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Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers
Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy.;CISM, Dept Fluid Mech, I-33100 Udine, Italy..
Univ Udine, Dept Engn & Architecture, I-33100 Udine, Italy..
Chalmers Univ Technol, Dept Mech & Maritime Sci, Div Fluid Dynam, S-41258 Gothenburg, Sweden..
KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-4346-4732
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2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 12, article id 124304Article in journal (Refereed) Published
Abstract [en]

In this work, we use direct-numerical-simulation-based Eulerian-Lagrangian simulations to investigate the dynamics of small gyrotactic swimmers in free-surface turbulence. We consider open-channel flow turbulence in which bottom-heavy swimmers are dispersed. Swimmers are characterized by different vertical stability, so that some realign to swim upward with a characteristic time smaller than the Kolmogorov timescale, while others possess a reorientation time longer than the Kolmogorov timescale. We cover one order of magnitude in the flow Reynolds number and two orders of magnitude in the stability number, which is a measure of bottom heaviness. We observe that large-scale advection dominates vertical motion when the stability number, scaled on the local Kolmogorov timescale of the flow, is larger than unity: This condition is associated to enhanced migration toward the surface, particularly at low Reynolds number, when swimmers can rise through surface renewal motions that originate directly from the bottom-boundary turbulent bursts. Conversely, small-scale effects become more important when the Kolmogorov-based stability number is below unity: Under this condition, migration toward the surface is hindered, particularly at high Reynolds, when bottom-boundary bursts are less effective in bringing bulk fluid to the surface. In an effort to provide scaling arguments to improve predictions of models for motile microorganisms in turbulent water bodies, we demonstrate that a Kolmogorov-based stability number around unity represents a threshold beyond which swimmer capability to reach the free surface and form clusters saturates.

Place, publisher, year, edition, pages
American Physical Society, 2019. Vol. 4, no 12, article id 124304
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Physical Sciences
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URN: urn:nbn:se:kth:diva-266247DOI: 10.1103/PhysRevFluids.4.124304ISI: 000501526100002Scopus ID: 2-s2.0-85076539056OAI: oai:DiVA.org:kth-266247DiVA, id: diva2:1382446
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QC 20200103

Available from: 2020-01-03 Created: 2020-01-03 Last updated: 2020-01-03Bibliographically approved

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Brandt, Luca

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