-text available
We present a Spectral Element Fully Nonlinear Potential Flow (FNPF-SEM) model developed for the simulation of wave-body interactions between nonlinear free surface waves and impermeable structures. The solver is accelerated using an iterative p-multigrid algorithm. Two cases are considered: (i) a surface piercing box forced into vertical motion cre...
Article
Full-text available
Parametric resonance is a non-linear phenomenon in which a system can oscillate at a frequency different from its exciting frequency. Some wave energy converters are prone to this phenomenon, which is usually detrimental to their performance. Here, a computationally efficient way of simulating parametric resonance in point absorbers is presented. T...
Article
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...
Article
Full-text available
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 b...
Article
Full-text available
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, w...
Article
Full-text available
Mooring systems often make use of submerged buoys (SBs) in order to make the moorings compliant. In this paper we present the dynamic effects of changing the buoy geometry or the buoy model fidelity on the mooring system response. Three cylindrical SBs with increasing slenderness (height/diameter) are studied for a mooring leg with two polyester ro...
Presentation
Full-text available
The nonlinear interaction between waves and floating wave energy converters (WECs) has become in the recent years one of the biggest challenges for wave power industry. Large displacements of real structures demand accurate and robust numerical tools to solve the problems more efficiently. However, all this nonlinear interactions between the floati...
Article
Full-text available
Mooring systems exhibit high failure rates. This is especially problematic for offshore renewable energy systems, like wave and floating wind, where the mooring system can be an active component and the redundancy in the design must be kept low. Here we investigate how uncertainty in input parameters propagates through the mooring system and affect...
Article
Full-text available
We present a new stabilised and efficient high-order nodal spectral element method based on the Mixed Eulerian Lagrangian (MEL) method for general-purpose simulation of fully nonlinear water waves and wave-body interactions. In this MEL formulation a standard Laplace formulation is used to handle arbitrary body shapes using unstructured – possibly...
Article
Full-text available
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...
Conference Paper
Full-text available
Mooring systems for floating wave energy converters often rely on floaters to allow for minimum restraints of the body motion in heavy. However, the inclusion of floaters also introduce possible slack-taut scenarios induced by the dynamic response of the floater in relation to the fair-lead point of the mooring. This can increase the occurrence of...
Article
Full-text available
POST-CONFERENCE ISOPE 2018 CONTRIBUTION: A 3D fully nonlinear potential flow (FNPF) model based on an Eulerian formulation is presented. The model is discretized using high-order prismatic - possibly curvi-linear - elements using a spectral element method (SEM) that has support for adaptive unstructured meshes. The paper presents details of the FNP...
Article
Full-text available
Results from Blind Test Series 1, part of the Collaborative Computational Project in Wave Structure Interaction (CCP-WSI), are presented. Participants, with a range of numerical methods, simulate blindly the interaction between a fixed structure and focused waves ranging in steepness and direction. Numerical results are compared against correspondi...
Conference Paper
Full-text available
Since time-domain simulations of wave energy converters are computationally expensive , how can we analyse their dynamics and test wide ranges of design variables, without simplifying the physics involved? One possible solution is the use of General Polynomial Chaos (gPC). GPC provides computationally efficient surrogate models for partial differen...
Article
Full-text available
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 [26] and recently analysed by Lannes [2...
Presentation
We present a fully nonlinear potential flow (FNPF) model for simulation of wave-body interaction in three spatial dimensions (3D) and apply it to the case of an axi-symmetric point absorber. The FNPF model is discretized is space by a C 0 spectral element method (SEM) using high-order prismatic-possibly curvilinear-elements. This SEM-FNPF model is...
Preprint
Results from Blind Test Series 1, part of the Collaborative Computational Project in Wave Structure Interaction (CCP-WSI), are presented. Participants , with a range of numerical methods, simulate blindly the interaction between a fixed structure and focused waves ranging in steepness and direction. Numerical results are compared against correspond...
Conference Paper
Full-text available
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 bodies using an efficient spectral/hp element depth-integrated unified Boussinesq model. The unified Boussinesq model treats also...
Chapter
Full-text available
A coupling between a dynamic mooring solver based on high-order finite element techniques (MooDy) and a radiation-diffraction based hydrodynamic solver (WEC-Sim) is presented. The high-order scheme gives fast convergence resulting in high-resolution simulations at a lower computational cost. The model is compared against a lumped mass mooring code...
