Air–Water Mass Transfer on a Stepped Waterway

Toombes, Luke; Chanson, Hubert

doi:10.1061/(asce)0733-9372(2005)131:10(1377)

Cited by 64 publications

(48 citation statements)

References 8 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Similarly, the lesser number of entrained bubble sizes in laboratory flumes does affect the rate of air-water mass transfer on the chute. Present results imply that the airwater interface area, hence the rate of air-water mass transfer, are underestimated in small-size physical-models, and extrapolation are not reliable unless working at full scale (Toombes and Chanson 2005).…”

Section: Froude Similitudementioning

confidence: 91%

Turbulence, dynamic similarity and scale effects in high-velocity free-surface flows above a stepped chute

Felder

Chanson

2009

Exp Fluids

View full text Add to dashboard Cite

In high-velocity free-surface flows, air entrainment is common through the interface, and intense interactions take place between turbulent structures and entrained bubbles. Two-phase flow properties were measured herein in high-velocity open channel flows above a stepped chute. Detailed turbulence measurements were conducted in a large-size facility, and a comparative analysis was applied to test the validity of the Froude and Reynolds similarities. The results showed consistently that the Froude similitude was not satisfied using a 2:1 geometric scaling ratio. Lesser number of entrained bubbles and comparatively greater bubble sizes were observed at the smaller Reynolds numbers, as well as lower turbulence levels and larger turbulent length and time scales. The results implied that small-size models did underestimate the rate of energy dissipation and the aeration efficiency of prototype stepped spillways for similar flow conditions. Similarly a Reynolds similitude was tested. The results showed also some significant scale effects. However a number of self-similar relationships remained invariant under changes of scale and confirmed the analysis of Chanson and Carosi (Exp Fluids 42:385-401, 2007). The finding is significant because self-similarity may provide a picture general enough to be used to characterise the airwater flow field in large prototype channels. Cross-correlation time scale ( List of symbols

show abstract

Section: Froude Similitudementioning

confidence: 91%

Turbulence, dynamic similarity and scale effects in high-velocity free-surface flows above a stepped chute

Felder

Chanson

2009

Exp Fluids

View full text Add to dashboard Cite

show abstract

“…Similarly, the lesser number of entrained bubble sizes in laboratory flumes must affect the rate of air-water mass transfer on the chute. Present results imply that the air-water interface area, hence the rate of air-water mass transfer, are underestimated in small-size physical-models, and extrapolation are not reliable unless working at full scale as shown by Toombes and Chanson (2005).…”

Section: Discussionmentioning

confidence: 99%

Physical Modelling, Scale Effects, and Self-Similarity of Stepped Spillway Flows

Chanson

2008

World Environmental and Water Resources Congress 2008

View full text Add to dashboard Cite

Stepped spillway flows are characterised by significant turbulent dissipation on the chute associated with strong free-surface aeration. Herein the validity of the Froude similitude is tested by re-analysing three experimental data sets corresponding to moderate slopes typical of embankment dams and storm waterways ( = 3.4 to 22°). The measurements included distributions of void fraction, bubble count rate, interfacial velocity, turbulence intensity, bubble chord sizes, and integral turbulent length and time scales which were scaled with a 2:1 undistorted geometric scaling ratio. The criterion selection for scale effects is a critical issue. Major differences (i.e. scale effects) were observed in terms of bubble count rates, bubble chord sizes, turbulence levels and integral turbulent length scales although little scale effects were seen in terms of void fraction and velocity distributions. The findings suggested that most physical results, including the rate of energy dissipation and re-aeration, cannot be extrapolated to prototype flow conditions without significant scale effects. A complementary approach may be based upon self-similarity, since self-similar relationships were observed systematically at both macroscopic and microscopic levels. Keywords : Stepped spillways, Air-water flow properties, Physical modelling, Scale effects, Self-similarity, Froude similitude. INTRODUCTIONStepped spillways are typically designed to increase the rate of energy dissipation on the (stepped) chute and to reduce the size of the downstream energy dissipator. At large discharges, skimming flows on stepped spillways are characterised by a significant rate of turbulent dissipation associated with strong free-surface aeration (Fig. 1). Although research in the hydraulics of stepped spillways has been very active during the last three decades (Chanson 1995,2001, Matos and Chanson 2006, the fundamental issues of dynamic similarity and scale effects were rarely discussed. How confidently can we extrapolate laboratory results to prototype spillways ? Detailed air-water flow measurements are re-analysed systematically. The data were recorded in three large-size stepped chute facilities with slopes = 3.4º, 16º and 22º, and step heights h = 0.05 m to 0.143 m. Detailed turbulence data were collected in the large-size channels with a stepped invert, including turbulence levels and turbulent time and length scales. The results are used to test the validity of the Froude similarity and to assess the scale effects and the experimental distortion to prototype stepped spillways.

