Hydrodynamic modelling of wave energy converter arrays

Wave energy from wind-generated waves in the ocean or sea is absorbed by wave energy converters (WECs). In this research, floating point absorber (FPA) WECs are studied which are floating devices on the water surface. FPA WECs installed in the ocean or sea respond to the incoming waves and start moving in six degrees of freedom. The WECs extract energy from the waves by using a power take-off (PTO) system which converts the WEC's motion into electricity. In order to absorb a considerable amount of wave energy at a location in a cost-effective way, a number of WECs are arranged in an array layout using a particular geometrical configuration. If the individual WECs are installed close to each other, they will interact with each other, affecting the overall electricity production of the array (near-field effects). Firstly, the presence of a WEC unit disturbs the incoming wave field by both wave reflection and wave diffraction. Secondly, the WEC's motion leads to the generation of waves, called radiated waves. The wave field around a WEC is thus perturbed by a combination of incoming, reflected, diffracted and radiated waves. This results in zones with higher or lower wave heights compared to the incident wave field. The case where one WEC is positioned in the wake region of another WEC where lower wave heights are observed must be avoided. By positioning the individual WECs in the zones with higher wave heights, the total energy extraction of the WEC array is significantly improved, increasing the electricity production. In addition to these near-field effects, a WEC array also influences the wave climate further away (far-field effects). The wave height reduction behind an entire WEC array affects other users in the sea, the environment or even the coastline. In this research, only the near-field effects are considered. The WECs are tested in a three-dimensional (3D) non-linear viscous numerical wave tank (NWT). The NWT is implemented in the computational fluid dynamics (CFD) toolbox OpenFOAM and consists of two fluid phases: water with air on top. The 3D incompressible Navier-Stokes equations, which represent the physics with a very high accuracy, are solved on a mesh in a computational domain. The interface between water and air is resolved by a conservation equation formulated by the volume of fluid (VoF) method. Compared to traditional linear potential flow solvers based on a boundary element method (BEM), CFD is necessary to resolve complex physical processes. Examples are survivability simulations of WECs subjected to breaking waves and WECs operating in resonance mode by applying control methods resulting in significant non-linear and viscous effects combined with large WEC motions. The present research focusses on filling two knowledge gaps for a NWT. The first one is related to enhanced turbulence modelling for NWTs using a two-phase fluid solver and therefore applicable for a wide range of coastal and offshore processes such as wave-structure interaction, wave-current interaction, wave breaking, sediment transport, etc. The second gap is related to fluid-structure interaction simulations of a floating body. Instabilities between the fluid solver and the motion solver might happen due to added mass effects. During this research, enhanced prediction tools for turbulence modelling and efficient fluid-structure interaction simulations in a NWT have been developed. All these developed methods are coupled and validated by using experimental data obtained in a physical wave flume or basin.status: publishe

Numerical simulation of an array of heaving floating point absorber wave energy converters using OpenFOAM

In this paper we use the CFD toolbox OpenFOAM to perform numerical simulations of multiple floating point absorber Wave Energy Converters (WECs) in a numerical wave basin. The two-phase Navier-Stokes fluid solver is coupled with a motion solver to simulate the wave-induced rigid body heave motion. The key of this paper is to extend numerical simulations of a single WEC unit to multiple WECs and to tackle the issues of modelling individual floating objects close to each other in an array lay-out. The developed numerical model is validated with laboratory experiments for free decay tests and for a regular wave train using two or five WECs in the array. For all the simulations presented, a good agreement is found between the numerical and experimental results for the WECs’ heave motions, the surge forces on the WECs and the perturbed wave field. As a result, our coupled CFD–motion solver proofs to be a suitable and accurate toolbox for the study of wave-structure interaction problems of multiple floating bodies in an array configuration

Towards the numerical simulation of 5 Floating Point Absorber Wave Energy Converters installed in a line array using OpenFOAM

