Dissolved oxygen transfer from oscillatory flows to microbes in a permeable organic sediment bed

Abstract We investigate high-Schmidt-number mass transfer of dissolved oxygen (DO) from oscillatory turbulent flows to permeable microbial sediment bed by large eddy simulations. The three-dimensional open channel flow is numerically solved for friction Reynolds number 180 and Schmidt number 373. In this numerical study, an external periodical pressure gradient is applied to drive intermittent turbulence over the streamwise direction. The bottom sediment layer populated with DO-absorbing bacteria is described by a modified Darcy-Brinkman-Forcheimer model which couples a bio-geochemical model. The dependence of mass transfer on amplitude and period is analyzed by means of phase-average statistical quantities. The goal is to investigate oxygen transport dynamics towards an underlying mass-absorbing sediment layer in the oscillating flow regime. The present study reveals that the turbulent diffusion is the major path in DO transport. The bulk velocity, the turbulent kinetic energy, DO concentration, and the diffusion flux all exhibit periodic and quasi-periodic patterns. Phase lags can be observed for these quantities compared with the oscillatory forcing signal. It reveals that the flow field as well as oxygen consumption requires a certain amount of time for feedback on the oscillatory force. A negative correlation exists between the oxygen consumption and DO concentration. Low frequency and high amplitude of oscillations could significantly influence the transfer of momentum and oxygen.