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A multi-fidelity (MF) active learning method is presented for design optimization problems characterized by noisy evaluations of the performance metrics. Namely, a generalized MF surrogate model is used for design-space exploration, exploiting an arbitrary number of hierarchical fidelity levels, i.e., performance evaluations coming from different models, solvers, or discretizations, characterized by different accuracy. The method is intended to accurately predict the design performance while reducing the computational effort required by simulation-driven design (SDD) to achieve the global optimum. The overall MF prediction is evaluated as a low-fidelity trained surrogate corrected with the surrogates of the errors between consecutive fidelity levels. Surrogates are based on stochastic radial basis functions (SRBF) with least squares regression and in-the-loop optimization of hyperparameters to deal with noisy training data. The method adaptively queries new training data, selecting both the design points and the required fidelity level via an active learning approach. This is based on the lower confidence bounding method, which combines the performance prediction and the associated uncertainty to select the most promising design regions. The fidelity levels are selected considering the benefit-cost ratio associated with their use in the training. The method’s performance is assessed and discussed using four analytical tests and three SDD problems based on computational fluid dynamics simulations, namely the shape optimization of a NACA hydrofoil, the DTMB 5415 destroyer, and a roll-on/roll-off passenger ferry. Fidelity levels are provided by both adaptive grid refinement and multi-grid resolution approaches. Under the assumption of a limited budget for function evaluations, the proposed MF method shows better performance in comparison with the model trained by high-fidelity evaluations only.

The performance of surrogate-based optimization is dependent on the surrogate training set, certainly for realistic optimizations where the high cost of computing the training set data imposes small training set sizes. This is especially true for multi-fidelity surrogate models, where different training sets exist for each fidelity. Adaptive sampling methods have been developed to improve the fitting capabilities of surrogate models, adding training points only where necessary or most useful to the optimization process (i.e., providing the highest knowledge gain) and avoiding the need for an a priori design of experiments. Nevertheless, the efficiency of the adaptive sampling is highly affected by its initialization. The paper presents and discusses a novel initialization strategy with a limited training set for adaptive sampling. The proposed strategy aims to reduce the computational cost of evaluating the initial training set. Furthermore, it allows the surrogate model to adapt more freely to the data. In this work, the proposed approach is applied to single- and multi-fidelity stochastic radial basis functions for an analytical test problem and the shape optimization of a NACA hydrofoil. Numerical results show that the results of the surrogate-based optimization are improved, thanks to a more effective and efficient domain space exploration and a significant reduction of high-fidelity evaluations.

This paper is devoted to the assessment of ship maneuvering simulation using different propeller models with the focus on a simplified propeller model that represents the action of the propeller by body force and uses propeller performance curve to determine propeller loading during ship maneuvering. Simulations are also performed with an actual propeller approach with which the propeller rotation is simulated directly with the Reynolds averaged Navier-Stokes equation (RANSE) solver. Both time accurate simulations using sliding grid and rotating frame approximations have been performed for comparison. The zigzag and turning circle maneuvers in calm water have been simulated for two different ship models, namely the ONR tumblehome (ONRT) test case and the KRISO (Korea Research Institute of Ship and Ocean) Container Ship (KCS) test case. Predicted ship motion is compared with measurement data to assess the accuracy of the numerical prediction using RANSE computations with different propeller models.

Cet article a pour premier objectif de faire un point sur les différents domaines de recherche en biohydrodynamique. Après une revue historique et bibliographique, l'accent sera mis sur la modélisation et les études numériques de la fonction locomotrice des poissons. La deuxième partie sera consacrée à la présentation du travail effectué au sein de l'E.M.N. dans le cadre du Projet Interdisciplinaire de Recherche CNRS Robea visant à concevoir un robot-anguille autopropulsé à locomotion anguilliforme. Les résultats des premiers calculs tridimensionnels recensés couplant la résolution de l'écoulement par les équations de Navier-Stokes en moyenne de Reynolds avec le Principe Fondamental de la Dynamique appliqué au corps à déformation imposée seront ainsi présentés. Un bilan et les perspectives de ces études concluront cette contribution.

ABSTRACTThe study describes a synthesis of a numerical and experimental work performed during the European research project EFFORT (European Full-scale FlOw Research and Technology) aiming at studying scale effects for hulls of high geometrical complexity by numerical and experimental means. During this project, the CFD group of Fluid Mechanics Laboratory (UMR6598) has performed a complete study of scale and appendage effects on an hopperdredger “Uilenspiegel”. These computations are discussed and compared with available local wake measurements obtained within EFFORT.

This article deals with Navier–Stokes simulations of multiphase flows around moving bodies coupled with an adaptive mesh refinement strategy. The numerical framework is considered first: the Navier–Stokes solver, the methodologies for handling multiphase flows and moving bodies, the remeshing techniques, and the adaptive procedure are explained and detailed. Then an application involving hydrodynamic impacts is presented in detail and studied to highlight the relevance of the whole global approach. Of particular interest is the accurate computation of pressure peaks arising during impacts.This article deals with Navier–Stokes simulations of multiphase flows around moving bodies coupled with an adaptive mesh refinement strategy. The numerical framework is considered first: the Navier–Stokes solver, the methodologies for handling multiphase flows and moving bodies, the remeshing techniques, and the adaptive procedure are explained and detailed. Then an application involving hydrodynamic impacts is presented in detail and studied to highlight the relevance of the whole global approach. Of particular interest is the accurate computation of pressure peaks arising during impacts.

