Fluid-Structure Interation of a Rigid Wing for Minesto Deep Green, a Tidal Energy Device

Abstract

Fluid structure interaction (FSI) analysis has been performed on the main wing structure of an underwater renewable energy tidal flow device using methods of different fidelity. A lower fidelity method represented the wing as a series of beams (1D line elements) which deflect and rotate under hydrodynamic load. These deflections and rotations were used to update the geometry for the hydrodynamic model where updated forces and moments were calculated using potential flow theory with integral boundary layer (2D panel methods). The higher fidelity method represented the wing using a combination of 3D solid elements for the foam structure inside the wing and 2D shell elements for the carbon fibre wing stiffening beams and fibre glass wing skin. The results of a modal analysis of the wing structure were used in a 3D CFD simulation that coupled the modal equations with the Navier-Stokes equations to compute the deformed shape of the wing under hydrodynamic load. In both cases, the maximum deformations of the wing were quite small (<25mm) compared to the wing size (12m span, 3.3m maximum chord) but the effect on hydrodynamic characteristics was quite different. The low fidelity analysis made the assumption that the wing cross-sectional shape profile did not change, although it could move and rotate. There was no significant change in the predicted hydrodynamic characteristics between the deformed and undeformed wing shapes. The contribution of the foam to the stiffness of the wing was not included in the low fidelity analysis as it was thought to be minor contributor due the much lower Young’s Modulus compared to the carbon fibre and fibre glass of the main wing structure. In contrast, the high fidelity method resulted in about a 15% reduction in lift and 6% in drag forces due to deformation of the wing cross-section profiles rather than bending and twisting of the wing which the low fidelity analysis showed not to be significant. The effect on energy yield of these changes is estimated to be very small.