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  • 51.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Billstein, M
    Cederström, Malte
    Vattenfall AB.
    Viklander, Peter
    Sjödin, Gunnar
    Dam safety and rebuilding – a Swedish engineering perspective2006Ingår i: Proceedings of Hydropower 2006 International Conference, Chinese National Committee on Large Dams , 2006, s. 67-79Konferensbidrag (Refereegranskat)
    Abstract [en]

    The large-scale development of hydropower in Sweden started immediately after the Second World War and reached its peak during the 1950’s and 1960’s. When the dams were built, there were no prescribed criteria for determination of their design floods, and the discharge capacity of many spillways was underestimated. Dam safety was at that time not a subject of state regulatory surveillance, and there were no national directives governing the dam design, construction and supervision. The responsibility rested with the dam developer. With the updated flood guidelines released 1990, many dams have been found to have up to 50% higher design floods than constructed for. As required by the dam-safety guidelines RIDAS, many existing dams, especially high-hazard ones, have to be rebuilt, an issue that involves large capital investment and takes many years to accomplish. The engineering measures include spillway modification, new spillway gates, raising dam crests and/or impervious cores, new erosion protection upstream embankment dams, reinforcement of dam body and re-shaping and strengthening of waterways including stilling basin enlargement. Quite often several rebuilding measures are combined so as to achieve a technically feasible and economical solution. A few dams are given as example to illustrate the rebuilding as imposed by RIDAS.

  • 52.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik. Vattenfall Research and Development, Sweden .
    Billstein, M.
    Engström, F.
    Strand, R.
    Experimental studies on intake headloss of a blasted lake tap2014Ingår i: IOP Conference Series: Earth and Environmental Science, 2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    In existing reservoirs, construction of an intake is sometimes achieved by so-called lake tapping, a submerged tunnel piercing by blasting out the rock plug at the intake. The blasting process involves phases of rock, water, air and gas released from the explosive charge; the resulting entrance profile often differs from design assumptions. The intake headloss is a factor of concern for power generation. For a vertical intake formed by lake tapping, experiments have been carried out in a 1:30 physical model to examine the effect of entrance shapes on intake headlosses. The purpose is that, if there is potential to reduce the headlosses, the originally blasted intake shape would be modified. In the model, five alternative shapes are evaluated. The test results show that to enlarge the vertical shaft area is the most effective way to reduce the intake headloss; to further blast out a narrow channel upstream does not give much effect. Bearing in mind the risk of free-surface vortex at the intake, the influence of the intake modifications on vortex is also checked.

  • 53.
    Yang, James
    et al.
    Vattenfall R&D.
    Cederström, Malte
    Vattenfall AB.
    Coping with extreme floods through dam refurbishment – a Swedish practice2006Konferensbidrag (Refereegranskat)
    Abstract [en]

    With the current guidelines of design floods, studies have shown that many high-hazard dams in Sweden have a higher design flood than constructed for. An extensive dam-safety program is currently going on to adapt the dams to the guidelines. The paper illustrates a number of dams undergoing refurbishment to withstand extreme flood events. Alternative engineering measures are mainly (1) modification of the existing spillway to increase its discharge capacity; (2) increase of the spillway capacity by allowing the water to rise above the legal retention level , (which usually creates a need to reinforce the dam); and (3) mitigation of the flood magnitude by temporary storage of flood water in large reservoirs of a river basin. For the safe discharge of the new design flood, there is a need to check such aspects of a dam as leakage potential, risk for inner erosion, dam slope stability, energy dissipation, risk for erosion in the waterways, etc.

  • 54.
    Yang, James
    et al.
    Vattenfall R&D.
    Cederström, Malte
    Vattenfall AB.
    Gallejaur dam safety – hydraulic concerns related to spillway flood discharge2006Konferensbidrag (Refereegranskat)
    Abstract [en]

    In dam-safety evaluations of Gallejaur dam, commissioned 1965, a few safety issues were identified. The main concern is associated with use of the spillway for discharge of higher floods in light of the updated design-flood criteria. The operation of the dam has indicated insufficient energy dissipation that gives rise to frequent damages in the erosion protection of blasted rock riprap in the spillway canal. The canal has also too low sidewalls that have to be heightened in order to prevent overtopping during higher floods. Hydraulic model tests are conducted to serve as a basis for examinations of related issues. Existing flow conditions of the spillway and in the canal are investigated. Potential solutions are tested for their technical suitability. For some issues, numerical calculations are made for comparison with the experimental results. The model studies prove to be a useful tool in the rebuilding of the dam.

