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Location : Ecole Centrale de Nantes
Results 1 - 5 of 5
Application of System-based Modelling and Simplified-FSI to a Foiling Open 60
The increasing number of foiling yachts in offshore and inshore races has driven engineers and researchers to significantly improve the current modelling methods to face new design challenges such as flight analysis and control (Heppel, 2015). Following the publication of the AC75 Class Rules for the 36th America's Cup (RNZYS, 2018) and since the brand new Open 60 Class yachts are all equipped with hydrofoils, the presented study will propose a system-based modelling coupled with a simplified FSI (fluid-structure interaction) method that leads to better understand the dynamic behavior of monohulls with deformable hydrofoils. The aim of the presented paper is to establish an innovative approach to assess appendage behavior in a dynamic VPP (velocity prediction program). For that purpose, dynamic computations are based on a 6DOF mathematical model derived from the general non-linear maneuvering equations (Horel, 2016). The force model is expressed as the superposition of 7 major force components expressed at the center of gravity of the yacht: gravity, hydrostatic, maneuvering, damping, propulsion (wind), control (rudders, daggerboard, foils ...) and wave (Froude-Krylov and diffraction phenomenon). As test cases, course keeping simulations are performed on an Open 60 yacht with control loops to simulate the wing trimmer, helmsman and foil trimmer when finding the optimal foil settings is needed. In first hand, IMOCA’s polar diagrams are used as reference. In calm water and in waves, the influence of foil’s shapes (foil with shaft pointing downward and tip pointing upward, foil with shaft pointing upward and tip pointing downward) and stiffness (non-deformable, realistic, flexible) on the global behavior of the yacht is presented.
In the last decades, the use of foils on sailing yacht has highly increased. Whether they are mono or multihull, yachts are using foils to reduce their drag forces and then, to increase their speeds in a large range of wind and sea conditions. Several CFD-based studies have already been carried out in order to optimize the foil’s shape and location on the hull, but feedbacks on the yacht’s behaviour is mainly given by the crew when sailing at sea. The aim of the presented paper is to propose a complementary and faster approach that could help to predict and quantify the yacht behaviour in calm water and in waves while sailing under foil’s action. This approach is well known as a system-based modeling and is a mathematical method that leads to understand the complexity of a system from the study of its interactions in their entirety. The paper will present the ability of the system-based approach to predict the attitude of a catamaran while performing maneuvers such as turning circles with 35 degrees of rudder deflection and zigzag tests 10-10 and 20-20 shapes.
Towards Unsteady Approach for Future Flutter Calculations
Surrogate models are simplified models for the behaviour of a complex system, based on a limited number of operating points where the system behaviour is simulated accurately. In hydrofoil design for sailing yachts, surrogate models can be used both for automatic shape optimisation of the foil and for performance evaluation of the entire yacht in the context of a VPP, as a response surface. We present an adaptive method for the construction of surrogate models which can be used for both these objectives, with a simple change of parameters. A Gaussian process regression (GPR) is used for the data fitting, while the adaptive choice of sample points is based either on the GPR variance or on a combination with a cross-validation error estimation. A first test on analytical functions shows that the cross-validation approach is superior for response surface creation, while both adaptation methods are equally suited for shape optimisation. A second test on the shape optimisation of a two-dimensional hydrofoil indicates thatfor moderate immersion depths, the optimum shape is not sensible to the distance to the free surface.
On the choice of CFD codes in the design process of planing sailing yachts
The discussion is based on results gathered during the first two years of a 3 years research program for the benefits of Groupe Finot-Conq, Naval Architects. The introduction presents the objectives of the program: Setting up a practical method using numerical and experimental available tools to design fast planing sailing yachts. The aim of this paper is to compare advantages and disadvantages of four different kinds of CFD codes which are linear and non-linear potential flow approach, RANSE solver using finite differences method and RANSE solver using volume of fluid method. The Fluid Mechanics Laboratory of the Ecole Centrale de Nantes (France) has developed those three approaches so those homemade codes will be used for this study. The first one is REVA, a potential flow code with a linearised free surface condition. ICARE is a RANSE solver using finite differences method with a non linear free surface condition. It is extensively used for industrial projects as for sailing yachts projects (ACC for example). ISIS-CFD is a RANSE solver using finite volume method to build the spatial discretization of the transport equations with unstructured mesh. The latter is able to compute sprays for fast planing ships but is also the slower in terms of CPU time. In addition, we had the opportunity to test FS-FLOW which is a potential flow code with a non linear free surface condition distributed by FRIENDSHIP CONSULTING. Numerical results for the four codes are compared with the other codes' results as with tank tests data. Those tank tests were made using captive model test technique on two Open60' models. Reasons of the choice of the captive model technique are explained and experimental procedures are briefly described. Comparisons between codes are mainly based on the easiness of use, the cost in CPU time and the confidence we can have in the results as a function of the boat speed. Flow visualizations, pressure maps, free surface deformation are shown and compared. Analysis of local quantities integrated or by zone is also presented. Results are analyzed focusing on the ability of each code to represent flow dynamics for every speed with a special attention to high speeds.The practical question raised is to know which kind of answers each code can bring in terms of tendencies evaluation or sensitivity to hull geometry modifications. The main goal is to be able to judge if those codes are able to make reliable and consistent comparisons of different designs. Conclusion is that none of the codes is perfect and gather all the advantages. It is still difficult to propose a definitive methodology to estimate hydrodynamic performances at every speed and at every stage of the design process. Knowing each code limitations, it appears more coherent to use each of them at different stages of the design process: the quickest and less reliable to understand the main tendencies and the longest and more precise to validate the final options.
