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Modelling air―water flows in bottom outlets of dams
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

If air is entrained in a bottom outlet of a dam in an uncontrolled way, the resulting air pockets may cause problems such as blowback, blowout and loss of discharge capacity. In order to provide guidance for bottom outlet design and operation, this study examines how governing parameters affect air entrainment, air-pocket transport and de-aeration and the surrounding flow structure in pipe flows. Both experimental and numerical approaches are used.

Air can be entrained into the bottom outlet conduit due to vortex formation at the intake if the intake submergence is not sufficient. The influent of the intake entrance profiles and channel width on the critical submergence were studied in the experiment.

The experimental study was performed to investigate the incipient motion of air pockets in pipes with rectangular and circular cross sections. The critical velocity is dependent on pipe slope, pipe diameter, pipe roughness and air-pocket volume. If the pipe is horizontal, air removal is generally easier in a rectangular pipe than in a circular pipe. However, if the pipe is downward-inclined, air removal is easier in a circular pipe.

When a bottom outlet gate opens, air can become entrained into the conduit in the gate shaft downstream of the gate. Using FLUENT software, the transient process of air entrainment into a prototype bottom outlet during gate opening is simulated in three dimensions. The simulations show in the flow-pattern changes in the conduit and the amount of air entrainment in the gate shaft. The initial conduit water level affects the degree of air entrainment. A de-aeration chamber is effective in reducing water surface fluctuations at blowout.

High-speed particle image velocimetry (HSPIV) were applied to investigate the characteristics of the flow field around a stationary air pocket in a fully developed horizontal pipe flow. The air pocket generates a horseshoe vortex upstream and a reverse flow downstream. A shear layer forms from the separation point. Flow reattachment is observed for large air pockets. The air―water interface moves with the adjacent flow. A similarity profile is obtained for the mean streamwise velocity in the shear layer beneath the air pocket.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2014. , xiv, 32 p.
Series
TRITA-LWR. PHD, ISSN 1650-8602 ; 2014:02
Keyword [en]
Air pocket, Air entrainment, Bottom outlet, Critical velocity, Critical submergence, CFD, Experiment, Vortex, PIV, Two-phase air―water flow
National Category
Civil Engineering
Research subject
Land and Water Resources Engineering; Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-141182ISBN: 978-91-7595-017-4 (print)OAI: oai:DiVA.org:kth-141182DiVA: diva2:695501
Public defence
2014-02-28, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140211

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2014-02-11Bibliographically approved
List of papers
1. Effects of intake-entrance profiles on free-surface vortices
Open this publication in new window or tab >>Effects of intake-entrance profiles on free-surface vortices
2014 (English)In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 52, no 4, 523-531 p.Article in journal (Refereed) Published
Abstract [en]

Intake free-surface vortices can cause efficiency losses, flow fluctuations and even structural damages. Experiments were performed to examine the effect of entrance shapes on the critical submergence. Seven entrance shapes were devised and tested, including a square-edged, a bell-mouthed, three symmetrical conical and two conical profiles with eccentricity. The focus of the study was on a range of Froude numbers from 0.25 to 0.65. The square-edged shape appeared to show the highest local head-loss compared to other shapes. Steady counter-clockwise vortices characterize all the intake profiles except in a narrow water tank. The experiments show both discrepancy and similarity between the intake profiles. The critical submergence of the bell-mouthed intake is lower when compared to the square-edged shape. For the other profiles, it is proportional to the Froude number. A closer sidewall may lead to larger critical submergence in the case of weak circulations. The results demonstrate that the intake-entrance profile has an important effect on the critical submergence.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2014
Keyword
Critical submergence, downward intake, entrance profile, free-surface vortex, Froude number
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-141190 (URN)10.1080/00221686.2014.905504 (DOI)000341847100009 ()2-s2.0-84912001646 (Scopus ID)
Note

QC 20160104

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved
2. Experimental studies of air pocket movement in a pressurized spillway conduit
Open this publication in new window or tab >>Experimental studies of air pocket movement in a pressurized spillway conduit
2013 (English)In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 51, no 3, 265-272 p.Article in journal (Refereed) Published
Abstract [en]

Undesired air entrainment in a bottom outlet conduit causes pressure transients, leading to conduit vibrations, blowbacks and discharge pulsations and thus endangers operational safety. In this study, the propagation velocity of a solitary air pocket and the characteristics of its critical velocity were examined in experiments conducted using a 240-mm-diameter pipe. Air pocket movement depends on the pipe diameter, slope, roughness and air pocket size. The critical pipe Froude number for initiating downstream movement of an air pocket is smaller in a larger pipe, most likely due to the scale effect and/or to a smaller reduction in the effective cross-sectional area. The critical velocity in rough pipes was found to be independent of the air pocket size. A minimum Froude number was suggested for a rough pipe instead of a critical pipe Froude number because the air removal process was found to involve successive air losses from the air pocket caused by turbulence.

Place, publisher, year, edition, pages
Taylor & Francis, 2013
Keyword
Air pocket, bottom outlet, critical velocity, large diameter pipe, roughness effect
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-52308 (URN)10.1080/00221686.2013.777371 (DOI)000320361500004 ()2-s2.0-84879825055 (Scopus ID)
Note

QC 20130806. Updated from submitted to published.

