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NP6.2

Turbulence and plankton

Planktonic organisms live in suspension in marine or fresh waters where they have adapted through the slow process of natural evolution (over hundred of thousands of generations) to the harsh turbulent currents of their environment. Therefore, contrary to what the meaning of their name “marine drifter” might let to speculate, their dynamics is potentially different from the one of material bodies passively transported by fluid flows. It is indeed known that these organisms developed many adaptive strategies involving shape and density regulation, swimming activity, aggregation and other mechanisms in order to be sheltered from or to take advantage of turbulent flow features.

Bloom inceptions, thin layers formation, motility, nutrient and light uptakes, specific Lagrangian dynamics, among others are topics involving phytoplankton and turbulence. Jumps, grazing, contact rates, and vertical migration are, among others, topics concerning zooplankton in turbulence. For all planktonic species, adaptive mechanisms in response not only to mechanical, but also chemical and electro-magnetic (such as luminous) cues are topics of great interest.

This interdisciplinary session will welcome works from marine ecologists, oceanographers, fluid-dynamicists, physicists and mathematical modellers. Contributions in the fields of observation, laboratory experimentations, numerical models (such as Computational Fluid Dynamics simulations of non-spherical or motile particles) are welcome. Both phytoplankton and zooplankton will be considered, as well as marine and freshwater studies.

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Co-organized by OS3

Convener: François G. Schmitt | Co-conveners: Filippo De Lillo, Enrico Calzavarini, I. Tuval, Martin Bees

Presentations

| Tue, 24 May, 15:55–18:30 (CEST)

Room 0.94/95

Tue, 24 May, 15:10–16:40

Chairpersons: Eric Climent, François G. Schmitt

15:55–15:58

Introduction to the session

15:58–15:59

Experiments and in situ observations

15:59–16:05

EGU22-13437

A litre-scale turbulence facility for microorganism-flow interactions

Jeanette D. Wheeler et al.

Bacteria and phytoplankton are abundant in aquatic environments, forming the base of the food web and mediating elemental cycling at a global scale. Understanding the interactions these microorganisms have with their turbulent fluid environments is an active area of research, largely conducted in laboratory-based flow experiments. In this work, we provide an open-source design and rigorous flow characterization for a 1L, dual oscillating grid turbulence facility, the smallest volume facility to date which produces near-isotropic, homogeneous turbulence. We optimized the tank geometry (grid-to-grid and grid-to-wall spacing), the grid geometry (for both classical and fractal grids: effective mesh size, blockage ratio, and fractal grid parameters), and the grid forcing regimes (for both coupled, antiphase and decoupled, randomized forcing: frequency range, stroke range, and randomized forcing parameters) to minimize mean flows and to produce acceptably homogeneous and isotropic turbulence within the unique constraints of a litre-scale volume. We acquired particle image velocimetry (PIV) measurements for both classical and fractal grids across a wide range of grid forcing regimes. We discuss the resulting length- and timescales relevant to microorganism-flow interactions, from the integral to the Kolmogorov scales. Finally, we discuss how the range of turbulent kinetic energy (TKE) dissipation rates achieved across the operational space of the facility mimics oceanographic turbulence in a range of in situ conditions, from the nearshore to the open ocean. This facility meets a long-standing need in the oceanography community in which feasible experimental working volumes are constrained by labor-intensive culturing requirements for large volumes of aquatic bacteria and phytoplankton.

How to cite: Wheeler, J. D., True, A. C., Michalec, F.-G., Holzner, M., Stocker, R., and Crimaldi, J. P.: A litre-scale turbulence facility for microorganism-flow interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13437, https://doi.org/10.5194/egusphere-egu22-13437, 2022.

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16:05–16:11

EGU22-7020

ECS

Effect of Light and Upwelling Intensity on the Phytoplankton Community Composition in the Peruvian Upwelling System

Jacqueline Behncke et al.

The coast of Perú hosts the largest and most productive Eastern Boundary Upwelling System. Climate change is predicted to increase stratification, thereby increasing light availability and lowering nutrient concentrations at the surface. Moreover, the winds causing upwelling in this area are predicted to change their intensity and migrate polewards.

