Prediction of cavitation damage

Abstract

A theory which gives a unified picture of (1) the bubble collapse, (ii) the transmission of the collapse pressure, and (iii) the deformation of the material due to the work done by the bubble is proposed, incorporating the ideas of some of the earlier investigations. Localized dents produced by the short-range shock waves have been photographed in support of this theory. A non-dimensional number called “Cavitation Number” has been proposed based on the concept of dynamic indentations produced by the short-range shock waves due to the collapse of the cavitation bubbles. The Cavitation Number gives the ratio of the energy absorbed by the material for complete fracture to the energy of collapse of the bubble. This ratio has been correlated with another non-dimensional factor named the “Severity Number”, which represents the number of cycles the indentation process has undergone in a given period. Within the experiments, there exists a unique correlation between these two numbers. It is demonstrated how this approach can be used to predict the cumulative average depth, the cumulative volume of erosion, and the cumulative weight loss. Some possible explanations are offered for the effect of:

properties of the material, effect of size and geometry of the flow system, effect of time, effect of pressure, effect of velocity (including power-law and linear flow), and effect of the properties of the liquid.

The concepts of incubation period and threshold velocity are discussed in the light of the present theory. A non-dimensional number known as the “Resistance Number” is derived, by which it is possible to specify the material’s resistance to cavitation damage. This number has been correlated with the critical cavitation, showing that it is possible to predict the length, breadth, and area of erosion if the geometry of the cavitating system and the value of the cavitation parameters are known. These results indicate the possibilities of predicting cavitation on a given material and thus aid in the proper choice of the material for fabricating any hydraulic machine or structure.