The Influence of Anisotropic Sediment Layer on Dissolved Oxygen Transfer in Turbulent Flows

The concentration and distribution of dissolved oxygen (DO) are critical to the self‐purification capacity of water and the survival of aquatic organisms, and they are important factors considered in the ecological balance system of water resources. In this work, the high‐Schmidt number DO transfer phenomenon in open channel flows with anisotropic sediment beds is studied using large eddy simulation. The volume‐averaged Navier‐Stokes equations coupled with the Monod equation is used to describe the in‐sediment flow and the sediment oxygen demand of micro‐organisms, so as to examine the transport characteristics of DO inside and outside the sediment layer. The effects of various permeable conditions of sediments (characterized by the anisotropy index of permeability, i.e., λ) on the distribution of DO concentration and flux, penetration depth, and mass transfer processes is studied by means of the time‐averaged statistics variations in flow and scalar fields. A larger anisotropy index λ is found can enhance the transport of DO from the water flowing into the sediment layer, which is caused by turbulent diffusions and turbulence bursting events. It suggests that the λ‐dependent distributions of the DO fluctuations and fluxes are highly correlated with the coherent structures in the turbulence over the sediment‐water interface (SWI). A key relationship between the Sherwood number and the anisotropy index is found by means of the current simulations and also is verified by our theoretical analysis. The dependence of λ on DO penetration depth is consistent with the hypothesis that coherent turbulence in the water column drives mass transport across the SWI. Key Points High‐Schmidt‐number dissolved oxygen transfer from near‐bed turbulent flows to anisotropic sediment bed is studied by large eddy simulation The volume‐averaged Navier‐Stokes equations describing the flow within the permeable sediment layer are coupled with a biogeochemical model The dependence of the anisotropy index of permeability on the mass transfer (dissolved oxygen concentration, penetration depth, and mass transfer efficiency) are evaluated quantitatively. The relationship between the Sherwood number and the anisotropy index, that is,Sh∼λ−0.3 is obtained by both numerical results and analytic derivation