Guillaume Ducrozet

A non-linear wave decomposition model for efficient wave-structure interaction. Part A

This paper deals with the development of an enhanced model for solving wave-wave and wave-structure interaction problems. We describe the application of a non-linear splitting method originally suggested by Di Mascio et al. 1], to the high-order finite difference model developed by Bingham et al. 2] and extended by Engsig-Karup et al. 3,4]. The enhanced strategy is based on splitting all solution variables into incident and scattered fields, where the incident field is assumed to be known and only the scattered field needs to be computed by the numerical model. Although this splitting technique has been applied to both potential flow and Navier-Stokes solvers in the past, it has not been thoroughly described and analyzed, nor has it been presented in widely read journals. Here we describe the method in detail and carefully analyze its performance using several 2D linear and non-linear test cases. In particular, we consider the extreme case of non-linear waves up to the point of breaking reflecting from a vertical wall; and conclude that no limitations are imposed by adopting this splitting. The advantages of this strategy in terms of robustness, accuracy and efficiency are also demonstrated by comparison with the more common strategy of solving the incident and scattered fields together.

Fully Nonlinear Potential/RANSE Simulation of Wave Interaction With Ships and Marine Structures

This paper documents recent advances of the SWENSE (Spectral Wave Explicit Navier-Stokes Equations) approach, a method for simulating fully nonlinear wave-body interactions including viscous effects. The methods efficiently combines a fully nonlinear potential flow description of undisturbed wave systems with a modified set of RANS with free surface equations accounting for the interaction with a ship or marine structure. Arbitrary incident wave systems may be described, including regular, irregular waves, multidirectional waves, focused wave events, etc. The model may be fixed or moving with arbitrary speed and 6 degrees of freedom motion. The extension of the SWENSE method to 6 DOF simulations in irregular waves as well as to manoeuvring simulations in waves are discussed in this paper. Different illlustative simulations are presented and discussed. Results of the present approach compare favorably with available reference results.Copyright

High-order finite difference solution for 3D nonlinear wave-structure interaction

This contribution presents our recent progress on developing an efficient fully-nonlinear potential flow model for simulating 3D wave-wave and wave-structure interaction over arbitrary depths (i.e. in coastal and offshore environment). The model is based on a high-order finite difference scheme OceanWave3D presented in [1, 2]. A nonlinear decomposition of the solution into incident and scattered fields is used to increase the efficiency of the wave-structure interaction problem resolution. Application of the method to the diffraction of nonlinear waves around a fixed, bottom mounted circular cylinder are presented and compared to the fully nonlinear potential code XWAVE as well as to experiments.

Simulation of breaking waves using the high-order spectral method with laboratory experiments: wave-breaking energy dissipation

We examine the implementation of a wave-breaking mechanism into a nonlinear potential flow solver. The success of the mechanism will be studied by implementing it into the numerical model HOS-NWT, which is a computationally efficient, open source code that solves for the free surface in a numerical wave tank using the high-order spectral (HOS) method. Once the breaking mechanism is validated, it can be implemented into other nonlinear potential flow models. To solve for wave-breaking, first a wave-breaking onset parameter is identified, and then a method for computing wave-breaking associated energy loss is determined. Wave-breaking onset is calculated using a breaking criteria introduced by Barthelemy et al. (J Fluid Mech https://arxiv.org/pdf/1508.06002.pdf, submitted) and validated with the experiments of Saket et al. (J Fluid Mech 811:642–658, 2017). Wave-breaking energy dissipation is calculated by adding a viscous diffusion term computed using an eddy viscosity parameter introduced by Tian et al. (Phys Fluids 20(6): 066,604, 2008, Phys Fluids 24(3), 2012), which is estimated based on the pre-breaking wave geometry. A set of two-dimensional experiments is conducted to validate the implemented wave breaking mechanism at a large scale. Breaking waves are generated by using traditional methods of evolution of focused waves and modulational instability, as well as irregular breaking waves with a range of primary frequencies, providing a wide range of breaking conditions to validate the solver. Furthermore, adjustments are made to the method of application and coefficient of the viscous diffusion term with negligible difference, supporting the robustness of the eddy viscosity parameter. The model is able to accurately predict surface elevation and corresponding frequency/amplitude spectrum, as well as energy dissipation when compared with the experimental measurements. This suggests the model is capable of calculating wave-breaking onset and energy dissipation successfully for a wide range of breaking conditions. The model is also able to successfully calculate the transfer of energy between frequencies due to wave focusing and wave breaking. This study is limited to unidirectional waves but provides a valuable basis for future application of the wave-breaking model to a multidirectional wave field. By including parameters for removing energy due to wave-breaking into a nonlinear potential flow solver, the risk of developing numerical instabilities due to an overturning wave is decreased, thereby increasing the application range of the model, including calculating more extreme sea states. A computationally efficient and accurate model for the generation of a nonlinear random wave field is useful for predicting the dynamic response of offshore vessels and marine renewable energy devices, predicting loads on marine structures, and in the study of open ocean wave generation and propagation in a realistic environment.

