Large‐eddy simulation and low‐order modeling of sediment‐oxygen uptake in a transitional oscillatory flow

Abstract

We have tested a dissolved oxygen (DO) transport model based on large‐eddy simulation (LES) of a transitional oscillatory flow observed in the bottom boundary layer of Lake Alpnach, Switzerland. The transition from a quasi‐laminar to a fully turbulent state makes this flow difficult to study with a Reynolds‐averaged Navier‐Stokes equation (RANSE) model. By resolving the full range of governing transport processes, LES provides a reliable prediction of the sediment‐oxygen uptake (SOU). The model biogeochemical and flow parameters have been calibrated against DO and velocity measurements from published in situ data at the earliest phase available in the cycle. The fully developed flow thus obtained is used as an initial condition for the imposed oscillatory forcing. Numerical predictions show that transport in the outer layer is in equilibrium with the main current throughout most of the cycle and that nonequilibrium effects are limited to the diffusive sublayer response to the external forcing. During flow deceleration, the concentration boundary layer slowly expands as turbulence decays; later, during re‐transition, mixing is restored by rapid and intense turbulent production events enhancing the SOU with a well‐defined time lag. An algebraic model for the SOU is proposed for eventual inclusion in RANSE biogeochemical management‐type models developed based on parameterizations used in turbulent mass transfer and with the support of published numerical data and the present simulation. The only input parameters required are the sediment oxidation rate, bulk temperature and DO concentration, and friction velocity.