A framework for the design by optimization of hydrofoils under cavitating conditions

Hydrodynamic shape optimization based on CFD calculations can dramatically improve the design of marine devices (i.e. propellers, rudders and appendages) by simultaneously considering opposite objectives and by modeling phenomena that well-established and still widely adopted design approaches (i.e. lifting line and lifting surface) cannot accurately deal with. Cavitation on propellers, for instance and among the others, is one of the most dangerous phenomena. It causes vibration, erosion and it is a source of radiated noise, consequently resulting incompatible with modern propeller design, continuously aimed for higher efficiency, comfort and environmentally safe operations. An accurate selection, firstly, of the most appropriate blade sections is, consequently, of crucial importance at least to limit the side effects of cavitation. In the present work, therefore, a numerical framework for the design by optimization of marine hydrofoils under cavitating conditions is proposed. By combining a parametric description of the hydrofoil shape, the NSGA-II multi-objective genetic algorithm and appropriate flow solvers, new hydrofoil shapes are derived. Objectives of the design are blade sections with enlarged cavitation buckets to increase the cavitation inception speed and to reduce the cavity volume (under the constraint of unchanged delivered lift) with respect to widely accepted NACA66 profiles. Boundary element methods and RANSE solvers (a proprietary Hess & Smith implementation and the open-source RANSE solver OpenFOAM) are applied in succession in order to verify the influence of the inviscid/viscous nature of the flow on the final optimal hydrofoil shape and of the additional maximum lift/drag ratio objective required in the case of viscous calculations.