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 [12] 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 [1, 2, 3, 8]. 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 [9] 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. The importance of these nuclei has led to the development of several detection and observation techniques [4, 6, 10, 11]. These include the microscopic observation of water samples, the 'coulter counter', 'single particle light scattering', the 'acoustic' techniques for detection of gas bubbles, holograms with microscopic observation of the reconstructed image, and finally the venturi 'liquid quality' meter [11]. Some results of the nuclei number density distribution function N(R) that were accumulated from various sources [6] in different experimental facilities and some open sea measurements are presented in Figure 2. The results include holographic observation in the Caltech HSWT which are found to be mostly solid particles and the Caltech LTWT which are found to be mostly bubbles. This difference in the type of nuclei may explain why is it easier to cavitate the same body in the LTWT (provided that the flow is non-separating) while the liquid in the HSWT can support a certain tension [6]. Note the several orders of magnitude difference in the populations shown and the correspondence between the oceanic acoustic measurements of Medwin [10] that display the population of bubbles and the holographic observations of plankton [5, 13]. It is interesting that some of these facilities are well above the 'natural' levels and it may be inferred that the Vacu-Tank may be well below. Thus, significant 'scaling' errors may occur while applying laboratory results to field cavitation phenomena [1, 2, 3].

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