Author(s): Makoto Higashino; Heinz G. Stefan
Linked Author(s): Heinz G. Stefan
Keywords: Boundary layer; Diffusion; K-ε turbulence model; Mass transfer; Sediment/water Interaction; Sedimentary oxygen demand; Turbulence
Abstract: Diffusional mass transfer, across a sediment/water interface can contribute significantly to the mass balances of dissolved oxygen, phosphorus, nitrogen and sulfate, especially in lakes, reservoirs and river impoundments. 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. To study this effect, boundary layer turbulence models can be applied. In this paper three such models are used: 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 projects stronger eddy diffusivity/viscosity near the sediment/water interface than the other two turbulence models. Simulated solute concentration profiles and vertical diffusive fluxes (Sherwood numbers Sh) at the sediment/water interface were only weakly dependent on the choice of turbulence model when sinks or sources in the sediments were not explicitly quantified. The mass transport model including a microbial oxygen sink in the sediments was also applied to the estimation of sedimentary oxygen demand (SOD). When microbial dissolved oxygen kinetics were included in the mass transport model the characterization/model of near-bed turbulence was of significance, but all three models were found to be equally appropriate, except at low shear velocity (U*<0. 5cm), and at high microbial activity inside the sediment. For all other conditions the simulated sedimentary oxygen uptake rate, i. e. SOD, was virtually the same for all three turbulence models, because microbial activity inside the sediment in addition to near-bed turbulence was a limiting process.
Year: 2005