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  • 1. Borghei, Seyed Mahmood
    et al.
    Nekooie, Mohammad Ali
    Sadeghian, Hadi
    Ghazizadeh, Mohammad Reza Jalili
    Parvaneh, Ali
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Javaheri, Amir
    Kabiri-Samani, Abdorreza
    Discussion: Triangular labyrinth side weirs with one and two cycles2016Ingår i: Proceedings of the Institution of Civil Engineers: Water Management, ISSN 1741-7589, E-ISSN 1751-7729, Vol. 169, nr 3, s. 111-114Artikel i tidskrift (Övrigt vetenskapligt)
  • 2.
    Ekström, Ingvar
    et al.
    Sweco.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Berg, Mikael
    Improving spillway discharge safety at Bergeforsen2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    Bergeforsen is a 35 m high embankment dam. The technical discharge capacity of the existing spillway is 2300 m3/s at full reservoir retention level (FRRL). To meet new design flood criteria a first major rehabilitation was carried out 2003, which allowed discharge of an extreme flood at a water level higher than the FRRL. Recent underwater inspections of the spillway and energy converter have however revealed a need for rehabilitation and adaptation of the concrete structures to the design flood flow conditions. To make it possible to take the spillway out of operation for the time period required for adequate repairs it has been decided to rebuild the dam, adding an alternative spillway structure that will serve as discharge facility during the upgrade of the existing spillway. For this reason, a new 25-m gated spillway, with a stilling basin followed by a curved tunnel section, will be constructed in the left river bank. Once the existing spillway is rehabilitated the combined discharge capacity will make it possible to safely pass the revised design flood at a reservoir level not exceeding the FRRL. A physical model was built to asssit in the design decision, the purpose of which is to verify and optimize engineering solutions for increased discharge capacity, so that the design flood is released without jeopardizing the stuctural integrity of the dam.

  • 3.
    Ekström, Ingvar
    et al.
    Sweco.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Mören, Lena
    Cederström, Malte
    Vattenfall AB.
    Adapting Ligga to higher design flood, spillway channel modification through physical & numerical modeling2007Konferensbidrag (Refereegranskat)
    Abstract [en]

    As part of Vattenfall’s dam-safety program, Ligga dam is to be rebuilt for higher safety level. Besides diverse measures, the spillway channel will be modified. The channel is bounded by natural bedrock. Spillway operations during the past years have evidenced severe erosion and damages in the fractured rock. Rock instability becomes visible even at moderate spillway discharges 800 – 900 m3/s. The updated design-flood of the dam is in the order of the magnitude of 2000 m3/s. To adapt the dam to the higher flood, the spillway channel has to be re-shaped, so that the flood is safety released and the structural integrity of the embankment dam is maintained. As a cost-effective alternative, excavation is recommended, as a large volume of rock material is required for stabilizing the dam and for replacing the slope protection upstream. To achieve satisfactory flow behaviors, hydraulic model tests are made to examine and optimize excavation possibilities. The re-shaping of the waterway is complicated by the fact that the existing channel bends in plan and the excavation must follow roughly the same curvature. Considerations are also given to exemption of the dam toe from erosion and the forest on the left riverside from being flooded. Computer simulations of water-rock interaction are made with the commercial code UDEC. Against previous flood release records, the numerical model of rock erosion is calibrated and uses measured water level, flow velocity and dynamic water pressure in the model as input data. Results show that the new waterway configuration provides proper flow patterns and the rock foundation after reinforcement is not subjected to unacceptable damage at spillway discharges up to the design flood.

  • 4.
    Ferdos, Farzad
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Wörman, Anders
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Characterization of hydraulic behaviours of coarse rock materials in a large permeameter2013Ingår i: Journal of Geoscience and Environment Protection, ISSN 2327-4336, Vol. 1, nr 3, s. 1-6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The hydraulic behavior of a rock material structure is a major feature for its design and safety assessment. Similar to all other physical prob-lems, in order to enclose the governing equations systems and achieve a solution, the hydraulic characteristics of these materials need to be determined experimentally and implemented then into adopted thermo-dynamical models. This paper covers the process of the design, con-struction and operation of an experimental rig built for this specific purpose. Using the constructed large-scale permeameter, tests have been conducted. The non-linear hydraulic behavior of various materials under extreme turbulent conditions, where Reynolds number reaches un-precedented values, has not been studied before. Preliminary results are presented and discussed.

  • 5. Kung, C. S.
    et al.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Energy interpretation of hydraulic transients in pipelines1993Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 31, nr 6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The energy concept sheds light upon the hydraulic system that comprises the throttled surge tank, the tunnel and the penstock. The mathematical manipulation of the governing differential equations of water hammer yields an energy (power) equation in point form. By integration with respect to pipe distance, this relation is extended first to an individual pipe, then to the surge-tank system with branching connection. The system energy (power) equation states the relationship between the total (internal plus kinetic) energy, the convective energy, the work done on the system and the energy dissipation. A case study identifies the conversion between the internal and kinetic energy, and quantifies the behaviour and magnitudes of related energy components of the system.

  • 6. Liang, S. D.
    et al.
    Zhang, Y. L.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    An experimental study on pile scour mitigating measures under waves and currents2015Ingår i: Science China Technological Sciences, ISSN 1674-7321, Vol. 58, nr 6, s. 1031-1045Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An experimental study has been carried out to investigate effects of four flow-altering scour countermeasures placed around a foundation pile under currents only as well as under coexisting waves and currents. The countermeasures are sacrificial piles, downstream bed sill, sleeve and slot in the pile. Their arrangements follow the suggested optimal configurations, with some of them slightly modified. In terms of the evolution of scour depth and bed topography around the pile, the scour countermeasures are evaluated, subjected to steady currents with or without irregular waves. A comparison between maximum scour depth with the countermeasures and one without the countermeasures demonstrates the efficiency of countermeasures. All the tested scour measures reduce the scour depth by 17.6%–42.6% under the action of currents only and 5.8%–24.0% under the combined action of both currents and waves. The results also show that it takes a shorter time for the scour depth to reach its equilibrium with the measures under either coexisting currents and waves or currents only than ones without the measures.

