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  • 1. Lin, C.
    et al.
    Kao, M. -J
    Wong, W. -Y
    Tsai, C. -P
    Chang, K. -T
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Characteristics of flow separation and hydraulic jump during run-down motion of shoaling solitary wave traveling over steep sloping bottom2016In: Proceedings of the International Offshore and Polar Engineering Conference, International Society of Offshore and Polar Engineers , 2016, p. 724-731Conference paper (Refereed)
    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.

  • 2. Lin, C.
    et al.
    Kao, M. -J
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    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 chute2018In: Journal of Marine Science and Technology (Taiwan), ISSN 1023-2796, Vol. 26, no 3, p. 309-326Article in journal (Refereed)
    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%.

  • 3.
    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, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. 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 Beach2019In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 11, no 8, article id 1556Article in journal (Refereed)
    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.

  • 4.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    CFD modeling of two-phase flow of a spillwaychute aerator of large width2016In: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 4, no 2, p. 163-177Article in journal (Refereed)
    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.

  • 5.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Flows over flip-bucket aerators, physical and CFD modeling with prototype testsManuscript (preprint) (Other academic)
    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. 

  • 6.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Modeling and Prototype Testing of Flows over Flip-Bucket Aerators2018In: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 144, no 12, article id 04018069Article in journal (Refereed)
    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.

  • 7.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Modeling and Prototype Testing of Flowsover Flip-Bucket Aerators2018In: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 144, no 12Article in journal (Refereed)
    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.

  • 8.
    Teng, Penghua
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Pfister, Michael
    Ecole Polytechnique Federale de Lausanne (EPFL).
    Studies of Two-Phase Flow at a Chute Aeratorwith Experiments and CFD Modelling2016In: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, article id 4729128Article in journal (Refereed)
    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.

  • 9. Xie, Q.
    et al.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Lundström, S.
    Dai, W.
    Understanding morphodynamic changes of a tidal river confluence through field measurements and numerical modeling2018In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, no 10, article id 1424Article in journal (Refereed)
    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.

  • 10.
    Xie, Qiancheng
    et al.
    Lulea Univ Technol, Div Fluid & Expt Mech, S-97187 Lulea, Sweden..
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    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 Load2019In: FLUIDS, ISSN 2311-5521, Vol. 4, no 1, article id 15Article in journal (Refereed)
    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.

  • 11.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering. Vattenfall AB, Sverige.
    Andreasson, P.
    Bending the spillway flow for safety upgrades2016In: International Water Power and Dam Construction, ISSN 0306-400X, E-ISSN 1538-6414, Vol. 68, no 11, p. 36-43Article in journal (Refereed)
  • 12.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. 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, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    The Tale of an Intake Vortex and Its Mitigation Countermeasure: A Case Study from Akkats Hydropower Station2018In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, no 7, article id 881Article in journal (Refereed)
    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.

  • 13.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Andreasson, Patrik
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Xie, Qiancheng
    The Past and Present of Discharge Capacity Modeling for Spillways-A Swedish Perspective2019In: FLUIDS, ISSN 2311-5521, Vol. 4, no 1, article id 10Article, review/survey (Refereed)
    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.

  • 14.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Lin, C.
    Kao, M. -J
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Raikar, R. V.
    Application of SIM, HSPIV, BTM, and BIV techniques for evaluations of a two-phase air-water chute aerator flow2018In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, no 11, article id 1590Article in journal (Refereed)
    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.

  • 15.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering. R&D Alvkarleby Lab, S-81426 Alvkarleby, Sweden.
    Liu, Ting
    Dai, Wenhong
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Transient Air-Water Flow and Air Demand following an Opening Outlet Gate2018In: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, article id 3194935Article in journal (Refereed)
    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.

  • 16.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    Lin, Chang
    Air-vent layouts and water-air flow behaviors of a wide spillway aerator2019In: Theoretical and Applied Mechanics Letters, ISSN 2095-0349, Vol. 9, no 2, p. 130-143Article in journal (Refereed)
    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.

  • 17.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE).
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE).
    Xie, Qiancheng
    Luleå University of Technology (LTU).
    MODELLING OF AIR DEMAND OF A SPILLWAY AERATOR WITH TWO-PHASE FLOW MODELS2018In: E-proceedings of the 2nd International Symposium onHydraulic Modelling and Measuring Technology Congress, 2018Conference paper (Refereed)
    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.

  • 18.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure. Vattenfall AB, R&D Älvkarleby Lab, Älvkarleby, Sweden.
    Teng, Penghua
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Resources, Energy and Infrastructure.
    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 facility2019In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 13, no 1, p. 48-66Article, review/survey (Refereed)
    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.

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