To better understand and predict the response of phytoplankton to changes in light and nutrient conditions, we recreated different light and nutrient scenarios in 9 off-shore mesocosms during the KOSMOS-Peru-2020 experiment in March-April 2020 off the coast of Callao (Perú). We recreated two light scenarios: high light (HL) and low light (LL); and four levels of upwelling by adding deep water (DW) in different proportions (0, 15, 30, 45 and 60 %). We monitored the phytoplankton composition every two days for 36 days. Photosynthetic pigments were measured using HPLC and the phytoplankton community composition was estimated using CHEMTAX and taxonomically determined by microscopic analyses, whereas chlorophyll-a (Chla) as a proxy for bulk phytoplankton biomass and particulate organic carbon, nitrogen and phosphorus (POC, PON and POP) provided information about biomass and stoichiometry of the total suspended matter.

The enclosed initial community was dominated by the red-tide forming raphidophyte Fibrocapsa japonica, detected for the first time off the coast of Perú during this experiment.

After an initial phase, during which F. japonica consumed the nutrients available, the DW was added and a second bloom, dominated by diatoms developed. As expected, more phytoplankton accumulated under HL and in higher DW treatments. The phytoplankton community under LL increased its Chla content per cell to maximize photosynthetic performance, whereas HL caused a significant increase in the POC:PON ratio.

Diatoms, coccolithophores and Phaeocystis were positively affected by HL, whereas the LL phytoplankton assemblage was dominated by smaller groups such as cryptophytes, prasinophytes, Synechococcus and especially the pelagophyte Octactis octonaria. F. japonica became more abundant under LL during the initial phase. Higher upwelling intensity favored diatoms as well as pelagophytes and chlorophytes under LL, whereas low nutrients conditions favored prasinophytes. Upwelling events were accompanied by high contributions of diatoms, whereas nutrient-depleted conditions were dominated by small phytoplankton groups and dinoflagellates.

From our results we conclude that although upwelling intensity did not affect stoichiometry significantly for the duration of the experiment, an intensification of stratification causing greater exposure to HL conditions might decrease the nutritional value of phytoplankton for upper trophic levels. Changes in light and nutrient availability caused by climate change will trigger a shift in the phytoplankton community composition. HL and intense upwelling areas might be dominated by diatoms and LL and low nutrient areas might be dominated by prasinophytes with distinct consequences for the trophic transfer and export efficiency of the Peruvian upwelling system.

How to cite: Behncke, J., Fernández-Méndez, M., and Riebesell, U.: Effect of Light and Upwelling Intensity on the Phytoplankton Community Composition in the Peruvian Upwelling System, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7020, https://doi.org/10.5194/egusphere-egu22-7020, 2022.

16:11–16:17

EGU22-8638

The Agiturb laboratory turbulence generation system and its application to plankton studies: zooplankton and phytoplankton

François G. Schmitt et al.

Plankton species live in a turbulent flow and are fully adapted to it. They have specific behaviour and responses related to turbulence characteristics and intensities, that are still largely unknown. Turbulence systems in the laboratory are needed to perform controled experiments with different zooplankton and phytoplankton species. Here we present the Agiturb turbulence generation system and some first results using different plankton species.

In the Agiturb system, the turbulent flow is produced using four contra-rotating agitators that are place under a cubic tank. The model for such flow is the so-called “four-roll mill” proposed by G.I. Taylor in 1934 to generate a statistically stationary, spatially inhomogeneous flow with compression and stretching. In our experiment, the flow close to the agitators is a free flow similar to the four-roll mill, without the cylindrical rolls. The injection of the energy in the flow is produced by 4 stirring bars activated by 4 magnetic stirrers situated at symmetric positions, the centers being placed at one-fourth of the width of the tank. The cubic tank is almost half-full with 15 liters of sea water. For each experiment, the magnitude of the rotation rate of each agitator was identical, with two agitators rotating clockwise and two anti-clockwise, the same directions being along the diagonal. Different values of the rotation rate were chosen to reach different turbulence levels, characterized by the microscale Reynolds number R_λ going from 130 to 360.

We present the result of two different experiments: the first one is a record, using a high speed camera in the infrared, of copepods trajectories, at different turbulent intensities, in order to see an optimal Reynolds number for copepods swimming activities (Acartia tonsa). The second one is a systematic study of the proliferation of diatoms under different turbulent intensities (Pseudo-nitzschia). In both cases different rotation rates of the system are considered, and an optimal turbulence level has been found, with maximum swimming activity for copepods and maximum growth rate for diatoms.

