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Optimization for interaction fluide-structure problems. Application to modelling floating breakwaters.

Abstract : The subject of this thesis concerns modelling and optimizing floating breakwaters, i.e., the study of the motion of a floating breakwater and its response to surface water waves, the analysis of the hydrodynamic behaviour of the floating breakwater through a comprehensive parametrical analysis, and finally to improve the performance and design of the floating breakwater through an optimization problem. It is an interdisciplinary problem, where it addresses the fluid mechanics, mechanical resistance, and structural optimization. A two dimensional modelling and optimisation process has to be developed to serve as a general design tool to determine the dimensions of an optimal floating breakwater capable of surviving in a significant wave height. A rectangular floating body with varying width, draft, mass, internal geometrical section, mooring line angle, and mooring stiffness constitutes the optimization problem. The hydrodynamic analysis was studied using the diffraction-radiation numerical model and extended so as to include the reflective sidewall characterizing the port terminal and assimilating a real practical problem for port sites. So it is different to the problems of structures oscillation on water surface with unbounded domain. In order to proceed forward and determine the transmission coefficient, an analytical modelling for the vibrating structure is developed using the Lagrangian mechanics. The equations of motions are solved to evaluate structure responses in the three modes of motion, and hence vibrational effects are determined and discussed. Finally, a parametrical analysis is developed to identify the influence of the structural parameters on the wave attenuating capacity of the moored floating breakwater. The complexity of the floating breakwater design due to repetitive resonance bands and the interference between the structural parameters makes an analytical optimal design somehow difficult if not impossible. This forces us to orient the problem towards an optimization approach. The main idea in this work is to address the optimization of floating breakwaters (shape and topology) in order to reduce its weight, or to represent a new resistive form, in accordance to the physical and mechanical constraints using various optimization methods. It starts with a simple approach summarized by optimizing a predefined geometry using its geometrical parameters or dimensions. Then, continues towards topology optimization, where we have elaborated a new contribution in this field. Two types of triangular meshes were used. One for indicating the number of variables in the optimization problem, and another refined mesh used for Finite element computations. Thus, we can use very fine meshes without affecting the scale of the optimization problem. Also, we have elaborated another idea in the domain of shape optimization based on arbitrary geometrical shape composed by introducing n variable points constituting a valid structure. This method yields to high flexibility in the optimization process since the points coordinates constitute the variables of the problem leading to unrestricted shapes. All these previously mentioned methods are applied for a simplified model for the wave structure interaction. Where we considered that to some extent, we can disregard or omit the dynamical vibration of the floating breakwater itself. This has permitted us to go thoroughly in structural optimization methods and their developments, where it was very hard to start the optimization problem with the complete dynamical model. It consumes an enormous computational time and especially for the topology problem. Finally, the optimisation problem of a real floating breakwater model is treated with the predefined geometrical shape method. In fact, it constitutes a multidisciplinary optimization problem, where in each iteration a problem of fluid mechanics, dynamic motion, and mechanical resistance are to be solved separately and then assembled through the imposed constraints. This yields to realistic results adaptable with the practical data and experience used in their construction, since it concerns the fluid flow propagation (diffraction-radiation), dynamic motion, mooring lines, and the structural demands.
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Submitted on : Friday, August 25, 2017 - 6:33:12 PM
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Ghassan Elchahal. Optimization for interaction fluide-structure problems. Application to modelling floating breakwaters. . Optimization and Control [math.OC]. Université de Technologie de Troyes - UTT, 2007. English. ⟨NNT : ⟩. ⟨tel-01577467⟩

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