  • 7.
    Liang, SD
    et al.
    Tsinghua University.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    An experimental study of scour countermeasures for offshore piles in marine environment2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    Four commonly used scour countermeasures, i.e. riprap, horizontal collar, tetrahedral frame and the combination of riprap and tetrahedral frames around a cylindrical pile are tested in a current-wave flume to examine their effects on scouring. The evolution of scour depth around the pile in regard of the scour countermeasures, subjected to the action of steady currents with or without irregular waves, and the bed topography change around the pile are evaluated. The range of scour and its erosion volume are estimated. A comparison between scour parameters with and without the countermeasures demonstrates the efficacy of the countermeasure in terms of the maximum scour depth. All of the tested countermeasures reduce the scour depth significantly. However, depending on the type of scour countermeasure adopted, the scour depth reduction efficacy varies considerably. The tests also show that it takes a longer time to reach the maximum scour depth in the current-wave conditions than only in the current conditions. For the majority of the cases, the scour range and volume subjected to the combined current-wave action is larger than only to the current action.

  • 8. Lin, C.
    et al.
    Kao, M. -J
    Wong, W. -Y
    Tsai, C. -P
    Chang, K. -T
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Characteristics of flow separation and hydraulic jump during run-down motion of shoaling solitary wave traveling over steep sloping bottom2016Ingår i: Proceedings of the International Offshore and Polar Engineering Conference, International Society of Offshore and Polar Engineers , 2016, s. 724-731Konferensbidrag (Refereegranskat)
    Abstract [en]

    The features of velocity fields for the evolution of shoaling solitary wave, having a wave-height to water-depth ratio of 0.363 and propagating over a 1:3 sloping bottom, are investigated experimentally. A flow visualization technique using particle trajectory method and a high-speed particle image velocimetry (HSPIV) system employing a high-speed digital camera were used. This study mainly focuses on the occurrence of separated shear layer from the sloping bottom, evolved vortex structure, subsequent hydraulic jump, and curling jet of the backward breaking wave impinging upon the free surface of retreated flow during the run-down motion of the shoaling solitary wave.

  • 9. Lin, C.
    et al.
    Kao, M. -J
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Teng, P. -H
    Raikar, R. V.
    Study on probabilistic mean features of lower and upper free-surface profiles and velocity fields of a highly fluctuating free jet over a chute2018Ingår i: Journal of Marine Science and Technology (Taiwan), ISSN 1023-2796, Vol. 26, nr 3, s. 309-326Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An optic-based method that utilizes the particle-laden images captured during high-speed particle image velocimetry (HSPIV) measurements is presented, aiming to explore the probabilistic mean characteristics of the free surface profiles and velocity fields of a free jet with high-frequency random fluctuations over a chute. The technique based on the gray-level gradients in the smoothed gray level distribution of the contrast-enhanced images is used to determine the probabilistic mean features of the free jet, right beneath and above which the water-air interfaces have I%/(100 I)% intermittent appearance of air/water phase and (100 I)% /I% fitful show-up of water/air phase. Further, the cross-correlation calculation for HSPIV measurements is employed to obtain the instantaneous and probabilistic mean velocity fields of the free jet. A target experiment of the free jet having a mean water-depth of 2.76 cm and a Froude number of 3.92 over a 17 chute model is performed in a re-circulating water channel to demonstrate the application of this method. The entire process for obtaining the probabilistic mean positions of the free surface profiles is elucidated step-by step. The lower/upper part of the free surface changing from the height at which the possibility of intermittent appearance of water phase is 3%/97%, via the counterpart for 50%/50%, to that for 97%/ 3% is identified precisely. In addition, the probabilistic mean velocity field is further categorized into the conditionally and overall time-averaged ones. Each streamwise velocity profile in the conditionally time-averaged velocity field is fairly uniform. However, the counterpart in the overall time-averaged velocity field evidently shows the non-uniform feature with prominent velocity gradient in the lower/upper part between the height at which the possibility of intermittent appearance of water phase is 3%/97% and the counterpart is 97%/3%.

  • 10. Lin, C
    et al.
    Kao, MJ
    Lo, LF
    Yeh, PH
    Raikar, RV
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Huang, H
    Similarity velocity profile of vortex flow in the cavities between girders under a partially inundated bridge deck2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper presents the experimental results on similarity velocity profile of vortices inside the cavities, formed between two neighboring girders, under a partially inundated bridge deck. Particle image velocimetry (PIV) and flow visualization technique are both employed to explore the flow field. The approaching flow is subcritical with Froude number varying in the range 0.137–0.381. The velocity characteristics of vortex structure inside the cavities under a partially inundated bridge deck, where water is fully occupied without air-pocket, are mainly investigated. The similarity profile of the azimuthal velocity along an arbitrary line, passing through the vortex core, is uniquely obtained using the measured azimuthal velocities for two different flow types. The selection of the characteristic length and velocity scales used for obtaining the similarity profile is discussed in this study.

  • 11. 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.

  • 12. Lin, C.
    et al.
    Yu, S. -M
    Wong, W. -Y
    Tzeng, G. -W
    Kao, M. -J
    Yeh, P. -H
    Raikar, R. V.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Tsai, C. -P
    Velocity characteristics in boundary layer flow caused by solitary wave traveling over horizontal bottom2016Ingår i: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 76, s. 238-252Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The characteristics of horizontal velocity in the bottom boundary-layer flow induced by a solitary wave propagating over a horizontal bottom are presented experimentally, using high-speed particle image velocimetry (HSPIV). The ratio of wave height to water depth varies from 0.096 to 0.386 and the flow inside the boundary layer is laminar. The results show that the horizontal velocity profiles can be mainly classified into two categories with respect to the passing of the solitary wave-crest at the measuring section: the pre-passing (or acceleration) phases under favorable pressure gradient and post-passing (or deceleration) phases under adverse pressure gradient. For the velocity distributions obtained during the pre-passing phases, a nonlinear regression analysis was used to precisely determine the time-dependent characteristic length and velocity scales underlying these profiles. A similarity profile of the horizontal velocity is established first using the time-dependent free-stream velocity and boundary layer thickness as the characteristic velocity and length scales, respectively. In addition, the displacement thickness, the momentum thickness, and the energy thickness are also considered as alternative length scales. All these four representative thicknesses are closely related to each other, demonstrating that any one amongst them can be regarded as the characteristic length scale. The forms of similarity profiles for the non-dimensional velocity distributions are somewhat analogous to the results of steady boundary layer flow over a thin plate under with pressure gradient, but with different coefficients or powers. While during the post-passing phases, flow reversal which acts like an unsteady wall jet and moves in the opposite direction against the wave propagation occurs after the passage of solitary wave-crest. The thickness of flow reversal layer increases with time. A similarity profile is proposed for the velocity distributions corresponding to occurrence of the extreme value in the maximum negative velocity of flow reversal. Variations of the maximum negative velocity and the thickness of flow reversal with the time right after the start of flow reversal are also discussed in detail. Moreover, the non-dimensional time leads of the horizontal velocities at different heights in the boundary layer over the free-stream velocity are evidenced to be more noticeable toward the bottom, and also in lower ratio of wave height to water depth. A similarity profile for the non-dimensional time lead versus the non-dimensional height above the bottom surface is also presented.

