Author(s): Makoto Higashino; Heinz G. Stefan
Linked Author(s): Heinz G. Stefan
Keywords: Boundary layer; Mass transfer; Sediment bed; Turbulent diffusion
Abstract: A model, i. e. the ‘pulse-model’, is presented to simulate the transfer of turbulence from flowing water across and below a sediment/water interface, and its effect on the diffusional mass transfer of a solute, e. g. oxygen, sulfate or phosphate into or out of the sediment bed. Turbulence above the sediment surface is described by sinusoidal variations of vertical velocity in time. It is shown that vertical velocity components dampen quickly inside the sediment when the frequency of velocity fluctuations is high and viscous dissipation is strong. Viscous dissipation (ν) inside the sediment is related to water viscosity (ν0) as well as inertial effects when the flow is turbulent. A value ν/ν0 between 1 and 20 has been considered. Turbulence penetration into the sediment is parameterized by the Reynolds number Re=UL/ν and the relative penetration velocity W/U, where U is the amplitude of the velocity pulse and W is the penetration velocity; L=WT is the wave length of the velocity pulse, and T is its period. Amplitudes of vertical velocity components inside the sediment and their auto-correlation functions are computed, and the results are used to estimate turbulent diffusivities inside the sediment. The results of the velocity pulse-model can be used to predict effective diffusion coefficient affected by near-bed turbulence, and hence, for water quality modeling.
Year: 2007