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Near-bed turbulence models: significance for diffusional mass transfer at the sediment/water interface / Makoto Higashino in Journal of hydraulic research, Vol. 46 n°3 (2008) 

Public ISBD [article]  in Journal of hydraulic research > Vol. 46 n°3 (2008) . - pp. 291-300 Titre : Near-bed turbulence models: significance for diffusional mass transfer at the sediment/water interface Titre original : Les modèles de turbulence en proximité de lit: importance pour le transfert de masse par diffusion à l'interface sédiment/eau Type de document : texte imprimé Auteurs : Makoto Higashino, Auteur ; Stefan, Heinz G., Auteur Article en page(s) : pp. 291-300 Note générale : Hydraulique Résumé en Français Langues : Anglais (eng) Mots-clés : Boundary  layer  Diffusion  K-£  turbulence  model  Mass  transfer  Sediment/water  interaction  Sedimentary  oxygen  demand  Turbulence Index. décimale : 627 Ingénierie des cours d'eau naturels, des ports, des rades et des cotes. Installations de navigation, de dragage, de récupération et de sauvetage. Barrages et centrales électriques hydrauliques Résumé : It is well-known that sediment/water interaction affects water quality in natural water bodies in a variety of ways. Diffusional mass transfer, across a sediment/water interface can contribute significantly to the mass balances of dissolved substances such as oxygen, phosphorus, nitrogen and sulfate, especially in lakes, reservoirs, detention basins, navigation canals and estuaries. At low flow velocities above a sediment bed, diffusional mass transfer across a sediment/water interface becomes limited by lack of turbulence in the boundary layer. In this paper three boundary layer turbulence models are used to quantify the limiting effect of turbulence near a sediment bed on diffusional mass transfer across a sediment/water interface: One is Dade’s formula, the second is a model by Myong and Kasagi (MK model), and the third is a model by Nagano and Tagawa (NT model). The latter two are low-Reynolds number k-ε turbulence models that have been successfully used for the prediction of turbulent heat transfer near a wall. Dade’s formula gives larger eddy diffusivity/viscosity values near the sediment/water interface than the other two turbulence models, but the velocity profile obtained by Dade’s formula is only slightly different from the universal velocity profile given by the linear law and the log-law. Simulated solute concentration profiles and vertical diffusive fluxes (Sherwood numbers Sh) at the sediment/water interface were found to be only weakly dependent on the choice of turbulence model when material sinks or sources in the sediments were not explicitly quantified. Sherwood numbers simulated by the NT model were less than 40% smaller than those obtained by Dade’s formula and the MK model, when the Schmidt number Sc＞10, which is the case for most materials in aquatic solution. The mass transport model was also applied to the estimation of sedimentary oxygen demand (SOD) by including a microbial oxygen sink in the sediments. Simulated oxygen uptake rate, i.e. SOD (gm-2d-1), was virtually the same for all three turbulence models, except at low shear velocity (U*<0.5cm), and at high microbial activity inside the sediment (Xmax>100mg/l). Even then the lowest SOD values obtained with the NT turbulence model were within 20% of the higher values estimated with the other two models. One can conclude that any of the three models used to describe turbulence near the sediment bed, is appropriate for SOD estimation, because microbial activity inside the sediment is as limiting as near-bed turbulence. DEWEY : 627 ISSN : 0022-1686 En ligne : http://www.journalhydraulicresearch.com [article] Near-bed turbulence models: significance for diffusional mass transfer at the sediment/water interface = Les modèles de turbulence en proximité de lit: importance pour le transfert de masse par diffusion à l'interface sédiment/eau [texte imprimé] / Makoto Higashino, Auteur ; Stefan, Heinz G., Auteur . - pp. 291-300. Hydraulique Résumé en Français Langues : Anglais (eng) in Journal of hydraulic research > Vol. 46 n°3 (2008) . - pp. 291-300 Mots-clés : Boundary  layer  Diffusion  K-£  turbulence  model  Mass  transfer  Sediment/water  interaction  Sedimentary  oxygen  demand  Turbulence Index. décimale : 627 Ingénierie des cours d'eau naturels, des ports, des rades et des cotes. Installations de navigation, de dragage, de récupération et de sauvetage. Barrages et centrales électriques hydrauliques Résumé : It is well-known that sediment/water interaction affects water quality in natural water bodies in a variety of ways. Diffusional mass transfer, across a sediment/water interface can contribute significantly to the mass balances of dissolved substances such as oxygen, phosphorus, nitrogen and sulfate, especially in lakes, reservoirs, detention basins, navigation canals and estuaries. At low flow velocities above a sediment bed, diffusional mass transfer across a sediment/water interface becomes limited by lack of turbulence in the boundary layer. In this paper three boundary layer turbulence models are used to quantify the limiting effect of turbulence near a sediment bed on diffusional mass transfer across a sediment/water interface: One is Dade’s formula, the second is a model by Myong and Kasagi (MK model), and the third is a model by Nagano and Tagawa (NT model). The latter two are low-Reynolds number k-ε turbulence models that have been successfully used for the prediction of turbulent heat transfer near a wall. Dade’s formula gives larger eddy diffusivity/viscosity values near the sediment/water interface than the other two turbulence models, but the velocity profile obtained by Dade’s formula is only slightly different from the universal velocity profile given by the linear law and the log-law. Simulated solute concentration profiles and vertical diffusive fluxes (Sherwood numbers Sh) at the sediment/water interface were found to be only weakly dependent on the choice of turbulence model when material sinks or sources in the sediments were not explicitly quantified. Sherwood numbers simulated by the NT model were less than 40% smaller than those obtained by Dade’s formula and the MK model, when the Schmidt number Sc＞10, which is the case for most materials in aquatic solution. The mass transport model was also applied to the estimation of sedimentary oxygen demand (SOD) by including a microbial oxygen sink in the sediments. Simulated oxygen uptake rate, i.e. SOD (gm-2d-1), was virtually the same for all three turbulence models, except at low shear velocity (U*<0.5cm), and at high microbial activity inside the sediment (Xmax>100mg/l). Even then the lowest SOD values obtained with the NT turbulence model were within 20% of the higher values estimated with the other two models. One can conclude that any of the three models used to describe turbulence near the sediment bed, is appropriate for SOD estimation, because microbial activity inside the sediment is as limiting as near-bed turbulence. DEWEY : 627 ISSN : 0022-1686 En ligne : http://www.journalhydraulicresearch.com