  • 13. Lin, Chang
    et al.
    Hsieh, SC
    Lo, LF
    Chang, SY
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Green aerator using periodically oscillatory flow generated inside a vertical dropping pool2012Konferensbidrag (Refereegranskat)
    Abstract [en]

    The flow conditions for generating periodically oscillatory flows and thus entraining a large amount of air bubbles into the water mass in a vertical dropping pool, which can be used potentially as a green aerator, were investigated experimentally. The approaching flows passing through upstream reach of a vertical dropping pool were studied under sub-critical condition. A wave gauge was used to measure the free surface fluctuations in the pool, and flow visualization technique was employed to reveal the flow structure of the dropping flows qualitatively. Under certain conditions, the falling flow over a vertical dropping pool forms a switching jet that oscillates up and down periodically and impinges on the bottom and the downstream corner of the pool alternately. The switching jet switches between an impinging jet (or napped flow) and a sliding jet (or skimming flow), causing it to oscillate periodically with a unique period and to entrain a large number of air bubbles into the switching jet, thus enhancing the dissolved oxygen quantity and turbulent mixing. The primary frequency of the periodic oscillation was determined by applying spectral analysis to the time history of wave-gauge measurements for the free surface elevation of the dropping flows. Variables influencing the fundamental oscillation frequency were carefully checked, and an empirical relation between a weighted Strouhal number and a grouped non-dimensional parameter was proposed to predict the primary frequency of the periodically oscillatory flow.

  • 14. Lin, Chang
    et al.
    Kao, MJ
    Tzing, GW
    Wong, WY
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Study on Flow Fields of Boundary-Layer Separation and Hydraulic Jump during Rundown Motion of Shoaling Solitary Wave2015Ingår i: Journal of Earthquake and Tsunami, ISSN 1793-7116, Vol. 9, nr 5Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The characteristics of flow fields for a complete evolution of the non-breaking solitary wave, having a wave-height to water-depth ratio of 0.363 and propagating over a 1:5 sloping bottom, are investigated experimentally. This study mainly focuses on the occurrences of both flow separation on the boundary layer under an adverse pressure gradient and subsequent hydraulic jump with the abrupt rising of free surface during run-down motion of the shoaling wave, together with emphasis on the evolution of vortex structures underlying the separated shear layer and hydraulic jump. A flow visualization technique with particle trajectory method and a high-speed particle image velocimetry (HSPIV) system with a high-speed digital camera were used. Based on the instantaneous flow images visualized and/or the ensemble-averaged velocity fields measured, the following interesting features, which are unknown up to date, are presented and discussed in this study: (1) Flow bifurcation occurring on both offshore and onshore sides of the explicit demarcation curve and the stagnation point during run-up motion; (2) The dependence of the diffuser-like flow field, being changed from the supercritical flow in the shallower region to the subcritical flow in the deeper counterpart, on the Froude number during the early and middle stages of run-down motion; (3) The positions and times for the occurrences of the incipient flow separation and the sudden rising of free surface of the hydraulic jump; (4) The associated movement and evolution of vortex structures under the separated shear layer, the hydraulic jump and/or the high-speed external main stream of the retreated flow; and (5) The entrainment of air bubbles from the free surface into the external main stream of the retreated flow.

  • 15. 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.

  • 16. 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.

  • 17.
    Lin, Chang
    et al.
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Wong, Wei-Ying
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Kao, Ming-Jer
    Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan..
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Betongbyggnad. Älvkarleby Lab, Vattenfall AB Res & Dev, SE-81426 Älvkarleby, Sweden..
    Raikar, Rajkumar V.
    KLE Dr MS Sheshgiri Coll Engn & Technol, Dept Civil Engn, Angol Main Rd, Belgaum 590008, Karnataka, India..
    Yuan, Juan-Ming
    Providence Univ, Dept Data Sci & Big Data Analyt, Taichung 43301, Taiwan..
    Hydrodynamic Features of an Undular Bore Traveling on a 1:20 Sloping Beach2019Ingår i: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 11, nr 8, artikel-id 1556Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The hydrodynamic characteristics, including local and convective accelerations as well as pressure gradient in the horizontal direction, of the external stream of an undular bore propagating on a 1:20 sloping beach are experimentally studied. A bore with the water depth ratio of 1.70 was generated downstream of a suddenly lifted gate. A high-speed particle image velocimetry was employed to measure the velocity fields during the run-up and run-down motions. The time series of free surface elevation and velocity field/profile of the generated bore, comprising a pure bore accompanied by a series of dispersive leading waves, are first demonstrated. Based on the fast Fourier transform (FFT) and inverse FFT (IFFT) techniques, the free surface elevation of leading waves and the counterpart of pure bore are acquired separately at a specified measuring section (SMS), together with the uniform horizontal velocity of the pure bore. The effect of leading-wave-induced velocity shift on the velocity profiles of the generated bore are then evaluated at the SMS. To understand the calculation procedure of accelerations and pressure gradient, three tabulated forms are provided as illustrative examples. Accordingly, the relationships among the partially depth-averaged values of the non-dimensional local acceleration, convective acceleration, total acceleration and pressure gradient of the generated/pure bore acquired at the SMS versus the non-dimensional time are elucidated. The trends in the non-dimensional accelerations and pressure gradient of the external stream of generated bore are compared with those of the pure bore. During the run-up motion from the instant of arrival of the bore front to the moment of the peak level at the SMS, continuous decrease in the onshore uniform horizontal velocity, and successive deceleration of the pure bore in the onshore direction are evidenced, exhibiting the pure bore under the adverse pressure gradient with decreasing magnitude. However, the pure bore once ridden by the leading waves is decelerated/accelerated spatially and accelerated/decelerated temporally in the onshore direction during the rising/descending free surface of each leading wave. This fact highlights the effect of pre-passing/post-passing of the leading wave crest on the velocity distribution of generated bore. It is also found that, although the leading waves have minor contribution on the power spectrum of the free surface elevation as compared with that of the pure bore, the leading waves do play an important role on the magnitudes of both accelerations and pressure gradient. The largest magnitude of the acceleration contributed by the leading waves is around 26 times the counterpart contributed by the pure bore. Further, during the run-down motion right after the moment for the peak level of the bore, a linear increase in the magnitude of the offshore uniform horizontal velocity and a constant local acceleration with increasing time are both identified. The partially depth-averaged value of the non-dimensional pressure gradient is equal to a small negative constant (-0.0115) in the offshore direction, indicating that the bore is subject to a constant favorable pressure gradient.

