Thermal stratification hinders gyrotactic micro-organism rising in free-surface turbulence

Thermal stratification in water bodies influences the exchange of heat, momentum, and chemical species across the air-water interface by modifying the sub-surface turbulence characteristics. Turbulence modifications may in turn prevent small motile algae (phytoplankton, in particular) from reaching...

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Bibliographic Details
Published in:Physics of fluids (1994) 2017-05, Vol.29 (5)
Main Authors: Lovecchio, Salvatore, Zonta, Francesco, Marchioli, Cristian, Soldati, Alfredo
Format: Article
Language:English
Subjects:
Aerodynamics
Channel flow
Computational fluid dynamics
Computer simulation
Confinement
Fluid dynamics
Heat exchange
Mathematical models
Oceans
Orientation
Physics
Phytoplankton
Swimming
Turbulence
Turbulent flow
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Summary:Thermal stratification in water bodies influences the exchange of heat, momentum, and chemical species across the air-water interface by modifying the sub-surface turbulence characteristics. Turbulence modifications may in turn prevent small motile algae (phytoplankton, in particular) from reaching the heated surface. We examine how different regimes of stable thermal stratification affect the motion of these microscopic organisms (modelled as gyrotactic self-propelling cells) in a free-surface turbulent channel flow. This archetypal setup mimics an environmentally plausible situation that can be found in lakes and oceans. Results from direct numerical simulations of turbulence coupled with Lagrangian tracking reveal that rising of bottom-heavy self-propelling cells depends strongly on the strength of stratification, especially near the thermocline where high temperature and velocity gradients occur: Here hydrodynamic shear may disrupt directional cell motility and hamper near-surface accumulation. For all gyrotactic re-orientation times considered in this study (spanning two orders of magnitude), we observe a reduction of the cell rising speed and temporary confinement under the thermocline: If re-orientation is fast, cells eventually trespass the thermocline within the simulated time span; if re-orientation is slow, confinement lasts much longer because cells align in the streamwise direction and their vertical swimming is practically annihilated.
ISSN:1070-6631
1089-7666
DOI:10.1063/1.4983345