How to cite: Schmitt, F. G., Le Quiniou, C., Huang, Y., Calzavarini, E., Houliez, E., and Christaki, U.: The Agiturb laboratory turbulence generation system and its application to plankton studies: zooplankton and phytoplankton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8638, https://doi.org/10.5194/egusphere-egu22-8638, 2022.

16:17–16:23

EGU22-51

ECS

OSPP

Novel Coccolithophores from the Lower Deep Photic Zone Off Bermuda

Josue Millan et al.

Coccolithophores are a ubiquitous oceanic phytoplankton group. Their unique ability to acquire carbon from different environmental sources and to produce calcareous body scales (coccoliths) make them an integral functional group in the biogeochemical cycling of carbon. Despite this, their vertical distribution, particularly in the lower photic zone (LPZ), species composition, and life cycles, are still poorly understood. Discrete water samples were examined from the LPZ during the 2020 fall overturn event occurring from October to November at hydrostation S of the Bermuda Atlantic Time-Series (BATS). This provided an opportunity to compare our results with a previous BATS survey of coccolithophore population dynamics taken 28 to 26 years earlier (1992-1994). This latter study demonstrated that coccolithophores exhibit seasonal changes in their vertical and horizontal distribution and showed that the coccolithophore population transition of the LPZ occurs primarily at overturn events. Here, we place particular emphasis on those LPZ coccolithophore species adapted to live between the deep chlorophyll maximum and the upper mesopelagic zone because of their potential for mixotrophic activity. We discovered numerous unidentified taxa in this region, which may be either new to science or alternate phases of already described species. Some of the holococcolithophores appear to be associated with the Papposphaeraceae, with similarities to the Turrisphaera-phase. In addition, we provide the first unquestionable evidence of Florisphaera profunda combination coccospheres, featuring both heterococcolith and holococcolith phases in the same sample.

How to cite: Millan, J., Winter, A., Jordan, R. W., and Blanco-Bercial, L.: Novel Coccolithophores from the Lower Deep Photic Zone Off Bermuda, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-51, https://doi.org/10.5194/egusphere-egu22-51, 2022.

OSPP judging

16:23–16:29

EGU22-12875

Impact of the winter convective event on gelatinous zooplankton in the open southern Adriatic

Mirna Batistić et al.

16:29–16:40

Questions and discussions

Tue, 24 May, 17:00–18:30

Chairperson: François G. Schmitt

17:00–17:01

Numerical models

17:01–17:07

EGU22-7811

Navigation of micro-swimmers in steady flow: the importance of symmetries

Bernhard Mehlig

Marine micro-organisms must cope with complex flow patterns and even turbulence as they navigate the ocean. To survive they must avoid predation and find efficient energy sources. A major difficulty in analysing possible survival strategies is that the time series of environmental cues in non-linear flow is complex, and that it depends on the decisions taken by the organism. One way of determining and evaluating optimal strategies is reinforcement learning. In a proof-of-principle study, Colabrese et al. [Phys. Rev. Lett. (2017)] used this method to find out how a micro-swimmer in a vortex flow can navigate towards the surface as quickly as possible, given a fixed swimming speed. The swimmer measured its instantaneous swimming direction and the local flow vorticity in the laboratory frame, and reacted to these cues by swimming either left, right, up, or down. However, usually a motile micro-organism measures the local flow rather than global information, and it can only react in relation to the local flow, because in general it cannot access global information (such as up or down in the laboratory frame). Here we analyse optimal strategies with local signals and actions that do not refer to the laboratory frame. We demonstrate that symmetry-breaking is required to find such strategies. Using reinforcement learning we analyse the emerging strategies for different sets of environmental cues that micro-organisms are known to measure. This talk is based on "Navigation of micro-swimmers in steady flow: the importance of symmetries" by Jingran Qiu, [Opens in a new win Navid Mousavi, Kristian Gustavsson[Opens in a new window], Chunxiao Xu, Bernhard Mehlig, and Lihao Zhao, Journal of Fluid Mechanics 932, A10. doi:10.1017/jfm.2021.978

How to cite: Mehlig, B.: Navigation of micro-swimmers in steady flow: the importance of symmetries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7811, https://doi.org/10.5194/egusphere-egu22-7811, 2022.

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17:07–17:13

EGU22-4812

Active gyrotactic stability of microswimmers using hydromechanical signals

Kristian Gustavsson et al.

