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 nonlinear 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.
Dealing with freely-floating bodies in the framework of non-linear potential flow theory may require solving Laplace's equation for the time derivative of the velocity potential. At present, there are two competing formulations for the body boundary condition. The first one was derived by Cointe [Cointe(1989)] in 2D. It was later extended to 3D by van Daalen [van Daalen(1993)]. The second formulation was derived by Tanizawa [Tanizawa(1995)] in 2D. It was extended to 3D by Berkvens [Berkvens(1998)]. In this paper, a proof is given that the Cointe-Van Daalen's and the Tanizawa-Berkvens' formulations are equivalent. It leads to a simplified version of Cointe-Van Daalen's formulation. The formulation is validated against the analytical solution for a moving sphere in an unbounded water domain.
In continuation of [1], this paper presents the progress made towards the development of a new modeling tool based on the Weak-Scatterer approaches. 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 [2] and Cointe’s [3] formulations for the acceleration condition on the body are revisited. Numerical prediction with the present method for a submerged body in vertical free motion is presented and energy conservation is verified. In order to adapt the mesh to the moving body geometry, advanced mesh moving schemes have been integrated based on radial basis functions [4] 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 wave diffraction and radiation predicted by linear theory, a fully nonlinear BEM solver and the present method are shown.
The present paper describes the coupling between two open source software packages, OpenFAST and FRyDoM (named FRyFAST), dedicated to the floating offshore wind turbine (FOWT) dynamics simulation. FRyDoM (Flexible and Rigid body Dynamic modelling for Marine operations) is a multi-body and multi-physics framework, mainly dedicated to complex offshore systems, developed by D-ICE Engineering and Ecole Centrale Nantes. This framework is coupled with OpenFAST in order to simulate FOWT with advanced platform system, corresponding to multi-body system, platform with high interaction with other bodies in close environment or in contact with those bodies. This coupling enables also the used of an advanced Finite Element Model (FEM) based on an iso-geometric representation of the mooring lines. A lot of dedicated models for the platform can also be added through this coupling thanks to the FRyDoM API. In the first part, the coupling between OpenFAST and FRyDoM is presented. Particular attention has been paid to the integration of the added mass term to enable stability of this coupling, and to the synchronization between the two time integration schemes. Verification and validation results of this coupling will be presented. Tension in the mooring lines, dynamic behaviour of the FOWT in different wind and sea conditions have been analysed and show the capacity of this model to perform FOWT design.
This study investigates the use of a linear discretization of the velocity potential in a frequency-domain potential flow based solver. The velocity potential is assumed to vary linearly over each panel. This approach differs from the Constant Panel Method (CPM), classically used in diffraction-radiation codes. The linear discretization is studied as a possible interesting strategy in terms of accuracy and CPU time.
The first goal of this study is the presentation of the impact of the linear discretization in the equations of the potential flow theory. The second goal is the quantification of its interest in terms of accuracy and CPU time compared to the Constant Panel Method.
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