495 



The following value is taken as a measure of the 

 accoustical effect of an elementary cavitation 

 process to an accuracy of the potential energy 

 transformation coefficient for the maximally expanded 

 cavity: 



G = 10 log 

 a 



(4) 



Here, R^ is the threshold value of the cavity radius 

 which, for the sake of convenience and without 

 limiting the generality of conclusions, is taken as 

 10-6 m. For large Rm/Ro values this measure differs 

 only slightly from a simpler measure used in Figure 1 



G' = 30£g 

 a 



(5) 



The threshold of the visual observation of cavita- 

 tion is taken as Rg = 10~3 m, which coincides with 

 the upper limit to the size of the cavitation nuclei 

 under study. For the chosen measure of acoustic 

 effect this threshold corresponds to 90 dB. Since 

 the resolution of vision is limited by angular 

 dimension, the measure of the visual effect where 

 the distance to the object of observation remains 

 constant is the first order, linear dimension of 

 the cavity. Hence, when the origins coincide 



B 



i=. 



and the processes below the level of G^^ = 90 dB are 

 out of visual observation. 



Thus, leaving out of account the actual signal- 

 noise ratios, the acoustical recording makes it 

 possible to penetrate much deeper (by 2-3 orders) 

 into the "microcavitation" region. 



Worthy of notice is the qualitative similarity 

 of the curves shown in Figure 1 to the experimental 

 curves of cavitation noise increase against velocity 

 which are given below, as well as by Sturman's data 

 (1974) . It is evident that at an early cavitation 



FIGURE 1. Calculated comparison of visual and acous- 

 tical effects of elementary cavitation process in a 

 limited region of negative pressure. 



stage the predicted levels drop by 20 dB with the 

 velocity decreasing 10-fold. This stage is usually 

 regarded as free of cavitation. 



With the increase of velocity there comes a stage 

 which is sometimes referred to as "true" cavitation 

 and in which the most intensive growth of cavities 

 and cavitation noise is observed. This stage corre- 

 sponds to a decrease and loss of static equilibrium 

 of the cavity. 



At the third stage the intensive cavity growth 

 ceases and asymptotic saturation of the acoustic 

 effect occurs due to the fact that the size of the 

 cavity is nearing that of the zone of negative 

 pressure. The asymptotic values of saturation shown 

 in Figure 1 correspond to the rough estimation 



G =15 + 15 ZqC + 30J,g - 

 as pm R 



(6) 



As to the relationship between visual and noise 

 manifestations of cavitation. Figure 1 allows one 

 to assert that: 



- at sufficiently high levels of ambient noise 

 the acoustic detection of cavitation may coincide 

 with the visual detection or takes place even later; 



- potentially, at a fairly low level of the 

 ambient noise, the acoustic manifestation of cavita- 

 tion must be detected much earlier than the visual 

 one. 



In particular, the acoustic effect of cavitation 

 can be rather strong (e.g., an increase of noisiness 

 by several dozens of decibels) in the case of "micro- 

 scopic" cavitation invisible to the eye. 



The indicated values are largely conditional as 

 the threshold of visual detection may differ under 

 different conditions. Nevertheless they are close 

 to those obtained under laboratory conditions. 



It is of interest that Figure 1 reveals such a 

 contradictory phenomenon as vagueness in respect 

 to cavitation inception. At high levels the curves 

 for various nuclei coincide, so for a more correct 

 determination of cavitation inception one should try 

 to reduce rather than to increase the accuracy of 

 recording methods. The increase of accuracy, as is 

 shown in Figure 1, brings about increasing ambiguity 

 of cavitation inception and expansion of the vague- 

 ness region to cover an increasing range of veloci- 

 ties. However, as the accuracy decreases, more and 

 more small zones of cavitation inception are left 

 out of control. 



The above analysis simplifies the actual processes 

 and can be at variance with them mainly due to the 

 fact that the coefficient of cavity potential energy 

 transformation into acoustic energy is not constant 

 being a complex function of many parameters [Benia- 

 minovich et al. (1975)]. Specifically it may have 

 a much greater value for small cavities as compared 

 to larger cavities. 



2. EXPERIMENTAL STUDY ON MODELS 



There is an urgent need for an effective and well- 

 founded classification of a great variety of forms 

 and types of cavitation which substantially differ 

 in the mechanism of nonstationarity giving rise 

 to noise and having other practical consequences 

 of cavitation. 



The following brief list of the forms and types 

 of cavitation represents a more or less established 

 practice with respect to marine propellers [Goncharov 

 et al. (1977) ] . 



