Free-Surface Viscous Flow Solution Methods for Ship Hydrodynamics

The simulation of viscous free-surface water flow is a subject that has reached a certain maturity and is nowadays used in industrial applications, like the simulation of the flow around ships. While almost all methods used are based on the Navier-Stokes equations, the discretisation methods for the water surface differ widely. Many of these highly different methods are being used with success. We review three of these methods, by describing in detail their implementation in one particular code that is being used in industrial practice. The descriptions concern the principle of the method, numerical details, and the method’s strengths and limitations. For each code, examples are given of its use. Finally, the methods are compared to determine the best field of application for each. The following surface descretisation methods are reviewed. First, surface fitting/mesh deformation in PARNASSOS, developed by MARIN; the description focuses on the efficient steady-state solution method of this code. Then surface capturing with Volume-of-Fluid in ISIS-CFD, developed by CNRS/Ecole Centrale de Nantes; the main topic of this review are the compressive flux discretisation schemes for the volume fraction that are used in this code. And fi- nally, the Level Set method in SURF, developed by NMRI; this description contains a modified formulation of the Level Set method that is optimised for ship flow computation

Simulation numérique d'un bateau amarré

International audienceCe papier présente les résultats numériques obtenus par le code de calcul ISIS-CFD pour un bateau amarré dans des vagues. Le cas choisi est celui d'un transporteur de gaz naturel liquéfié (LNG Carrier) en faible profondeur. Les amarres sont modélisées par quatre ressorts. En tenant compte des amarres, les forces exercées sur le bateau sont plus faibles que si le bateau était libre et sans amarres. Le mouvement de cavalement présente une double période alors que les mouvements de pilonnement et de tangage sont réguliers

AN ADAPTIVE N -FIDELITY METAMODEL FOR DESIGN AND OPERATIONAL-UNCERTAINTY SPACE EXPLORATION OF COMPLEX INDUSTRIAL PROBLEMS

International audienceAn adaptive N-fidelity (NF) metamodel is presented for the solution of simulation-based design optimization and uncertainty quantification problems. A multi-fidelity approximation is built by an additive correction of a low-fidelity metamodel with metamodels of hierarchical differences (errors) between higher-fidelity levels. The metamodel is based on the expected value of an ensemble of stochastic radial-basis functions, which also provides the uncertainty associated to the prediction. New training points are added to the appropriate fidelity level, based on the NF prediction uncertainty and the computational cost. The method is demonstrated for an analytical test function, the shape optimization of a NACA hydrofoil, and the operational-uncertainty quantification of a RoPax ferry. The fidelity levels are defined by adaptive-grid refinement and multi-grid approach, for the NACA hydrofoil and the RoPax ferry, respectively. The generalization of the multi-fidelity concept to N fidelities shows promising results both in terms of accuracy and computational cost

Multi-Fidelity Machine Learning from Adaptive-and Multi-Grid RANS Simulations

International audienceA generalized multi-fidelity (MF) metamodel of CFD (computational fluid dynamics) computations is presented for design-and operational-space exploration, based on machine learning from an arbitrary number of fidelity levels. The method is based on stochastic radial basis functions (RBF) with least squares regression and in-the-loop optimization of RBF parameters to deal with noisy data. The method is intended to accurately predict ship performance while reducing the computational effort required by simulation-based optimization (SBDO) and/or uncertainty quantification problems. The present formulation here exploits the potential of simulation methods that naturally produce results spanning a range of fidelity levels through adaptive grid refinement and/or multi-grid resolution (i.e. varying the grid resolution). The performance of the method is assessed for one analytical test and three SBDO problems based on CFD simulations, namely a NACA hydrofoil, the DTMB 5415 model, and a roll-on/roll-off passenger ferry in calm water. Under the assumption of a limited budget of function evaluations, the proposed MF method shows better performance in comparison with its single-fidelity counterpart. The method also shows very promising results in dealing with and learning from noisy CFD data

ASSESSING BI-STABLE VENTILATION FOR SURFACE-PIERCING HYDROFOILS THROUGH NUMERICAL SIMULATION

International audienceThis paper studies the inception and stability of ventilation in bi-stable conditions. It is shown that spontaneous inception depends on the path to the steady state. Therefore, it is more relevant to provoke ventilation by artificial perturbation and to consider its stability, than to simulate spontaneous inception. At low angles of attack, the simulated ventilation is eliminated by a wave crest breaking and closing the ventilation pocket from the top. The tests show that this behaviour depends on the detailed physical and numerical modelling of the water sheet which forms the edge of the ventilating pocket