  • 55.
    Yang, James
    et al.
    Vattenfall R&D.
    Cederström, Malte
    Vattenfall.
    Modification of spillways for higher discharge Capacity2007Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 45, nr 5Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In Sweden, many dams are to be refurbished in light of the new dam-safety guidelines that lead to higher design floods. To modify the spillway of a dam is often one of the alternatives proposed to increase its discharge capacity. Spillway rebuilding proposals for Ajaure, Bergeforsen, Midskog and Stenkullafors have been examined in hydraulic models. This paper deals with modification of the bottom outlet at Ajaure and proposed crest lowering at the other three overflow spillways. The hydraulic characteristics after the modifications are summarized and compared with the existing situation. The results are shown mainly in terms of water stage - discharge relationship and discharge coefficient, reasons for discrepancy are discussed and explained for each case.

  • 56.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Cederström, Malte
    Spillway hydraulic issues in rebuilding embankment dams – experiences from Vattenfall’s dam-safety program2009Ingår i: Proceedings New Progress on Roller Compacted Concrete Dams, Chinese National Committee on Large Dams , 2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    In the light of the revised design floods and higher dam-safety requirements, many dams, owned by Vattenfall, have been rebuilt and many others undergo dam-safety evaluations or an upgrading process. Depending upon the dam in question, hydraulic concerns arising from the dam rebuilding may cover dam-slope erosion close to the spillway, increase of spillway discharge capacity, construction of new spillway or rebuilding of existing spillway, re-shaping spillway channel, enlargement of stilling basin or plunge pool and risk for erosion in the dam toe or in the river channels downstream. In this paper, experiences and hydraulic issues of attention are summarised and discussed in the rebuilding of a number of embankment dams.

  • 57.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Cederström, Malte
    Vattenfall AB.
    Upgrading of Stornorrfors dam for safe release of extreme floods2005Konferensbidrag (Refereegranskat)
    Abstract [en]

    Comprehensive dam-safety evaluations of the Stornorrfors dam were made with regard to new Swedish dam-safety guidelines including new design flood. This resulted in classification of the scheme as high hazard and identified inadequacy ofin the need to increase the spillway discharge capacity. To adapt to the Swedish dam-safety guidelines RIDAS, Vattenfall AB intendsand to refurbish the dam to raise its safety level in some other respects. The safety requirements in relation to discharge capacity are to be met by an additional spillway., chosen from three rehabilitation proposals and tThree alternatives have been evaluated in detail. Hydraulic model studies are made to assist in the decision-making. The old reserve spillway opening, sealed at present by a salmon ladder and a concrete wall, will be rebuilt into an gated overflow spillway to accommodate the increased design flood, a decision made by taking into account of a range of technical, environmental, social and economic issues.

  • 58.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Engström, Magnus
    Nilsson, Carl-Oskar
    Hydraulic safety of handling extreme floods of at Edensforsen dam, Sweden2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    Edensforsen is a high-hazard dam. Along with other engineering measures, a significant increase in its spillway discharge capacity is needed in order to meet the existing dam-safety criteria. A number of design options are examined in order to achieve cost-effective rebuilding. The options includes (A) the use of the existing spillway in combination with extra storage of water above the legal reservoir level, (B) modifications of the abandoned log flume into a spillway, (C) threshold lowering in the existing sector spillway (D) a new, wider spillway with the log flume removed. Some of the options might be combined. To evaluate the measures, hydraulic model studies are made of discharge capacity, energy dissipation and flow patterns downstream. The final choice of refurbishment depends mainly upon the technical feasibility and economical considerations.