Article
Full-text available
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 bodies using an efficient spectral/hp element depth-integrated unified Boussinesq model. The unified Boussinesq model treats also...
Conference Paper
Full-text available
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 aga...
Article
Full-text available
This paper analyses the nonlinear forces on a moored point-absorbing wave energy converter (WEC) in resonance at prototype scale (1:1) and at model scale (1:16). Three simulation types were used: Reynolds Averaged Navier–Stokes (RANS), Euler and the linear radiation-diffraction method (linear). Results show that when the wave steepness is doubled,...
Conference Paper
Full-text available
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 characteristic...
Conference Paper
Full-text available
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...
Preprint
Full-text available
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 OpenFOAM R for two body shapes: (i) a truncated cylinder; and (ii) a cylinder with a smooth hemispherical bottom. Flow characteristics...
Preprint
Full-text available
We present recent progress on the development of a new fully nonlinear potential flow (FNPF) model for estimation of nonlinear wave-body interactions based on a stabilised unstructured spectral element method (SEM). We introduce new proof-of-concepts for forced nonlinear wave-body interaction in two spatial dimensions to establish the methodology i...
Conference Paper
Full-text available
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 elemen...
Conference Paper
Full-text available
We present recent progress on the development of a new fully nonlinear potential flow (FNPF) model for estimation of nonlinear wave-body interactions based on a stabilised unstructured spectral element method (SEM). We introduce new proof-of-concepts for forced nonlinear wave-body interaction in two spatial dimensions to establish the methodology i...
Preprint
Full-text available
A coupling between a dynamic mooring solver based on high-order finite element techniques (MooDy) and a radiation-diffraction based hy-drodynamic solver (WEC-Sim) is presented. The high-order scheme gives fast convergence resulting in high-resolution simulations at a lower computational cost. The model is compared against a lumped mass mooring code...
Technical Report
The present report summarizes all work conducted in the EUDP project "Mooring Solutions for Large Wave Energy Converters" (MSLWEC) running in the period June 2014 - July 2018. The project aimed at improving the mooring design for large floating wave energy converters, using four Danish devices as case studies: Floating Power Plant, KNSwing, LEANCON...
Technical Report
Full-text available
Moody is developed as a module for computing cable dynamics. It is primarily aimed at solving problems in marine applications. All examples in this manual are therefore different types of mooring configurations for an offshore structure. This manual aims to explain the usage of Moody as a stand-alone solver for mooring dynamics, and as a plug-in mo...
Chapter
Full-text available
Recently CorPower Ocean AB presented laboratory tests of a point absorber wave energy converter equipped with a novel technique for passive phase control. The technique, known as WaveSpring, widens the response bandwidth by a negative spring arrangement, and in the tank experiment an up to threefold increase in delivered power as compared to pure l...
Preprint
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 nonlinear potential flow spectral element...
Preprint
We present a spectral/hp element method for a depth-integrated Boussinesq model for the efficient simulation of nonlinear wave-body interaction. The model exploits a 'unified' Boussinesq framework, i.e. the flow under the body is also treated with the depth-integrated approach, initially proposed by Jiang [25] and more recently rigorously analysed...
Preprint
Full-text available
PRE-CONFERENCE ISOPE 2018 CONTRIBUTION: 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 -...
Article
Full-text available
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 Legen...
Article
Full-text available
The wave loads and the resulting motions of floating wave energy converters are traditionally computed using linear radiation–diffraction methods. Yet for certain cases such as survival conditions, phase control and wave energy converters operating in the resonance region, more complete mathematical models such as computational fluid dynamics are p...
Conference Paper
Full-text available
Computational fluid dynamics (CFD) is becoming an increasingly popular tool in the wave energy sector, and over the last five years we have seen many studies using CFD. While the focus of the CFD studies have been on the validation phase, comparing numerically obtained results against experimental tests, the uncertainties associated with the numeri...
Article
Full-text available
This paper focuses on modelling snap loads in mooring cables. Snap loads are a known problem for the established oil and gas industry, and they pose a major challenge to robust mooring design for the growing industry of wave energy conversion. We present a discontinuous Galerkin formulation using a local Lax-Friedrich Riemann solver to capture snap...