show abstract

“…Elle est souvent calculée à une température de référence de 20 °C en utilisant l'équation de GULLIVER et al (1990) : La majorité des études existantes (BAYLAR et al, 2006(BAYLAR et al, , 2007TOOMBES et CHANSON, 2005) n'intègrent pas la totalité des paramètres significatifs. De plus, les débits d'eau étudiés sont assez élevés et ne correspondent pas aux débits dans les petites rivières.…”

Section: Transfert D'oxygène Air-eauunclassified

Autoépuration en eau courante : évaluation de l’oxygénation dans les cascades en marches d’escalier

Khdhiri

Potier

Leclerc

2014

rseau

View full text Add to dashboard Cite

Dans les écosystèmes aquatiques, l’oxygène dissous est consommé par les processus chimiques et biologiques de l’autoépuration (oxydoréduction, dégradation de la matière organique,...) ainsi que par la respiration des espèces aquatiques. L’autoépuration sera donc favorisée par une amélioration de l’aération et le taux d’oxygène dissous constituera un indicateur de la qualité de l’eau. Les structures hydrauliques telles que les cascades jouent le rôle de système d’aération en favorisant l’absorption dans l'eau de l’oxygène atmosphérique. Afin d’évaluer le potentiel d’aération de ces ouvrages aux faibles débits, une étude a été réalisée sur une cascade de laboratoire représentative des petits cours d’eau et équipée de plusieurs marches modulables en nombre et en taille. Le transfert d’oxygène dans les cascades est fortement dépendant de l’aire de l’interface d’échange rapporté au volume d’eau, du type d’écoulement diphasique et de la turbulence dans le fluide. Du point de vue hydrodynamique, trois régimes d’écoulement ont été identifiés pour un débit inférieur à 3 L•-s-1, deux régimes de type nappe et un régime de transition. L’efficacité du transfert d’oxygène mesurée varie de 15 % à 40 % en fonction du débit d’eau, du nombre et de la longueur des marches de la cascade. Les résultats ont montré une amélioration de l’aération avec le débit (en régime nappe) et avec le nombre de marches. Sur les différents systèmes de cascades étudiés, une relation de linéarité entre le nombre de marches et l’efficacité d’aération a été constatée. L’ajout de graviers sur les marches provoque une légère amélioration de la capacité d’aération, dépendant du niveau d’immersion des graviers. À hauteur donnée, l’augmentation de la longueur des cascades défavorise le transfert d’oxygène.In aquatic ecosystems, dissolved oxygen is consumed by the chemical and biological processes involved in natural self-purification (oxidation-reduction, degradation of organic matter,…) and by the respiration of the aquatic species. The self-purification can be enhanced by water aeration improvement and the dissolved oxygen level is thus an indicator of the water quality. Hydraulic structures such as cascades play the role of an aeration system by improving the dissolution of atmospheric oxygen in water. To estimate the aeration potential of these structures in low discharges corresponding to small natural streams, a study was realized on a laboratory cascade model equipped with a modular number of size-adjustable steps. Oxygen transfer in stepped cascades is strongly dependent on the interfacial exchange area per water volume, the multiphase flow structure and the turbulence in the fluid. From the hydrodynamic point of view, three flow regimes were identified for a flow rate lower than 3 L•s-1: two nappe regimes and one transition regime. The measured oxygen transfer efficiency varies between 15% and 40% depending on water flow rates, and the number and length of steps in the cascade. The results showed an improvement of the aeration with the flow rate (in ...

show abstract

Air–Water Mass Transfer on a Stepped Waterway

Cited by 64 publications

References 8 publications

Turbulence, dynamic similarity and scale effects in high-velocity free-surface flows above a stepped chute

Turbulence, dynamic similarity and scale effects in high-velocity free-surface flows above a stepped chute

Physical Modelling, Scale Effects, and Self-Similarity of Stepped Spillway Flows

Autoépuration en eau courante : évaluation de l’oxygénation dans les cascades en marches d’escalier

Contact Info

Product

Resources

About