  • 59.
    Yang, James
    et al.
    Vattenfall R&D.
    Eriksson, Hans
    Sweco.
    Gustafsson, A
    Stenmark, M
    Mikaelsson, Jan
    Vattenfall.
    Långbjörn dam - adaptation for safe discharge of extreme floods2007Konferensbidrag (Refereegranskat)
    Abstract [en]

    Långbjörn is a 33 m high embankment dam; its updated design flood is 30% higher than the existing spillway capacity. Based on dam-safety evaluations, several measures are now implemented in order to rebuild the dam according to the updated dam-safety guidelines. The design flood is accommodated by allowing a temporary rise of the water level by 1,3 m above the legal retention reservoir level, which requires heightening and reinforcement of the dam. The large increase in the flood magnitude calls also for examinations of the energy dissipation and flow pattern in the river downstream. The impervious core and dam crest of the dam are heightened to account for the higher reservoir level during extreme floods. To increase stability, new drainage system is added to the existing one, and increased instrumentation of seepage water, slope movement and groundwater stage downstream is suggested. Stability analysis is made for the spillway, which is to be reinforced with e.g. pre-stressed anchors in the dam foundation to increase its stability. The stilling basin with baffle blocks needs repair and reinforcement to safely discharge the higher flood. Extension of he left wall may be necessary to protect the left embankment dam from erosion. Part of the rebuilding measures depends to great extent on how the flow behaves during extreme floods. Hydraulic model tests are therefore made to investigate the energy dissipation and flow pattern downstream. Experiments are made in both fixed and movable bed. Model testing proves to be helpful in the decision making of the dam rebuilding and leads to cost-effective engineering measures.

  • 60.
    Yang, James
    et al.
    Vattenfall R&D.
    Halvarsson, Anders
    WSP.
    Bond, Håkan
    WSP.
    Cederström, Malte
    Vattenfall AB.
    Modification of flood discharge structure for updated design flood at Letsi dam2006Konferensbidrag (Refereegranskat)
    Abstract [en]

    In its safety evaluations of the Letsi dam in northern Sweden, a number of safety issues were identified. The main concern was associated with the safe discharge of its updated design flood, which was some 25% higher than that the dam was originally designed for. To adapt the dam to the higher safety standard, the flood discharge structure, consisting of upper spillway channel, plunge pool and canal downstream, had to be modified. To guarantee satisfactory flow behaviors and cost efficiency, the re-shaping of the waterway was optimized through hydraulic model tests. The final refurbishment measures included sidewall heightening of the upper channel, addition of partition wall in its middle, enlargement and deepening of the plunge pool and a new canal downstream with erosion protection. When the rebuilding of the dam was completed, the spillway was tested to release a flood of about 1500 m3/s, followed by visual and diver inspections. From the prototype release, several auxiliary measures were recommended, including some that were tested in the model but not realized at the time of the release.

  • 61.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Hellstadius, Karin
    Cederström, Malte
    Vattenfall AB.
    Adapting the spillway for updated design flood at Stenkullafors dam, North Sweden2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    New Swedish design-flood and dam-safety guidelines require a higher level of structural and hydraulic safety of the existing dams in the country. The guidelines initiated a review of each river to find good solutions – combinations of retention of water and increased spillway capacity. To comply with the guidelines, a dam-safety program undergoes in the country to rebuild a large number of dams to improve their safety – for higher design floods and also in many other respects. Technical solutions include dam reinforcement, spillway capacity increase and temporary storage of water in large reservoirs. Stenkullafors has a spillway with two gated overflow openings. Its revised design flood is 40 % higher than its existing spillway capacity. Based on a cost-benefit analysis, modifying one spillway opening to a lower crest was chosen as the main measure to cope with the flow. Through hydraulic model tests, the spillway crest profile was optimized so as to achieve the required capacity on one hand and to minimize the cost of the spillway crest and the new gate on the other. The sill elevation in the right spillway opening was lowered by 4.3 m and a new higher tainter gate was installed. The dam has now been rebuilt to higher safety level. This paper addresses hydraulic model tests and the risk of flood occurrence during the construction time. Also, the construction process and difficulties that were encountered are described. Other engineering measures are strengthening of the dam body, new erosion protection on upstream slope, energy dissipater and channel downstream.

