The trim optimisation is nowadays a common practice for all ship owners. Reduction of fuel consumption and improvement of ship energy efficiency concern all sectors of the maritime industry. The trim optimisation consists in finding the best trim angle with regards to the lowest power i.e. fuel consumption for a given operating condition (loading condition and speed). Although the operational efficiency of trim optimisation based on calm water resistance computations databases could be proven at sea, it is well known that added resistance due to wave can significantly impact fuel consumption. The increase of cluster computing power make it today possible to evaluate the added resistance in waves using state of the art free surface RANSE solvers. This study of the optimal trim depending on the seastates show that taking into account the waves could have a significant impact on the optimal trim that have been identify on still water. Although the optimal trim angle trends are the same on still water and in waves for low sea-states, for highest sea-states the trends can be the perfect opposite. This study shows that the optimal trim mainly depends on the draft and the sea-state. The speed seems to have an effect on the gains / losses amplitude but not on the best trim value. The in waves databases will enable operators to predict their ship efficiency with better accuracy, taking into account weather predictions. The trim needs to be therefore adapted regularly depending on the weather conditions the ship encounter during her voyage.

This paper is part of the Bassin Numerique project managed by IRT Jules Verne (French Institute in Research and Technology in Advanced Manufacturing Technologies for Composite, Metallic and Hybrid Structures). The authors wish to associate the industrial and academic partners of this project; respectively DCNS Research/SIREHNA, HYDROCEAN, STX France, Bureau Veritas.Abstract. A numerical towing tank needs to efficiently estimate the ship performances in both calm water and in regular waves. The knowledge of ship performances are mandatory during the design phases in order to provide architects with values helping in technological choices. In this context, numerical towing tanks appear as a more versatile solution than time consuming and costly model tests. The French Technical Research Institute IRT Jules Verne conducts studies to assess and validate methodologies based on CFD simulations to evaluate added resistance in regular waves.The present work conducted in the "Bassin Numerique" project provides a preliminary sensitivity analysis which aims to validate the numerical settings necessary to model the wave propagation. The main result of this preliminary study enables to specify accurate meshes for wave propagation.The present paper focuses on the validation study done by three different members of the IRT Jules Verne using three CFD solvers on four test cases: one static vertical cylinder [1] and three ships in head wave condition [2], [3] and [4]. For each case, numerical results are compared with towing tank experiments in terms of added resistance and motions.The different wave conditions and test cases allow covering the wide range of encountered wave frequencies and dealing separately with the cases of diffraction at zero speed, diffraction with forward speed and finally including radiation. Most of the results correctly fit the experimental data, especially in terms of heave and pitch. The added resistance is also accurately simulated for sufficiently high wave lengths.

The purpose of this paper is to present combination of the SWENSE (Spectral Wave Explicit Navier-Stokes Equations – [1]) method — an original method to treat fully nonlinear wave-body interactions — and a free surface RANSE (Reynolds Averaged Navier-Stokes Equations) solver using a single-phase Level Set method to capture the interface. The idea is to be able to simulate wave-body interactions under viscous flow theory with strong deformations of the interface (wave breaking in the vicinity of the body, green water on ship decks…), while keeping the advantages of the SWENSE scheme. The SWENSE approach is based on a physical decomposition by combining incident waves described by a nonlinear spectral scheme based on potential flow theory and an adapted Navier-Stokes solver where only the diffracted part of the flow is solved, incident flow parameters seen as forcing terms. In the single-phase Level Set method [2, 3], the air phase is neglected. Thus, only the liquid phase is solved considering a fluid with uniform properties. The location of the free surface is determined by a Level Set function initialised as the signed distance. The accuracy of simulation depends essentially on the pressure scheme used to impose free surface dynamic boundary condition. Comparisons of numerical results with experimental and numerical data for US navy combatant DTMB 5415 in calm water and in head waves are presented.

