Comparison of measured URN levels with those predicted by CRS developed software

Extending on erosion The costs of repairing erosive damage to a propeller or rudder are high, whereas fuel costs rise when propeller blades are roughened by erosion. Early in the 21st century the EU-sponsored cavitation erosion research project EROCAV was conducted. One deliverable was the “Cavitation observation handbook”, of which the importance was such that the EROCAV membership approved dissemination of it beyond the limits of its sponsoring members.

Within CRS it was considered important to investigate basic physics of hydrodynamics, material response and cathodic protection. This became possible by applying new experimental techniques. One of the CRS members developed an Acoustic Emission (AE) technique by which crack growth in materials can be related to the energy of imploding cavities and applied a sonotrode technique to test the response of different materials, material treatments and cathodic protection. Another new technique was high-speed video. The AE technique was simultaneously applied with high-speed video recordings of the cavitation to a number of rudders at full-scale. Fundamental knowledge on cavitation dynamics was increased by model-scale experiments in

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a high-speed cavitation tunnel on a foil. Recordings of up to 50k frames per second were made of imploding cavities, synchronised with AE measurements. The erosion studies were concluded by issuing jointly written design guidelines for rudders and propellers, providing concrete practical guidance.

Broadband excitation and noise The work done on broadband hull excitation in the 1990s was continued. In addition to the issues addressed then, Underwater Radiated Noise (URN) of merchant ships was studied because of its effect on the behaviour of marine mammals and fish. New prediction methods for broadband noise emitted by either tip vortex cavitation (ETV model) or sheet cavitation were developed, making use of results obtained by the boundary element method PROCAL that was also developed through CRS. Available empirical methods were evaluated, and the structural response of the ship by broadband excitation was investigated using finite element methods, as well as statistical energy analysis type computations. Computational models were validated using data obtained from sea trials that were sponsored by CRS. The URN was measured from a military research

vessel, a general cargo vessel and a containership. Of these latter two vessels, the hull pressure fluctuations and structural vibrations were also measured, from which the relation between hull-pressure fluctuations and URN could be investigated. The developed computational models were used in a propeller design study to evaluate efficiency, hull pressure fluctuations and URN, and became part of the CRS propeller software suite. This software suite is currently being extended with a semi- empirical method to predict the noise inboard of the vessel by cavitation.

The CRS research line on cavitation spans a period of some 35 years. Based on knowledge gained about basic aspects of cavitation and related hull-excitation, research expanded on more specific aspects, like higher harmonics, broadband hull excitation and cavitation erosion. Recent work includes URN. CRS members contributed by carrying out parts of the research and by making data and research methods available. Practical guidelines and validated prediction methods were the result, and these contribute to achieving the higher goal: the reduction of risk for the member organisations in their daily business by gaining more control of cavitation.

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