  • 62.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Hellstadius, Karin
    Sweco.
    Cederström, Malte
    Vattenfall.
    Case study of spillway modifications – a means to a higher level of dam safety2008Ingår i: Journal of Geotechnical Engineering, ISSN 0733-9410, Vol. 30, nr 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    New Swedish design-flood and dam-safety guidelines require a higher level of structural and hydraulic safety of the existing dams in the country. The guidelines initiated a review of each river to find good solutions – combinations of retention of water and increased spillway capacity. To comply with the guidelines, a dam-safety program undergoes in the country to rebuild a large number of dams to improve their safety – for higher design floods and also in many other respects. Technical solutions include dam reinforcement, spillway capacity increase and temporary storage of water in large reservoirs. Stenkullafors has a spillway with two gated overflow openings. Its revised design flood is 40 % higher than its existing spillway capacity. Based on a cost-benefit analysis, modifying one spillway opening to a lower crest was chosen as the main measure to cope with the flow. Through hydraulic model tests, the spillway crest profile was optimized so as to achieve the required capacity on one hand and to minimize the cost of the spillway crest and the new gate on the other. The sill elevation in the right spillway opening was lowered by 4.3 m and a new higher tainter gate was installed. The dam has now been rebuilt to higher safety level. This paper addresses hydraulic model tests and the risk of flood occurrence during the construction time. Also, the construction process and difficulties that were encountered are described. Other engineering measures are strengthening of the dam body, new erosion protection on upstream slope, energy dissipater and channel downstream.

  • 63.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Johansson, N
    Cederström, Malte
    Handling reservoir floating debris for safe spillway discharge of extreme floods – laboratory investigations2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    Floating debris that obstructs the spillway reduces its discharge capacity and could thus constitute a dam-safety risk in an extreme flood situation. Tests are made in a 1:60 Laxede dam model, with about 130 small trees, whose median length is selected on the basis of the prototype data. Without any countermeasure, the spillway capacity is reduced by up to 27%. To prevent debris clogging, two types of debris racks or visors are devised and examined in the laboratory. The visors are composed of sloping beams placed directly on the spillway or on a semi-circular platform. No matter which type of visor one uses, it stops effectively the floating debris and the spillway openings are kept free from the debris. With the same model trees, the reduction in the spillway discharge capacity is only 2 – 3%. The study illustrates the concept of deploying the visors to stop the floating debris, maintain relatively free spillway flow and a marginal reduction in the discharge capacity.

  • 64.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Kung, C. S.
    Nonlinear stability of differential surge chambers1992Ingår i: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 118, nr 11Artikel i tidskrift (Refereegranskat)
  • 65.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Kung, C. S,
    Stability of air-cushion surge tanks with throttling1992Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 30, nr 6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The throttling orifice is introduced into air-cushion surge chambers to improve stability. The presence of the orifice adds another nonlinear term to the dynamic system, and its effect becomes predominant for oscillations with large displacements. By means of linearization, the type of singularities in the phase plane and their stability criteria in case of small oscillations are identified. The nonlinear analysis by direct numerical integration indicates that the system manifests itself as Hopf bifurcation with the surge-chamber size as its controlling factor. The bifurcation point corresponds to Svee's stability criterion. Before bifurcation, an unstable limit cycle may occur around the equilibrium state of practical interest, and it defines the domain of asymptotic stability. After bifurcation, two limit cycles may occur. Because of the existence of the stable one, the chamber size can be smaller than that specified by the Svee criterion. The orifice has stabilizing effect on surge motions.

  • 66.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Kung, C. S.
    Cederwall, Klas
    Large-amplitude oscillations in closed surge chamber1992Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 30, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The governing equations for surge oscillations in a closed surge chamber yield a second-order nonlinear differential equation for constant power. The surge stability is investigated by the direct method of Liapunov, which introduces the Liapunov function from the energy consideration. The nonlinear terms arising from large surges are included. The resulting stability criterion is demonstrated on a phase plane. For the equilibrium point of practical interest, the stability diagram specifies domains of asymptotic stability as a function of the safety factor of the surge-chamber. The diagram indicates that large-amplitude surges necessitate larger chamber size than that laid down by Svee for damping and the surge stability deteriorates with downward oscillations. By postulating sinusoidal surge motion, the critical chamber area is equal to the product of the critical area in case of small oscillations (Svee's formula) and a factor greater than 1.0. The factor is a function of surge amplitude and air cushion parameters for given turbine head. Stability criteria for open surge tanks can be obtained as special cases of those for closed ones.