Diffraction-radiation codes enable to model the behaviour of Wave Energy Converters (WEC) and seakeeping of ships on many sea-states with very little computational time. However, the viscous effects are neglected and therefore the simulations lead to relatively inaccurate values. The inaccuracy mainly occurs at the resonance frequency, especially in roll motions for which viscous effects are of major importance. Classically, the viscous effects are represented by adding viscous damping coefficients obtained either from experimental data or analytical approaches based on numerous approximations.In order to improve the accuracy of the diffraction-radiation solvers, the damping coefficients can also be calculated from Computational Fluid Dynamics (CFD) simulations. The first part of this paper presents the three CFD solvers and turbulence models used in this validation study: ICARE and ISIS-CFD are developed by Ecole Centrale de Nantes and Star-CCM+ is a general purpose solver developed by CD-adapco. For each case, a preferred solver is chosen and a second solver is used for verification in most cases.The second part briefly presents the theory that obtains drag coefficients in oscillatory flows, which are closely related to damping coefficients in waves. Each of the three following parts introduces the experimental test cases to which numerical results are compared to. The numerical parameter convergence study leads to a choice of around 200 timesteps per period with an adapted mesh enabling to obtain drag coefficients with errors lower than 5%. A mesh convergence study in the wake area leads to a mesh refinement of around 2 to 2.5 % of the body characteristic length. In order to reduce the computational time, the total number of cells can be decreased by mainly refining locations where specific flow detachment occurs, such as body corners or sharp edges. Turbulence models are also varied. Validation results are finally presented in terms of single or coupled damping coefficients and added mass coefficients. They are presented for various non-dimensional numbers such as Keulegan-Carpenters and Reynolds number.

Cette étude est effectuée dans le cadre du projet collaboratif MAREVA (MAquette de REalisation Virtuelle des Aménagements portuaires), porté par BlueRing et SCE Aménagement & Environnement, labellisé par le Pôle Génie Civil Ecoconstruction et soutenu financièrement par l'Etat à travers le Fonds Unique Interministériel et par des Collectivités Locales.Dans cette note sont abordés les aspects environnementaux, hydrauliques et sédimentaires liés au concept d'aménagement portuaire breveté BlueRing ® .Dans sa configuration de base, il s'agit d'un port circulaire avec dans sa partie centrale un puits d'accès à un espace souterrain aménagé.

The discussion is based on results gathered during the first two years of a 3 years research program for the benefits of Groupe Finot-Conq, Naval Architects. The introduction presents the objectives of the program: Setting up a practical method using numerical and experimental available tools to design fast planing sailing yachts. The aim of this paper is to compare advantages and disadvantages of four different kinds of CFD codes which are linear and non-linear potential flow approach, RANSE solver using finite differences method and RANSE solver using volume of fluid method. The Fluid Mechanics Laboratory of the Ecole Centrale de Nantes (France) has developed those three approaches so those homemade codes will be used for this study. The first one is REVA, a potential flow code with a linearised free surface condition. ICARE is a RANSE solver using finite differences method with a non linear free surface condition. It is extensively used for industrial projects as for sailing yachts projects (ACC for example). ISIS-CFD is a RANSE solver using finite volume method to build the spatial discretization of the transport equations with unstructured mesh. The latter is able to compute sprays for fast planing ships but is also the slower in terms of CPU time. In addition, we had the opportunity to test FS-FLOW which is a potential flow code with a non linear free surface condition distributed by FRIENDSHIP CONSULTING. Numerical results for the four codes are compared with the other codes' results as with tank tests data. Those tank tests were made using captive model test technique on two Open60' models. Reasons of the choice of the captive model technique are explained and experimental procedures are briefly described. Comparisons between codes are mainly based on the easiness of use, the cost in CPU time and the confidence we can have in the results as a function of the boat speed. Flow visualizations, pressure maps, free surface deformation are shown and compared. Analysis of local quantities integrated or by zone is also presented. Results are analyzed focusing on the ability of each code to represent flow dynamics for every speed with a special attention to high speeds. The practical question raised is to know which kind of answers each code can bring in terms of tendencies evaluation or sensitivity to hull geometry modifications. The main goal is to be able to judge if those codes are able to make reliable and consistent comparisons of different designs. Conclusion is that none of the codes is perfect and gather all the advantages. It is still difficult to propose a definitive methodology to estimate hydrodynamic performances at every speed and at every stage of the design process. Knowing each code limitations, it appears more coherent to use each of them at different stages of the design process: the quickest and less reliable to understand the main tendencies and the longest and more precise to validate the final options.

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.