Motion of air cavities in long horizontal ducts

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Abstract
This paper describes an experimental study of the flow associated with the intrusion of an air cavity into a long horizontal duct as water was allowed to drain from one end. Flows of this nature were discussed by Benjamin (1968), who showed that throttling of the flow of water from the end of the duct would cause both the celerity and the depth of the cavity to reduce. However, the experiments described in this paper revealed that the celerity of the cavity was not reduced from its unthrottled value until the water depth beneath the cavity was 0·78 of the duct depth. For values of this depth ratio between 0·5 and 0·78, the flow as a whole was unsteady. It is shown that Benjamin's model can be modified to allow for the unsteady nature of the flow. Benjamin's original model was found to describe accurately the form and behaviour of the cavity in the case of unthrottled flow, when the flow was steady, and also when the depth beneath the cavity exceeded 0·78 of the duct height, when the flow was again steady. Surface-tension effects were found to reduce the celerity of the cavity and to modify its shape as described by Gardner & Crow (1970).

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