Propeller geometry optimization for pressure pulses reduction: an analysis of the influence of the rake distribution

The evaluation of pressure pulses is a current issue for any high-performance propeller design. It has been addressed experimentally, by means of model tests, and numerically but in most cases the analysis has been limited to the verification of a given geometry (or, at least of few configurations) identified at the end of a traditional design loop. A more direct inclusion of pressure pulses evaluation in the design procedure, for instance by very attractive multi-objective optimization approaches, could be beneficial, especially if more accurate codes may be exploited. Among the others, BEM represent an acceptable compromise between computational costs and accuracy with the further advantage, with respect to lower fidelity approaches, to account for effects of geometrical haracteristics (such as rake distribution) which are often defined only according to designer experience and special needs. However, if the ability of the BEM methods to predict propeller performance and cavitation extension is well documented, the direct computation of pressure pulses may be less reliable, especially in correspondence to heavy cavitating conditions, requiring further validations in particular when the influence of characteristics such as rake distribution, hardly addressed in literature also from the experimental point of view, are considered. Cavitation tunnel test, BEM and RANS calculations have been consequently carried out for two propellers, designed for the same functioning conditions with different rake distributions, in order to stress the capabilities and the limitations of the numerical approaches in dealing with cavitation, pressure pulses predictions and the capability to discriminate between slightly different geometries in the light of their possible application in a design by optimization procedure.