  • 18. Lindblom, E.
    et al.
    Arnell, M.
    Flores-Alsina, X.
    Stenström, F.
    Gustavsson, D. J. I.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Jeppsson, U.
    Dynamic modelling of nitrous oxide emissions from three Swedish sludge liquor treatment systems2016Ingår i: Water Science and Technology, ISSN 0273-1223, E-ISSN 1996-9732, Vol. 73, nr 4, s. 798-806Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The objective of this paper is to model the dynamics and validate the results of nitrous oxide (N2O) emissions from three Swedish nitrifying/denitrifying, nitritation and anammox systems treating real anaerobic digester sludge liquor. The Activated Sludge Model No. 1 is extended to describe N2O production by both heterotrophic and autotrophic denitrification. In addition, mass transfer equations are implemented to characterize the dynamics of N2O in the water and the gas phases. The biochemical model is simulated and validated for two hydraulic patterns: (1) a sequencing batch reactor; and (2) a moving-bed biofilm reactor. Results show that the calibrated model is partly capable of reproducing the behaviour of N2O as well as the nitritation/nitrification/denitrification dynamics. However, the results emphasize that additional work is required before N2O emissions from sludge liquor treatment plants can be generally predicted with high certainty by simulations. Continued efforts should focus on determining the switching conditions for different N2O formation pathways and, if full-scale data are used, more detailed modelling of the measurement devices might improve the conclusions that can be drawn.

  • 19.
    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.

  • 20.
    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.

  • 21.
    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.

  • 22.
    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.

  • 23.
    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.

  • 24.
    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.

  • 25. Lundell, Dan
    et al.
    Jin, Feng
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Some aspects of rock-filled concrete in hydraulic structures2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    The study addresses several aspects of Rock-Filled Concrete (RFC) – a new construction material developed for use in hydraulic structures. Some of the latest results of compressive strength, tensile strength and compaction level of the material are presented. In RFC, Self-Compacting Concrete (SCC) is used together with large aggregates (minimum of 30 cm in size), to produce concrete with a high content of aggregate material and a low content of cement. The large aggregates in RFC complicate the material testing. In this study, the aggregate size is limited to 150 mm in size and the strength parameters have been evaluated using cubic samples with a surface area of 500 * 500 mm. The results imply that RFC does not lead to reduced material strength as compared to SCC. Based on the latest performance results of the material, together with results from earlier studies of the production costs and environmental impacts of RFC, the conclusion is that RFC has advantages of lower material cost, faster construction and less environmental impacts as compared to both conventional concrete (CC) and Roller-Compacted Concrete (RCC) when used in large concrete structures such as dams.

  • 26. Petti, Olivier
    et al.
    Wang, C
    Cervantes, M
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Nilsson, H
    Numerical and experimental investigations of a hydraulic pipe during a gate closure at high Reynolds number2015Ingår i: 6th IAHR meeting of the Working Group Cavitation and dynamic problems, Faculty of technologies and systems , 2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    The role of hydropower to provide regulated power is important to the Swedish power system. This becomes even more accentuated with the expansion of intermittent renewable electricity sources, such as wind power. The variation of hydropower operation ranges over a large spectrum of time scales, from seconds to years. For scales larger than a minute, the flow may be considered as quasi-steady from a hydrodynamic point of view. The present work addresses the shorter time scales. Such scales are manifested mainly as pressure transients, which is an issue of concern in design and operation of hydropower plants.

    The objective of the study is to address rapid pressure transients with a special focus on detailed 3D processes interacting with transients travelling in an essentially 1D geometry. The test case is a gate closing in a long rectangular pipe, where a high-Reynolds number flow is driven by a pressure difference between upper and lower water levels. Experimental time-resolved static pressure and PIV data are gathered for validation of the numerical results.

    In a first stage the computational domain is modelled in 3D with an incompressible volume of fluid method that includes the prediction of the free surfaces. The domain includes the upper and lower water tanks with free water surfaces, a pipe in-between and a closing and opening gate. The gate movement is modelled with a dynamic mesh that removes the cells as the gate closes. The block-structured mesh is generated in ICEM CFD, and parallel simulations are performed using the OpenFOAM open source software. The numerical results are compared with the experimental data, and it is shown that the experimentally observed pressure fluctuations after gate closure are not an effect of the free surfaces.

    In a second stage, the upper tank and the pipe are modelled using a compressible 1D code based on the method of characteristics (MOC). A comparison with the experimental data shows that the correct unsteady behavior of the system is captured by the 1D approach if the losses and the gate characteristics are correctly accounted for, at the same time as the compressibility is adapted to the air contents of the water and flexibility of the structure.

