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Visualizing Conduit Flows around Solitary Air Pockets
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.
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. Vol. 141, no 5
Keyword [en]
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
URN: urn:nbn:se:kth:diva-141192DOI: 10.1061/(ASCE)EM.1943-7889.0000867ISI: 000354095000008ScopusID: 2-s2.0-84927936703OAI: diva2:695570

QC 20160104

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2016-02-18Bibliographically approved
In thesis
1. Modelling air―water flows in bottom outlets of dams
Open this publication in new window or tab >>Modelling air―water flows in bottom outlets of dams
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.
TRITA-LWR. PHD, ISSN 1650-8602 ; 2014:02
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
urn:nbn:se:kth:diva-141182 (URN)978-91-7595-017-4 (ISBN)
Public defence
2014-02-28, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)

QC 20140211

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

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