54 



available. However, visible light has been observed in the interior of liq- 

 uids cavitated by high-frequency acoustic pressures 85 so that the above ob- 

 jection may not be valid. When the results of Reference 85 were brought to 

 the writer's attention, tests were made in the same nozzle used by Crump 37 

 wherein the high tensions mentioned previously were obtained. Observations 

 were made under conditions of tension with individual bubbles of the type 

 which formed occasionally and disappeared so that the cavitation was inter- 

 mittent. The rate of growth of the bubble is very high and this rate is cer- 

 tainly toward the upper limit of rate of growth which might occur in practice. 

 Despite extreme precautions as to darkness of the laboratory and darkness 

 adaptation, no light flashes were visible to the eye nor could any be recorded 

 on a photographic plate. In view of the results of Reference 85, perhaps the 

 emission of light is associated with the frequency of formation of cavities. 

 Nevertheless, that damage is somehow associated with this occurrence seems 

 untenable since damage appears to occur in the regions of collapse rather than 

 formation. 



A much more tenable proposal relating electrical phenomena to dam- 

 age is outlined by Petracchi. 86 Petracchi indicates that additional corrosion 

 not accounted for entirely by fatigue is associated with the corrosive effects 

 of the electrical currents generated in adjacent crystals of the solid as a 

 result of the alternating mechanical stresses and deformations produced by the 

 forces associated with the collapse of cavitating regions. This would account 

 for the lack of correlation between hardness and ductility of metals and re- 

 sistance to corrosion. 87 Petracchi points out that the presence of the oil 

 in Poulter's experiments may prevent crystal deformation and, thus the asso- 

 ciated electrical phenomena and damage. However, this problem is evidently 

 one of the physics of crystal lattices and will not be elaborated here. 



The conclusion which may be drawn from the above discussion is that 

 the origin of cavitation erosion is in the high collapse pressures, whether 

 the damage is directly a result of fatigue caused by impacts or by corrosive 

 action due to electrical currents generated by crystal deformation in the 

 solid under these impacts. 



SOME REMARKS ON SCALING OP CAVITATING SYSTEMS 



The use of scaled models in reproducing flows with cavitation is 

 well known. However, it might be well to point out a few precautions which 

 must be observed in such tests. The fundamental parameter relating the model 



P - Pir 



and prototype is the cavitation number a = x „^ v . This parameter estab- 

 lishes the condition of similarity of pressures and is, thus, merely the Euler 

 number or another form of pressure coefficient. Although the vapor pressure 