  • 27. Stenström, Petter
    et al.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Andersson, Patrick
    Henriksson, Gunnar
    Enlarging the undersized spillway of Höljes dam2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    Höljes dam is one of the largest embankment dams in Sweden. Studies have shown that it suffers from several deficiencies that would jeopardize the safety of the dam during extreme floods. The major deficiencies include insufficient spillway discharge capacity in relation to the updated design flood and potential risk of erosion in the dam toe in case of large spillway discharges. To safely pass the higher design flood, four major rehabilitation options are evaluated with the help of hydraulic model tests. Option 1 refers to the use of the existing spillway in combination with extra water storage above the full reservoir retention level (FRRL). Option 2 includes construction of a new, separate tunnel spillway. In Option 3, a new gated spillway, 17 m in width, is added to the right of the existing one. Option 4 refers to construction of a new spillway in the form of a 155 m long overflow weir in the reservoir in combination with moderate storage above the FRRL. In Option 1 and 2, the width of the existing spillway channel remains the same and the stilling basin is enlarged, while in Option 3 and 4, the spillway channel is widened and the stilling basin volume is also increased. This paper addresses the advantages and disadvantages of those options. Together with other investigations, hydraulic model testing provides necessary basis for finalization of each design option in terms of spillway discharge capacity and waterway configuration. After weighing the pros and cons, the dam owner has chosen Option 3 as the final rehabilitation alternative. Further investigations are made to optimize the design.

  • 28. Stenström, Petter
    et al.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Bond, Håkan
    Sjödin, Anders
    Steiner, Rolf
    Increasing the discharge capacity at the Höljes dam in Klarälven, Sweden2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    The Höljes dam was constructed during 1959-62 and is one of Sweden’s largest embankment dams, with a height of 80 m. The dam is classified in the highest category with regard to dam break consequences. There are three spillways: one bottom outlet, one primary spillway and one log flume. Their discharge capacity at full reservoir level is 185 m3/s, 1290 m3/s and 120 m3/s, respectively. The function of the spillways at full discharge has however been questioned. Two issues are of particular concern: (i) The stilling basin at the end of the chute from the primary spillway was originally designed for a discharge of less than 900 m3/s, and is now in poor condition. One of the concerns is whether the walls of the stilling basin could collapse at high discharge, which could lead to accelerating erosion towards the dam toe. (ii) The discharge from the bottom outlet is conveyed via the old diversion tunnel, in which a hydraulic jump occurs. There is a risk that the diversion tunnel outlet may be inundated at the design flood. The free aeration for the hydraulic jump would then be cutoff, which could lead to large-scale air pulsations and dangerous shock pressures. The design flood has also been questioned and recent recalculations give a 20-30 % increase in the design flood. During 2008, different measures are evaluated for increasing the discharge capacity so as to safely pass a higher design flood. Initially, three alternatives will be analyzed through hydraulic model tests:

  • 29.
    Sundqvist, Per
    et al.
    Vattenfall .
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Analytical solutions of two-dimensional turbulent seepage in rock-fill dams2007Konferensbidrag (Refereegranskat)
    Abstract [en]

    Turbulent seepage in porous media, often in rock-fill dams, represents a classical topic. Easy estimation of the seepage flow characteristics facilitates the slope stability analysis of a dam. In this paper, we revisit the analytical approach in two dimensions for solving turbulent seeping flow in homogenous isotropic rock-fill dams lying on an impervious foundation. The derivation of its analytical solution is simplified. The usefulness of the analytical formulations is improved in a sense that explicit expressions of the seepage point location in the downstream slope and the phreatic surface are worked out. The seeping discharge can be easily computed, although its accuracy relies on a good knowledge of the turbulent permeability of a dam. The analytical approach is useful when an approximate answer is needed.

  • 30.
    Teng, Penghua
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE).
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    CFD modeling of two-phase flow of a spillwaychute aerator of large width2016Ingår i: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 4, nr 2, s. 163-177Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An aerator is frequently used to prevent cavitation damages in high-velocity spillways. To understand its characteristics,one often resorts to physical model tests. To complement physical model tests, computation fluid dynamics simulations areused to determine water–air flow behaviors.With Bergeforsen’s 35 m wide aerator, numerical modeling has been performedto evaluate its performance and improve its configuration. The parameters of interest include spillway discharge capacity,air entrainment rate, duct subpressure and air concentration in the aerated flow. The simulated discharge capacity agreesreasonably with experimental data. Due to the larger chute width, empirical formulas do not reasonably predict the airdemand. To provide the air required by the aerator, its distribution in the cavity must be guaranteed. We thus looked into theair supply system and the air flux in the cavity to improve the aerator function. Larger vent openings in the middle of thechute are preferable for large-width aerators.

  • 31.
    Teng, Penghua
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    CFD modelling of two-phase flow of a spillway chute aerator of large width2016Ingår i: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An aerator is frequently used to prevent cavitation damages in high-velocity spillways. To understand its characteristics, one often resorts to physical model tests. To complement physical model tests, CFD simulations are used to determine water-air flow behaviors. With Bergeforsen’s 35 m wide aerator, numerical modeling has been performed to evaluate its performance and improve its configuration. The parameters of interest include spillway discharge capacity, air entrainment rate, duct subpressure and air concentration in the aerated flow. The simulated discharge capacity agrees reasonably with experimental data. Due to the larger chute width, empirical formulas do not reasonably predict the air demand. To provide the air required by the aerator, its distribution in the cavity must be guaranteed. This is challenging for wide spillways. We thus looked into the air supply system and the air flux in the cavity to improve the aerator function. Larger vent openings in the middle of the chute are preferable for large-width aerators.

  • 32.
    Teng, Penghua
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Flows over flip-bucket aerators, physical and CFD modeling with prototype testsManuskript (preprint) (Övrigt vetenskapligt)
    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. 

  • 33.
    Teng, Penghua
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Modeling and Prototype Testing of Flows over Flip-Bucket Aerators2018Ingår i: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 144, nr 12, artikel-id 04018069Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper deals with a unique spillway which incorporates an aerator in each flip bucket with the intention to aerate the flow and avoid subatmospheric air cavities enclosed by the jets. In terms of jet breakup and stability, the physical models and the prototype lead to contradicting conclusions. With sealed aerators, the models exhibit intact air cavities featuring negative air pressure, suggesting the aeration need. Computational fluid dynamics (CFD) is performed to determine the reason for the discrepancy. Both the prototype observations and CFD indicate that the jets break up as a result of air entrainment; the resulting cavity air-pressure drops are insignificantly small. The discrepancy is due to the small model scale, in which the threshold flow velocity for air entrainment is not met and the prerequisite for jet breakup does not exist. To correctly reproduce similar water-air flow phenomena, the model should be large enough to meet the air-entrainment criterion. When questioning a small-scale model with air-cavity formation, CFD simulations should be performed to check the model results and make corrections, if needed.

