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  • 1. Lin, C.
    et al.
    Lu, C. H.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Liu, Ting
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Study on water flow field around a stationary air bubble attached at the top wall of a Circular Pipe2013Ingår i: Computational methods in multiphase flow VII, WIT Press, 2013, s. 323-338Konferensbidrag (Refereegranskat)
    Abstract [en]

    The presence of bubbles in a pipeline is thought to be one of the reasons to cause the hydraulic-electrical and hydraulic-mechanical facility systems to lose their efficiency. From previous research, the bubble also reduces the effective pipe cross section, which results in a reduction in pipe capacity. The efficiency and service life of pumps and turbines are reduced and shortened consequently. It may even create the interruption of the flow field within blowout phenomenon. As a result, the presence of a bubble in the pipeline is anticipated to create potential hazards. Therefore, it is very interesting to make clear the corresponding variation of a water flow field around a stationary air bubble attached at the top inner-wall of pipe due to the surface problems in contact mechanism of these three phases among the solid wall of pipe, stationary air bubble, and ambient water flow. This study applied flow visualization techniques and high time-resolved PIV to investigate the characteristics of a flow field around a stationary bubble in a fully-developed horizontal pipe flow. Experiments were carried out in a pipe having a constant inner diameter of 9.60 cm and a length of 260.0 cm, yet varied with different bubble volumes (or lengths). Two settling water chambers with different still water levels were connected to both ends of the circular pipe. Titanium dioxide powder being uniformly dispersed in the pipe flow was used as a tracer both for flow visualization tests and for PIV measurements. The results show that a horseshoe vortex and reverse flow at the upstream and downstream of the bubble respectively are commonly seen in all test cases. The experimental results also show that the shape and volume of a bubble highly affect the flow field in the surroundings of the stationary air bubble. Since the bubble surface is slippery, flow velocity exists on the surface of a bubble. As a result, the reverse flow at the end of a long-flat bubble would not affect the velocity on the bubble surface.

  • 2. Lin, Chang
    et al.
    Liu, Ting
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Lu, Chia-Hsun
    Visualizing Conduit Flows around Solitary Air Pockets2014Ingår i: Journal of engineering mechanics, ISSN 0733-9399, E-ISSN 1943-7889, Vol. 141, nr 5Artikel i tidskrift (Refereegranskat)
    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.

  • 3. Lin, Chang
    et al.
    Lu, CH
    Liu, Ting
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Characteristics of air-water interface of air pockets in a conduit2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    The presence of air pockets in a pipeline system often causes reduction in its efficiency and shortens its service life. Potential safety hazards arise in some cases from air blowout or blowback. It is thus of interest to examine the water flow field at air pockets and the feature of water-air interface. This study applied flow visualization technique and high-speed particle image velocimetry (HSPIV) to investigate characteristics of flow fields at stationary solitary air pockets in a fully-developed horizontal pipe flow. Experiments were performed in a Plexiglas pipe having an inner diameter of 9.6 cm, with Titanium dioxide powder as tracer for measurements. The results show that a horseshoe vortex and reverse flow pattern existed both up- and downstream of the air pockets. A deformable air pocket in the turbulent flow caused streamwisely a random movement of both stagnation and separation points around their mean positions. An intermittent flow re-attachment occurred also downstream of the mean separation point. The air-water interface was not stationary but moved with the adjacent water flow.

  • 4.
    Liu, Ting
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Air-pocket transport in conjunction with bottom-outlet conduits for dams2011Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Undesired air entrainment in bottom outlet conduits of dams may cause pressure transients, leading to conduit vibrations, blowback, discharge pulsation and even cavitation, and jeopardize the operational safety. Due to design limitations or construction costs, it is impossible to create an air free environment in a pressurized pipe. Therefore, it is essential to understand the air transport in enclosed pipes in order to provide guidance in bottom outlet design and operation. The commonly used criterion of the air-pocket movement in pipe flow is the water flow velocity for starting moving an air pocket, the so-called critical velocity.

    In this thesis, the classical Volume of Fluid (VOF) model combined with the k-ε turbulence model is adopted for the computation of the critical velocity of a 150-mm pipe. The computed critical velocities are compared with the experimental results. The governing parameters investigated in this study include pipe slope and diameter, wall shear stress and air-pocket volume. Meanwhile, the carrying capacity (air-pocket velocity/ flow velocity) at all pipe slopes are analyzed. The simulation results of air pockets with different volumes in the bottom outlet conduit of Letten Dam in Sweden are presented in this study.

    Moreover, experimental study was conducted to measure the critical velocity for a 240-mm Plexiglas pipe. The results are in agreement with the experiments performed by HR Wallingford (HRW) in 2003 in terms of the effects of pipe slope and air-pocket volume; however, the critical Froude pipe number is slightly smaller in this study. In rough pipes, a larger critical velocity is required compared with that in the smooth pipe. The removal mechanism in the rough pipe involves the successive loss of air caused by turbulence. This explains that the air-pocket size, with the dimensionless air-pocket volume n < 0.015, has little impact on the critical velocity for the rough pipe. In addition, roughness has little impact on the air-pocket velocity when it moves upstream in the downward inclined pipe. The trapped air bubbles most likely remain permanently in the rough pipe.

  • 5.
    Liu, Ting
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Modelling air―water flows in bottom outlets of dams2014Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    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.

  • 6.
    Liu, Ting
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Air-pocket movement in an 18.2 degree downward 240 mm conduit, experimental studies2012Ingår i: Procedia Engineering, ISSN 1877-7058, E-ISSN 1877-7058, Vol. 28, s. 791-795Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Experiments are carried out in a test rig, consisting of a Plexiglas pipe with an inner diameter of 240 mm and an inclination of 18.2o, to investigate air-water two-phase flows in conjunction with bottom spillways. Results show that the critical velocity, which is the minimal water velocity to start moving an air pocket, in the rough pipe, is independent of the air-pocket volume; in the smooth pipe it doesn’t increase with increasing diameter as much as the previous researchers indicated. Pipe roughness doesn’t affect the velocity of the air-pocket when it moves upstream in the downward inclined pipe.