17:13–17:19

EGU22-3902

ECS

OSPP

Lagrangian connectivity of marine plankton under thermal constraints

Darshika Manral et al.

The biogeographic distribution of marine planktonic communities in the global ocean and its drivers has been a topic of great interest in the scientific community. Some of these drivers can be abiotic: ocean currents, temperature, salinity, nutrients, and others biotic: presence of predators and competitive species. In our study, we focus on the distribution mediated by ocean currents and temperature. Combining Lagrangian modeling and network theory approaches, we estimate the pathways and timescales that establish the surface connectivity for passive i.e., freely floating plankton between stations in the Atlantic Ocean where plankton have been sampled during Tara Oceans & Tara Oceans Polar Circle (2009-2013) and Tara Pacific (2016-2018) expeditions.

We obtain these estimates using a transition matrix approach derived from surface ocean simulations. Given the high rates of reproduction of many planktonic species and that only a few organisms are needed to establish connectivity, we make use of the minimum time path between different stations. To obtain plankton connectivity, two types of constraints are applied on the passive connectivity model: thermal niche and thermal adaptation rate, based on data for a given planktonic species from the literature. From the preliminary analysis, we find that, using minimum time paths, passive particles representative of foraminifera can connect all the stations in less than 3 years. Application of thermal niche constraints increases the minimum connectivity time between stations by approximately 10%, suggesting that plankton can keep to within their favorable thermal conditions by advecting via slightly longer paths. Main pathways of connectivity between these stations are also highlighted in this study. The developed approach can be applied for other plankton species, for any location in the Atlantic and can also be further expanded to derive seasonal connectivity.

How to cite: Manral, D., Amaral-Zettler, L., and van Sebille, E.: Lagrangian connectivity of marine plankton under thermal constraints, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3902, https://doi.org/10.5194/egusphere-egu22-3902, 2022.

OSPP judging

17:19–17:25

EGU22-7710

ECS

Copepods counter dispersion to maintain high mating-encounter rates

Ron Shnapp and Markus Holzner

Finding mating partners can be challenging for copepods: the ocean is vast and turbulent, the average animal's concentration is sparse, and their swimming ability is limited. Therefore, the probability for locating a mate assuming a homogeneous distribution of animals and a random motion leads to low mating encounter rates. However, zooplankton distribution is not homogeneous; field observations since the 1950s have shown that plantkon have patchy distributions over multiple scales - from thousand of kilometers down to the millimeter scale¹. Of relevance to mating is patchiness at small scales (on the order of the animal's size), leading to increased probability for sexual encounters due to higher local concentrations. Indeed, such mating clusters have been identified in ship transect observations². However, how such clusters form in the diffusive turbulent environment is not fully understood.

In certain species, males actively search for females to achieve sexual encounters. When a male locates a female it pursuits her to achieve contact³, and this behavior is thought to drive small-scale clustering⁴. Nevertheless, the details of this process are not so straightforward. Specifically, the random swimming pattern males perform in their search, and the (super) diffusive nature of turbulence⁵, both increases the animals' dispersion, thus opposing patch formation. Therefore, the existence of mating clusters requires a detailed balance between diffusion and pair-interactions. However, this equilibrium in zooplankton patch formation was not examined in the past.

Our study examines the equilibrium between diffusion and pair-interactions in zooplankton. Specifically, we have formulated a numerical framework, the pair-interaction model, which allows to study patch formation. Remarkably, we observe that pair-interactions can lead to patches of numerous particles, similar to the field observations². We thus explore the model's parameter space, to reveal what is required for patchiness to be sustained. Furthermore, we compare our model's results with laboratory measurements of calanoid copepod trajectories³ and show good agreement between the model and the experiment. Our results support the hypothesis that small-scale patchiness is driven by the animal's behavior and thus explain the details of how zooplankton achieve high mating encounter rates in their complex environment.

¹ B. Pinel-Alloul and A. Ghadouani (2007). Spatial heterogeneity of planktonic microorganisms in aquatic systems, Springer Netherlands, Dordrecht.

² C. S. Davis, S. M. Gallager and A. R. Solow (1992). Science 257, 230-232.

³ F.-G. Michalec et al. (2017). Proc. Natl. Acad. Sci. U.S.A. 114 ; F.-G. Michalec et al. (2020). eLife 9, e62014.

⁴ C. L. Folt and C. W. Burns (1999). Trends in Ecology and Evolution, 14, 300–305.

