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Sedimentation of inertia-less prolate spheroids in homogenous isotropic turbulence with application to non-motile phytoplankton
KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0003-4328-7921
KTH, School of Engineering Sciences (SCI), 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
School of Marine Sciences, University of Maine.
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(English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
Abstract [en]

Phytoplankton are the foundation of aquatic food webs. Through photosynthesis, phytoplankton draw down $CO_2$ at magnitudes equivalent to forests and other terrestrial plants and convert it to organic material that is then consumed by other organisms of phytoplankton 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 down-welling 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.   

National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-204164OAI: oai:DiVA.org:kth-204164DiVA: diva2:1084201
Note

QC 20170328

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-04-04Bibliographically approved
In thesis
1. Numerical study of non-spherical/spherical particles in laminar and turbulent flows
Open this publication in new window or tab >>Numerical study of non-spherical/spherical particles in laminar and turbulent flows
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The presence of solid rigid particles alters the global transport and rheological properties of the mixture in complex (and often unpredictable) ways. In recent years a few studies have been devoted to investigating the behavior of dense suspensions in the turbulent/inertial regime with the majority of theses analyses limited to mono-disperse rigid neutrally-buoyant spheres. However, one interesting parameter that is rarely studied for particles with high inertia is the particle shape. Spheroidal particles introduce an anisotropy, e.g. a tendency to orient in a certain direction, which can affect the bulk behavior of a suspension in an unexpected ways. The main focus of this study is therefore to investigate the behavior of spheroidal particles and their effect on turbulent/inertial flows.

We perform fully resolved simulations of particulate flows with spherical/spheroidal particles, using an efficient/accurate numerical approach that enables us to simulate thousands of particles with high resolutions in order to capture all the fluid-solid interactions.

Several conclusions are drawn from this study that reveal the importance of particle's shape effect on the behaviour of a suspension e.g. spheroidal particles tend to cluster while sedimenting. This phenomenon is observed in this work for both particles with high inertia, sedimenting in a quiescent fluid and inertialess particles (point-like tracer prolates) settling in homogenous isotropic turbulence. The mechanisms for clustering is indeed different between these two situations, however, it is the shape of particles that governs these mechanisms, as clustering is not observed for spherical particles. Another striking finding of this work is drag reduction in particulate turbulent channel flow with rigid oblate particles. Again this drag reduction is absent for spherical particles, which instead increase the drag with respect to single-phase turbulence. 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. 31 p.
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-204421 (URN)978-91-7729-333-0 (ISBN)
Presentation
2017-04-20, E51, Osquars backe 14, Stockholm, 14:00 (English)
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Note

QC 20170328

Available from: 2017-03-28 Created: 2017-03-24 Last updated: 2017-03-28Bibliographically approved

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