488 



3. SUMMARY AND CONCLUSIONS FROM EXPERIMENTS WITH 

 AN OSCILLATING HYDROFOIL 



1. The generation of sharp pulses was dependent 

 of the oscillation frequency. At low 

 frequencies no high and sharp pulses were 

 generated and above a certain frequency very 

 high pulses were generated. 



2. The sharpest and highest pulses were generated 

 by cavities which separated from the main 

 cavity and underwent a rather symmetrical 



and orderly collapse. Detailed studies 

 showed, however, that a series of pulses was 

 often generated, indicating that the collapse 

 was not always simple at the very end. 



3. Very high pulses could also be generated by 

 cavities that were attached to the leading 

 edge during the whole collapse . 



4. The highest pressure generation efficiency 

 was observed for spherical bubbles , which 

 despite their smallness generated rather 

 strong pulses. 



5. The sharp pulses were generated during the 

 very last part of the collapse. 



5. Rebound of cavities was an important process 

 for generation of sharp pulses . The most 

 violent rebounds were obtained for separated 

 cavities . 

 7. Low frequency noise was generated during the 

 growth, near the time of maximum cavity 

 extent and during the rather late stage of 

 collapse. Because of a disturbing resonance 

 the importance of collapse was, however, 

 difficult to determine. 

 The basis of existing scaling laws for cavitation 

 noise is mainly [see for example Levkovskii (1968) 

 and Baiter (1974) ] : 



1. Ideas from theory and experiment concerning 

 the dynamics and radiation properties of a 

 single cavity. 



2. Ideas concerning statistical properties of 

 the pulse-generating events. 



The dynamics and radiation depend on cavity 

 geometry, cavity size, and the surrounding pressure. 

 Scaling laws based on simple theory deal with model 

 scale and magnitude of surrounding pressure, while 

 similarity has to be assumed in cavitation behavior. 



It has to be accepted that complete similarity 

 in cavitation behavior will not occur, but if it is 

 known which events in the cavitation process are 

 crucial for generation of important pulses this 

 will provide an indication of to what extent 

 similarity is necessary for proper application of 

 scaling laws. 



Of course these introductory experiments cannot 

 supply the final and complete answer, but the results 

 indicate that one of the most important factors is 

 that the separation of a cavity into parts is 

 correctly scaled, the reason being that these 

 separations are often the starting points for violent 

 collapses. Especially when large parts are separated, 

 this often begins at an early stage of the collapse, 

 or is even initiated by disturbances during the 

 growth of the main cavity. 



Parameters that determine tendencies to separation 

 of cavities have only been studied to a limited 

 extent, but it is clear that the combination of a 

 long (chord-wise) cavity and high reduced frequency 

 causes extensive separation of large parts from the 

 main sheet. From the plots of collapse times and 

 pressure generation efficiency, p'*'r/APJ!,_ , as 



functions of reduced frequency it can be concluded 

 that within special regions it is important that 

 the time variations of the surrounding pressure be 

 properly scaled. Such a scaling may be critical 

 for the onset of separation of large cavity parts 

 from the main cavity. 



4. NOISE FROM DIFFERENT CAVITATION SOURCES 



Introduction 



In order to gain more information concerning the 

 noise emitted from a cavitating source, tests with 

 four axisymmetric head forms and two hydrofoils 

 have been carried out in SSPA cavitation tunnel No. 

 1. The aim of these tests was to obtain well-defined 

 and unambiguous types of cavitation, as biibble, 

 sheet, and vortex cavitation. Comparisons of the 

 noise levels from these different types of cavitation 

 were made , as well as some investigations of the 

 effect of free-stream velocity and gas content. 

 The results reported here will only concern effects 

 of the type of cavitation. 



Test Set-Up 



The tests were carried out in SSPA cavitation tunnel 

 No. 1 test section, 0.5 m ^ 0.5 m. The noise was 

 measured using arrangement 4 (hydrophone in water- 

 filled box) , see also Figure 1. In some of the 

 later tests a flush-mounted hydrophone in the 

 tunnel wall (arrangement 2) was used as well as a 

 hydrophone in the flow field. Signals from the 

 hydrophone { s ) were registered by a tape recorder, 

 but also directly analysed by a 1/3 octave band 

 analyser and a narrow-band analyser. Main results 

 given here are from the 1/3 octave band analysis. 



Tests were carried out for a water speed 9 m/s, 

 but with some additional tests at 7.5 m/s and 11 

 m/s . The gas content of the water at the tests 

 was 10? and 40?, with some additional tests at 

 higher gas content. 



Test Set-Up 



The first series of tests was carried out with 

 axisymmetric head forms. The reason for this 

 choice was that cavitation patterns for these bodies 

 were well-known and well-defined from rather exten- 

 sive tests [Johnsson (1972)]. The head forms used 

 are given below, see also Figure 33. 



Head form Shape 

 SSPA iden- of nose 

 tification contour 



Cavitation Type of 

 number for cavita- 

 cav inception tion 



The head forms were attached to a cylinder and 

 a faired afterbody, which were suspended from the 

 tunnel roof via a thin wing. The main difficulty 

 at the tests was the low cavitation numbers needed. 

 At cavitation numbers below 0.4 fairly extensive 

 cavitation occurred at the wing- tunnel roof junction 

 and at other imperfections along the tunnel walls. 

 This cavitation caused rather excessive background 



