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CFD MODELLING AND EXPERIMENTS ON AERATOR FLOW IN CHUTE SPILLWAYS
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering. (Hydraulic and Hydrologic Engineering)ORCID iD: 0000-0001-7631-3503
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A chute spillway is a typical component of large dams for discharging floods. Because of the high water head, the flow velocity in the chute is often in excess of 20 m/s. Consequently, the structure is usually prone to cavitation damages. Flow aeration is evidenced to efficiently eliminate or to mitigate the damages. An aerator is a device that entrains air into the water flows and is an effective technical measure to counter the cavitation damages.

Aerator flow includes intense air-water exchange and involves a process of air entrainment, transport, and detrainment. Because of the complex phenomena, it is still a challenge to investigate the behaviors of interaction between air and water. It is fundamental to understand the flow behaviors downstream of the aerator. This thesis investigates the aerator flow features using both the Computational Fluid Dynamics (CFD) and advanced measurement techniques.

The CFD method presents three two-phase flow models to describe the aerator flows, namely, the Volume of Fluid Model, the Mixture Model, and the Two-Fluid Model. They are applied and evaluated via practical engineering projects and experimental data. The Volume of Fluid model leads to reasonable results regarding the water flow discharge and flow fields. For predicting the air concentration distribution and air bubble transport processes, the Two-Fluid Model is superior to others because it includes forces acting on the air bubbles. However, the model still overestimates the air content near the chute bottom. Based on the aerator flow from a chute spillway in Sweden, three two-phase flow models are applied and compared.

Physical model tests are commonly conducted to investigate aerator flow features. Because of the scale effects, the results may lead to a discrepancy in the flow behaviors compared with the prototype. Thus, CFD modeling becomes an alternative tool when seeking the reason for the difference. Based on the aerator flow in a real spillway, CFD is applied to reproduce the flow; the discrepancy between the model tests and prototype observations is evidenced. The results show similar flow features with the prototype but differ from those of the model tests. An explanation for the discrepancy is discussed in terms of flow features, effect of surface tension in model tests, and the prerequisite for air entrainment of the free-surface flow.

Laboratory experiments are conducted to study the aerator flow in a chute. Four image-based measurement techniques-i.e., high-speed particle image velocimetry (HSPIV), shadowgraphic image method (SIM), bubble tracking method (BTM), and bubble image velocimetry (BIV)-are employed. The study focuses on issues of exploring characteristic positions of water-air interfaces, interpreting the evaluation process of air bubbles shed from the tip of the air cavity, identifying the probabilistic means for characteristic positions near the fluctuating free surface, and obtaining the flow field both water flow and air bubbles features of the aerator flow. The application of these techniques leads to a better understanding of two-phase flow characteristics of the chute aerator.

Abstract [sv]

Ett flodutskov är en typisk komponent hos vattenkraftdammar och som syftar till att släppa ut dimensionerande vattenföring. På grund av den stora fallhöjden hos många höga dammar är flödeshastigheten i utskovskanalen ofta högre än 20 m/s. Följaktligen är utskovsstrukturen oftast sårbar för kavitationsskador. Att lufta vattenströmmen är ett effektivt sätt att eliminera eller mildra skadorna. En luftningsramp är en anordning som blandar in luft i vattenflödet och är en ingenjörsmässig åtgärd för att motverka kavitationsskadorna. 

Luftinblandning avser intensivt luft-vattenutbyte och involverar en process med luftinträngning, transport och infångning av luft i immobila fickor i strömningsvägen. På grund av det komplexa fenomenet är det fortfarande en utmaning att undersöka det växelvisa mekaniska beteendet mellan luft och vatten. Det är grundläggande att förstå flödesbeteenden nedströms av luftningsrampen eftersom luften blandas i vattnet och kan sedan avges till atmosfären. Denna avhandling undersöker egenskaperna hos luftningsflödet med både beräkningsmetoder (Computational Fluid Dynamics – CFD) och avancerade labbförsök.

CFD-metoden presenterade tre stycken tvåfasmodeller för att beskriva luftningsflödena, nämligen s.k. Volume of fluid (VOF), Mixture Model och Two-Fluid Model (TFM). De tillämpas och utvärderas med data från prototyputskov och även genom jämförelser med experimentella data. VOF modellen leder till rimliga resultat avseende både vattenflöde och luftmängd. För att förutsäga luftkoncentrationsfördelning och luftbubblors transportprocesser är TFM överlägsen andra metoder (modeller) eftersom den inkluderar krafter som verkar på luftbubblorna. Modellen överskattar dock fortfarande luftinnehållet nära utskovsbotten. Baserat på uppgifter av ett prototyputskov i Sverige tillämpas och jämförs de tre modellerna.

