484 



^ity area (cm') 



FIGURE 23. Cavity area and generated pres- 

 sure. Oscillation period 14. 



rebounded cavity (often a group of small 

 cavities) collapsed after three to four 

 milliseconds. Compared with the main cavity 

 the area of the rebounded cavity was small 

 (Figure 18 cavity C, Figure 22 cavity C and 

 Figure 23 cavity B) . The rebounded cavity 

 often generated pulses of nearly the same 

 height as the main cavity. 

 The equipment was not designed to measure 

 small and fast collapsing cavities such as 

 small bubbles , but an example .of a diameter 

 measurement of a bubble is shown in B'igure 

 24. The area (ird /4) of the same cavity is 

 plotted in Figure 19 (cavity A) , where the 

 sharp collapse pulses are also visible. 

 Other examples of bubble collapses are shown 

 in Figure 17 (time = t = 5 ms) , 18 (t = 10) , 

 20 (t = 0, cavity A), and 23 (t = 0). Bubble 

 collapses are also shown in Figures 9-13. 



Diameter (mm) 

 8 



-4 -3 -2 -1 



Time (milliseconds ) 



The bubbles studied appeared just before or 

 during the growth of the main cavity and the 

 pressure pulses were then easy to identify. 

 The bubbles normally rebounded once or twice. 

 From the size of the bubbles and the generated 

 pressure it is obvious that the bubbles are 

 very effective as sources of high frequency 

 noise. During the first life cycle, the 

 bubble surface was smooth, but in the rebound 

 cycles it became rough as reported by other 

 authors . 



Dimensionless Presentation of Some Results 



The pressure generation at collapse is related 

 to the violence of the collapse and it is then 



natural to study the collapse time. 



for cavities 



generating different types of pressure pulses. Tc, 

 given in Figures 9-13, is measured for the complete 

 cavity, but in several cases it is only a separted 

 part of the cavity that generates the main pressure 

 pulse. Because of this simplification T^ is probably 

 not significant for the generated pressure in all 

 cases. The intention was, however, to study the 

 relevance of parameters for the complete cavity. 



In Figure 25 T^/(T|, + T )., (T = growth time), 

 is plotted for the cavities shown in Figures 9-13. 

 As seen the steepness of the curves tends to 

 stabilize at a lower value for fosc ^^esulting in 

 sharp pulses. The growth and collapse are, however, 

 not generally related to each other and Figure 25 

 may thus give a distorted picture of T^-behaviour. 

 In an effort to remove this drawback T /T^,' also 

 was plotted, where T^ is a hypothetical collapse 

 time given by the formula for spherical cavities 

 (Rayleigh 1917) : 



T ' = 0.915 «, 

 c max \Fo - p 



FIGURE 24. Diameter of a spherical cavity. (Cavity A 

 in Figure 19. ) 



0.915 a 



