(368h) Biogenic Mixing in a Stratified Fluid

The goal of this work is to answer the intriguing question vigorously argued in the literature during the past few years that whether the combined motion of marine organisms contributes to the global ocean mixing.  In the aphotic ocean (i.e. regions that are 200m beneath the sea surface), zooplanktons are the most abundant organisms that are engaged in the diel vertical migration and their swimming Reynolds number is in the range of Re ∼ O(1 − 100). Therefore, it is important to examine the potential contribution to the ocean mixing in this inertial regime, which requires a fully resolved motion of organisms. We utilize the squirmer model to resolve the hydrodynamic interactions of a suspension of swimmers in a density-stratified fluid. Our study shows that the eddy diffusivity, a measure of vertical mass flux, within a suspension of squirmers increases with Reynolds number and the maximum value we achieve is comparable to the value reported for the ocean turbulent mixing, even though the corresponding mixing efficiency is smaller than the typical value of 0.2 reported for the turbulent mixing. We have also considered a suspension of squirmers in a decaying isotropic turbulence. We find that the eddy diffusivity in the presence of a suspension of squirmers enhances due to the strong viscous dissipation generated by squirmers as well as the interaction of squirmers with the background turbulence.