The behaviour of Siphon Spillways, with particular reference to air entrainment and model scale effects

  • David Alan Ervine

    Student thesis: Doctoral ThesisDoctor of Philosophy


    This work is a broad study of the mechanisms of air entrainment and air regulation in self-priming, air-controlled siphon spillways. The early chapters of the work provide the reader with the history and general design principles of siphon spillways as well as a comprehensive review of the research carried out to date in the field of air entrainment and air regulation in hydraulic structures. An attempt is made to outline the main purpose of air - controlled siphon spillways and how they can be modified to function in a great variety of engineering situations.

    In the experimental work, six siphon models are constructed, three models of a high head siphon suitable for a reservoir or dam project, and the remaining three models of a low head siphon suitable for a lake , river or canal project . The models are constructed to the scales 1:20, 1:10 and 1:7. The model scale effects, the rate of air entrainment, the air flow under the inlet lip, the negative air pocket pressures, the priming characteristics , the 'hunting' effect , the energy at inlet and the drawdown velocity are studied in each model for various configurations of inlet and outlet geometry, providing a deep study of the interrelationships existing in siphon spillways .

    The most fundamental and the least understood aspect of siphon behaviour is shown to be the entrainment of air by the falling rectangular nappe. This is studied in controlled conditions independently of the siphon models ~ by relating the amount of air entrained by a rectangular jet plunging into a downstream pool of water to various depths of free fall jet sizes, and a wide range of velocities. The tests reveal an empirical relationship between the rate of entrainment and the jet parameters, providing many applications in engineering practice.

    The results of the air entrainment jet tests are used to attempt a prediction of model and prototype air flows. The predictions are compared with the measured model air flows, showing a close correlation.

    The work concludes with a discussion on the application of the results to future siphon design criteria, and also to other air- entraining structures such as dropshafts, stepped spillways and oxygen uptake at weirs.
    Date of Award1974
    Original languageEnglish
    Awarding Institution
    • Queen's University Belfast

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