Turbulence-plankton interactions. A mechanistic approach.

Computer simulations developed for process engineering purposes have been carried over to marine sciences. An aim has been to bridge the gap between engineering and natural sciences. The knowledge gained from simulations of turbulence - plankton interactions is likely to have relevance for marine ecological systems and the ocean's ability to serve as CO2 storage. The overall objective of the project has been to explore turbulence-plankton interactions by means of computer experiments performed together with TU (Tsinghua University in Beijing) and supported by laboratory experiments at UCB (University of California at Berkeley). The new insight into different particle transport mechanisms may also assist in improved tools to control and/or predict avian pollution and microplastic debris in the ocean. We have used a tri-axial ellipsoid or an axisymmetric spheroid as a plankton prototype to mimic either disk-like or rod-like shapes. Translational and rotational motions of plankton in oceanic-like turbulence have been explored. The influence of shape and inertia on plankton dispersion, clustering, and sinking velocities has been considered. Specific goals have been to (i) to devise a mathematical model for the translation and rotational motion of plankton particles; (ii) to demonstrate that the rotational dynamics of plankton particles in the core of a turbulent channel flow closely resemble particle dynamics in isotropic turbulence; (iii) to examine the role of gravity on rotation and settling of ellipsoidal plankton over a two-dimensional parameter space (shape and inertia), including settling velocity measurements from UCB. We parameterized shape by means of a particle aspect ratio and inertia by means of a Stokes number. The project focused on some different activities. A new turbulent flow field was created, which resembled a near-surface layer in the ocean. We explored the drift of inertial spheres and examined how they were preferentially concentrating. Voronoi analysis and Shannon entropies were used to quantify the clustering. A concept adopted from continuum mechanics was used to analyze how inertialess non-spherical particles orient themselves. To effectively inspect and characterize particle rotation modes, we developed a diagnostic tool based on invariants of the rotation correlation tensor by means of a so-called Lumley triangle. This tool can be used not only to diagnose rotational modes but also particle orientation modes and can be used to characterize inertialess as well as inertial particles. In another activity we examined the influence of gravity on the horizontal drift and vertical sinking of differently shaped inertial particles. Rod-like particles orient themselves rather differently than disk-like particles in presence of gravity. This is associated with the shape-effect on orientation of spheroidal particles, which adds to the effect of inertia. Together with collaborators at UCB and UT, we analyzed rotational modes of plankton particles in the core of a turbulent channel and found the rotation modes to be almost indistinguishable from those in isotropic turbulence. We incidentally observed that even very nearby rods could orientate surprisingly different. The unexpected phenomenon observed in our computer simulations was also be seen in refined laboratory measurements at Wesleyan University in Connecticut and predicted by a theoretical model at Gothenburg University. Simulations and analysis of plankton-like spheroids have also been developed in another direction. The community recently observed that a central quiescent core (QC) exists in turbulent channel flows at higher Reynolds numbers. We believe that such QCs may exist also in oceanic turbulence affected by wind shear. We explored how inertialess and inertial spheroidal particles behaved in a QC and how the orientation and rotation of the spheroids were affected by the interface between the QC and the ambient flow. This research will be further advanced together with TU after termination of this project. A new way to compute unsteady TGV (Taylor-Green vortex) flow has been developed, in which the embedded Poisson equation is solved by the Sherman-Morrison algorithm. The velocity and vorticity are decaying in time. In this statistically unsteady flow, the suspended particles experience three fundamentally different flow stages. After verification of the computed flow field, we explored how inertial spheres were carried along by the fluid flow and how the spheres clustered preferentially in certain parts of the flow. We again used Voronoi volumes to quantify the local particle concentration, which we found to be strongly dependent on particle inertia. Inertial spheres preferentially clustered in shear-dominated regions whereas vortex regions were left void. The PhD candidate is also considering how non-spherical particles are behaving in this ocean-like flow field.

Kunnskapen vi har ervervet om turbulens-plankton vekselvirkning kan bidra til bedre å forstå marine økosystemer og spesielt havets rolle som CO2 lager. Prosjektet har vært avgrenset til rene mekanistiske undersøkelser. Forskningsresultatene har blitt behørig publisert og kan benyttes og videreføres av andre forskere innen samme og tilgrensende fagfelt, men også av forskningsgrupper innen marin biologi. Den nye kunnskapen om partikkeltransport-mekanismer kan dessuten bidra til bedre metoder for å forutsi og kontrollere luftforurensinger og spredning av mikroplastikk i vassdrag og i havet. Postdok stipendiaten har vært førsteforfatter av hele 6 artikler publisert i anerkjente internasjonale fagtidsskrift, noe som styrker hennes kvalifikasjoner med tanke på en videre forskerkarriere. To PhD stipendiater ved NTNU og Tsinghua University vil dra direkte nytte av både metodene og resultatene som er oppnådd i prosjektet.

The overall objective of this generic project is to explore turbulence-plankton interactions by means of extensive computer experiments supported by complementary laboratory experiments at UC Berkeley. As a plankton prototype we consider tri-axial ellipsoids which can mimic disk-like and rod-like plankton depending on their axes ratio. Translational and rotational motion of nearly-neutrally buoyant plankton in oceanic-like turbulence will be explored. A wide range of plankton morphologies will be studied and compared with findings for isotropically-shaped particles. The influence of shape and inertia on plankton clustering, turbulent diffusion, and sinking velocities will be investigated. The latter is of primary concern as a controlling factor of the net vertical flux of carbon in the open ocean.