Pit clustering in cavitation erosion

An investigation of the erosion effects of cavitation on a thin aluminium foil was made. Cavitation was generated in a small tank with capacity of about 500 ml of clean water. The growth and collapse of bubbles was triggered by means of ultrasound. The sustained damage was measured by evaluating the area of the damaged surface in time intervals of 4 s. Also the length of the boundary between the damaged and the undamaged surface and the characteristic pit/hole size were measured. The goal of the study was to investigate the phenomenon of pit clustering within the incubation period and the influence of the already eroded surface on appearance of new pits. Additionally, the possibility of self-amplification of the erosion due to the presence of small deformations (pits) was studied. The results evidently show that pits tend to appear in clusters and near already damaged surface and this can be explained by the fact that the already present deformations on otherwise polished alloy surface act as the “cavitation generators” and cause more bubbles to appear and collapse in that region. Determining the physics of these phenomena is of great importance for engineering applications (preventing erosion in water turbines, pumps, diesel engine nozzles, etc.), also because most studies, that deal with predicting the actual mass loss, investigate the erosion in the incubation period and then extrapolate the results to the time scale of a real machine operation (B. Bachert, G. Ludwig, B. Stoffel, S. Baumgarten, Comparison of different methods for the evaluation of cavitation damaged surfaces, ASME Fluid Engineering Division Summer Meeting and Exhibition, Houston, 2005; J.P. Franc, J.M. Michel, Fundamentals of Cavitation, Kluwer Academic Publishers, 2004).