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

Efficient and multi-objective cavitating propeller optimization: An application to a high-speed craft

The design of a propeller for a high-speed craft is addressed by using a multi-objective numerical opti-mization approach. By combining a fast and reliable Boundary Elements Method (BEM), a viscous flowsolver based on the RANSE approximation, a parametric 3D description of the blade and a genetic algo-rithm, the new propeller shape is designed to improve the propulsive efficiency, reduce the cavitationextension, increase the cavitation inception speed and maximize, at the same time, the ship speed. Ratherthan by constraining the propeller delivered thrust, indeed, the proposed procedure works together withan engine-propeller matching algorithm that, each time a new propeller is defined, identifies the achiev-able maximum ship speed and the resulting engine functioning point that turn in additional goals for themulti-objective optimization. A set of optimal propellers, obtained through the design by optimizationbased on potential flow calculations (via the Boundary Elements Method), are selected for additionalviscous analyses (RANSE calculations) in order to further validate the results of the BEM calculationsand provide a deeper insight into the complex flow fields of high speed propellers. Among this subsetof optimal configurations, a final geometry is selected to verify the reliability of the design procedure bymeans of dedicated cavitation tunnel tests and full-scale measurements on a high-speed craft providedby Azimut|Benetti

Optimization based design of high speed craft propellers

The state of the art of fast propeller design codes is mainly based on classical vortical theories by Lerbs. Their crude simplifications make them often insufficiently accurate in the case of modern fast propellers geometries having highly skewed blades, non-conventional profiles, mixed type of cavitation and strict constraints about pressure pulses and radiated noise. Advanced design tools, based on (more) accurate flow solvers able to deal with the multi-objective nature of any modern design are, definitely, required. In light of this, the design of a propeller for a high-speed craft is addressed by using a multi-objective numerical optimization approach, based on a fast and reliable BEM, a parametric description of the geometry and a genetic algorithm. The new propeller is designed to improve the propulsive efficiency, reduce the cavitation extension, increase the inception speed and simultaneously maximize the ship speed. A final geometry is selected to verify the reliability of the design procedure by means of dedicated RANSE analyses, cavitation tunnel tests and full-scale measurements on a high-speed craft provided by Azimut|Benetti

Experimental investigation of pressure pulses and radiated noise for two alternative designs of the propeller of a high-speed craft

The present paper is focused on an experimental investigation of pressure pulses and radiated noise for two alternative designs of a propeller of a high-speed craft. The propellers have been designed in the context of a research project starting from two different rake distributions (forward rake to increase thrust and efficiency, backward rake to reduce cavitation), using different techniques (traditional lifting line / lifting surface and optimization algorithm coupled with a panel code), leading thus to rather different geometries. Propellers have been tested through cavitation tunnel experiments. The activity represents an interesting case study for this kind of measurement in presence of rather large cavitation extensions. The effects of cavitation on different components of pressure pulses and noise are investigated for the different rake distributions adopted. Results clearly shows the effects of this geometrical characteristic on cavitation and pressure pulses pointing out that, in some cases, propeller hydrodynamic performances may determine pressure pulses intensity more than cavitation extensions. A simplified numerical approach, adopting stationary RANS calculations, for the evaluation of the effects of propeller geometry, has been proposed. Results show a good correlation with measurements allowing to have an insight into the phenomenon and confirming the effect of the rake

Application of multi-objective optimization based design to high-speed craft propellers

The design of a propeller for a high-speed craft is addressed by using a multi-objective numerical optimization approach. By combining a fast and reliable Boundary Elements Method, a viscous flow solver based on the RANSE approximation, a parametric 3D description of the blade and a genetic algorithm, the new propeller shape is designed to improve the propulsive efficiency, reduce the cavitation extension, increase the cavitation inception speed and maximize, at the same time, the ship speed. Rather than by constraining the propeller delivered thrust, indeed, the proposed procedure works together with an engine-propeller matching algorithm that, each time a new propeller is defined, identifies the achievable maximum speed and the resulting engine functioning point that turn in additional goals for the optimization. A set of optimal propellers, obtained through the design by optimization based on potential flow calculations, are preliminary selected for additional viscous analyses in order to further validate the results of the BEM calculations and provide a deeper insight into the complex flow fields of high-speed propellers useful for choosing the optimal geometry. The improvements observed at the cavitation tunnel and the substantial increase of the maximum ship speed during sea trials on a high-speed craft provided by Azimut|Benetti prove the reliability of the design procedure