Fysiska modellförsök genomförs vanligtvis för att undersöka egenskaper hos luftningsrampen. På grund av skaleffekter kan resultaten leda till avvikelser i flödesbeteenden jämfört med prototypen. Således blir CFD-modellering ett alternativt verktyg när orsaken till skillnaden söks. Baserat på fältförsök i ett flodutskov appliceras CFD för att reproducera flödesbeteenden; avvikelser mellan modellförsöken och prototypobservationer visas. CFD, som utförs i prototypmått, visar liknande flödesegenskaper som i prototypen men skiljer sig från dem i modellen. En förklaring till avvikelserna diskuteras i termer av flödesegenskaper, ytspänningens effekt i modellförsöken och förutsättningen för luftinträngning genom den fria vattenytan.

Experiment utförs i labbmiljö för att studera luftningsflödet i en ränna. Fyra bildbaserade mättekniker – det vill säga hastighetsmätning med höghastighetspartikelfotografering (High-Speed Particle Image Velocimetry, HSPIV), skuggrafisk bildmetod (Shadowgraphic Image Method, SIM), bubbelspårningsmetod (Bubble Tracking Method, BTM) och bubbelbildshastighetsmetod (Bubble Image Velocimetry ,BIV) – används. Studien fokuserar på frågor som rör karakteristiska lägen för vatten-luftgränssnitt, tolkning av utvärderingsprocessen för luftbubblor som släpper från spetsen av luftkaviteten, identifiering av sannolika medelvärden för karakteristiska lägen av nära den fluktuerande fria vattenytan och att erhålla hastighetsfältet från både vatten- och luftflöde och egenskaper hos luftbubblor i strömmen. Tillämpningen av dessa tekniker leder till en bättre förståelse av tvåfasflödets egenskaper hos luftningsrampen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 82
Series
TRITA-ABE-DLT ; TRITA-ABE-DLT-1937
Keywords [en]
Spillway Aerator, Cavitation, Air Entrainment, Two-phase Flow, VOF, Mixture Model, Two-Fluid Model, Air Cavity, Cavity Air Pressure, Air Concentration, Shadowgraphic Image Method (SIM), High-speed Particle Image Velocimetry (HSPIV), Bubble Tracking Method (BTM)
Keywords [sv]
luftningsramp, kavitation, luftinträngning, tvåfasflöde, VOF, Mixture Model, Two-Fluid Model, luftkavitet, kavitetslufttryck, luftkoncentration, skuggrafisk bildmetod, hastighetsmätning med höghastighetspartikelfotografering, bubbelspårningsmetod
National Category
Engineering and Technology Civil Engineering Water Engineering
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-259591ISBN: 978-91-7873-313-2 (print)OAI: oai:DiVA.org:kth-259591DiVA, id: diva2:1352384
Public defence
2019-10-21, F3, Lindstedtsvägen 26, KTH Campus, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

QC 20190919

Available from: 2019-09-19 Created: 2019-09-18 Last updated: 2019-09-19Bibliographically approved
List of papers
1. CFD modeling of two-phase flow of a spillway chute aerator of large width
Open this publication in new window or tab >>CFD modeling of two-phase flow of a spillway chute aerator of large width
2016 (English)In: Journal of Applied Water Engineering and Research, ISSN 2324-9676, E-ISSN 2324-9676, Vol. 4, no 2, p. 163-177Article in journal (Refereed) Published
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 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. 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.

Place, publisher, year, edition, pages
TAYLOR & FRANCIS LTD, 2016
Keywords
spillway, chute aerator, air demand, air-water flow, CFD, subpressure
National Category
Fluid Mechanics and Acoustics Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:kth:diva-214922 (URN)10.1080/23249676.2015.1124030 (DOI)000409034800008 ()2-s2.0-85045835884 (Scopus ID)
Note

QC 2017-09-25

Available from: 2017-09-25 Created: 2017-09-25 Last updated: 2019-09-19Bibliographically approved
2. Air-vent layouts and water-air flow behaviors of a wide spillway aerator
Open this publication in new window or tab >>Air-vent layouts and water-air flow behaviors of a wide spillway aerator
2019 (English)In: Theoretical and Applied Mechanics Letters, ISSN 2095-0349, Vol. 9, no 2, p. 130-143Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Spillway, Wide aerator, Air entrainment, Air-vent layout, Jet trajectory, CFD
National Category
Water Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-258934 (URN)10.1016/j.taml.2019.02.009 (DOI)000470901800010 ()2-s2.0-85066931897 (Scopus ID)
Note

QC 20190917

Available from: 2019-09-11 Created: 2019-09-11 Last updated: 2019-09-19Bibliographically approved
3. Studies of Two-Phase Flow at a Chute Aerator with Experiments and CFD Modelling
Open this publication in new window or tab >>Studies of Two-Phase Flow at a Chute Aerator with Experiments and CFD Modelling
2016 (English)In: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, article id 4729128Article in journal (Refereed) Published
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 high-velocity water flow. Correctly predicting the air entrainment and two-phase flow pattern at the aerator would contribute to reliable spillway operation. Based on experimental data, 2D numerical simulations are preformed to predict streamwise air concentrations in the aerated flow, in which a two-fluid model is used. Depending on the air bubble size, relatively good agreement is seen with the experiments in the air cavity zone. The simulations give rise to higher air concentration downstream of the cavity, which is presumably due to underestimation of the interfacial forces in the two-fluid model.