Conference Paper
The present paper describes the work carried out in the project ’Mooring Solutions for Large Wave Energy Converters’, which is a Danish research project carried out in a period of three years from September 2014, with the aim of reducing cost of the moorings for four wave energy converters and improving the applied design procedure. The paper prese...
Technical Report
This report covers a preliminary assessment of available numerical tools to be used in upcoming full dynamic analysis of the mooring systems assessed in the project _Mooring Solutions for Large Wave Energy Converters_. The assessments tends to cover potential candidate software and subsequently cover their capabilities. The result of the assessment...
Article
Full-text available
A major challenge in next-generation industrial applications is to improve numerical analysis by quantifying uncertainties in predictions. In this work we present a formulation of a fully nonlinear and dispersive potential flow water wave model with random inputs for the probabilistic description of the evolution of waves. The model is analyzed usi...
Conference Paper
Full-text available
In a number of experiments and field tests of point absorbers, snap loads have been identified to cause damage on the mooring cables. Snap loads are basically propagating shock waves, which require special care in the numerical modeling of the mooring cable dynamics. In this paper we present a mooring cable model based on a conservative formulation...
Article
Mooring systems are required to keep floating wave energy converters (WECs) on station. The mooring concept might impact the performance of the WEC, its cost and its integrity. With the aim of clarifying the pros and cons of different mooring designs, we present the results from physical model experiments of three different mooring concepts in regu...
Conference Paper
Full-text available
Air cavity and air chamber concepts have been proven to be an efficient way for drag reduction in low-speed ships. Series of experiments were conducted in the SSPA cavita-tion tunnel to simulate the working conditions of an air filled cavity under the hull of a ship. In this paper, study is extended with a numerical validation using a CFD Open Sour...
Conference Paper
Full-text available
We introduce a new stabilized high-order and unstructured numerical model for modeling fully nonlinear and dispersive water waves. The model is based on a nodal spectral element method of arbitrary order in space and a σ-transformed formulation due to Cai, Langtangen, Nielsen and Tveito (1998). In the present paper we use a single layer of quadrati...
Conference Paper
For tidal turbines mounted on floating structures the possibility of cavitation occurring on the blades is higher than for seabed mounted tidal turbines. In this study we present Reynolds-Averaged Navier-Stokes (RANS) solutions of the well-studied Southampton three bladed horizontal axis tidal turbine (HATT). The numerical simulations were carried...
Conference Paper
Boussinesq-type equations are used to describe the propagation and transformation of free-surface waves in the nearshore region. The nonlinear and dispersive performance of the equations are determined by tunable parameters. Recently the authors presented conditions on the free parameters under which a Nwogu-type equations would yield bounded eigen...
Article
Full-text available
Floating wave energy converters (WECs) operating in the resonance region are strongly affected by non-linearities arising from the interaction between the waves, the WEC motion and the mooring restraints. To compute the restrained WEC motion thus requires a method which readily accounts for these effects. This paper presents a method for coupled mo...
Article
Full-text available
We present an arbitrary-order spectral element method for general-purpose simulation of non-overturning water waves, described by fully nonlinear potential theory. The method can be viewed as a high-order extension of the classical finite element method proposed by Cai et al (1998), although the numerical implementation differs greatly. Features of...
Article
Full-text available
The dynamic response of mooring cables for marine structures is scale-dependent, and perfect dynamic similitude between full-scale prototypes and small-scale physical model tests is difficult to achieve. The best possible scaling is here sought by means of a specific set of dimensionless parameters, and the model accuracy is also evaluated by two a...
Conference Paper
Full-text available
We present an initial study of using nonlinear and dispersive Boussinesq-type equations for simulating the response of a point absorber in waves. Boussinesq-type models are expressed in horizontal dimensions only, resulting in computationally efficient models with a real possibility to simulate entire sea states in a fully nonlinear sense. The wave...
Conference Paper
Full-text available
Mooring systems are required to keep floating wave energy converters (WECs) on station. The mooring concept chosen for this has an impact on the performance of the WEC, on its cost and on its integrity. To clarify the pros and cons of different mooring designs, we present the results of experiments on physical models of three different mooring conc...