Fluid structure interaction of hydrofoils

Hydrofoils are used in the marine industry to produce enough lift to raise the boat and crew out of the water, therefore reducing resistance on the hull and enabling increased speeds. The interaction between the hydrofoil and water puts severe stress and strain on the hydrofoil. Fluid-structure interaction (FSI) is a multi-physics coupling of both fluid dynamics and structural mechanics into one simulation. When a fluid flow interacts with a structure, stresses and strains are applied within the structure which can lead to a deformation, which can change the flow field, giving a revised pressure loading. This change in pressure loading can lead to either an increase or decrease in lift, which is dependant on the location of the elastic axis of the hydrofoil. If the pressure loading is increased and left unchecked, the deformation could lead to failure of the structure. A symmetrical hydrofoil is studied and good agreement to within 1% variation in pressure is found between the simulated fluid and experimental results found in literature. Good agreement is essential for FSI as any differences can be amplified in subsequent iterations of the FSI. The FSI effects of lift are reported with varying material properties for the NACA0012 hydrofoil. The lift was found to be highly dependent on structural rigidity. The FSI effects are reported for a particular case with a tip deflection of 45cm which is 23% of span. This results in an increase of lift by a factor of 19%, although much larger deformations are possible. In addition, the effects of an FSI on the more complex geometry of the daggerboard on the AC45F foiling boat used in America’s Cup are presented. Here, due to FSI effects, the tip deflection of 32cm changes the coefficient of lift by a factor of 10%. All FSI simulations are found to be stable and give an indication of material strengths needed. However, in all analyses we simplify the structural simulation by treating the structure as a solid volume with isotropic material properties. Future work including the use of anisotropic material properties are highlighted.