478 



Slow pressure increase at cavity collapse 

 (normally obtained at fosc = 1~3 Hz) 

 Fast pressure increase (fosc = ^~^ Hz) 



Very fast pressure increase, 

 pulses (fosc ~ V-15 Hz) 



sharp 



Generation of High Frequency Noise 



Sharp pulses (i.e., high frequency noise) were 

 generated in three main ways : 



A. By violent collapse of the main cavity (or 

 a large part of it) . 



B. By collapse of small spherical bubbles 

 occurring independently of the main cavity. 

 The bubbles generated rather strong pulses. 



C. By collapse of rather small irregular cavities 

 separating continuously from the main cavity. 



Of greatest interest is the generation process 

 A, which was obtained at high fosc- '^^^ high and 

 sharp pulses were generated in three somewhat 

 different ways: 



Al. Separation of a rather large part of the 

 main cavity at an early stage of the 

 collapse. Thick cavity formations often 

 separated in this way, especially if the 

 cavity was long (large ^rnax' ^^^ broken up 

 by disturbances. At the end the collapse 

 was often very violent and often followed 

 by a violent rebound. Also the rebounded 

 cavities (complex in form) cometimes 



collapsed violently. An example of this 

 behaviour is shown in Figure 13 for fosc ~ 

 7 Hz (oscillation period 5) . 



A2 . Sharp pulses were also generated when a 



sheet collapsed towards the leading edge. 

 The upstream cavity boundary was attached 

 to the leading edge during the whole collapse. 

 This process was normal at the conditions 

 shown in Figures 11 and 12 and especially 

 in cases where the main cavity was rather 

 small. In these cases the whole collapse 

 was orderly and without extensive separa- 

 tions of cavity parts from the main sheet. 

 After the collapse was completed a rebound 

 of small cavities occurred about 10 iran 

 downstream from the leading edge and not at 

 the center of collapse as in the case of 

 more symmetrical collapses. Also in cases 

 where large cavities separated from the 

 main cavity the remaining, rather smooth 

 sheet often collapsed in this way (Figure 

 10, 10 and 14 Hz, Figure 13, 7 and 10 Hz). 



A3 . In cases where the smooth sheet attached 



to the leading edge was long and narrow it 

 was also cut off from the leading edge. For 

 the downstream part, the collapse then be- 

 came more symmetric and violent and with 

 a violent rebound (Figure 11, 10 Hz and 

 Figure 13, 7 Hz). This process often 

 occurred near the end of collapse. 



Spherical bubbles were very effective as genera- 



Pressure signals 

 I — I — I — I — I — I 50 scale units (su) 



Cavitation 



Max. extent. 



Collapse (-max p) 



'osc 

 (Hz) 



Tq (ms) 



Tc (ms) 



Tc/(Tc^-Tq) 



(D 



■^ A A... 







-»- tinne 



cav. starts 



55 



0.23 



50 

 (71) 



0.A8 

 (0.56) 



I I 



Non - cavitating case 



» 

 » 



/iO 



0.53 



21 



0.3i, 



13 



CM 

 O) 



I I 



® 



"^ 







»( 



30 



D.75 



10 



15 



0.33 



s. 



CM 

 CT) 





■©■ 



^/v>J''v/^\A^vAv^ 



M 



-r 



17 



^^ 



1.13 



15 



0.32 



FIGURE 11. Oscillating hydrofoil. Pressure signals and cavitation. 



