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Flows over flip-bucket aerators, physical and CFD modeling with prototype tests
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The paper deals with a unique spillway structure, in which an aerator is incorporated in each flip bucket to aerate the flow jet and avoid sub-pressure in the air cavity below. In terms of jet breakup and stability, the physical models and the prototype lead to contradicting conclusions. CFD is performed to help seek the reason for the discrepancy. With sealed aerators, the model studies exhibit intact flow jets featuring negative cavity air pressure and oscillations, suggesting the need for jet aeration. Both the field observations and CFD indicate that the jets break up, allowing air to penetrate into the air cavities. The resulting cavity air pressure drops are small. The discrepancy is due to the effect of surface tension in the physical models leading to the formation of the enclosed air cavities with negative air pressure, which together with air entrainment in the tail-water gives rise to the jet oscillations. It is suggested, for similar flow phenomena, that compound modelling be performed so as to make amends for physical model test results. 

Keywords [en]
spillway, aerator, model tests, prototype observations, CFD
National Category
Other Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-202390OAI: oai:DiVA.org:kth-202390DiVA, id: diva2:1076503
Note

QC 20170224

Available from: 2017-02-23 Created: 2017-02-23 Last updated: 2017-03-03Bibliographically approved
In thesis
1. CFD MODELLING OF TWO-PHASE FLOWS AT SPILLWAY AERATORS
Open this publication in new window or tab >>CFD MODELLING OF TWO-PHASE FLOWS AT SPILLWAY AERATORS
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Due to the high-speed flow in a chute spillway, cavitation damages often occur. This undesired phenomenon threatens the safety of the structure. For the purpose of eliminating the damages, an aerator is often installed in the spillway. To understand its characteristics, physical model tests are a popular method. To complement the model tests, computation fluid dynamics (CFD) simulations are used to study aerator flows. To represent the two-phase flows, multiphase models should be employed. This thesis examines two of them, namely, the Volume-Of-Fluid model (VOF) and Two-Fluid model.

Based on the background of the Bergeforsen dam, the aerator flow is modelled by means of the VOF model. The simulated spillway discharge capacity is in accordance with the experimental data. Compared with the results, empirical formulas fail to evaluate the air supply capacity of aerator as it is wider than the conventional width. A hypothetical vent modification is proposed. For the original and proposed layouts, the study illustrates the difference in the air-flow conditions. The results show that a larger vent area is, for a large-width aerator, preferable in the middle of the chute.

To study the flip bucket-shaped aerators in the Gallejaur dam, physical model tests and prototype observations are conducted. The results lead to contradicting conclusions in terms of jet breakup and air entrainment. A CFD model is, as an option, employed to explain the reason of the discrepancy. The numerical results coincide with the prototype observations. The jet breakup and air entrainment are evaluated from air cavity profiles; the air-pressure drops are small in the cavity. The discrepancy is due to overestimation of the surface-tension effect in the physical model tests.

Based on the experimental data of an aerator rig at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, the Two-Fluid model is used to predict air concentration distributions in the aerated flow. The model includes relevant forces governing the motion of bubbles and considers the effects of air bubble size. The numerical results are conformable to the experiments in the air cavity zone. Downstream of the cavity, the air concentration near the chute bottom is higher, which is presumably caused by the fact that the interfacial forces in the Two-Fluid model are underestimated.

Place, publisher, year, edition, pages
Stockholm: KTH, 2017. p. 44
Series
TRITA-HYD ; 2017:02
Keywords
spillway; cavitation; aerator; air entrainment; air–water flow; VOF model; Two-Fluid model
National Category
Other Civil Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-202392 (URN)978-91-7729-304-0 (ISBN)
Presentation
2017-03-20, B24, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170224

Available from: 2017-02-24 Created: 2017-02-23 Last updated: 2017-02-24Bibliographically approved

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Flows over flip-bucket aerators, physical and CFD modeling with prototype tests(1972 kB)141 downloads
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Teng, PenghuaYang, James

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