  • 67.
    Yang, James
    et al.
    Vattenfall R&D.
    Larsson, Per
    Dath, Jörgen
    Löwen, K-L
    Halvfari dam – hydraulic concerns and rebuilding for higher safety standard2007Konferensbidrag (Refereegranskat)
    Abstract [en]

    Halvfari is a 43 m high embankment dam. In its safety evaluations made in the light of the revised dam-safety guidelines, several safety issues have been identified. The major concerns are insufficient spillway discharge capacity, inadequate energy dissipation and risk for erosion in the right riverbank downstream of the dam. The updated design flood of the dam turned out to be some 50% higher than its exiting spillway capacity. Structural modifications are necessary to adapt the dam to a higher safety standard. Among a few potential measures, construction of a new bottom spillway, located below the existing free-drop spillway, is chosen to increase the discharge capacity. To guarantee satisfactory hydraulic behavior, physical model studies are made to help in re-shaping the waterway. Due to the large increase in the flood magnitude, energy dissipation has to function properly even at higher discharges. To reduce the flow velocity and wave motions in the channel downstream, modifications are made to the spillway structure. The spillway piers are extended downstream and deflectors are added to direct the floodwater into the plunge pool that unfortunately has a very limited volume. Tests of movable bed are also conducted for the right riverbank downstream to investigate the risk for erosion and verify potential engineering measures.

  • 68.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Lin, C.
    Kao, M. -J
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Raikar, R. V.
    Application of SIM, HSPIV, BTM, and BIV techniques for evaluations of a two-phase air-water chute aerator flow2018Ingår i: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, nr 11, artikel-id 1590Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Four image-based techniques-i.e., shadowgraphic image method (SIM), high-speed particle image velocimetry (HSPIV), bubble tracking method (BTM), and bubble image velocimetry (BIV)-are employed to investigate an aerator flow on a chute with a 17° inclination angle. The study focuses on their applications to the following issues: (1) to explore the characteristic positions of three water-air interfaces; (2) to interpret the evolution process of air bubbles shed from the wedged tip of the air cavity; (3) to identify the probabilistic means for characteristic positions near the fluctuating free surface; (4) to explore the probability distribution of intermittent appearance of air bubbles in the flow; (5) to obtain the mean streamwise and transverse velocity distributions of the water stream; (6) to acquire velocity fields, both instantaneous and mean, of air bubbles; (7) to construct a two-phase mean velocity field of both water flow and air-bubbles; and (8) to correlate the relationship among the probability distribution of air bubbles, the mean streamwise and transverse velocity profiles of air bubbles, and water stream. The combination of these techniques contributes to a better understanding of two-phase flow characteristics of the chute aerator.

  • 69.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Lin, Chang
    Li, YD
    Fu, YH
    Tseng, W
    Visualization and measurement of two-phase flow field around an aerator using PIV and BIV techniques2014Konferensbidrag (Refereegranskat)
  • 70.
    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.

  • 71.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik. R&D Alvkarleby Lab, S-81426 Alvkarleby, Sweden.
    Liu, Ting
    Dai, Wenhong
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Transient Air-Water Flow and Air Demand following an Opening Outlet Gate2018Ingår i: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, artikel-id 3194935Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In Sweden, the dam-safety guidelines call for an overhaul of many existing bottom outlets. During the opening of an outlet gate, understanding the transient air-water flowis essential for its safe operation, especially under submerged tailwater conditions. Three-dimensional CFD simulations are undertaken to examine air-water flow behaviors at both free and submerged outflows. The gate, hoisted by wire ropes and powered by AC, opens at a constant speed. A mesh is adapted to follow the gate movement. At the free outflow, the CFD simulations and model tests agree well in terms of outlet discharge capacity. Larger air vents lead to more air supply; the increment becomes, however, limited if the vent area is larger than 10 m(2). At the submerged outflow, a hydraulic jump builds up in the conduit when the gate reaches approximately 45% of its full opening. The discharge is affected by the tailwater and slightly by the flow with the hydraulic jump. The flow features strong turbulent mixing of air and water, with build-up and break-up of air pockets and collisions of defragmented water bodies. The air demand rate is several times as much as required by steady-state hydraulic jump with free surface.

  • 72.
    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.

