Journal
JOURNAL OF THE ROYAL SOCIETY INTERFACE
Volume 10, Issue 81, Pages -Publisher
ROYAL SOC
DOI: 10.1098/rsif.2012.1041
Keywords
algae; swimming micro-organisms; Taylor dispersion; direct numerical simulation; turbulent transport; photobioreactors
Categories
Funding
- Carnegie Trust
- EPSRC [EP/D073398/1, EP/J004847/1]
- Economic and Social Research Council [RES-168-26-0125] Funding Source: researchfish
- Engineering and Physical Sciences Research Council [EP/D073308/1, EP/J004847/1] Funding Source: researchfish
- EPSRC [EP/D073308/1, EP/J004847/1] Funding Source: UKRI
- ESRC [RES-168-26-0125] Funding Source: UKRI
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Shear flow significantly affects the transport of swimming algae in suspension. For example, viscous and gravitational torques bias bottom-heavy cells to swim towards regions of downwelling fluid (gyrotaxis). It is necessary to understand how such biases affect algal dispersion in natural and industrial flows, especially in view of growing interest in algal photobioreactors. Motivated by this, we here study the dispersion of gyrotactic algae in laminar and turbulent channel flows using direct numerical simulation (DNS) and a previously published analytical swimming dispersion theory. Time-resolved dispersion measures are evaluated as functions of the Peclet and Reynolds numbers in upwelling and downwelling flows. For laminar flows, DNS results are compared with theory using competing descriptions of biased swimming cells in shear flow. Excellent agreement is found for predictions that employ generalized Taylor dispersion. The results highlight peculiarities of gyrotactic swimmer dispersion relative to passive tracers. In laminar downwelling flow the cell distribution drifts in excess of the mean flow, increasing in magnitude with Peclet number. The cell effective axial diffusivity increases and decreases with Peclet number (for tracers it merely increases). In turbulent flows, gyrotactic effects are weaker, but discernable and manifested as non-zero drift. These results should have a significant impact on photobioreactor design.
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