⁵ J. P. Salazar and L. R. Collins (2009). Ann. Rev. Fluid Mech. 41, 405-432.

How to cite: Shnapp, R. and Holzner, M.: Copepods counter dispersion to maintain high mating-encounter rates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7710, https://doi.org/10.5194/egusphere-egu22-7710, 2022.

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17:25–17:31

EGU22-897

Phytoplankton-zooplankton dynamics in three-dimensional turbulent flows behind an idealized island

Stefano Berti et al.

Plankton constitutes the productive base of aquatic ecosystems and plays a key role in climate dynamics, by taking part in the global carbon budget. Understanding how turbulent flows affect the distributions of planktonic species is a complex problem that has attracted considerable interest in the past, with particular emphasis on the scaling behavior of plankton variance spectra. The issue is relevant to assess the relative importance of fluid and biological dynamics, and to quantify the patchiness of plankton spatial distributions. Indeed, were the spectra of the, reactive, planktonic fields different from those of a passive (non-reactive) scalar, this would point to predominant biological activity in the corresponding range of scales. Furthermore, spectral slopes give information on the scale-by-scale intensity of the fluctuations of biological population densities and, hence, could allow to quantify the typical size of structures of highest plankton concentration.

Previous numerical studies provided interesting insight into plankton bloom formation and patchiness. However, they relied on simplified kinematic flow settings or on turbulence parametrizations. By means of direct numerical simulations, in this work we investigate the dynamics of interacting phytoplankton and zooplankton populations in two and three-dimensional turbulent wakes behind a cylinder. We mainly aim at identifying the minimal flow ingredients needed to sustain a bloom, and at characterizing how the latter could be affected by multiscale fluid properties. Notwithstanding its idealized character, the system we consider allows us to avoid any bias possibly coming from the modeling of small-scale fluid motions. Our analysis focuses on the impact of the space dimensionality of the advecting velocity field on the variance spectra, and spatial distributions, of the planktonic species.

In spite of the different statistical properties of the two-dimensional and three-dimensional carrying flows, we find that the qualitative biological dynamics in the two cases share important common features, mostly independent of the space dimensionality. This observation suggests that, in both cases, the emergence of persistent blooms is controlled by the ratio between the typical timescales of the biological activity, and of the fluid flow at large length scales. Similarly, in both two and three dimensions, we find that the spectral properties of the planktonic populations are essentially indistinguishable from those of an inert tracer. This result then hints at the prevailing role of turbulent transport over biological mechanisms in the generation of plankton patchiness. The main difference, instead, that arises from the comparison of our two and three-dimensional configurations concerns the local spatial distribution of plankton density fields. In fact, the three-dimensional turbulent dynamics tend to destroy the localized coherent structures characterizing the two-dimensional flow, in which the planktonic species are mostly concentrated, thus reducing the phytoplankton average biomass in the system.

How to cite: Berti, S., Jaccod, A., Calzavarini, E., and Chibbaro, S.: Phytoplankton-zooplankton dynamics in three-dimensional turbulent flows behind an idealized island, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-897, https://doi.org/10.5194/egusphere-egu22-897, 2022.

17:31–17:32

Direct numerical simulations (DNS) and plankton dynamics

17:32–17:42

EGU22-895

solicited

Gyrotactic plankton cells in turbulence: the effects of motility, shape, fluid acceleration and inertia

Eric Climent et al.

A detailed understanding of the physical mechanisms driving gyrotactic species to migrate vertically towards the surface allows better quantification of biogeochemical fluxes across the ocean. We focus on marine phytoplankton cells that are motile under gyrotactic forcing. Some species spontaneously swim in the direction opposite to gravity [1]. Gyrotaxis is originating either from morphological aspects (elongated shape, density heterogeneity) or the coupled effect of swimming and settling which results in an inertial torque. Indeed, fluid inertial torque may have a potential impact on the gyrotaxis for elongated planktonic swimmers, especially for those forming long chains and thus having large swimming and settling speeds
Based on numerical simulations of hundreds of thousands of micro-organisms swimming in homogeneous isotropic turbulence, we will comment on the different sources of gyrotactic induced spatial clustering [2, 3] and vertical migration [4]. Some specific configurations lead to the accumulation of elongated plankton cells in upwelling flow regions enhancing their ability to move across turbulence through the water column.

