Cyclic Instabilities at Circular Crested Weirs

The movie presents a time lapse of the cyclic overflow pattern above a half-round circular weir. The experiment was conducted at the University of Queensland, in the Hydraulics Laboratory of the School of Civil Engineering. The movie presents a sideview of the nappe overflow. Sometimes, the nappe is attached to the weir and the flow streamlines present a marked curvature, highlighted by the free-surface that is a streamline itself Other times, the nappe is detached with an air cavity underneath. The experiment was conducted with a constant flow rate, left constant for several hours before, and remaining constant for the full duration of the recording. With a such constant flow rate, we would not expect to see such a drastic change in nappe overflow pattern. The changes are caused by the absence of ventilation of the nappe and associated non linear hydrodynamic instabilities (Chanson 2020,2023).The time lapse record includes a frame very 0.5 s, for 60 minutes, and the movie is replayed at 20 fps. Thus, the movie speed to 10 times faster than in real-life. The accelerated movie replay emphasises the cyclic nappe overflow pattern. These cyclic transient instabilities cause very rapid change in hydrodynamic properties, upstream and downstream of the weir, as well as transient loads and vibrations to the weir structure. During a cyclic pattern, the formation of the air cavity occurs very rapidly, often with some loud noise. It is followed by the progressive filling of the cavity, then the air cavity disappearance, and again a sudden cavity re-opening into a large air cavity while keeping a constant flow rate. The periods of the cyclic pattern were typically between 3 and 10 min, although some unusually wider range of periods, from 1 to 12 min, was observed in laboratory.
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ACKNOWLEDGEMENTS
Jason Van Der Gevel
Stewart Matthews
School of Civil Engineering at the University of Queensland (Australia)
References
Chanson, H., and Montes, J.S. (1998). "Overflow Characteristics of Circular Weirs: Effect of Inflow Conditions." Journal of Irrigation and Drainage Engineering, ASCE, Vol. 124, No. 3, pp. 152-162 (DOI: 10.1061/(ASCE)0733-9437(1998)124:3(152))
Tullis, B.P., Crookston, B.M., and Bung, D.B. (2019). "Weir head-discharge relationships: A multilab exercise." Proc. 38th IAHR World Congress, Panama City, 1-6 Sept., IAHR Publication, Lucas Calvo Editor, pp. 486-500 (DOI: 10.3850/38WC092019-0806).
Chanson, H. (2020). "Half-round Circular Crested Weir: On Hysteresis, Instabilities and Head-Discharge Relationship." Journal of Irrigation and Drainage Engineering, ASCE, Vol. 146, No. 6, Paper 04020008, 7 pages & 3 video movies (DOI: 10.1061/(ASCE)IR.1943-4774.0001473).
Chanson, H., and Memory, O. (2022). "Hysteresis, Non-Linearity and Instabilities on Circular Crested Weir." Proceedings of 30th Hydrology and Water Resources Symposium HWRS2022, Brisbane, Australia, 30 November-2 December, Published by Engineers Australia, Brisbane, Australia, Paper 156, pp. 1116-1124 (ISBN 9781925627640).
Chanson, H. (2023). "Half-Rounded Circular Overflow - on Cavity Transients and Non Linear Instabilities." Proc. 40th IAHR World Congress, Vienna, 21-25 August, Helmut Habersack; Michael Tritthart, Lisa Waldenberger Editors, Paper 29340, 1830-1836 (DOI: 10.3850/978-90-833476-1-5_iahr40wc-p0028-cd).