  • 73.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Liu, Ting
    Lin, Chang
    Lu, CH
    Raikar, RV
    Similarity Profile of Shear Layer in Water Flow Field beneath an Air Pocket at Inner Top-Wall of a Horizontal Pipe2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    The characteristics of water flow field around an air pocket stuck to the inner top-wall of a horizontal pipe, with approaching flows having fully developed turbulent boundary layer situation, are investigated experimentally. Flow visualization technique using particle trajectory photography and high-speed particle image velocimetry were employed to explore the water flow field around the air pocket in the plane of symmetry both qualitatively and quantitatively. The Reynolds number Re of the pipe flow (= UD/v, where U and D denote the cross-sectional mean streamwise velocity and pipe diameter being equal to 9.60 cm, respectively, and v is the kinematic viscosity of water) is 17,100. The volume of the air pocket tested varies from 1.0 ml to 10.0 ml. The fully developed boundary layer flow in the pipe is examined at first to assure the water flow field around the air bubble is independent of the streamwise position at which the air pocket adhered to the top-wall of the pipe. Based on the measurement results obtained by high-speed particle image velocimetry, the characteristics of water flow fields around the air pocket are presented using the mean velocity vector fields as well as utilizing the distributions of mean streamwise velocity measured at different streamwise sections. In addition, evolution of the mean streamwise velocity measured beneath the air pocket is demonstrated consequently to highlight the formation of shear layer with a reverse flow region inside and extending to the air-pocket surface. Using the non-linear regression analysis with curve fitting to the measured mean streamwise velocity, the appropriate characteristic velocity and length scales are determined precisely to attain the similarity profiles in the shear layer beneath the air pocket. The proposed characteristic length and velocity scales do provide a promising similarity profile as indicated by the data collapse and regression coefficients.

  • 74.
    Yang, James
    et al.
    Vattenfall R&D.
    Lovoll, A
    Norconsult, Norway.
    Turbulent seepage in a 6-m rock-fill dam – field measurements, analytical & numerical Solutions2006Konferensbidrag (Refereegranskat)
    Abstract [en]

    Field experiments of seepage, stability and dam break have been made in several embankment dams of various materials in North Norway. One of the dams is a homogenous rock-fill dam, 35 m long, 6.2 m high and 2.9 m wide on crest. Turbulent seepage occurs in the dam body. Assuming isotropic porous media, an analytical solution in two dimensions is developed of flow discharge and phreatic surface in turbulent seepage. Using the CFD code FLUENT, numerical simulations are made to find the best-fitted turbulent permeability. The analytical and numerical results are compared with data from the field measurements. To be able to correctly predict the turbulent seepage is important, an issue that affects calculations of slope stability, particle erosion and downstream reinforcement measures.

  • 75.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Persson, Jonas
    Norconsult.
    Moustafa, S
    Fortum.
    Upgrade of Långströmmen dam for higher safety standard2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    Långströmmen, a high-hazed facility, is planned for rehabilitation in the light of the existing dam-safety guidelines. The safety evaluations made during the recent years have revealed a number of deficiencies with respect to its spillway discharge capacity, rock erosion downstream, dam spillway stability, status of embankment dams and functions of mechanical and electrical systems. After investigations of a number of rebuilding alternatives, economic and technical considerations have reduced to one option, i.e. construction of a new gated spillway with a stilling basin. Three reservoir levels are considered, which also result in different requirements for the spillway stability and freeboards of the embankment dams. The new 18 m spillway is placed separately, with an 80 m sloping chute followed by a stilling basin. To assist in the rebuilding design, hydraulic model tests have been performed. For the reservoir area, the main issues of concern are flow patterns and head losses from the reservoir to the new spillway. For the existing spillway, attentions are paid to the river bank erosion downstream and energy dissipation. For the new spillway, the model study focuses mainly on its orientation, threshold elevation, discharge capacity and energy dissipation.

  • 76.
    Yang, James
    et al.
    Vattenfall R&D.
    Stefansson, F
    Vatnsfell dam, Iceland – experimental study of flood discharge structures.2006Konferensbidrag (Refereegranskat)
    Abstract [en]

    The main dam of Vatnsfell hydroelectric scheme is a 30 m high rock-fill dam with concrete slab facing. The design adopts a bottom spillway and a so-called converging overfall spillway. They share a energy dissipation structure of the type spatial stilling basin. Hydraulic model studies are made to evaluate and optimize the hydraulic behavior of the converging spillway, the bottom spillway and the common stilling basin. The layout offers a unique combination of interesting hydraulic features. A cover is suggested on the converging spillway chute to suppress the cross-wave. A splitter wall, centrally placed under the cover, is used to achieve uniform approach flow to the stilling basin. The basin is modified in terms of baffle-block layout and location of the end sill and side-walls. The energy dissipation should behave satisfactorily no matter when the overfall spillway and the bottom outlet are used separately or simultaneously. Other aspects examined include the spillway discharge capacity, water-surface profile in the chute, water pressure on the chute and side-walls, forces on the baffles and end sill, risk for erosion downstream the rip-rap and the behaviour of hydraulic jump at the radial gate.