[1] Kessler J.O. (1985), Nature - 313, 218–220.
[2] Durham W. M., et al. (2013) Nat. Commun. - 4, 2148.
[3] De Lillo F., et al. (2014) Phys. Rev. Lett. – 112, 044502
[4] Lovecchio S., et al. (2019) Sci. Adv. - 5: eaaw7879

How to cite: Climent, E., Qiu, J., Cui, Z., and Zhao, L.: Gyrotactic plankton cells in turbulence: the effects of motility, shape, fluid acceleration and inertia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-895, https://doi.org/10.5194/egusphere-egu22-895, 2022.

17:42–17:48

EGU22-3718

Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers

Cristian Marchioli et al.

Using DNS-based Eulerian-Lagrangian simulations, we investigate the dynamics of small gyrotactic swimmers in free-surface turbulence. We consider open channel flow turbulence in which bottom-heavy swimmers are dispersed. Swimmers are characterized by different vertical stability, so that some realign to swim upward with a characteristic time smaller than the Kolmogorov time scale, while others possess a re-orientation time longer than the Kolmogorov time scale. We cover one order of magnitude in the flow Reynolds number, and two orders of magnitude in the stability number, which is a measure of bottom heaviness. We observe that large-scale advection dominates vertical motion when the stability number, scaled on the local Kolmogorov time scale of the flow, is larger than unity: This condition is associated to enhanced migration towards the surface, particularly at low Reynolds number, when swimmers can rise through surface renewal motions that originate directly from the bottom-boundary turbulent bursts. Conversely, small-scale effects become more important when the Kolmogorov-based stability number is below unity: Under this condition, migration towards the surface is hindered, particularly at high Reynolds, when bottom-boundary bursts are less effective in bringing bulk fluid to the surface. In an effort to provide scaling arguments to improve predictions of models for motile micro-organisms in turbulent water bodies, we demonstrate that a Kolmogorov-based stability number around unity represents a threshold beyond which swimmer capability to reach the free surface and form clusters saturates.

How to cite: Marchioli, C., Bhatia, H., Sardina, G., Brandt, L., and Soldati, A.: Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3718, https://doi.org/10.5194/egusphere-egu22-3718, 2022.

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17:48–17:54

EGU22-2386

ECS

Accumulation and alignment of elongated gyrotactic swimmers in turbulence

Linfeng Jiang et al.

Understanding the dynamics and transport of elongated gyrotactic swimmers in a flow is essential for the ecology of aquatic plankton. We study their dynamics in turbulence, whose orientation is governed by gravitational torque and local fluid velocity gradient. The gyrotaxis strength is measured by the ratio of the Kolmogorov time scale to the reorientation time scale due to gravity, and a large value of this ratio means the gyrotaxis is strong. By means of direct numerical simulations, we investigate the effects of swimming velocity and gyrotactic stability on spatial accumulation and alignment. Three-dimensional Voronoi analysis is used to study the spatial distribution and time evolution of the particle concentration. We study spatial distribution by examing the overall preferential sampling and where clusters and voids (subsets of particles that have small and large Voronoi volumes respectively) form. Compared with the ensemble particles, the preferential sampling of clusters and voids is found to be more pronounced. The clustering of fast swimmers lasts much longer than slower swimmers when the gyrotaxis is strong and intermediate, but an opposite trend emerges when the gyrotaxis is weak. In addition, we study the preferential alignment with the Lagrangian stretching direction, with which passive slender rods have been known to align. We show that the Lagrangian alignment is reduced by the swimming velocity when the gyrotaxis is weak, while the Lagrangian alignment is enhanced for the regime in which gyrotaxis is strong.

How to cite: Jiang, L., Liu, Z., and Sun, C.: Accumulation and alignment of elongated gyrotactic swimmers in turbulence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2386, https://doi.org/10.5194/egusphere-egu22-2386, 2022.

17:54–18:00

EGU22-13293

Orientation of swimmers in turbulent flows

Filippo De Lillo et al.

18:00–18:06

EGU22-13031

ECS

Orientation of anisotropic particles in stratified turbulent flows

Alessandro Sozza and Alain Pumir

Small-scale turbulence and density stratification are two major ingredients shaping the life of marine micro-organisms in the pycnocline. Such tiny particles are rarely spherical, ranging from flat disks to elongated rods. Particle orientation with respect to the flow or to density gradients plays a crucial role in many aspects of phytoplankton's life, e.g. light harvesting for photosynthesis, enhancement of nutrient uptake, optimal navigation and vertical migration. However, it's still unclear how anisotropic particles align in a turbulent pycnocline and how they are able to cope with density stratification.

