Unsteady hydrodynamics of flexible submerged foils

The wave induced hydrodynamics of rigid and flexible foils are investigated by means of experimentation, including Particle Image Velocimetry (PIV), and analytically through Theodorsen’s unsteady loading theory. High-amplitude high-frequency fluctuations were investigated in order to explore extreme flow fluctuations, which cannot be matched by a pitch control system, and to assess the limits of Theodorsen’s linear approach. We found a strong non-linear interaction between the separated boundary layer and the trailing edge vortex, with circulation counter to the bounded case present. This resulted in low lift-to-drag ratio (i.e. low efficiency) and poor agreement between Theodorsen and experiments for the rigid foil. Conversely, the flexible foil was very promising: preventing large flow separation, it resulted in 25% higher lift-to-drag ratio, 30% lower flow fluctuations and reasonable agreement with Theodorsen’s prediction.