Mixing by microorganisms in stratified fluids

TitleMixing by microorganisms in stratified fluids
Publication TypeJournal Article
Year of Publication2014
AuthorsWagner G.L, Young W.R, Lauga E.
JournalJournal of Marine Research
Date Published2015/03
Type of ArticleArticle
ISBN Number0022-2402
Accession NumberWOS:000347185800001
Keywordsdiffusion; oceans; propulsion; regularized stokeslets; turbulence

We examine the vertical mixing induced by the swimming of microorganisms at low Reynolds and Peclet numbers in a stably stratified ocean, and show that the global contribution of oceanic microswimmers to vertical mixing is negligible. We propose two approaches to estimating the mixing efficiency, eta, or the ratio of the rate of potential energy creation to the total rate-of-working on the ocean by microswimmers. The first is based on scaling arguments and estimates eta in terms of the ratio between the typical organism size, a, and an intrinsic length scale for the stratified flow, l = (nu kappa/N-2)(1/4), where. is the kinematic viscosity, kappa the diffusivity, and N the buoyancy frequency. In particular, for small organisms in the relevant oceanic limit, a/l << 1, we predict the scaling eta similar to (a/l)(3). The second estimate of. is formed by solving the full coupled flow-stratification problem by modeling the swimmer as a regularized force dipole, and computing the efficiency numerically. Our computational results, which are examined for all ratios a/l, validate the scaling arguments in the limit a/l << 1 and further predict eta approximate to 1.2 (a/l)(3) for vertical swimming and eta approximate to 0.15 (a/l) 3 for horizontal swimming. These results, relevant for any stratified fluid rich in biological activity, imply that the mixing efficiency of swimming microorganisms in the ocean is at very most 8% and is likely smaller by at least two orders of magnitude.

Short TitleJ. Mar. Res.
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