A design of a hydrofoil family for current-driven marine-hydrokinetic turbines

The natural kinetic motion of oceans, rivers, and other bodies of water is a promising resource for sustainable power production. Rotor-based marine and hydrokinetic (MHK) turbines generate electricity from river, tidal, and other water currents, operating analogously to wind turbines in air. An MHK rotor designer can draw upon a vast body of general purpose and wind power specific airfoils, but application specific hydrofoils can more optimally meet the needs of MHK power. We present the MHKF1 family of hydrofoils, designed upon experience drawn from wind turbine airfoils and incorporating hydro-specific considerations. The MHKF1 hydrofoils were developed to balance the following design objectives: (1) basic hydrodynamic performance with lift to drag ratio (l/d) as a key metric, (2) limited sensitivity to soiling because of biofouling concerns and the high cost of maintenance in the marine environment, (3) sufficient thickness for structural efficiency, (4) good stall characteristics, (5) hydrodynamic and geometric compatibility such that the different hydrofoils of the family can be applied on the same rotor blade, (6) low susceptibility to cavitation, and (7) low susceptibility to singing. While the first five criteria are common to wind turbine airfoil design, the last two are specific to operation in water. Cavitation, the formation of bubbles within a fluid, can have numerous detrimental effects including erosion of impinged surfaces, degraded performance, vibration, and noise. The minimum surface pressure of the MHKF1 hydrofoils were managed to reduce the likelihood of cavitation. Singing, a hydroacoustic/hydroelastic phenomenon of the trailing edge of hydrofoils, results in noise and vibration. To suppress singing, trailing edge thicknesses were increased and hydrofoil variants were designed with "anti- singing" profiles. The MHKF1 hydrofoils were developed with a combination of inverse and direct design methods using XFOIL and various routines for parameterizing hydrofoil geometries and surface velocity distributions. Performance was further evaluated with OVERFLOW, a Reynolds averaged Navier Stokes computational fluid dynamics code.