Experimental Investigations 

 of Cavitation Noise 



Goran Bark and Willem B. van Berlekom 

 The Swedish State Shipbuilding Experimental Tank, 

 Coteborg, Sweden 



ABSTRACT 



The requirement of low or acceptable noise levels 

 onboard ships as well as low levels of radiated 

 noise for special purpose ships can cause large 

 problems for the naval architect. Low noise levels 

 onboard ships are required in living quarters and 

 also in some working spaces. The radiated noise 

 field is of concern for instance for fishing vessels 

 and ships with acoustical dynamic positioning systems. 



One important source of noise in ships is cavita- 

 tion and especially cavitating propellers. The 

 cavitation noise can have a quite varying character. 

 It may for example sound like a hiss or like sharp 

 hammer blows. For the naval architect it is impor- 

 tant to be able to predict and, if possible, to 

 reduce undesired cavitation noise. 



In this paper some of the research and develop- 

 ment work on cavitation noise at the Swedish State 

 Shipbuilding Experimental Tank (SSPA) will be 

 described. This work at SSPA is mainly experimental 

 and two projects will be described here in detail. 

 One concerns the relation between cavity dynamics 

 and cavitation noise . This work was carried out 

 using an oscillating hydrofoil in the No. 1 SSPA 

 cavitation tunnel. The other project concerns the 

 relation between types of cavitation and cavitation 

 noise. Different types of cavitation were generated 

 in the tunnel using axisymmetric head forms and 

 hydrofoils. 



A great deal of effort has been made at SSPA to 

 develop adequate methods for measuring cavitation 

 noise in cavitation tunnels. A short review of 

 the measuring techniques now in use is given in an 

 introductory chapter. Besides the two projects 

 mentioned above several other projects are, or 

 have been, carried out at SSPA. 



1. REVIEW OF MEASUREMENT TECHNIQUES AT SSPA 



Measurements of cavitation noise started at SSPA 



as early as 1958. The first tests concerned cavita- 



ting axisymmetric head forms and were carried out 

 in the SSPA cavitation tunnel No. 1. The measuring 

 equipment was a waterfilled box attached to one of 

 the plexiglass windows of the tunnel. A hydrophone 

 was lowered into this box and could thus pick up 

 the noise emanating from the source (propeller etc) . 



The transmission path from the noise source is 

 through water, plexiglass, and water to the hydro- 

 phone. The transmission loss due to the presence 

 of the plexiglass window is low. The drawbacks to 

 this arrangement are reflected acoustic waves and 

 vibrations in the box. The problem with the 

 reflected waves may partly be overcome by carefully 

 calibrating, or rather comparing, results from the 

 hydrophone in a free field and in the box using the 

 same known noise source. Vibration problems (from 

 the vibrating tunnel plating) may be cured by using 

 a pair of rubber bellows between the box and the 

 window '(see Figure 1). 



The signal from the noise source is, however, 

 still distorted as can be seen in Figure 2. This 

 figure shows the noise from a cavitating propeller, 

 as measured by the hydrophone in the box and a 

 hydrophone near the propeller. The differences in 

 the curves are striking and show that the general 

 shape is seriously altered by the box. It is in 

 fact almost impossible to analyse the signal in 

 time-domain using the hydrophone in the box. Com- 

 paring results from 1/3 octave band analysis also 

 shows differences, especially as regards the 

 frequency dependence. These differences are, 

 however, not as striking as those for signals in 

 time-domain. 



The arrangements for noise measurements at SSPA 

 are at present: 



1. Flush mounted pressure tranducers on the hull 

 (Figure 3) 



2. Flush mounted pressure transducers on the tunnel 

 wall 



3. Hydrophones in the flow field near the propeller 

 (Figure 3) 



4. Hydrophone in the water-filled box outside the 

 tunnel 



470 



