Quantifying the impact of the free surface on the performances of a fully passive oscillating-foil turbine via numerical simulations*

This presentation aims to quantify the impact of the free surface on the performances of a fully passive oscillating-foil turbine operating in a horizontal configuration. Since the movement of such turbine is passive, it is sensitive to operating conditions and flow variations. A numerical approach to evaluate the impact of the free surface on the performances of the turbine in realistic conditions is presented along with the associated design guidelines. The turbine's mathematical model with gravity was implemented within a fluid-structure coupling algorithm. Simulations were conducted under different flow conditions and installation depths. Efficiencies of over 50 % are obtained with a turbine undergoing a stall-flutter instability, whereas in previous studies turbines undergoing a coupled-flutter instability were reported to be better. The improvement of the stall-flutter configuration is achieved by taking advantage of the free surface and the confinement it provides. The results also show that these turbines are more robust to changes in the flow than one operating through a coupled-flutter instability. Indeed, the former is much less sensitive to gravitational and buoyancy effects, whereas the fundamental principles of the coupled-flutter instability make it not suitable for operation with a horizontal axis when these effects are important.