  • 77.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Stenström, Petter
    Ensuring spillway discharge capacity in case of reservoir floating debris2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    Reservoir floating debris can clog up the spillway and constitutes, during extreme floods, a potential danger to dam safety. Efforts must be made to mitigate the reduction in the spillway discharge capacity, especially for dams with only two or three spillway openings, as is often the case in Sweden. In connection with the upgrade of a dam, two countermeasures are examined in hydraulic model studies, i.e. a debris visor and an overflow debris weir. A total of 100 small pine trees with roots are used, with a density varying between 650-1000 m3/s and corresponding prototype lengths varying between 15-25 m. The visor is placed obliquely upstream of the spillway where the water depth is 8-10 m. The visor stops effectively 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. For the overflow weir, if the weir water depth exceeds certain limit, solitary trees or a few trees knitted together can pass the weir without much difficulty. However, for the weir water depth allowed (less than 3 m), if clogging up along the weir starting from the existing spillway and extending upstream, the debris would get wedged and remain on the weir. As a result, the overflow capacity would be restricted, which limits the use of such a fixed weir as countermeasure against debris.

  • 78.
    Yang, James
    et al.
    Vattenfall R&D.
    Sundqvist, Per
    Uppsala University.
    Modeling two-dimensional turbulent seepage in rock-fill dams2007Ingår i: Journal of Dam Engineering, Vol. 18, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Turbulent seepage in porous media, often rockfill dams, represents a classical topic. Easy estimation of the seepage flow characteristics facilitates the slope stability analysis of a dam. An analytical solution in two dimensions is revisited for turbulent seepage flow in homogenous isotropic rockfill dams on an impervious foundation and its derivation is simplified. The usefulness of the analytical approach is improved in that explicit expressions of the seepage point location in the downstream slope and the phreatic surface are worked out. The seepage discharge can be easily computed, although its accuracy relies on a good knowledge of the turbulent permeability. Laboratory experiments involving turbulent seepage in a 1.9 m high rockfill dam were previously made in a flume at Vattenfall R&D, Älvkarleby. The flow rate varied from 60 to 80 l/s/m. CFD computations in 2D are also performed with the commercial code FLUENT, where the dam in question is modelled as a porous medium and the phreatic surface is determined with a two-phase (water-air) model. Results from the analytical solution and CFD computations are compared with the experimental data, indicating that the agreement is generally good. CFD modelling gives better agreement with the experimental results. The analytical approach is useful when an approximate answer is needed quickly.

  • 79.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Lin, Chang
    Air-vent layouts and water-air flow behaviors of a wide spillway aerator2019Ingår i: Theoretical and Applied Mechanics Letters, ISSN 2095-0349, Vol. 9, nr 2, s. 130-143Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A spillway aerator should guarantee favorable flow conditions in the coupled water-air system even if the aerator is unconventionally wide. Eight air-vent configurations are devised and incorporated into a 35-m wide chute aerator for a generalized study. Computational fluid dynamics (CFD) simulations are performed to explore their effects on water-jet and air-cavity features. The Re-normalisation group (RNG) k - epsilon turbulence model and the two-fluid model are combined to predict the two-phase flow field. The results demonstrate appreciable influences of the vent layouts on the water-air flow. The air vents stir the air motion and re-distribute the cavity air pressure. Once the vent layout is modified, reciprocal adjustments exist between the jet behavior and air-pressure field in the cavity, thus leading to considerable differences in air-flow rate, jet-trajectory length, vent air-flow distribution across the chute, etc. The large width plays a discernable role in affecting the aerated flow. Telling differences exist between the near-wall region and the central part of the chute. To improve the duct pressure propagation, a gradual augment of the vent area should be assigned towards the chute center. Relative to single-slot vents across the flow, the layouts with segregated vents gain by comparison. A designer should see to it that a vented aerator operates satisfactorily for a given range of flow discharges.

