Observations of nuclei in cavitating flows

Abstract

The present report focuses on the source of the large difference between the theoretical strength of pure liquid and the actual tension that is required to initiate cavitation in technical fluids such as test facilities and natural waters. This discrepancy is commonly explained by the existence of nuclei, either solid particles or vapor and gas bubbles that permit phase transition to take place near equilibrium. The existence of these nuclei, their source and lifetimes have occupied much space in the technical literature for decades. Yet, direct observations of tests in applications to naval hydrodynamics and hydraulic machinery flows has not provided much information about these nuclei. Their existence, however, and their effect on cavitation is in no doubt as is demonstrated by the series of photographs of a propeller that were taken in the ‘Vacu-Tank’ of NSMB shown in Figure 1. There, addition of ‘nucleating’ sources to the water by electrolysis clearly increases the number of visible cavitating bubbles on the blade surfaces.