Cavitation Erosion: A critical review of physical mechanisms and erosion risk models

Abstract

This work aims at a postprocessing procedure for the assessment of the cavitation erosion risk based on multiphase CFD results an on experimental observations. Existing procedures often use available information, such as the rms value of the vapour fraction in a particular area, without thorough justification of this criterion. This paper aims at linking the available information that comes from multiphase RANS or experimental observations with High Speed Video, with the many publications on fundamental mechanisms of cavitation dynamics. The first objective of this paper is to review physical mechanisms for cavitation erosion loads that have been suggested in the literature. These mechanisms are evaluated with observations that are available from full scale ships where cavitation has lead to erosion damage on the rudder or the propeller. A second objective is to review risk assessment models that use CFD results or experimental results as input for the prediction of the risk of cavitation erosion. A detailed phenomenological description of the process leading to cavitation erosion from sheet cavitation and vortex cavitation is hypothesized. The process is based on the conversion of potential energy contained in the cavity and a focusing of this energy in space and in time that is governed by ring vortices or horseshoe vortices in case the ring vortices attach to a surface. It is concluded from experiments by Kawanami etal. [1] that horseshoe vortices tend to concentrate the vorticity toward the material surface. It is hypothesized in this paper that this concentrated vorticity forms a mechanism to break a monolithic cavity up into bubbles due to instabilities caused by the high fluid velocities and to concentrate all microbubbles in space by centrifuging out the heavier liquid particles. The radiated shockwaves caused by the implosion of one microbubble is then hypothesized to be sufficient to initiate a synchronized implosion of the cloud of microbubbles in the immediate vicinity. A selection of cavitation erosion models available in open literature is reviewed in this paper. It is concluded that the models by Bark et al. [2] and Fortes-Patella et al. [3] appear to offer the best frameworks to be coupled to the mechanisms hypothesized in this paper.