Development and Validation of a Highly Nonlinear Model for Wave Propagation Over a Variable Bathymetry

Liu and Yue [1] developed a numerical scheme for propagating waves over a variable bathymetry with a High-Order Spectral (HOS) Method. The development of this nonlinear model is detailed and validated on three different test cases. They intend to demonstrate that such a model may be applied to small bottom variations as considered in [1] but also on cases where the bottom variation may be important. In this concern, the very well documented test case of a 2D underwater bar is studied in details. Comparisons are provided with both experimental and numerical results.Copyright

An Integrated Approach for the Representation of Concrete Gravity Based Foundations for Offshore Wind Turbines

This paper describes a novel approach to efficiently simulate the structural dynamics of a concrete Gravity Based Foundation (GBF). In this time-domain analysis, the GBF is subjected to loads applied by the turbine, wave loads and the influence of the soil structure interaction is taken into account. Wind turbine loads are computed using the aeroelastic software FAST and expressed at the connection point between the turbine and the GBF Wave loads on the GBF are computed using a potential, nonlinear wave model. Nonlinear soil-structure interaction is modelled with the use of a macro-element specifically developed for shallow foundations. Finally, the structure itself is modelled using an Euler-Bernoulli multifiber beam, which allows representing the reinforced concrete sections.It is shown that the numerical model is able to efficiently simulate the behaviour of a GBF foundation under nonlinear irregular wave forces and loads transmitted by the turbine. It reproduces nonlinear phenomena such as a decrease in material stiffness due to damage and permanent strains but also the GBF displacements considering soil structure interaction.© 2013 ASME

Efficient Hybrid-Spectral Model for Fully Nonlinear Numerical Wave Tank

A new hybrid-spectral solution strategy is proposed for the simulation of the fully nonlinear free surface equations based on potential flow theory. A Fourier collocation method is adopted horisontally for the discretization of the free surface equations. This is combined with a modal Chebyshev Tau method in the vertical for the discretization of the Laplace equation in the fluid domain, which yields a sparse and spectrally accurate Dirichlet-to-Neumann operator. The Laplace problem is solved with an efficient Defect Correction method preconditioned with a spectral discretization of the linearised wave problem, ensuring fast convergence and optimal scaling with the problem size. Preliminary results for very nonlinear waves show expected convergence rates and a clear advantage of using spectral schemes.Copyright

Deterministic Reconstruction and Prediction of a Non-Linear Wave Field Using Probe Data

The accurate simulation of non-linear sea states evolution over long time periods represents a great challenge, with number of applications in oceanography, marine engineering, security of people or marine transportation, etc... The aim of this study is to develop an efficient deterministic prediction model for irregular wave fields based on the exploitation of wave elevation time series given by one or more probes. We use the High-Order-Spectral model (HOS) to simulate numerically the wave field evolution, in order to take the non-linear effects up to a desired order into account. In this paper, we report on the development of an effective reconstruction scheme, for two dimensional wave fields and using one wave record, that allows us to get proper initial conditions for numerical simulations and nonlinear wave fields forecast.Copyright

Progresses in the developement of a weakly-nonlinear wave body interaction model based on the weak-scatterer approximation

Recent development in wave energy converter technology brings some new challenges in fluid structure interaction modeling and seakeeping analysis. Designs and dimensions of oscillating wave energy systems imply that the amplitude of their motion response will be large. [2]. Usual approach based on linear wave theory is not well-suited in this case because they are limited to small amplitude because of the linearity assumptions. In principle, CFD codes are able to deal with large amplitude motion response, but their computational cost is still too expensive for design purpose [3]. Combining non linear features and computational efficiency of BEM approaches, the Weak-Scatterer [4] approximation is believed to be a promising alternative [5]. In the continuity of [5], this paper presents (i) the progresses made towards the development of a new modeling tool based on the Weak-Scatterer approaches and (ii) quantitative comparisons of numerical prediction using usual linear theory vs the present approach for a submerged heaving wave energy converter. Recent developments are the coupling of the fluid and body solver in order to predict the free motion response of the body. Pressure field over the wetted area is obtained by solving an additional boundary value problem for the time derivative of the velocity potential. Tanizawa’s [6] and Cointe’s [7] formulations for the acceleration condition on the body are revisited. The solver is verified by energy conservation considerations. In order to adapt the mesh to the moving body geometry, advanced mesh moving schemes have been integrated based on radial basis functions [1] and spring analogy methods. In this way it is possible to solve the problem with an Arbitrary Euler Lagrangian formalism and preserve the order of the numerical scheme. However moving mesh methods are limited in time and automatic remeshing generation algorithms have been integrated in order to enable simulating longer durations. Finally, comparisons of the body response predicted by a fully linear BEM solver and the present method are shown.

Comparison of Fully Nonlinear and Weakly Nonlinear Potential Flow Solvers for the Study of Wave Energy Converters Undergoing Large Amplitude Motions

We present a comparison between two distinct numerical codes dedicated to the study of wave energy converters. Both are developed by the authors, using a boundary element method with linear triangular elements. One model applies fully nonlin-ear boundary conditions in a numerical wavetank environnment (and thus referred later as NWT), whereas the second relies on a weak-scatterer approach in open-domain and can be considered a weakly nonlinear potential code (referred later as WSC). For the purposes of comparison, we limit our study to the forces on a heaving submerged sphere. Additional results for more realistic problem geometries will be presented at the conference. INTRODUCTION Among the marine renewable energy sources, wave energy is a promising option. Despite the great number of technologies that have been proposed, currently no wave energy converter (WEC) has proven its superiority over others and become a technological solution. Usual numerical tools for modeling and designing WECs are based on boundary elements methods in linear potential theory [1-4]. However WECs efficiency relies on large amplitude motions [5], with a design of their resonance frequencies in the wave excitation. Linear potential theory is thus inadequate to study the behavior of WEC in such configuration.