  • 34.
    Teng, Penghua
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE).
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Modeling and Prototype Testing of Flowsover Flip-Bucket Aerators2018Ingår i: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 144, nr 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper deals with a unique spillway which incorporates an aerator in each flip bucket with the intention to aerate the flow andavoid subatmospheric air cavities enclosed by the jets. In terms of jet breakup and stability, the physical models and the prototype lead tocontradicting conclusions. With sealed aerators, the models exhibit intact air cavities featuring negative air pressure, suggesting the aerationneed. Computational fluid dynamics (CFD) is performed to determine the reason for the discrepancy. Both the prototype observations andCFD indicate that the jets break up as a result of air entrainment; the resulting cavity air-pressure drops are insignificantly small. The discrepancyis due to the small model scale, in which the threshold flow velocity for air entrainment is not met and the prerequisite for jet breakupdoes not exist. To correctly reproduce similar water–air flow phenomena, the model should be large enough to meet the air-entrainmentcriterion. When questioning a small-scale model with air-cavity formation, CFD simulations should be performed to check the model resultsand make corrections, if needed.

  • 35.
    Teng, Penghua
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE).
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Pfister, Michael
    Ecole Polytechnique Federale de Lausanne (EPFL).
    Studies of Two-Phase Flow at a Chute Aeratorwith Experiments and CFD Modelling2016Ingår i: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, artikel-id 4729128Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The chute aerator of a spillway is a structure in such a sense that air is, in the intense emulsification, entrained into the highvelocitywater flow. Correctly predicting the air entrainment and two-phase flow pattern at the aerator would contribute to reliablespillway operation. Based on experimental data, 2D numerical simulations are preformed to predict streamwise air concentrationsin the aerated flow, in which a two-fluid model is used. Depending on the air bubble size, relatively good agreement is seen withthe experiments in the air cavity zone. The simulations give rise to higher air concentration downstream of the cavity, which ispresumably due to underestimation of the interfacial forces in the two-fluid model.

  • 36. Wu, YH
    et al.
    Sund, M
    Magnusson, F
    Zhang, JX
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Numerical and experimental investigations of closure of an emergency bulkhead gate2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    The design of hydraulic gates is crucial for safely operating a hydropower dam. An emergency gate is especially important as it protects structures or turbines situated downstream. In this study, both numerical and experimental investigations were performed of an emergency bulkhead gate closure. The gate has a size of 4.978 m (w) by 6.166 m (h) and a thickness of 0.94 m and is subjected to a water head of 80.8 m and a flow rate of 207 m3/s. In an earlier stage, a 1:18 scale model was constructed. The CFD simulations were then made using the RNG k−ε turbulence model and Volume of Fluid method (VOF). A moving dynamic mesh was adopted to follow the gate movement. The goal was to extract the pressure distribution around the gate, subsequently to obtain the hydrodynamic forces acting on the gate and to analyze the flow pattern. The CFD model was validated against the experimental data. The closing speed of the gate and its bottom edge angle were examined in order to reduce the down-pull force and to avoid undesirable flow phenomena. It was found that to lower the gate speed to 8.1 m/min would have a positive effect. The gate would close slower, with reduced forces and less induced vibrations. To change the gate bottom edge angle from 9° to 20-30°, would considerably reduce gate vibrations.

  • 37. Xie, Q.
    et al.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Lundström, S.
    Dai, W.
    Understanding morphodynamic changes of a tidal river confluence through field measurements and numerical modeling2018Ingår i: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, nr 10, artikel-id 1424Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A confluence is a natural component in river and channel networks. This study deals, through field and numerical studies, with alluvial behaviors of a confluence affected by both river run-offand strong tides. Field measurements were conducted along the rivers including the confluence. Field data show that the changes in flow velocity and sediment concentration are not always in phase with each other. The concentration shows a general trend of decrease from the river mouth to the confluence. For a given location, the tides affect both the sediment concentration and transport. A two-dimensional hydrodynamic model of suspended load was set up to illustrate the combined effects of run-offand tidal flows. Modeled cases included the flood and ebb tides in a wet season. Typical features examined included tidal flow fields, bed shear stress, and scour evolution in the confluence. The confluence migration pattern of scour is dependent on the interaction between the river currents and tidal flows. The flood tides are attributable to the suspended load deposition in the confluence, while the ebb tides in combination with run-offs lead to erosion. The flood tides play a dominant role in the morphodynamic changes of the confluence.

  • 38.
    Xie, Qiancheng
    et al.
    Lulea Univ Technol, Div Fluid & Expt Mech, S-97187 Lulea, Sweden..
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Lundström, T. Staffan
    Lulea Univ Technol, Div Fluid & Expt Mech, S-97187 Lulea, Sweden..
    Field Studies and 3D Modelling of Morphodynamics in a Meandering River Reach Dominated by Tides and Suspended Load2019Ingår i: FLUIDS, ISSN 2311-5521, Vol. 4, nr 1, artikel-id 15Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Meandering is a common feature in natural alluvial streams. This study deals with alluvial behaviors of a meander reach subjected to both fresh-water flow and strong tides from the coast. Field measurements are carried out to obtain flow and sediment data. Approximately 95% of the sediment in the river is suspended load of silt and clay. The results indicate that, due to the tidal currents, the flow velocity and sediment concentration are always out of phase with each other. The cross-sectional asymmetry and bi-directional flow result in higher sediment concentration along inner banks than along outer banks of the main stream. For a given location, the near-bed concentration is 2-5 times the surface value. Based on Froude number, a sediment carrying capacity formula is derived for the flood and ebb tides. The tidal flow stirs the sediment and modifies its concentration and transport. A 3D hydrodynamic model of flow and suspended sediment transport is established to compute the flow patterns and morphology changes. Cross-sectional currents, bed shear stress and erosion-deposition patterns are discussed. The flow in cross-section exhibits significant stratification and even an opposite flow direction during the tidal rise and fall; the vertical velocity profile deviates from the logarithmic distribution. During the flow reversal between flood and ebb tides, sediment deposits, which is affected by slack-water durations. The bed deformation is dependent on the meander asymmetry and the interaction between the fresh water flow and tides. The flood tides are attributable to the deposition, while the ebb tides, together with run-offs, lead to slight erosion. The flood tides play a key role in the morphodynamic changes of the meander reach.

