Scale Effects on Cavitation Inception in Submerged Jets

Abstract

The present work is an investigation into the scale effects on cavitation inception in submerged water jets. Four scale effects were studied: (i) jet size, (ii) jet velocity, (iii) dissolved air content, and (iv) the nuclei population in the flow. The nuclei population was artificially altered by "electrolysis seeding." Holography and schlieren photography were used to observe the flow. Direct measurements of the nuclei population were also accomplished by holography. In addition, the instantaneous pressure field in the jet was successfully mapped out using specially tailored bubbles as pressure sensors. It was found that inception did not generally occur in the cores of the turbulent eddies and that the region in which the cavities were first seen were dependent on the size of the jet. Pressure measurements showed that negative peak pressure fluctuation intensities of as high as 120 percent of the dynamic head existed in the jet. The results also revealed that the instantaneous pressure fluctuations have a slightly skewed bell shape probability distribution. For the present tests, the inception index was independent of the exit velocity for a constant size jet. However, when the flow was seeded, the inception number showed a linear dependence on velocity and this dependence increased with increased number of seeded nuclei. The effects of the nuclei number density and pressure fluctuations are incorporated in a "probable cavitation occurrence" parameter which shows promise in reducing scatter in this type of experiment.

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