Available from: 2011-12-15 Created: 2011-12-15 Last updated: 2017-12-08Bibliographically approved
3. Incipient motion of solitary air pockets in a rectangular pipe
Open this publication in new window or tab >>Incipient motion of solitary air pockets in a rectangular pipe
2013 (English)In: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 1, no 1, 58-68 p.Article in journal (Refereed) Published
Abstract [en]

The operation of bottom-outlet gates often gives rise to entrained air in the form of air pockets in the conduit under full-flow conditions. If unexpectedly released, it would cause problems for both personnel security and operational function. The present study addresses, through experimentation, the incipient movement of solitary air pockets in a rectangular pipe. A horizontal pipe and a 9.6° downward-inclined pipe are examined. The cross-section of the pipe measures 200 mm (width) by 250 mm (height). As distinct from a circular pipe, an air pocket in the rectangular pipe exhibits, at its incipient motion, a shape that depends mainly on factors such as the sloping angle of the pipe, cross-sectional location of the air pocket and its volume. These factors also determine the critical velocity of the air pocket. The experiments have shown that only small air pockets can exist under the roof. The corner is a cross-sectionally equilibrium position for larger air pockets. The air pocket in the corner position takes the shape of an elongated rectangular prism in the horizontal pipe and a triangular prism in the sloping one. When compared with a circular pipe, the critical velocity of air pockets in the rectangular pipe is lower if the pipe is horizontal and higher if it has a downward inclination.

Place, publisher, year, edition, pages
Taylor & Francis, 2013
Keyword
bottom outlet, rectangular conduit, air entrainment, incipient motion, critical velocity, experiments
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-141191 (URN)
Note

QC 20140211

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved
4. Visualizing Conduit Flows around Solitary Air Pockets
Open this publication in new window or tab >>Visualizing Conduit Flows around Solitary Air Pockets
2014 (English)In: Journal of engineering mechanics, ISSN 0733-9399, E-ISSN 1943-7889, Vol. 141, no 5Article in journal (Refereed) Published
Abstract [en]

Understanding flow characteristics around air pockets is fundamental in the study of air entrainment and transport in pipelines. This study deals with the use of flow visualization technique (FVT) and high-speed particle image velocimetry (HSPIV) in exploration of the characteristics around stationary air pockets in horizontal-pipe flow. The air-pocket volume varies from 0 to 10.0 mL, and the air pocket is injected into a fully developed turbulent flow with Reynolds numbers between 17,000 and 18,400. In the plane of symmetry, the main flow features include (1) a horseshoe vortex upstream, (2) a stagnation point on the frontal interface, (3) a separation point and a separated shear layer beneath, (4) a reattached shear layer downstream of the reattachment point (for air-pocket volumes greater than 2.0 mL), and (5) a reverse-flow region downstream. The deformable air pocket in the turbulent flow causes streamwise random movements of both the stagnation and separation points around their mean positions. The flow pattern is categorized based on the occurrence of either separated flow or flow reattachment. Fully separated flow (Mode I) occurs at air-pocket volumes less than 2.0 mL. Intermittently reattached flow (Mode II) occurs if the volume is within 2.0–5.0 mL. Fully reattached flow (Mode III) is evident at volumes greater than 5.0 mL. Water particles on the air-pocket surface move with the adjacent flow, thus forming a slip boundary. The evolution of mean streamwise velocity beneath the air pocket demonstrates the formation of either a separated or a reattached shear layer. Using nonlinear regression analysis, appropriate characteristic velocity and length scales are determined to obtain similarity profiles in the separated shear layer beneath.

Place, publisher, year, edition, pages
American Society of Civil Engineers (ASCE), 2014
Keyword
Air-pocket, Flow visualization, High-speed particle image velocimetry (HSPIV), Air-water surface, Flow separation, Flow reattachment, Similarity profile, Shear layer
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-141192 (URN)10.1061/(ASCE)EM.1943-7889.0000867 (DOI)000354095000008 ()2-s2.0-84927936703 (Scopus ID)
Note

QC 20160104

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved
5. Three-Dimensional Computations Of Water-Air Flow In A Bottom Spillway During Gate Opening
Open this publication in new window or tab >>Three-Dimensional Computations Of Water-Air Flow In A Bottom Spillway During Gate Opening
2014 (English)In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 8, no 1, 104-115 p.Article in journal (Refereed) Published
Abstract [en]

Undesired entrainment of air in a bottom spillway often leads to problems in both safety and operational functions. A numerical analysis of a transient process of air entrainment into bottom spillway flows when a spillway gate is opened was conducted in this study. The Volume of Fluid (VOF) model was used. The 3D computational domain consisted of a spillway conduit, a moving bulkhead gate, a gate shaft, an upstream reservoir and a downstream outlet. The large number of cells, together with the dynamic mesh modelling of the moving gate, required substantial computational resources, which necessitated parallel computing on a mainframe computer. The simulations captured the changes in the flow patterns and predicted the amount of air entrainment in the gate shaft and the detrainment downstream, which help in the understanding of the system behaviour during opening of the spillway gate. The initial conduit water level and the gate opening procedure affect the degree of air entrainment in the gate shaft. To release the undesired air, a de-aeration chamber with a tube leading to the atmosphere was added to the conduit. Despite the incomplete air release, the de-aeration chamber was found to be effective in reducing water surface fluctuations in the downstream outlet.

Place, publisher, year, edition, pages
Taylor & Francis, 2014
Keyword
bottom spillway, moving gate, air entrainment, two-phase flow, CFD
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-141194 (URN)000332216500009 ()2-s2.0-84896998070 (Scopus ID)
Note

QC 20140211

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved

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