  • 39.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Accuracy of spillway discharge capacity determination, the past & present of physical hydraulic modeling2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    The discharge capacity of most dam spillways has been determined through physical hydraulic model tests prior to construction. In Sweden, the majority of dams were built between 1945 and 1975. When the design-flood guidelines were updated, hydrological investigations have shown that many existing dams have a spillway capacity lower than the revised design flood. This leads to the need of structural rehabilitation for a higher level of dam safety. This has necessitated many dams and spillways being re-tested in the laboratory to examine their discharge capacity and the efficiency of energy dissipation at the new higher design flood. For the same spillway layout, differences exist in the spillway discharge capacity between the previous and present model tests. A total of 17 dam spillways are included, including both surface and bottom spillway types. The new experimental test data has shown that the spillway capacity can be greater than, less than or approximately equal to the capacity determined in previous experiments. For some dams, the deviation can be as large as 8-12%. The test data has been analyzed and the possible reasons for the noted discrepancy have been explored. The primary reasons for the discrepancies are thought to be due to the model construction quality and flow measurement method. In some cases, it is found that the dam was not constructed as originally tested and there are certain, albeit minor, differences in the spillway layout, which affects the discharge.

  • 40.
    Yang, James
    Vattenfall, Sweden.
    Interception of spillway floating debris following extreme floods2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    During extreme floods, a large quantity of floating debris can be produced due to erosion, drifting with the flood water and eventually accumulating upstream of the dam. In Sweden, the debris is typically composed of spruce and pine trees of lengths up to 25 m. The debris may clog up the spillway openings and reduce the discharge capacity. As this is a dam-safety risk, proper countermeasures need to be taken to handle the debris. An innovatively designed visor is devised to prevent the spillway from clogging with debris. It is formed to have a horizontally semicircular shape, with sloping beams supported on a platform. In one project, a concept model is tested, while in the other, a more realistic model according to scale is examined. Taking account of prototype tree lengths, 100-130 small trees with a density of between 650-1000 m3/s are used. Without the visor, the trees block all the spillway openings from the threshold to the water surface and the discharge capacity is reduced by some 30%. The tests have shown that the visor effectively stops the floating debris and the spillway openings are kept free from the debris. Trees with low density approach the visor afloat and are intercepted in the surface water. Debris with higher density is dragged down when approaching the visor. As a result, it covers the visor height, leading to a reduction in the flow passage. The study demonstrates the use of the visor to maintain relatively free spillway flow and a marginal reduction in the discharge capacity.

  • 41.
    Yang, James
    Vattenfall R&D.
    Investigations at Vatnsfell2007Ingår i: International Water Power and Dam Construction, ISSN 0306-400X, E-ISSN 1538-6414, Vol. 59, nr 9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hydraulic model studies were conducted to evaluate and optimise the hydraulic behaviour of the converging spillway, the bottom spillway and the common stilling basin at Vatnsfell hydroelectric scheme in Iceland

  • 42.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Stilling basin at Vatnsfell dam, Iceland - problem of water cascade and its solution2005Ingår i: HYDRO 2005, The international journal on hydropower and dams , 2005Konferensbidrag (Refereegranskat)
    Abstract [en]

    A converging overflow spillway and a bottom outlet share a stilling basin at the Vatnsfell dam, Iceland. When commissioned in 2001, the stilling basin experienced problems of water cascade onto its right rock slope at the discharge interval 70 – 130 m3/s from the bottom outlet. Beyond this scope, the stilling basin functions satisfactorily as designed. Corresponding to the discharge interval, the opening of the radial gate of the outlet varies between 1.0 – 2.5 m, depending on the reservoir water level. The height of the baffle blocks immediately downstream the gate is 1.5 and 2.0 m. An improper relationship between the flow depth and the block height was considered to account for its occurrence. Hydraulic model studies were made to study the reason and to investigate potential countermeasures. The test results led to the suggestion of cutting two or alternatively four baffle blocks located immediately downstream the outlet. To completely remove any blocks or raise the end sill of the stilling basin was not advisable. According to the recommendation, two baffle blocks adjacent to the right sidewall were cut by a half meter in the prototype. Flow release from the outlet showed that the stilling basin functioned satisfactorily and without cascading up to the full discharge capacity. No other countermeasures were considered necessary.

  • 43.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Underwater tunnel piercing in refurbishment of Akkats power station2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    Akkats has suffered a number of incidents that were complicated to repair and led to frequent shutdowns. Its existing unit is to be replaced with two smaller units. Underwater tunnel piercing is needed when one of the units is to be put in operation. The open type of piercing is chosen, in which the tunnel and the gate shaft are pre-filled with water. Hydraulic model studies are made to examine involved hydraulic parameters. The deposit profile of the blasted rock in the stone pit and the upsurge in the gate shaft are strongly affected by the plug thickness and shaft water level. To reduce upsurge height, the reservoir and the shaft should have the same water level. A few countermeasures are discussed for upsurge suppression. The stone pit should be optimized for the countermeasure in question so as to reduce intake head losses and prevent the blasted rock from entering the tunnel.

  • 44.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik. Vattenfall AB, Sverige.
    Andreasson, P.
    Bending the spillway flow for safety upgrades2016Ingår i: International Water Power and Dam Construction, ISSN 0306-400X, E-ISSN 1538-6414, Vol. 68, nr 11, s. 36-43Artikel i tidskrift (Refereegranskat)
  • 45.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik (flyttat 20130630), Vattendragsteknik (flyttat 20130630). Vattenfall RandD, Sweden.
    Andreasson, Patrik
    LTU.
    Headword erosion in embankment dams caused by spillway flood discharge2012Ingår i: International journal on hydropower and dams, ISSN 1352-2523, Vol. 19, nr 6, s. 68-72Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An embankment dam with conventional stability requirements should be able to withstand so- called design seepage flow, which is often turbulent. To meet this requirement, many dams in the country have been reinforced with stabilizing measures. Another type of instability issue for embankment dams is headward erosion, which is less common and subsequently receives less attention in the design. The bedrock elevation downstream of the energy dissipator and in its immediate vicinity is another important factor in erosion studies, and should be identified in advance. The bedrock elevation determines how deep the erosion down-cut will develop. Hydraulic model tests prove to be an easy means of evaluating issues of this type. All the models are based on Froude scaling laws without vertical scale distortion. Based on the prototype data, sand and gravel of appropriate sizes are chosen as erodible material.

