character of higher blade rate orders was intermittent, which tended to be reflected in the frequency domain as a broadband character. This suggested that the intensity and the phasing of cavitation events lay at the origin of the broadband problem. Next, a general method for decomposing a pressure signal into its continuous harmonic and impulsive components was developed and this meant that the characteristics of different cavity structures, in terms of their signal generation, could be understood. Detailed model testing and efficient software, allowing simultaneous visualisation of high-speed video images, pressure time series and corresponding pressure signals were vital here.
Screenshot of CRS software application for the analysis of pressure signals due to cavitating propellers
gaps in the knowledge identified and subsequently studied. Much work was devoted to the influence of the velocity distribution in the ship’s wake, as this is strongly related to cavitation dynamics, which are determining for erosion and vibration excitation issues. The sensitivity of analytical prediction methods, regarding effects of variations in propeller geometry or wake, was also studied. This was verified by experiments on model- and full-scale.
The effects of propeller manufacturing tolerances on cavitation were also investigated. It was concluded that, in order to obtain better control of cavitation for specific applications, tighter tolerances than the standard ones should be specified. In addition to unique datasets, a major deliverable of the project was a set of guidelines for the reduction of excitation forces due to cavitating propellers. This provided a practical help for ship operators, shipyards, classification societies and propeller designers, enabling them to discover potential cavitation-related issues in the early design stage, as well as how to mitigate such problems once they occurred on a ship.
Extending on vibration excitation In the 1990s CRS cavitation-related research focused on cavitation-induced hull excitation, in particular at higher harmonic and broadband frequencies. The experiences of several CRS members made it clear that there was a need to develop guidance and prediction methods that could be applied at the design stage of a ship. A study of model propeller test data showed that the harmonic
In the last phase of this research the knowledge gained and the tools developed were validated. A large amount of valuable ship scale data was made available by CRS members for no less than eight ships. Maximum hull pressures and source strengths computed showed reasonable results for the first blade rate component but failed to produce useful results at the higher harmonics. Further study of the excitation caused by cavitating tip vortices was therefore necessary.
Example of damage to a propeller blade due to cavitation-induced erosion
report 17
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