A CFD investigation on scale effects of open and ducted propellers

A systematic numerical investigation on the scale effects on the open water performance of open and ducted propellers is proposed. To this aim, two well-known open propellers, the conventional VP1304 and the tip-raked P1727 from the PPTC benchmark, and one ducted propeller in an accelerating nozzle and one ducted propeller in a decelerating nozzle, both from the BESST EU Project, are considered in the study. Results are obtained with the aid of RANSE-based simulations, using Star-CCM+ (open and ducted propellers) and OpenFOAM (open propeller only). Both are employed with the adoption of fully turbulent (model-scale and full-scale) and transition-sensitive (model-scale) turbulence models. A systematic variation of the model-scale rates of revolution (i.e. model-scale Reynolds number), encompassing usual values of towing tank and cavitation tunnel experiments, as well as the ambient turbulence intensity influencing turbulent boundary layer onset, are addressed for relatively large ranges of loading conditions for a complete overview of the phenomena influencing model-scale performance and scaling trends. Calculations show scale effects that are substantially different (higher, sometimes opposite) from those suggested by ITTC procedures and a non-negligible dependency on the flow conditions (in other words, the choice of the numerical model, initial values of calculations and loading conditions) that could make the definition of a scaling procedure, based on numerical calculations only, a very complex task.