In the present work, we aim to characterize the effects of stratification on the orientation of inertialess non-spherical particles. To achieve this purpose, we performed direct numerical simulations of a mixed Eulerian-Lagrangian model. The flow is described by the Boussinesq equations, which evolve fluid velocity and density fluctuations in a triply periodic cubic domain. The space is initially seeded with spheroidal particles of different shapes (from rods to disks) transported by the flow as passive tracers. Particle orientation evolves in response to velocity gradients according to Jeffery’s dynamics.

We have explored different configurations of the parameters' space by changing particle shape, density stratification and turbulence intensity. The statistical properties of orientation are then unveiled by characterizing the particles' distributions and their mean behavior. Moreover, we have inspected the alignment of particles with respect to the flow and to the iso-density surfaces. We have analyzed rotation rates of the particles and compared our results with the case of spherical particles and homogeneous isotropic turbulence. Such outcomes provide a clear picture of the influence of stratification on the orientational dynamics and on its transition from non-stratified to strongly stratified turbulence. Finally, we conclude by discussing the implications of our results for oceanic applications.

How to cite: Sozza, A. and Pumir, A.: Orientation of anisotropic particles in stratified turbulent flows, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13031, https://doi.org/10.5194/egusphere-egu22-13031, 2022.

18:06–18:12

EGU22-7195

ECS

Numerical study of collisions between settling non-spherical particles in turbulence

Anđela Grujić et al.

The dynamics of microplastics in the ocean can be modeled similarly to natural particles such as sediment grains, marine snow, phyto- and zooplankton. The settling of the particle is important not only for the individual particle motion, but it also affects the encounter rate, which is important for several physical processes such as nutrient uptake, biofouling, the degradation of microplastics and transport of pollutants into the food chain in the marine environment.

Some of the factors that determine the collision and accumulation of the particles are the level of turbulence, buoyancy, particle shape and diffusivity. Microplastics are often elongated in shape, whereas phytoplankton often form long chains colonies and filaments, even if these are unicellular, which makes the investigation as nonspherical particles in turbulent flows relevant. The objective of this study is to quantify how turbulence affects collision kernels of the nonspherical settling particles. This work is motivated by recent studies in laminar flows showing how collisions between fiber-like particles are much more frequent than those between spherical particles, even in the presence of turbulence (Slomka, J., Stocker, R., 2020. On the collision of rods in a quiescent fluid, Proceedings of the National Academy of Sciences 117, 3372-3374). To this end, we shall consider particles as elongated spheroids. Given the low-density ratios, close to 1, and the size, order of microns, inertia can be neglected, and the particle velocity is assumed to be equal to the sum of the fluid velocity at the particle position and the settling speed. The settling speed is taken to be the Stokes settling velocity for oblate spheroids, function of the object orientation and aspect ratio; note that this is not parallel to gravity for any general orientation. We report results from simulations of sinking inertia-less elongated spheroids in homogeneous isotropic turbulence (HIT). The velocity field is assumed to be incompressible and to obey the Navier-Stokes and continuity equation. To maintain the turbulent velocity in a statistically steady state, a random forcing field is needed. The elongated spheroids studied here are small compared to the Kolmogorov length scale of the turbulence and have different aspect ratios: 1 (spheres), 2, 5, 10 and 20. We will present results for two different settling velocities – equal to 1 Kolmogorov and 3 times the Kolmogorov velocity, velocity scale of the smallest vortices in the flow. In order to quantify clustering in fully three-dimensional isotropic turbulent flows, the radial pair distribution function (r.d.f.) is used, which provides information about the collision rates when combined with the relative particle velocity at distances of the order of the particle size.

We show that the effect of the different collisional relative velocity has a greater impact than the patchiness on the increase of the collision rate. For larger settling velocities, i.e. larger particle sizes, the collision rates of elongated particles increase with the aspect ratio, an increase however smaller than that observed in quiescent flows. Results obtained for the collision of particles of different buoyancy will also be presented.

How to cite: Grujić, A., Brandt, L., and Sardina, G.: Numerical study of collisions between settling non-spherical particles in turbulence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7195, https://doi.org/10.5194/egusphere-egu22-7195, 2022.

18:12–18:30

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