Geometric Modelling and Deformation for Shape Optimization of Ship Hulls and Appendages

International audienceThe precise control of geometric models plays an important role in many domains such as computer-aided geometric design and numerical simulation. For shape optimization in computational fluid dynamics (CFD), the choice of control parameters and the way to deform a shape are critical.In this article, we describe a skeleton-based representation of shapes adapted for CFD simulation and automatic shape optimization. Instead of using the control points of a classic B-spline representation, we control the geometry in terms of architectural parameters. We assure valid shapeswith a strong shape consistency control. Deformations of the geometry are performed by solving optimization problems on the skeleton. Finally, a surface reconstruction method is proposed to evaluate the shape's performances with CFD solvers. We illustrate the approach on two problems: thefoil of an AC45 racing sail boat and the bulbous bow of a fishing trawler. For each case, we obtained a set of shape deformations and then we evaluated and analyzed the performances of the different shapes with CFD computations

Geometric model for automated multi-objective optimization of foils

This paper describes a new generic parametric modeller integrated into an auto- mated optimization loop for shape optimization. The modeller enables the generation of shapes by selecting a set of design parameters that controls a twofold parameterization: geometrical - based on a skeleton approach - and architectural - based on the experience of practitioners - to impact the system performance. The resulting forms are relevant and effective, thanks to a smoothing procedure that ensures the consistency of the shapes produced. As an application, we propose to perform a multi-objective shape optimization of a AC45 foil. The modeller is linked to the fluid solver AVANTI, coupled with Xfoil, and to the optimization toolbox FAMOSA

Parametric shape modeler for hulls and appendages

International audienceThis paper describes a parametric shape modeler tool for deforming hulls and appendages, with the purpose of being integrated into an automatic shape optimization loop with a CFD solver. The modeler allows generating shapes by controlling the parameters of a twofold parameterization: geometrical – based on a skeleton approach – and architectural – based on the design practice and effects on the object's performance. The resulting forms are relevant and valid thanks to a smoothing term to ensure shape consistency control. Thanks to this approach, architects can directly use a NURBS CAD model in the modeler tool and will obtain variations of the initial design to improve performance without additional work. The methodology developed can be applied to any shape that can be described by a skeleton, e.g. hulls, foils, bulbous bows, but also wind turbines, airships, etc. The skeleton consists of a set of B-Spline curves composed of a generating curve and section curves. The deformation of the shape is performed by changing explicit parameters of the representation or implicit parameters such as architectural parameters. The new shape is obtained by minimizing a distance function between the current parameters and the target's in combination with a smoothing term to assure shape consistency control. Finally, the 3D surface wrapping the skeleton is rebuilt using surface network technics. This paper presents the general methodology and an example of application to a bulbous bow on a fishing trawler, with RANSE CFD computations to determine the best design

Geometric modelling and deformation for automatic shape optimisation

Le contrôle précis des modèles géométriques joue un rôle important dans de nombreux domaines. Pour l’optimisation de forme en CFD, le choix des paramètres de contrôle et la technique de déformation de forme est critique. Nous proposons un modeleur paramétrique avec une nouvelle méthode de déformation d’objets, ayant pour objectif d’être intégré dans une boucle d’optimisation automatique de forme avec un solveur CFD. Notre méthodologie est basée sur une double paramétrisation des objets : géométrique et architecturale. L’approche géométrique consiste à décrire les formes par un squelette, composé d’une famille de courbes B-Splines, appelées courbes génératrice et courbes de section. Le squelette est paramétré avec une approche architecturale. Au lieu d’utiliser les points de contrôle de la représentation classique par courbes B-Splines, la géométrie est contrôlée par ces paramètres architecturaux. Cela permet de réduire considérablement le nombre de degrés de liberté utilisés dans le problème d’optimisation de forme, et permet de maintenir une description haut niveau des objets. Notre technique intègre un contrôle de forme et un contrôle de régularité, permettant d’assurer la génération de nouvelles formes valides et réalistes. Les déformations de la géométrie sont réalisées en posant un problème inverse : déterminer une géométrie correspondant à un jeu de paramètres cibles. Enfin, une technique de reconstruction de surface est proposée. Nous illustrons le modeleur paramétrique développé et intégré dans une boucle d’optimisation automatique de forme sur trois cas : un profil d’aile d’avion, un foil AC45 d’un voilier de course et un bulbe de chalutier de pêche.The precise control of geometric models plays an important role in many domains. For shape optimisation in CFD, the choice of control parameters and the way to deform a shape are critical. In this thesis, we propose a new approach to shape deformation for parametric modellers with the purpose of being integrated into an automatic shape optimisation loop with a CFD solver. Our methodology is based on a twofold parameterisation: geometrical and architectural. The geometrical approach consist of a skeleton-based representation of object. The skeleton is made of a family of B-Spline curves, called generating curve and section curves. The skeleton is parametrised with an architectural approach: meaningful design parameters are chosen on the studied object. Thus, instead of using the control points of a classical B-spline representation, we control the geometry in terms of architectural parameters. This reduce the number of degrees of freedom and maintain a high level description of shapes. We ensure to generate valid shapes with a strong shape consistency control based on architectural considerations. Deformations of the geometry are performed by solving optimisation problems on the skeleton. Finally, a surface reconstruction method is proposed to evaluate the shape’s performances with CFD solvers. We illustrate the parametric modeller capabilities on three problems, performed with an automatic shape optimisation loop: the wind section of an plane (airfoil), the foil of an AC45 racing sail boat and the bulbous bow of a fishing trawler