In this paper we use the CFD toolbox OpenFOAM to perform numerical simulations of multiple floating point absorber Wave Energy Converters (WECs) in a numerical wave basin. The two-phase Navier-Stokes fluid solver is coupled with a motion solver to simulate the wave-induced rigid body heave motion. The purpose of this paper is twofold. The first objective is to extend numerical simulations of a single WEC unit to multiple WECs and to tackle the issues of modelling individual floating objects close to each other in an array layout. The second objective aims to include all the physical processes (e.g. friction forces) observed during experimental model tests in the numerical simulations. The achievements are verified by validating the numerical model with laboratory experiments for free decay and regular wave tests using a line array of two and five WECs. For all the simulations presented, a good agreement is found between the numerical and experimental results for the WECs’ heave motions, the surge forces on the WECs and the perturbed wave field. As a result, our coupled CFD–motion solver proves to be a suitable and accurate toolbox for the study of wave-structure interaction problems of WEC arrays.location: Cork, Irelandstatus: publishe

Marine renewable energy production

status: publishe

Performance of a buoyancy-modified k-ω and k-ω SST turbulence model for simulating wave breaking under regular waves using OpenFOAM ®

© 2018 Elsevier B.V. In this work, the performance of a buoyancy-modified turbulence model is shown for simulating wave breaking in a numerical wave flume. Reynolds-Averaged Navier-Stokes (RANS) modelling is performed by applying both a k-ω and a k-ω SST turbulence model using the Computational Fluid Dynamics (CFD) toolbox OpenFOAM. In previous work of the authors (Devolder et al., 2017), the observed significant decrease in wave height over the length of the numerical wave flume based on RANS turbulence modelling for the case of propagating waves has been avoided by developing a buoyancy-modified k-ω SST model in which (i) the density is explicitly included in the turbulence transport equations and (ii) a buoyancy term is added to the turbulent kinetic energy (TKE) equation. In this paper, two buoyancy-modified turbulence models are applied for the case of wave breaking simulations: k-ω and k-ω SST. Numerical results of wave breaking under regular waves are validated with experimental data measured in a wave flume by Ting and Kirby (1994). The numerical results show a good agreement with the experimental measurements for the surface elevations, undertow profiles of the horizontal velocity and turbulent kinetic energy profiles. Moreover, the underlying motivations for the concept of a buoyancy-modified turbulence model are demonstrated by the numerical results and confirmed by the experimental observations. Firstly, the buoyancy term forces the solution of the flow field near the free water surface to a laminar solution in case of wave propagation. Secondly in the surf zone where waves break, the buoyancy term goes to zero and a fully turbulent solution of the flow field is calculated. Finally and most importantly, the buoyancy-modified turbulence models significantly reduce the common overestimation of TKE in the flow field.status: publishe

Numerical simulation of a single floating point absorber wave energy converter using OpenFOAM

status: publishe

Accelerated numerical simulations of a heaving floating body by coupling a motion solver with a two-phase fluid solver

© 2018 Elsevier Ltd This paper presents a study on the coupling between a fluid solver and a motion solver to perform fluid–structure interaction (FSI) simulations of floating bodies such as point absorber wave energy converters heaving under wave loading. The two-phase fluid solver with dynamic mesh handling, interDyMFoam, is a part of the Computational Fluid Dynamics (CFD) toolbox OpenFOAM. The incompressible Navier–Stokes (NS) equations are solved together with a conservation equation for the Volume of Fluid (VoF). The motion solver is computing the kinematic body motion induced by the fluid flow. A coupling algorithm is needed between the fluid solver and the motion solver to obtain a converged solution between the hydrodynamic flow field around and the kinematic motion of the body during each time step in the transient simulation. For body geometries with a significant added mass effect, simple coupling algorithms show slow convergence or even instabilities. In this paper, we identify the mechanism for the numerical instability and we derive an accelerated coupling algorithm (based on a Jacobian) to enhance the convergence speed between the fluid and motion solver. Secondly, we illustrate the coupling algorithm by presenting a free decay test of a heaving wave energy converter. Thirdly and most challenging, a water impact test of a free falling wedge with a significant added mass effect is successfully simulated. For both test cases, the numerical results obtained by using the accelerated coupling algorithm are in a very good agreement with the experimental measurements.keywords: Modelling and Simulation,Computational Theory and Mathematics,Computational Mathematics location: Ghent Univ, Ghent, BELGIUMstatus: publishe

Survivability simulation of a wave energy converter in a numerical wave tank

location: Shanghai, Chinastatus: publishe