  • 46.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Andreasson, Patrik
    LTU.
    Instability of embankment dams due to headward erosion caused by spillway flood discharge2012Konferensbidrag (Refereegranskat)
    Abstract [en]

    Discharge of high spillway floods can give rise to serious headward erosion in the dam toe, which endangers the downstream slope stability of an embankment dam. In conjunction with dam rehabilitations, hydraulic model tests are made to identify this type of erosion, examine its extent and provide a basis for cost-effective rebuilding design. Four dams are included, i.e. Porsi, Långbjörn, Halvfari and Höljes. Spillway channels are often surrounded by embankments of earth/rock materials grounded on bed rock. The bed rock forms the lower boundary of erosion. Concrete structures limit the erosion development. However, not all the structures will remain in place during high floods. Driven by the spillway water, flow circulations are formed in the downstream area, as is the case with Porsi, Halvfari and Höljes. The headward erosion is a direct consequence of back flow circulations characterized by relatively high-flow velocity and sometimes strong wave motions. As the erosion develops, the main flow direction in the spillway channel shifts somewhat towards the eroded area. The circulation zone increases, which in turn aggravates the headward erosion. The limiting structures play a role in the toe erosion development. Although certain conditions may indicate a predisposition to headward erosion, the risk does not necessarily exist in all dams. The risk for toe erosion has been identified for Porsi and Halvfari, but not for Långbjörn and Höljes. Despite of serious erosion downstream, relatively calm water with insignificant wave motion are characteristic of the flow pattern at the dam toe of the latter two dams.

  • 47.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Andreasson, Patrik
    Cederström, Malte
    Experiences from modifications of curved spillway channels in dam rebuilding projects2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    To safely discharge higher design floods, a number of curved spillway channels need to be modified. The design flood of a dam is often released at higher reservoir level, thus giving rise to higher flow velocity in the channel. The combination of more water and higher velocity results in accentuated non-uniform channel flows. The unfavorably distributed flow into a stilling basin or plunge pool aggravates also the energy dissipation. The curved spillway channels of several existing dams are modified in physical model tests. The adopted countermeasures include heightened sidewalls, prolonged spillway piers, new partition walls, streamlining discontinuities in the waterway, the use of differential bottom elevations in cross section, addition of deflector on sidewalls or a combination of several measures. The project experiences are summarized in this paper, with emphasis on flow patterns at high discharges and countermeasures.

  • 48.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur. Vattenfall AB Res & Dev, Alvkarleby Lab, SE-81426 Alvkarleby, Sweden.
    Andreasson, Patrik
    Vattenfall AB Res & Dev, Alvkarleby Lab, SE-81426 Alvkarleby, Sweden.;Lulea Univ Technol, Div Fluid & Expt Mech, SE-97187 Lulea, Sweden..
    Högström, Carl-Maikel
    Vattenfall AB Res & Dev, Alvkarleby Lab, SE-81426 Alvkarleby, Sweden..
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    The Tale of an Intake Vortex and Its Mitigation Countermeasure: A Case Study from Akkats Hydropower Station2018Ingår i: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, nr 7, artikel-id 881Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The upgrade of Akkats power station in Sweden included a new, separate waterway for the addition of a 75 MW generating unit. The vertical intake of its headrace was formed by means of lake tapping. A physical model was used to help understand the blasting process involving fragmented rock, water, air, and gas. Upon commissioning of the unit, swirling flows occurred unexpectedly at the intake, which gave rise to negative consequences including limitations in power output. Echo-sounding showed that the blasted piercing resulted in an irregular intake. A hydraulic model, as part of the design process, was built to examine potential countermeasures for vortex suppression. The final solution was a segmented barrier between the intake and the dam. It effectively suppressed the intake flow circulations; only minor intermittent vortices were left. The fabricated steel segments were anchored into the bedrock, stretching to 1.0 m below the lowest legal reservoir level. The local intake headloss was also reduced. The implemented solution was tested under full turbine loading and the result was satisfactory. Even during winter seasons with ice cover above the wall, the power station ran normally. The case study is expected to provide guidance for solving similar problems with vortex formation.

  • 49.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Andreasson, Patrik
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Xie, Qiancheng
    The Past and Present of Discharge Capacity Modeling for Spillways-A Swedish Perspective2019Ingår i: FLUIDS, ISSN 2311-5521, Vol. 4, nr 1, artikel-id 10Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    Most of the hydropower dams in Sweden were built before 1980. The present dam-safety guidelines have resulted in higher design floods than their spillway discharge capacity and the need for structural upgrades. This has led to renewed laboratory model tests. For some dams, even computational fluid dynamics (CFD) simulations are performed. This provides the possibility to compare the spillway discharge data between the model tests performed a few decades apart. The paper presents the hydropower development, the needs for the ongoing dam rehabilitations and the history of physical hydraulic modeling in Sweden. More than 20 spillways, both surface and bottom types, are analyzed to evaluate their discharge modeling accuracy. The past and present model tests are compared with each other and with the CFD results if available. Discrepancies do exist in the discharges between the model tests made a few decades apart. The differences fall within the range -8.3%-+11.2%. The reasons for the discrepancies are sought from several aspects. The primary source of the errors is seemingly the model construction quality and flow measurement method. The machine milling technique and 3D printing reduce the source of construction errors and improve the model quality. Results of the CFD simulations differ, at the maximum, by 3.8% from the physical tests. They are conducted without knowledge of the physical model results in advance. Following the best practice guidelines, CFD should generate results of decent accuracy for discharge prediction.

  • 50.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Ascila, R
    Vattenfall.
    Nilsson, CO
    E.ON.
    Adding extra spillway discharge capacity, a few dam refurbishment examples in Sweden2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    As required by the revised dam-safety guidelines, the spillway discharge capacity of a large number of the existing dams must be increased so as to release the higher design floods without jeopardizing the structural safety. The extra capacity required falls usually within the scope of 2030%, but in some cases it can as high as 50%. To modify an existing spillway is often the most inexpensive alternative to increase its capacity. When this is difficult due to practical restraints, a new spillway, placed either separately or adjacent to the existing one, needs to be constructed. This paper illustrates several practical examples of dam rebuilding in Sweden that involve either a new spillway or modifications of an existing one.

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