Modèle géométrique déformable pour la simulation et l’optimisation automatique de forme

The precise control of geometric models plays an important role in many domains. For shape optimisation in CFD, the choice of control parameters and the way to deform a shape are critical. In this thesis, we propose a new approach to shape deformation for parametric modellers with the purpose of being integrated into an automatic shape optimisation loop with a CFD solver. Our methodology is based on a twofold parameterisation: geometrical and architectural. The geometrical approach consist of a skeleton-based representation of object. The skeleton is made of a family of B-Spline curves, called generating curve and section curves. The skeleton is parametrised with an architectural approach: meaningful design parameters are chosen on the studied object. Thus, instead of using the control points of a classical B-spline representation, we control the geometry in terms of architectural parameters. This reduce the number of degrees of freedom and maintain a high level description of shapes. We ensure to generate valid shapes with a strong shape consistency control based on architectural considerations. Deformations of the geometry are performed by solving optimisation problems on the skeleton. Finally, a surface reconstruction method is proposed to evaluate the shape’s performances with CFD solvers. We illustrate the parametric modeller capabilities on three problems, performed with an automatic shape optimisation loop: the wind section of an plane (airfoil), the foil of an AC45 racing sail boat and the bulbous bow of a fishing trawler.Le contrôle précis des modèles géométriques joue un rôle important dans de nombreux domaines. Pour l’optimisation de forme en CFD, le choix des paramètres de contrôle et la technique de déformation de forme est critique. Nous proposons un modeleur paramétrique avec une nouvelle méthode de déformation d’objets, ayant pour objectif d’être intégré dans une boucle d’optimisation automatique de forme avec un solveur CFD. Notre méthodologie est basée sur une double paramétrisation des objets : géométrique et architecturale. L’approche géométrique consiste à décrire les formes par un squelette, composé d’une famille de courbes B-Splines, appelées courbes génératrice et courbes de section. Le squelette est paramétré avec une approche architecturale. Au lieu d’utiliser les points de contrôle de la représentation classique par courbes B-Splines, la géométrie est contrôlée par ces paramètres architecturaux. Cela permet de réduire considérablement le nombre de degrés de liberté utilisés dans le problème d’optimisation de forme, et permet de maintenir une description haut niveau des objets. Notre technique intègre un contrôle de forme et un contrôle de régularité, permettant d’assurer la génération de nouvelles formes valides et réalistes. Les déformations de la géométrie sont réalisées en posant un problème inverse : déterminer une géométrie correspondant à un jeu de paramètres cibles. Enfin, une technique de reconstruction de surface est proposée. Nous illustrons le modeleur paramétrique développé et intégré dans une boucle d’optimisation automatique de forme sur trois cas : un profil d’aile d’avion, un foil AC45 d’un voilier de course et un bulbe de chalutier de pêche

Geometric model for automated multi-objective optimization of foils

This paper describes a new generic parametric modeller integrated into an automated optimization loop for shape optimization. The modeller enables the generation of shapes by selecting a set of design parameters that controls a twofold parameterization: geometrical - based on a skeleton approach - and architectural - based on the experience of practitioners - to impact the system performance. The resulting forms are relevant and effective, thanks to a smoothing procedure that ensures the consistency of the shapes produced. As an application, we propose to perform a multi-objective shape optimization of a AC45 foil. The modeller is linked to the fluid solver AVANTI, coupled with Xfoil, and to the optimization toolbox FAMOSA

Surrogates and Classification Approaches for Efficient Global Optimization (EGO) with Inequality Constraints

In this work, we compare the use of Gaussian Process (GP) models for the constraints with a classification approach relying on a Least-Squares Support Vector Machine (LS-SVM). We propose several adaptations of the classification approach in order to improve the efficiency of the EGO procedure, in particular an extension of the binary LS-SVM classifier to come-up with a probabilistic estimation of the feasible domain. The efficiencies of the GP models and classification methods are compared in term of computational complexities, distinguishing the construction of the GPmodels or LS-SVM classifier from the resolution of the optimization problem. The effect of the number of design parameters on the numerical costs is also investigated.The approaches are tested on the optimization of a complex non-linear Fluid-Structure Interaction system modeling a two dimensional flexible hydrofoil. Multi-design variables, defining the unloaded geometry of the foil and the characteristics of its elastic trailing edge, are used in the minimization of the foil’s drag, under constraints set to ensure minimal lift force and prevent cavitation at selected boat-speeds