This paper firstly aims at explaining numerical methods to achieve the integration of body motion into a Navier-Stokes solver. One of the peculiar features is to be able to deal not only with classical solids but also with flexible bodies (with an imposed deformation). One gives details concerning the resolution of the Newton's law (parameterization, use of quaternions to describe rotations), the regridding strategies linked to body motion and deformation and of the fluid-motion coupling methods. In the second part, the studies performed in the framework of the CNRS Project named ROBEA will be presented. This project aims at designing an efficient flexible underwater robot based on fish-like locomotion. The results of the first tridimensional simulations coupling a CFD code and the resolution of the Newton's law for a flexible body are shown. Cet article a pour objectif premier d'exposer les m?thodes num?riques li?es au mouvement de corps mises en ?uvre au sein d'un code de simulation d'?coulements turbulents de fluides visqueux incompressibles. Une des particularit?s est de pouvoir traiter, outre les solides classiques, des corps d?formables ? d?formation impos?e. On d?taillera les techniques utilis?es concernant la r?solution des ?quations du mouvement (param?trage, utilisation d'un quaternion), la gestion du maillage (li? au mouvement et ? la d?formation des corps) et le couplage ?coulement-mouvement. La deuxi?me partie sera consacr?e ? la pr?sentation des travaux effectu?s dans le cadre du Projet Interdisciplinaire de Recherche CNRS ROBEA visant ? concevoir un robot-anguille autopropuls? ? locomotion anguilliforme. Les r?sultats des premiers calculs tridimensionnels recens?s couplant la r?solution de l'?coulement par les ?quations de Navier-Stokes en moyenne de Reynolds avec le Principe Fondamental de la Dynamique appliqu? au corps ? d?formation impos?e seront ainsi expos?s.

Cet article a pour objectif premier d'exposer les méthodes numériques liées au mouvement de corps mises en œuvre au sein d'un code de simulation d'écoulements turbulents de fluides visqueux incompressibles. Une des particularités est de pouvoir traiter, outre les solides classiques, des corps déformables à déformation imposée. On détaillera les techniques utilisées concernant la résolution des équations du mouvement (paramétrage, utilisation d'un quaternion), la gestion du maillage (lié au mouvement et à la déformation des corps) et le couplage écoulement-mouvement. La deuxième partie sera consacrée à la présentation des travaux effectués dans le cadre du Projet Interdisciplinaire de Recherche CNRS ROBEAvisant à concevoir un robot-anguille autopropulsé à locomotion anguilliforme. Les résultats des premiers calculs tridimensionnels recensés couplant la résolution de l'écoulement par les équations de Navier-Stokes en moyenne de Reynolds avec le Principe Fondamental de la Dynamique appliqué au corps à déformation imposée seront ainsi exposés.

Cet article a pour premier objectif de faire un point sur les différents domaines de recherche en biohydrodynamique. Après une revue historique et bibliographique, l'accent sera mis sur la modélisation et les études numériques de la fonction locomotrice des poissons. La deuxième partie sera consacrée à la présentation du travail effectué au sein de l'E.M.N. dans le cadre du Projet Interdisciplinaire de Recherche CNRS Robea visant à concevoir un robot-anguille autopropulsé à locomotion anguilliforme. Les résultats des premiers calculs tridimensionnels recensés couplant la résolution de l'écoulement par les équations de Navier-Stokes en moyenne de Reynolds avec le Principe Fondamental de la Dynamique appliqué au corps à déformation imposée seront ainsi présentés. Un bilan et les perspectives de ces études concluront cette contribution.

Abstract:  The practical use of automated computational fluid dynamics (CFD)-based design tools in the ship-building industry requires powerful flow solvers which are able to take into account realistic geometries as well as complex physical phenomena, such as turbulence. A shape optimization tool is developed in this framework. A derivative-free optimizer, yielding both flexibility and robustness, is preferred to the classical gradient-based method, which is more difficult to implement and is still limited to only moderately complex problems. The flow solver included in the design procedure solves the incompressible Reynolds-averaged Navier–Stokes equations on unstructured grids using a finite-volume formulation involving several near-wall low-Reynolds-number turbulence models. The design tool is used to optimize the stern of a modern hull shape at model and full scale, with different purposes being considered. More precisely, the drag reduction and the homogenization of the flow in the wake are expected by controlling the longitudinal vortex generated. Our interest is particularly focused on the influence of turbulence modeling in the design process. The effects of a two-equation model based on the eddy-viscosity assumption and a second-order closure relying on the Reynolds stress transport equations are compared.

In this article, the finite element simulation of cables is investigated for future applications to robotics and hydrodynamics. The solution is based on the geometrically exact approach of Cosserat beams in finite transformations, as initiated by Simo in the 1980s. However, the internal basic kinematics of the beam theory is not those of Reissner–Timoshenko but rather those of Kirchhoff. Based on these kinematics, the dynamic model adopted is a nonlinear extension of the so-called linear model of twisted and stretched Euler–Bernoulli beams. In agreement with the investigated applications, one or both of the ends of the cable are submitted to predefined motions. This model is also implemented into a computational fluid dynamics code, which solves the Reynolds-averaged Navier–Stokes equations. Regarding this last point, an implicit/iterative algorithm including a conservative load transfer for the variable hydrodynamic forces exerted all along the beam length has been used to reach a stable coupling. The relevance of the approach is tested through three advanced examples. The first is related to the prediction of cable motion in robotics. Then, the two last illustrations deal with fluid-structure interaction (FSI). A 2D classical benchmark in FSI is first investigated, and, at last, a computation illustrates the procedure in a 3D case.