Two-objective optimisation of an intake diffuser for outboard dynamic-inlet waterjets

The Outboard Dynamic-inlet Waterjet (ODW) represents one of the latest advances in marine propulsion, providing several propulsive benefits over the existing technologies. Similar to aero-engines, the ODW consists of a single axisymmetric nacelle housing a pump which pressurises the flow that is accelerated through a nozzle. This study progresses current research on the system, improving the efficiency of the existing design through a two-objective optimisation of the propulsor inlet, whose shape directly affects drag. Adapting a thrust/drag bookkeeping scheme from aeronautical studies, the investigation optimises the system net thrust by balancing two conflicting targets: minimum inlet drag and maximum pressure recovery. Propulsive statistics are extracted by modelling the 2D axisymmetric geometry with Reynolds-Averaged Navier–Stokes (RANS) equations and processed by a genetic algorithm, controlling the manipulation of the baseline parameterisation. Optimised individuals indicate that decreasing the intake length and thickness can reduce the initial drag by 18% with marginal improvements on the pressure recovery, not exceeding 0.3%. Highlighting the specific role of each force component on the two objectives, the study shows that, adopting the present bookkeeping scheme, the proposed approach can optimise ODW performance through a fine control of the correlations between the geometrical parameters and the multiple drag terms.