It is shown that the gravity-induced draining of liquid from a tank does not cease when the pressure differential at the outlet is zero if the liquid level is still above the outlet. Instead, either air or exterior liquid is ingested, depending on whether or not the tank outlet is submerged. Analytical models are developed to predict both the onset of ingestion and the subsequent discharge rates; experimental results are used to guide these developments. The models are shown to agree well with tests of draining during air ingestion and during water ingestion when the tank liquid is denser than water. When the tank liquid is less dense than water, however, the predictions of water ingestion are only in qualitative agreement with tests. Some possible reasons for this discrepancy are advanced.

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