Analysis of downwind sail structures using non-linear shell finite elements: wrinkle development and fluid interaction effects

Abstract

Downwind yacht sails are subjected to fluid structure interaction effects which can slightly change the initial design shape, with a direct impact on the overall performances. The turbulent flow acting on downwind sails is separated for the larger part of the device, with large vortices and recirculating regions. The sail is made of a thin fabric, the deformation of which is affected by wrinkling, which produces out-of-plane oscillations of the surface of the fabric, and locally changes the stress/strain distribution. Because of the interactions of these fluid and structural phenomena, the detailed analysis of downwind sails requires sophisticated approaches able to capture the structural deformations, the generation of the wrinkles and the unsteady fluid structure interactions. This is not achieved in conventional sail analysis, the state of the art of which consist, for the most advanced applications, in steady fluid structure analysis adopting membrane structural elements, which are unable to reproduce the wrinkling. The turbulent flow is here analysed with a Reynolds Averaged Navier Stokes method implemented in the finite volume solver OpenFOAM, and case studies are presented regarding the detailed description of the flow/wrinkle interactions, as well as the flow generation on full 3D sail-type devices. This approach is a good compromise between accuracy and computational expense, allowing the investigation of unsteady fluid structure interactions. The work presented here in fact primarily concentrates on the structural response and its influence on the fluid flow rather than the analysis of the fine details of an isolated unsteady flow.

Shell finite elements of the Mixed Interpolation Tensorial Components (MITC) family are used for simulating the fabric. The use of these sophisticated Finite Elements allows for capturing the greater detail of the structural behaviour and the generation of the wrinkles. Comparisons are presented between the results obtained with the shells and the membrane finite elements, traditionally adopted for the structural analysis of fabrics. The performances of the method are demonstrated with simplified validation test cases and applications are shown for realistic 3D devices.

Unsteady fluid structure interaction analysis is performed using the Arbitrary Lagrangian Eulerian (ALE) framework, providing a conservative environment. Validation test cases are compared with reference solutions and the inflation of a sail-type device is analysed. The flow development is accurately captured, and the presence of wrinkles on the cross-flow determine a substantial decrease of the lift and an increase in the drag. Inducing unsteadiness in the flow produces a general increase of the performances of the device. Using shell elements the wrinkling can be directly reproduced, while using membrane models require additional wrinkling models. The prediction performances of the MITC shells are substantially higher than those of the Constant Strain Triangles (CST) membranes, traditionally adopted for simulating the sail fabric. Unsteady fluid structure interaction analysis are validated against reference solutions with good agreement. When applying the method to yacht-sail type geometries, results are coherent and consistent with the sailing practise.

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ORCID for D.J. Taunton: orcid.org/0000-0002-6865-089X

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