Introduction
Trying to meet demands for more renewable energy, research is undertaken to develop Wave Energy Converters (WECs). To
harvest a sufficient amount of energy, WECs must be placed in arrays. In order to capture energy from waves, a Power Take Off (PTO) mechanism is used in a WEC. The PTO absorbs energy from the waves by applying a damping force on the floating WEC
buoy, which on its turn causes changes in the surrounding wave fields. At Ghent University a “Master WEC” is developed for the upcoming WECfarm project that aims to investigate these near- and far-field effects of different WEC array geometric layouts and different distances between individual WECs. The “Master WEC”
is a cylindrically shaped, heaving point absorber that can only move in heave.
The goal of this thesis is to implement the “Master WEC” in the numerical domain of DualSPHysics, which is a Smoothed Particle Hydrodynamics (SPH) software. The target is to simulate the
involved physical processes and study the changes in the surrounding wave field.
Smoothed Particle Hydrodynamics – DualSPHysics: background
SPH is a meshfree method which implies that the physical properties of the fluid are concentrated in particles. In meshed methods the mesh elements must be connected to each other, while in meshfree methods the behaviour of the particles is based on the interaction of a particle with its neighbouring particles and connectivity is no longer required. This can solve problems that can occur when modelling rapidly deforming geometries and free surfaces.
The SPH formulation used in DualSPHysics is as follows:
• Discretization of domain in a set of Lagrangian points, or particles.
• Approximation of a generic function F at position ra :
• Summation over all neighboring particles b that fall within the support of the Kernel, defined by the smoothing length h.
• W is defined as the Kernel function and is usually cubic or quintic.
• Momentum conservation equation in SPH:
References
[1 A.J.C. Crespo, J.M. Domínguez, B.D. Rogers, M. Gómez-Gesteira, S. Longshaw, R. Canelas, R. Vacondio, A. Barreiro and O. García-Feal, (2015),
“DualSPHysics: Open-source parallel CFD solver based on Smoothed Particle Hydrodynamics (SPH)”, Computer Physics Communications no.
187: 204-216
[2] R.B. Canelas, M. Brito, O.G. Feal, J.M. Domínguez, and A.J.C. Crespo, (2018), “Extending DualSPHysics with a Differential Variational Inequality: modeling fluid-mechanism interaction”, Applied Ocean Research no. 76: 88-97
[3] P. Ropero-Giralda, A.J.C. Crespo, R.G. Coe, B. Tagliafierro, J. M. Domínguez, G. Bacelli, and M. Gómez-Gesteira, (2021), “Modelling a Heaving Point-Absorber with a Closed-LoopControl System Using the DualSPHysics Code”, Energies 14(3), 760, https://doi.org/10.3390/en14030760
[4] T. Verbrugghe, J.M. Domínguez, A.J.C. Crespo, C. Altomare, V. Stratigaki, and P. Troch, (2018), “Coupling methodology for smoothed particle hydrodynamics modelling of non-linear wave-structure interactions,” COASTAL ENGINEERING, vol. 138, pp. 184–198
[5] N. Quartier, P. Ropero-Giralda, J.M. Domínguez, V. Stratigaki, and P. Troch, (2021), ”Influence of the Drag Force on the Average Absorbed Power of Heaving Wave Energy Converters Using Smoothed Particle Hydrodynamics”, Water 13, 384. https://doi.org/10.3390/w13030384
• Continuity equation in SPH:
Objectives and methodology
Basic simulations of the “Master WEC” in DualSPHysics (illustrated in Figure 2):
• Numerical wave tank with attention to wave generation, propagation, reflection and absorption (passive and active wave absorption).
• Implement the “Master WEC” in the numerical wave tank.
• Use post-processing tools for representation of motion data of the “Master WEC”, data of water surface elevation and data of acting forces. A Response Amplitude Operator (RAO), which is the ratio of the WEC heave and the incoming wave height, will be computed and compared to the existing dataset from wave flume tests at Ghent University.
Advanced simulations of the “Master WEC” in DualSPHysics:
• Coupling with the multi-physics engine of Project Chrono: Allows modelling of mechanical constraints, such as friction coefficients and spring and damper systems, which is useful for PTO system modelling [2].
• Closed loop modelling, which allows the PTO force to be in function of e.g. the WEC position and acceleration, is also possible [3]. This is necessary because the control strategy of the PTO has an important effect on the amount of energy that can be absorbed. This control strategy determines the damping force of the PTO which can be in function of e.g. the WEC velocity or WEC position.
Validation and Results
In order to ensure that the numerical models correspond to the physical reality these models must be validated. This is done by comparing the experimental results from previously performed tests and tests to be performed in the wave flume in Ghent University with the results from the numerical simulations. This validation process allows tweaking of parameters related to friction, viscosity, … in the numerical model to get a better correspondence between numerical and experimental data.
The numerical modelling of the “Master WEC” is a step towards the goals of the WECfarm Project. At the end of this thesis a detailed analysis will be made on the near field-effects of the
“Master WEC”. These results will be further employed to improve the WECfarm experimental set-up which will be soon tested at the new Coastal & Ocean Basin in Ostend, Belgium.
NUMERICAL MODELLING OF A WAVE ENERGY CONVERTER
FOR THE WECFARM PROJECT
DEPARTMENT OF CIVIL ENGINEERING, GHENT UNIVERSITY, GHENT, BELGIUM
1Koenraad Van hulle
1, Nicolas Quartier
1, Vasiliki Stratigaki
1, Peter Troch
1Acknowledgements
• Nicolas Quartier, is Ph.D. fellow (fellowship 1SC5419N) of the FWO (Fonds Wetenschappelijk Onderzoek - Research
Foundation Flanders), Belgium.
• Vasiliki Stratigaki is a postdoctoral researcher (fellowship 1267321N) of the FWO (Fonds Wetenschappelijk Onderzoek - Research Foundation Flanders), Belgium and has also been granted an ‘FWO Research Grant’ for constructing the WEC experimental set-up (FWO-KAN-DPA376).
VLIZ Marine Science Day 3rd of March, 2021
Fig. 1: Illustration of the “Master WEC” used for the WECfarm
project. The WEC buoy is in yellow, the central rod is the rack and pinion PTO. In blue there are 3 bearings that together with the 2
outer guide rods limit the motion to heave.
Contact: [email protected] [email protected]
awww.ugent.be
Fig. 2: Visualization of the numerical wave basin with the “Master WEC” in DualSPHysics
