479 



Pressure signals 

 J — I — I — I 50 scale units (su) 



Cavitation 



Max. extent. 



Co 1 1 apse ("max p) (Hz) 



fcscKc 



Tglms) 



Tc(ms) 



Tc/(Tc«Tg 



89 



3.23 



57 

 (98) 



0.39 

 (0.52) 



3 



in 



s. 



> 



0.53 



A6 



19 



0.29 



»i 



» 



D.75 



10 



39 



15 



0.28 



I I I I I I I 'iy I I 1 I 



£ 



1.13 



15 



28 



0.24 



FIGURE 12. Oscillating hydrofoil. Pressure signals and cavitation. 



tors of high frequency noise. This is discussed 

 later in the text together with cavity area measure- 

 ments . 



The generation of high frequency noise by small 

 irregular cavities , continuously separating from 

 the main cavity is the only generation process when 

 ^osc ~ 0. Also at low fosc (ai>out 1-2 Hz) this 

 process generated pulses. The separation of small 

 cavities from the main cavity decreased with 

 increasing fosc- 



When the high frequency noise was obtained it was 

 always generated during the last part of collapse 

 of the generating cavity (i.e., a bubble could col- 

 lapse and generate high frequency noise during the 

 growth of the main cavity) . This is not surprising, 

 but it should be mentioned that at studies of pro- 

 peller cavitation it has been noticed that the growth 

 of cavities in some cases also generates rather fast 

 pressure variations which indicates that high volume 

 acceleration can also occur during growth. 



Generation of Low Frequency Noise 



The generation of low frequency noise (vibration 

 generating pressure disturbances at multiples of 

 propeller blade frequency) can be identified by 

 inspection of signals from non-cavitating conditions. 



cavitating conditions, and the schematic pressure 

 behavior shown in Figure 16. This is especially 

 easy in cases where cavitation start is marked 

 (Figure 9, 3 Hz, Figure 10, 7 Hz, Figure 11, 3 Hz, 

 Figure 12, 7 Hz, Figure 13, 3 Hz) or where a non- 

 cavitating period is followed by a cavitating one. 

 In several cases it can be seen that a rather slow 

 pressure increase is generated during the growth. 



When the volume acceleration is directed inwards , 

 during a period around the maximum cavity volume , 

 negative pressure is generated (for example Figure 9, 

 3 Hz) . This pressure variation is rather slow and is 

 an essential part of the low frequency disturbance. 

 Because of inertia effects in the motion of cavity 

 walls this part of the motion will probably never 

 contribute to really high frequencies. 



In most of the figures it can be seen that con- 

 tribution to the low frequency pressure is also 

 obtained from the collapse. Especially at low fgsc 

 the collapse seems important. The pressure increase 

 during collapse is due to the outward-directed 

 voliome acceleration existing during the final part 

 of collapse. This acceleration depends on the 

 cavity geometry and the velocity of the cavity walls 

 and it is in principle possible to obtain a collapse 

 with constant volume velocity (no pressure generation) , 

 as well as a collapse with decreasing volume velocity, 

 in which case a pressure increase is generated. It 

 is supposed that both types of collapse can occur 



