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DISCUSSION 

 S. Byard 



I would just like to describe in a very few words the results of some simple 

 laboratory experiments which have been carried out by Mr. Hey at the Admiralty 

 Research Laboratory, and which seem to illustrate some of the characteristics of pro- 

 peller cavitation noise. 



These experiments have been made with quite small hydrofoils, the smallest 

 being less than one inch by one inch, either mounted in a miniature water tunnel with 

 transparent sides, or mounted at the periphery of a disc which is rotated under water. 

 By suitably orienting the foil, a tip vortex cavity can be formed at the trailing edge, 

 or alternatively blade cavitation induced at the leading edge, and the noise spectra of 

 the two forms of cavitation are markedly different. 



In these model tests the noise spectrum associated with the tip vortex cavity is 

 localized around 2 to 3 kilocycles /second, whereas the spectrum of the noise due to 

 blade cavitation is more generally distributed, extending to the higher frequencies. 



In the case of a ship's propeller, where cavitation is well established, both forms 

 of cavitation are normally present and give rise to a continuous noise spectrum with a 

 marked peak. 



Some simple scaling experiments in which the effect of doubling the size of the 

 hydrofoil was observed, showed an approximate doubling of the diameter of the tip 

 vortex, and a lowering of the frequency of the peak in the noise spectrum. It was also 

 possible to show experimentally that the source of the noise associated with the tip 

 vortices is located close to the hydrofoil, and does not extend along the vortices. 



It is thought that experiments along these lines, which can be regarded as 

 supplementing and extending the work on the single collapsing cavity which has been 

 carried out by Kendrick some time ago at the Admiralty Research Laboratory, can 

 help us understand the mechanism of propeller cavitation noise. 



G. K. Batchelor 



I should like to ask a question of Mr. Fitzpatrick. He remarked that, in his 

 experiments, no noise generated by the turbulence in jets could be detected. Such noise 



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