Comparison of open source code Nemoh with Wamit for cargo ship motions in shallow water
The resolution of linear hydrodynamic problems is often a basic subject to deal with for analysis and development of marine technologies. This paper presents comparison results in addition to the benchmarking study reported in Gourlay, von Graefe, Shigunov and Lataire (2015) The focus of this study is mainly on the validation of analytical results produced with the radiation diffraction code Nemoh developed at Ecole Centrale de Nantes, against data results from commercial code Wamit. The Nemoh code is based on Boundary Element Methods (BEM) is open source, and is described in Babarit and Delhommeau (2015) Three different ship models were used in the study: a post panamax container ship, a panamax container ship and a panamax bulk carrier, corresponding to the numerically identical geometries adopted for the Wamit computations in Gourlay et al. (2015) Hydrodynamic coefficients, excitation forces and motion RAOs are computed and compared in order to provide further evidence of the Nemoh code reliability solving shallow water hydrodynamic problems, for example the prediction of ship motions for a vessel sailing through ports or channels. The analysis results presented in this memo show a good agreement between the code's computations across the different parameters observed. The study contributes to a number of positive verification cases already available for the open source code, and remark the consistency and the compatibility of the output produced. The benefits of a robust, open source BEM code for solving seakeeping problems are vast, and will certainly contribute to the exploration and the optimization of both existing and new marine technologies. INTRODUCTION Both software, Wamit and Nemoh, are designed to solve the three dimensional radiation diffraction problem using panel methods. Therefore, they allow to compute wave loads and hydrodynamic characteristics of bodies interacting with ocean surface waves. A very brief description of the resolution of vessel motions in regular waves is given below, while a complete dissertation can be found in literature. The solution of the hydrodynamic problem of a floating body subject to ocean waves can be obtained by applying linear potential theory to the fluid domain, treated as non viscous, incompressible and homogeneous. The surface waves are assumed to be regular (harmonic) waves of small amplitude. Linear boundary conditions on the domain bottom floor, fluid free surface and body surface are also applied. The set of differential equations with boundary conditions is known as Boundary Value Problem (BVP) Eventually, the computation of the velocity potential as solution of the BVP allows to derive hydrodynamic coefficients and excitation forces caused by the waves on the geometry surface. Ship motions are subsequently derived applying linear equations for floating body's responses in regular waves. VESSEL DESCRIPTION Hull meshes, namely Ship D, Ship F and Ship G, were obtained from the earlier Wamit analysis. Those gdf files were re arranged in a Nemoh format, which is described in Nemoh (2016) No manipulation or modification of the actual mesh points was performed for consistency with the input data and comparison purposes. Table 1 summarises main particulars of the considered vessels. The coordinate system is set to have origin in longitudinal centre of gravity, centreline and freesurface ~waterplane) The x axis is positive toward bow, y axis is positive to port and z axis pointing upward. Table 1 Vessel main particulars Item Ship D Ship F Ship G Length between perp. [m] 291.13 190.00 180.00 Beam at WL [m] 40.25 32.00 33.00 Design draft [m] 15.00 11.60 11.60 Modelled draft [m] 11.60 Displacement (computed) [m] 75169 42274 57724 Radii of inertia (x, y, z) [m] 13.3, 72.8, 72.8 11.7, 47.5, 47.5 14.8, 45.0, 45.0 Number of panels 216