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Sedimentation of inertia-less prolate spheroids in homogenous isotropic turbulence with application to non-motile phytoplankton

Published online by Cambridge University Press:  13 October 2017

M. Niazi Ardekani*
Affiliation:
Linné Flow Centre and SeRC (Swedish e-Science Research Centre), KTH Mechanics, SE-100 44 Stockholm, Sweden
G. Sardina
Affiliation:
Linné Flow Centre and SeRC (Swedish e-Science Research Centre), KTH Mechanics, SE-100 44 Stockholm, Sweden Department of Mechanics and Maritime Sciences, Chalmers University of Technology, 412 96 Gothenburg, Sweden
L. Brandt
Affiliation:
Linné Flow Centre and SeRC (Swedish e-Science Research Centre), KTH Mechanics, SE-100 44 Stockholm, Sweden
L. Karp-Boss
Affiliation:
School of Marine Sciences, University of Maine, Orono, ME 04469, USA
R. N. Bearon
Affiliation:
Department of Mathematical Sciences, University of Liverpool, Liverpool L69 7ZL, UK
E. A. Variano
Affiliation:
Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720, USA
*
Email address for correspondence: mehd@mech.kth.se

Abstract

Phytoplankton are the foundation of aquatic food webs. Through photosynthesis, phytoplankton draw down $\text{CO}_{2}$ at magnitudes equivalent to forests and other terrestrial plants and convert it to organic material that is then consumed by other planktonic organisms in higher trophic levels. Mechanisms that affect local concentrations and velocities are of primary significance to many encounter-based processes in the plankton, including prey–predator interactions, fertilization and aggregate formation. We report results from simulations of sinking phytoplankton, considered as elongated spheroids, in homogenous isotropic turbulence to answer the question of whether trajectories and velocities of sinking phytoplankton are altered by turbulence. We show in particular that settling spheroids with physical characteristics similar to those of diatoms weakly cluster and preferentially sample regions of downwelling flow, corresponding to an increase of the mean settling speed with respect to the mean settling speed in quiescent fluid. We explain how different parameters can affect the settling speed and what underlying mechanisms might be involved. Interestingly, we observe that the increase in the aspect ratio of the prolate spheroids can affect the clustering and the average settling speed of particles by two mechanisms: first is the effect of aspect ratio on the rotation rate of the particles, which saturates faster than the second mechanism of increasing drag anisotropy.

Type
Papers
Copyright
© 2017 Cambridge University Press 

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