FLEXSAIL Fluid Structure Interaction
For the design of Spinnakers sail designers are often using design shapes and cloth selections evolved through experience instead of analysis. Unfortunately these design shapes often have little resemblance to the flying shape the sail assumes, making it difficult or impossible to design for a specific behaviour of the flow around the sail. There are two methods to investigate the flying shape apart of building and testing the full scale sail: model-testing in a wind-tunnel and simulation using fluid-structure-interaction.
Model-testing requires a suitable facility, captures the flow’s behaviour only to a limited extend due to scale effects and disregards the model to full-scale difference of the sailcloth’s structural behaviour. Opposed to that simulations are carried out at full scale Reynolds number and takes into account real sail cloth and its structural behaviour.
The Yacht Research Unit Kiel has developed a program that allows to calculate the flow around a spinnaker incorporating full-scale viscous flow effects as well as the full scale properties of the sailcloth considering orthotropy and orientation. This program, called FlexSail, is based on the commercial RANSE-code CFX11, using its extensive capabilities of flow modelling. The Finite Element module is linked to CFX using an interface provided by CFX giving complete two-way coupling. This means that the external loads on the sail as calculated by CFX are transmitted to the FE-code which calculates the deformations and feeds these
back into the flow-code giving an adapted sail-surface shape. The FE-code allows isotropic as well as orthotropic materials with the latter being oriented within the sail according to the sail-designers panel layout. The structural data calculated by the flying shape enables the sail designer to orient the sail’s panels in accordance with the load lines and select the right sailclothes for the expected loads.
Validation studies carried out using the YRU-Kiel Twisted Flow Wind Tunnel showed good consistency of the calculated and measured data for the forces as well as the flying shapes.