Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec


to generate a new mesh instead of using a grid moving technique. This choice is more time consuming than moving grid techniques but it is more robust for large displacement membranes and small cells necessary on sail boundary layers for accurate


pressure field


prediction. The CPU time necessary to automatically generate a new mesh is about 10% of the total simulation time.


4.3 LIMITS For


This type of loose coupling FSI is not a perfect solution [31]. Maintaining accuracy in data exchanges between structural and aerodynamic software is important to obtain relevant aeroelastic results. Stability can also be a problem, in particular when aerodynamic stiffness and structural stiffness are of the same order. Tests and comparisons


are


coupling techniques. 5.


COMPUTATIONAL FRAMEWORK


Fluid motion around deforming and interacting sails in their real environment is a complex non linear problem. This may be more complex if separated flow sail configurations with unsteady phenomena related to deformations and wrinkling are considered. Because there are a lot of parameters that define a complete rig design, there is a crucial need to integrate and automate the entire simulation process. If this is done, it will be easier to understand flow physics and gain insight for better


rig design and trim. Turnaround time of the


simulation process is a major constraint in common use software. ADONF is a response to this problem. It gives us the ability to analyse or optimize a large number of rig configurations. It opens a new way to the design process


by


provides a method to enhance the classical design process,


which is problem.


ADONF is a computational framework which integrates and automates the entire computational environment for flow simulation from CAD definition, to mesh generation, flow simulation, flow analysis and design modifications


using an optimization loop. 6. OPTIMIZATION ALGORITHM


Many optimization algorithms may be used depending on the properties of the objective


function in the This


optimization loop is symbolically described in Figure 3. The main bottleneck is the mesh generation process automation. However, it is also a critical advantage over handmade mesh generation as it generates meshes of high reliability and reproducibility. This specific property of automated meshes increases the ability to compare and rank different sail designs and trims.


As will be shown through examples in the next section, it becomes possible to investigate and resolve new


explored design space. Gradient or simplex methods are well known optimization algorithms. They have been used but they have some disadvantages. They may be slow to converge close to the optimum and this may be a problem when the evaluation cost of the objective function is high. This is precisely the case for CFD applications. Another disadvantage is the zigzag down valley problem, but the more important problem is their dependence on initial conditions


optimization problems. It is not easy to show if a given CFD problem may


have a multi-modal


for multi-modal objective


function when the objective function evaluation is high ©2011: The Royal Institution of Naval Architects B-107


using a computational rather


based


experience and a trial and error through the


on framework.


Optimization Algorithms


It


the designer process, by a


computational design process able to explore the design space


resolution of an optimization


Objective Vector (f0, f1, …, fp)


Flow Analysis Figure 3: ADONF optimization diagram


Flow Solver


the second level of questions, optimization


algorithms have been implemented in ADONF. With optimization


algorithms, a second set of questions necessary to evaluate different


becomes open for sail researchers or sail designers. How to change the rig design or the deck plan to increase the performance of that particular sailing boat in given wind conditions? How to change rig trimming to increase boat speed in given wind conditions? What is the best camber and trim of these two interacting sails to maximize driving force or driving to heeling force ratio? Etc… This will be illustrated in more detail through examples in the results sections.


Design Vector (x0, x1, …, xn)


New Design Vector CAD Design Mesh Generation


questions about fluid motion around designed bodies and their related performances. The first level of new questions that can be investigated is the “what-if” questions. What will be the performance of this rig design if I change the mast section? What will be the performance of this rig if I change the genoa overlap, g, preserving a constant sail surface, S? Etc… Only the sail designer’s imagination and time limits the number of what-if questions.


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