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Author : Jean-Marie Finot
Results 1 - 3 of 3
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.
A Research Program on Performance of Planing Sailing Yachts
The aim of this paper is to expose the results gathered at the end of the first year of a three years research program
concerning the planing of sailing ships. The practical objective of this program is to characterize planing hull's behaviour
with a method as simple as possible like Savitsky did for motor boats with V-shaped hull. The general methodology of
the project is to combine the use of experimentation techniques (towing tank tests or sea-trials) and CFD calculations.
The paper shows first comparisons between CFD codes (REVA and ISIS) and towing tank tests data. Fixed model test
method has been chosen to compare with CFD results and to be used in the six degrees of freedom simulator which is a
new kind of VPP. Sea trials for further exploitation are also described.
Strongly Coupled VPP and CFD RANSE Code for Sailing Yacht Performance Prediction
During the design phase of a high performance sailing yacht, naval architects use tools such as VPPs (Velocity Prediction Programs). A VPP solves the equations of motion of the boat using hydrodynamic and aerodynamic force models and allows determination of the speed and the attitude of a sailing yacht. Solving the equations of motion is relatively simple but estimating aerodynamic and hydrodynamic forces is more complex and extremely costly since each boat configuration requires a set of forces that may be derived from more or less accurate empirical, numerical or experimental methods. In general at the beginning of the design phase, architects use empirical methods (quick but not so accurate) to obtain a first result. Then the databases are enriched using computed and/or experimental data around the operating points estimated by the empirical method. Recent developments in RANSE based CFD software FINE™/Marine enable us to reduce the number of steps in performing complete modelling of a sailing yacht in motion. The software is able to simulate the speed and the attitude of a sailing boat by solving the Unsteady Reynolds Averaged Navier-Stokes equations with free surface in 6 DOF (here 5 solved DOF with an imposed zero yaw motion) and by towing the vessel using an aerodynamic wrench computed by the software ARAVANTI. Both air and water flow fields are solved with the help of an Interface Capturing Method. Thus, the boat accelerates and adopts an attitude (angle, drift, trim and sink) and a speed that balance hydrodynamic and aerodynamic forces. This development is part of a long term collaboration aiming at modelling a full scale boat in motion subject to hydro and aero-elastic forces. As an illustration, a validation campaign on calm water of the RANSE code and recent results obtained with this new type of VPP applied to an AC 90 boat type will be presented.