  • 7.
    Liu, Ting
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    CFD Modeling of Air Pocket Transport in Conjunction with Spillway Conduits2011Ingår i: 11th International Conferenceon Fluid Control, Measutements and Visualization, Keeling, Taiwan, December 2-9 2011, 2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper focuses on simulations of enclosed air pocket movements in conjunction with bottom outlet operations. The critical velocity of water for air pocket transport in pipe is the minimal flow velocity for the air pocket start to move downstream. A numerical model is developed to simulate the critical velocity of air pocket transport in pipe flow and to discuss the impacts of tunnel slope, size of the air pocket and wall roughness. The computations are performed in FLUENT using Volume of Fraction (VOF) model combined with k-epsilon model. Parallel computing is adopted for high computational performance.

    The modeled critical velocity is compared with experimental results and they increase with increasing slopes. However, as the roughness height defined in the model is not big enough to represent the reality and no wall shear stress is applied in the upper wall where air pocket and wall contact, the modeled critical velocity is smaller than the experimental ones. Therefore, wall roughness contributes to keep the air pocket from moving downstream which is important in modeling critical velocity. However, by assuming a constant wall shear stress for the air phase the same as the water phase will overestimate the shear stress on the air pocket.

    Two air pocket volumes are simulated at the slope 0.8 degrees which shows the bigger the air pocket is the higher the critical velocity is. Modeling results also show that the critical velocity is non-zero in horizontal pipe and there is a limit for the carrying capacity at all slopes. The simulations of air pockets with different volumes in the bottom tunnel of Letten dam in North of Sweden is shown in this paper as well.

  • 8.
    Liu, Ting
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Experimental studies of air pocket movement in a pressurized spillway conduit2013Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 51, nr 3, s. 265-272Artikel i tidskrift (Refereegranskat)
    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.

  • 9.
    Liu, Ting
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik, Vattendragsteknik.
    Experiments of Air-pocket Movement in an 18.2 degrees downward 240-mm Conduit2012Ingår i: 2012 International Conference On Modern Hydraulic Engineering, Elsevier, 2012, s. 791-795Konferensbidrag (Refereegranskat)
    Abstract [en]

    Experiments are carried out in a test rig, consisting of a Plexiglas pipe with an inner diameter of 240 mm and an inclination of 18.2o, to investigate air-water two-phase flows in conjunction with bottom spillways. Results show that the critical velocity, which is the minimal water velocity to start moving an air pocket, in the rough pipe, is independent of the air-pocket volume; in the smooth pipe it doesn't increase with increasing diameter as much as the previous researchers indicated. Pipe roughness doesn't affect the velocity of the air-pocket when it moves upstream in the downward inclined pipe.

  • 10.
    Liu, Ting
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik (flyttat 20130630), Vattendragsteknik (flyttat 20130630).
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik (flyttat 20130630), Vattendragsteknik (flyttat 20130630).
    Incipient motion of solitary air pockets in a rectangular pipe2013Ingår i: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 1, nr 1, s. 58-68Artikel i tidskrift (Refereegranskat)
    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.

  • 11.
    Liu, Ting
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Three-Dimensional Computations Of Water-Air Flow In A Bottom Spillway During Gate Opening2014Ingår i: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 8, nr 1, s. 104-115Artikel i tidskrift (Refereegranskat)
    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.

  • 12.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Liu, Ting
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Bottacin Busolin, Andrea
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Lin, Chang
    National Chung Tsing University, Taiwan.
    Effects of intake-entrance profiles on free-surface vortices2014Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 52, nr 4, s. 523-531Artikel i tidskrift (Refereegranskat)
    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.

  • 13.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Liu, Ting
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Lin, Chang
    Kao, MJ
    Characteristics of water flow field around an air bubble attached at top of a downward-inclined pipe2013Ingår i: ISTP 24: Proceedings of the 24th International Symposium on Transport Phenomena, Yamaguchi, Japan, 1-5 November 2013, 2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    Flow visualization techniques and high time-resolved PIV were used to investigate the characteristics of water flow field around a stationary air bubble attached at the top of inner-wall of a fully-developed, downward-inclined pipe. Experiments were carried out in a downward-inclined pipe having a slope of 4o, a constant inner diameter of 9.6 cm, and a length of 260.0 cm. Two settling water chambers with different still water levels were connected to the inlet and outlet of the downward-inclined pipe. A pump having a power of 4 Hp was installed between two chambers and used to drive the flow through the inclined pipe. A tilting, honeycomb-like flow regulator made of many straws was placed in front of the pipe entrance in order to smooth the inlet flow. Titanium dioxide powder being uniformly dispersed in the pipe flow was used as tracer both for flow visualization tests and for PIV measurements. The results not only show that horseshoe vortex and reverse flow generated, respectively, at the upstream and downstream of the air bubble can be easily observed in all test cases; but also depict that the flow bifurcates around the stagnation point located at the leading edge of air bubble and prominent formation of the shear layer starts from the separation point and evolves right beneath the air bubble. Based on the precise determination of the specific length scale bs (indicating the representative thickness of a shear layer with the center position being located at ysc) and the specific velocity scale (u – us2) (showing the velocity deficit between the lower and upper bounds of the shear layer), a similarity profile can be obtained with the form of (u – us2)/(us1 – us2) versus the dimensionless shifted height, (y – ysc)/bs , for the mean streamwise velocity in the shear layer beneath the air bubble.

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