487 



. p*r [Pam] 



2000 



1000 



20 30 1.0 50 60 70 



Maximum cavity area (cm ) 



oscper -) 7 6 13 14 



FIGURE 30. Pressure p from different oscillation 



periods f = 15 Hz, ao = a = 3° . 

 osc 



reached the low value region (Figure 26) . There 

 is considerable scatter in generation efficiency. 

 It must, however, be remembered that the plot is 

 based on single cavitation events probably not 

 always typical, the results must only be seen as a 

 first hint of tendencies. The coefficient (3) gave 

 results rather similar to those from (2) but with 

 somewhat smaller dispersion. In Figure 29 it can 

 be seen that with coefficient (5) the dispersion 

 of the points was considerably decreased. 



In Figures 30-32 results from Figures 17-23 are 

 plotted. Only the dimensionless coefficients (2) 

 and (6) are shown and it is seen that both attain 

 approximately the same values for similar pulses, 

 but neither of them brings the values of oscillation 

 periods 6 and 7 into agreement with the others. 

 The other coefficients give similar results. Also 

 if the coefficients are based on values of area, 

 time, etc. closer to the final collapse, the scatter 

 is not decreased drastically. The conclusion of 

 this is that, in the prediction of noise by theory 

 or model tests, good similarity in certain cavitation 

 events is important, and that these important events 

 are not generally described by such simple parameters 

 as Tc and V^^^. 



Because it was not possible to estimate d^V/dt^ 

 directly from measured values of V(t) functions of 

 the type : 



p+r T^ /A «, p 

 c max p max max 



V(t) = const[l - cos 



(6) 



In Figures 27, 28, and 29 results are shown for 

 the different conditions shown in Figures 9-13. 

 p"''r is shown in Figure 27 only to provide a reference 

 for the other parameters . 



Figure 28 shows that the generation efficiency 

 increased strongly at a certain fosc (°^ reduced 

 frequency) . The increase normally coincided with 

 generation of very sharp pressure pulses and at 

 these fosc ^^^ relative collapse time had also 



)(t)] [(t>(t) is a polynomial 



with six variable para- 

 meters 1 



were closely matched to nearly the whole collapse. 

 The pressures then calculated by use of these 

 functions agreed fairly well with measured values 

 in many cases. These simple computations also 

 demonstrated how sensitive the generated pressure 

 often was to the final behavior of V(t) and it was 

 easy to realize that parameters of the types dis- 

 cussed above can only be "universal" if they are 

 applied to fairly similar cavitation events. 



2.0 



1.0 



0.5 



01 



0.05 



30 40 50 60 70 



Maximum cavity area (cm^) 



P'rtl 



20 



30 



40 50 BO 70 



Maximum cavity area [cm ) 



oscpen -17 6 



osc. per -17 6 



FIGURE 31. 

 periods f 



Pressure p from different oscillation 

 = 15 Hz, a =5=3°. 



FIGURE 32. Pressure p from different oscillation 



periods f = 15 Hz, a =5=3°. 

 osc o 



