Dynamic Fluid Structure Interaction of Racing Sailboats

Sailboats are complex dynamic systems affected by wind and waves. In recent decades foiling boats that fly over the water surface lifted by hydrofoils have become increasingly popular. The foiling adds even more dynamics and complexity to the boats. Foils, sails and rigging are flexible and deform under loading from water and air flow. This deformation affects not only the forces, but also the motions of the boats. The present work seeks to develop a dynamic Fluid Structure Interaction (FSI) model of the NACRA 17, a small foiling catamaran. Flow is modeled by a Computational Fluid Dynamics (CFD) model based on the Finite Volume Method (FVM), and the solid deformations are solved by the Finite Element Method (FEM). The work is published in three papers included in this thesis.

The first paper investigates the dynamic forces of a single hydrofoil with CFD. A rigid foil is moved in prescribed motion in all six degrees of freedom, one at a time. The resulting forces are correlated with the motion to obtain dynamic force and moment coefficient, added mass and damping. Amplitude and frequencies of the motion is varied to find the dependencies. The coefficients are used to predict motion of the boat subjected to a time varying external force. The resulting motions are compared to direct simulations in CFD including a free body motion solver. The predictions are close to the direct simulation and show significant improvement over the commonly used quasi-steady models.

In the second paper the effect of deflections of the foil are investigated by steady and dynamic FSI. The FSI model is validated by comparison to experimental data from a cavitation tunnel. Validation is partially successful, but uncertainties are high. Foil performance and the impact of deflection on the performance is investigated for wide range of operating conditions. Deflections are large and affect vertical lift and side forces significantly. A full hydrodynamic FSI model with both foils and rudder is used to evaluate motions and forces sailing in waves. Mass and momentum of the full boat is included. Inclusion of the flexibility of the foils increase motion and make the boat less stable in waves. It could also indicate that stability is better with a stiffer foil.

In the final paper an aerodynamic FSI model of sail, mast and rigging is developed. It is a multi component model including a solid model of mast, rigging and sails to account for the trimming options on the NACRA 17 sail plan. The model is verified by mesh convergence studies of both solid and flow models. The FSI model suffers from occasional instabilities of the solid solver and the overlapping overset meshes. The computational expense of the model is large and should be reduced to efficiently compute the aerodynamics.

The presented work in this thesis is not an exact copy of the papers as modification have been made to improve readability and quality. Besides the work from the three papers several unpublished studies are presented. They cover FSI test case for selection of software, validation of the structural model, sailcloth FSI validation, hull resistance and on-water measurement system.

Dynamic fluid structure interaction is a complex topic and development of the FSI model for the NACRA 17 still needs more work. The work so far has given valuable insight into the dynamic fluid structure interaction of foils. The aerodynamic FSI model needs more work and potentially solver upgrades, such as introduction of a shell model, before it can be used to predict forces and deformations reliably and efficiently.