  • 80.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE).
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE).
    Xie, Qiancheng
    Luleå University of Technology (LTU).
    MODELLING OF AIR DEMAND OF A SPILLWAY AERATOR WITH TWO-PHASE FLOW MODELS2018Ingår i: E-proceedings of the 2nd International Symposium onHydraulic Modelling and Measuring Technology Congress, 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    Air demand is an issue of concern for a spillway aerator. To numerically map its air-water flow behavior has a bearing on its design. The recently completed spillway at Bergeforsen features a 35-m wide chute aerator with 13 air vents. With this in background, CFD modelling is performed with three commonly used two-phase flow models, i.e. the Volume-of-Fluid (VOF) Model, Two-Fluid Model and Mixture Model. The purpose is to evaluate these models in terms of water-air flow properties. The simulations have shown that the VOF Model generates the lowest air demand, while the Two-Fluid Model points to a 34% higher value, which is attributable to the differences in the two-phase flow formulations. The resulting air pressure in the air cavity including the air groove leads also to considerable discrepancy in the vent air-flow distribution across the chute and spatial air concentration. Evaluations of two-phase models are necessary, so that a reliable model is adopted for engineering design.

  • 81.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur. Vattenfall AB, R&D Alvkarleby Lab, Alvkarleby, Sweden..
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Zhang, Hongwei
    Inst Water Resources & Hydropower Res IWHR, Dept Hydraul, Beijing, Peoples R China..
    Experiments and CFD modeling of high-velocity two-phase flows in a large chute aerator facility2019Ingår i: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 13, nr 1, s. 48-66Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    Mathematical formulations of two-phase flows at an aerator remain a challenging issue for spillway design. Due to their complexities in terms of water-air interactions subjected to high flow velocities, experiments play an essential role in evaluations of numerical models. The paper focuses on the underlying influence of the air-water momentum exchange in the two-phase Two-Fluid Model. It is modified to better represent the drag force acting on a group of air bubbles and the wall lubrication force accounting for near-wall phase interactions. Based on data from a large aerator rig with an approach velocity of 14.3 m/s, the models are evaluated for calculations of entrained air characteristics of a flow mixture. The air bubble diameter used in the modeling ranges from 0.5 to 4 mm as suggested by the experiments. In terms of air cavity configurations and aerator air demand, smaller air bubbles lead to better agreement with the test results. As far as air concentrations are concerned, the modified model gains by comparison. In the air cavity zone, smaller bubble sizes also provide better matches with the experiments. However, the near-base air concentration remains overestimated downstream from the impact area. The fact that the program user must pre-define a single air bubble size in simulations presumably limits the correct reproduction of near-base air concentrations and of their decay.

  • 82.
    Yang, James
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Teng, Penghua
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE).
    Zhang, Hongwei
    Institute of Water Resources and Hydropower Research (IWHR).
    Experiments and CFD modeling of high-velocity two-phase flows in a large chute aerator facility2019Ingår i: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 13, nr 1, s. 48-66Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Mathematical formulations of two-phase flows at an aerator remain a challenging issue for spillwaydesign. Due to their complexities in terms of water–air interactions subjected to high flow velocities,experiments play an essential role in evaluations of numerical models. The paper focuses on theunderlying influence of the air–water momentum exchange in the two-phase Two-Fluid Model. Itis modified to better represent the drag force acting on a group of air bubbles and the wall lubricationforce accounting for near-wall phase interactions. Based on data from a large aerator rig withan approach velocity of 14.3 m/s, the models are evaluated for calculations of entrained air characteristicsof a flow mixture. The air bubble diameter used in the modeling ranges from 0.5 to 4mmassuggested by the experiments. In terms of air cavity configurations and aerator air demand, smallerair bubbles lead to better agreement with the test results. As far as air concentrations are concerned,the modified model gains by comparison. In the air cavity zone, smaller bubble sizes also provide bettermatches with the experiments. However, the near-base air concentration remains overestimateddownstream from the impact area. The fact that the program user must pre-define a single air bubblesize in simulations presumably limits the correct reproduction of near-base air concentrations andof their decay.

  • 83. Ye, F.M.
    et al.
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Vattendragsteknik.
    Wang, S. R.
    Turbine governing and surge-tank stability1992Ingår i: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 30, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The stability of water-surface oscillations is investigated for the case of a tailrace surge chamber shared by two or more generating units in a hydroelectric power plant. Two kinds of turbine governing are applied for the condition of small-amplitude surge perturbations. Critical stability conditions for the surge chamber are derived by linearizing the ordinary differential equations describing the lumped system. As compared with the result achieved by previous researchers, the common surge chamber requires smaller horizontal crosssection for surge stability. A combination of the proposed manners of governing can be utilized in certain power plants in order to achieve an economical design of the surge chambers.

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