480 



Pressure signals 

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



Cavitation 



Max extent 



Collapse (=max.FJIHz) 



Kc 



Tq ( ms ) 



Tc (ms) 



Tc/(Tc-Tg) 



Non -cavitating case 







103 



0.23 



56 

 (82) 



0.35 

 (O.U) 



3 



in 



O 

 Q. 





krl 



i\ 



\ 



61 



D.53 



20 



0.25 



3 



in 



r2 



o 



I I I 



® 



I I I I 



% 



»>l 



43 



0.75 



10 



10 



0.19 



FIGURE 13. Oscillating hydrofoil. Pressure signals and cavitation. 



a = 5 



on propellers, depending on cavity geometry and 

 time variation of the surrounding pressure. 



The contribution from collapse obviously exists 

 (see Figure 9, 1 and 2 Hz) but the quantitative 

 results especially at fosc ~ ^"^ ^^ must be used 

 with much prudence, because of the resonant character 

 of the signal in these conditions. This is discussed 

 in the Appendix. 



,9rea Measurements of Some Cavities 



some results from measurements of cavity area are 

 shown in Figures 17-23. The main cavity includes 

 the sheet and some small bubbles at the downstream 

 edge, which follow the behavior of the sheet. 

 Although the cavities in this condition were rather 

 simple, with no large separations from the sheet, 

 quite complex events often occurred during the 

 last 1/2 millisecond of the collapse. 



Some comments on the figures will be made: 



1. From the shape of the area curves it can be 

 seen that the growth of cavities was rather 

 similar in all cases , while there are 

 differences in the collapses. Compare, for 

 example. Figures 17 and 20. 



2. It is seen that 1-2 milliseconds before 

 final collapse a slow or moderately fast 

 pressure increase was obtained. During this 

 time collapse is fast, but measurable. This 

 pressure fluctuation corresponds to low or 



4. 



medium-high frequencies from a propeller 

 (5-20 X blade frequency) . The pressure 

 fluctuation seems related to the dynamics 

 of the main cavity, which at this stage was 

 quite orderly. 



During the last part of collapse very sharp 

 pulses with durations less than 0.1 milli- 

 second were generated. At this scale of 

 time, measurements and detailed observations 

 of cavity behavior were not possible. Some 

 observations indicated, however, that the 

 sharp pulses sometimes were generated by a 

 rather well-ordered collapse. Figure 17 

 shows an example of this behavior. The 

 cavity was in this case attached to the 

 leading edge during the whole collapse. 

 More complex cases are shown in Figures 18 , 

 21, 22, and 23. Several pulses were generated 

 during a short time and it is impossible to 

 separate the generating events (collapses 

 and rebounds of several small cavities) . 

 Typical of these oscillation periods is 

 that when the downstream cavity wall moves 

 towards the leading edge, the cavity separates, 

 into two parts , both attached to the leading 

 edge. This separation was caused by a growing 

 disturbance on the cavity surface. The 

 disturbance grew from the downstream edge 

 towards the leading edge. (See also Figure 

 11) . During the collapse some bubbles also 

 separated from the downstream cavity edge 

 and the disturbed area. These three cavity 



