Aero/Hydro-elastic instabilities occurrence in naval architecture: strategies to approach yacht appendages design

Fluid Structure Interaction (FSI) methods are still elite and barely diffused in yacht engineering, mainly because of the specific knowledge required to treat these problems and often because of the important computational burden they required to be studied: the diffusion of simplified methods should be promoted, providing practical guidelines for FSI integration in traditional sailing yacht engineering. The main aim of this thesis is to give a contribution to the development of design strategies which allow and simplify the prevision of fluid-structure interaction phenomena in sailing yacht appendages. The author proposes a Design Strategy to solve a hydro-elastic problem on a hydrofoil, based on the simultaneous implementation of three main approaches: analytical methods, numerical computation and experimental campaigns. Special interest is given to the application of the proposed strategy for the prevision of a specific hydro-elastic instability: the phenomenon of flutter on hydrofoils. The main tool implemented to analytically predict the flutter condition, discussed in Part A of this thesis, is Theodorsen theory: this model can be considered semi-analytical since its analytical implementation requires the use of a CAD (Computer Aided Design) software for structure geometrical modelling and mass properties calculation, and Finite Element (FE) models to compute the pure natural frequencies of vibration of the structure. Since the flutter limit speed is strongly dependent on these variables, the FE models needed to be validated against experimental model assessment tests, based on static and dynamic dry testing. Simultaneously, the flutter speed was experimentally measured in INM-CNR (Institute of Marine Engineering) towing tank in Rome. The experimental campaign, aimed to encounter the flutter phenomenon, allows validating both analytical and numerical approaches: the condition of instability encountered experimentally is compared against the flutter limit computed with Theodorsen theory outcomes; FSI numerical simulations are not discussed within this thesis, but the presented experimental findings are clearly addressed to be compared with future numerical simulations outcomes. In order to develop and to present the Design Strategy here proposed, a pilot case is needed. The design process of the hydrofoil pilot case, discussed in the main body of this thesis, is aimed to find the optimised combination of structural parameters, in order to meet facilities speed range, construction issues and Theodorsen approach application field. The hydrofoil pilot case is conceived to encounter flutter at a speed compatible with the range of velocity imposed by the water tank facilities. The thesis is divided in four parts: the Main body is introduced by a wide literature review intended to build a theoretical base for fluid-structure interaction problem solving. Within the Main body, the pilot case design and construction processes are described, the structure of the Design Strategy is presented, and a results comparison is investigated. Part A, B and C report the three main pillar of the Design Strategy: respectively analytical methods, experimental campaign and numerical FSI simulations. Within these parts, the author described the proposed methods, the used tools, and the obtained results.