Place, publisher, year, edition, pages
Hindawi Publishing Corporation, 2016
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-193272 (URN)10.1155/2016/4729128 (DOI)000382691500001 ()2-s2.0-84986285599 (Scopus ID)
Note

QC 20161007

Available from: 2016-10-07 Created: 2016-09-30 Last updated: 2019-09-19Bibliographically approved
4. Experiments and CFD modeling of high-velocity two-phase flows in a large chute aerator facility
Open this publication in new window or tab >>Experiments and CFD modeling of high-velocity two-phase flows in a large chute aerator facility
2019 (English)In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 13, no 1, p. 48-66Article, review/survey (Refereed) Published
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.

Place, publisher, year, edition, pages
HONG KONG POLYTECHNIC UNIV, DEPT CIVIL & STRUCTURAL ENG, 2019
Keywords
Aerator, two-phase flow, two-fluid model, drag force, wall lubrication force, air cavity
National Category
Environmental Engineering
Identifiers
urn:nbn:se:kth:diva-239964 (URN)10.1080/19942060.2018.1552201 (DOI)000451548900001 ()
Note

QC 20181211

Available from: 2018-12-11 Created: 2018-12-11 Last updated: 2019-09-19Bibliographically approved
5. MODELLING OF AIR DEMAND OF A SPILLWAY AERATOR WITH TWO-PHASE FLOW MODELS
Open this publication in new window or tab >>MODELLING OF AIR DEMAND OF A SPILLWAY AERATOR WITH TWO-PHASE FLOW MODELS
2018 (English)In: E-proceedings of the 2nd International Symposium onHydraulic Modelling and Measuring Technology Congress, 2018Conference paper, Published 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.

Keywords
spillway, aerator, air demand, two-phase model
National Category
Water Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-258939 (URN)
Conference
2nd International Symposium on Hydraulic Modelling and Measuring Technology Congress, 2018
Note

QC 20190930

Available from: 2019-09-11 Created: 2019-09-11 Last updated: 2019-09-30Bibliographically approved
6. Modeling and Prototype Testing of Flows over Flip-Bucket Aerators
Open this publication in new window or tab >>Modeling and Prototype Testing of Flows over Flip-Bucket Aerators
2018 (English)In: Journal of Hydraulic Engineering, ISSN 0733-9429, E-ISSN 1943-7900, Vol. 144, no 12, article id 04018069Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
ASCE-AMER SOC CIVIL ENGINEERS, 2018
Keywords
Spillway, Aerator, Jet oscillation, Model tests, Prototype tests, Computation fluid dynamics (CFD)
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-238106 (URN)10.1061/(ASCE)HY.1943-7900.0001531 (DOI)000447254300004 ()2-s2.0-85053633622 (Scopus ID)
Note

QC 20190111

Available from: 2019-01-11 Created: 2019-01-11 Last updated: 2019-09-19Bibliographically approved
7. Application of SIM, HSPIV, BTM, and BIV techniques for evaluations of a two-phase air-water chute aerator flow
Open this publication in new window or tab >>Application of SIM, HSPIV, BTM, and BIV techniques for evaluations of a two-phase air-water chute aerator flow
Show others...
2018 (English)In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 10, no 11, article id 1590Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
MDPI AG, 2018
Keywords
Aerator, Air bubbles, Air cavity, Bubble image velocimetry (BIV), Bubble tracking method (BTM), Flow impingement point, High-speed particle image velocimetry (HSPIV), Shadowgraphic image method (SIM), Two-phase flow, Air, Flow of water, Flow visualization, Phase interfaces, Transpiration, Velocimeters, Velocity, Velocity distribution, Velocity measurement, Water aeration, Bubble tracking method, High-speed particles, Image method, Image velocimetry, Two phase flow, air bubble, cavity, detection method, flow velocity, free surface flow, groundwater flow, instrumentation, numerical method, particle image velocimetry, two-dimensional flow, water flow
National Category
Water Engineering
Identifiers
urn:nbn:se:kth:diva-247072 (URN)10.3390/w10111590 (DOI)000451736300097 ()2-s2.0-85055960679 (Scopus ID)
Note

QC 20190625

Available from: 2019-06-25 Created: 2019-06-25 Last updated: 2019-09-19Bibliographically approved

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