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inception. Rood [9], in his paper entitled “Mechanisms of Cavitation Inception”, makes a precise review of research progress during the last twenty years. He underlines that the scaling laws to predict the full-scale cavitation behavior from the model tests, are based on empirical rules, defined on a correlation between measurements at model scale and observations of full-scale characteristics. The need to better understand the scale phenomena and to develop physical prediction laws has led to focus studies on real fluid characteristics. For an engineer, the purpose of any model scale test is the accurate prediction of the full-scale behavior.

Experimental observations have shown the persistence of microbubbles in a real fluid. These free microbubbles, called nuclei, have been found to be a major parameter in the cavitation inception process. According to the study of Billet [10] and Gindroz [11], natural waters such as rivers and oceans contain free microbubbles.

The influence of the nuclei on the cavitation behavior has been written in numerous publications [12 to 30]. To summarize all these studies, the role of free cavitation nuclei was found to be of major importance in the cavitation inception and developments processes. More particularly, we must be able to measure the nuclei content in a facility, when performing cavitation tests on a model.

Thus, in order to relate nuclei size distributions with inception cavitation in cavitation facilities, a test program was conducted in 1992 at the Grand Tunnel Hydrodynamique (GTH) of the Bassin d 'Essais des Carènes. The GTH, which has a complete air control system including dissolved gas and nuclei (microbubbles) control, offered the opportunity to answer this question.

The tests were conducted on three 34 mm diameter propellers used by Kuiper [6], each of these propellers being characterized by a different cavitation type: bubble, sheet and vortex cavitation.

Four different nuclei distributions were generated (Figure 1): strong degassed water (maximum tension, T4), low injection of medium size nuclei (medium tension-low content, T3), large injection of medium size nuclei (medium tension-high content, T2) and large injection of large nuclei (minimum tension, T1). By injecting medium size nuclei for a low content and a high content, we were able to examine the influence of the number of nuclei on the cavitation inception characteristic. During all the tests, the dissolved air content was kept constant.

The GTH on-line Cavitation Nuclei Counter (Centerbody Venturi) was used to measure both the water nuclei distribution and the liquid tension.

Figure 1 summarizes the influence of the nuclei characteristic distributions on the cavitation inception parameter, σi, referred to its value for the T4 nuclei distribution (maximum tension case). The nuclei measurements are presented as a cumulative nuclei distribution, in Number of nuclei per ccm versus their critical pressure referred to the vapor pressure, Pcr—Pv (Figure 1).

FIGURE 1: NUCLEI INFLUENCE ON THE CAVITATION INCEPTION CHARACTERISTIC FOR SHIP PROPELLER

These tests at the GTH clearly demonstrate the direct influence of water quality on propeller cavitation inception (1, 2, 3). With a large increase in liquid tension, little change in cavitation inception was found for blade surface cavitation (sheet cavitation), but, in the case of both tip vortex cavitation and travelling bubble cavitation, the nuclei measurements from the GTH standard Centerbody Venturi correlated with many observed trends. Moreover, for the case of tip vortex cavitation inception, it appears that the measurement of both nuclei size (liquid tension) and nuclei number distribution (event rate) are necessary to correlate the data. Indeed, for the medium tension case, a strong influence on the inception of tip vortex cavitation can be observed from the two different nuclei concentrations (high and low).

The cavitation inception generally corresponds to an event rate of the order of magnitude of 1 event per second. When analyzing the cavitation inception results from the point of view of event rates Gindroz and Billet [3] show that the so-called “water tension” leading to cavitation inception



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