scholarly journals Investigating microscale patchiness of motile microbes driven by the interaction of turbulence and gyrotaxis in a 3D simulated convective mixed layer.

2021 ◽  
Author(s):  
Alexander Christensen ◽  
Matthew Piggott ◽  
Erik van Sebille ◽  
Maarten van Reeuwijk ◽  
Samraat Pawar
Keyword(s):  
Mixed Layer ◽  
Fluid Model ◽  
Spatial Dynamics ◽  
Critical Factor ◽  
Trophic Levels ◽  
Small Scale ◽  
Swimming Velocity ◽  
Primary Role ◽  
Fluid Surface ◽  
Convective Mixed Layer

Abstract Microbes play a primary role in aquatic ecosystems and biogeochemical cycles. Spatial patchiness is a critical factor underlying these activities, influencing biological productivity, nutrient cycling and dynamics across trophic levels. Incorporating spatial dynamics into microbial models is a long-standing challenge, particularly where small-scale turbulence is involved. Here, we combine a fully 3D direct numerical simulation of convective mixed layer turbulence, with an individual-based microbial model to test the key hypothesis that the coupling of gyrotactic motility and turbulence drives intense microscale patchiness. The fluid model simulates turbulent convection caused by heat loss through the fluid surface, for example during the night, during autumnal or winter cooling or during a cold-air outbreak. We find that under such conditions, turbulence-driven patchiness is depth-structured and requires high motility: Near the fluid surface, intense convective turbulence overpowers motility, homogenising motile and non-motile microbes approximately equally. At greater depth, in conditions analogous to a thermocline, highly motile microbes can be over twice as patch-concentrated as non-motile microbes, and can substantially amplify their swimming velocity by efficiently exploiting fast-moving packets of fluid. Our results substantiate the predictions of earlier studies, and demonstrate that turbulence-driven patchiness is not a ubiquitous consequence of motility but rather a delicate balance of motility and turbulent intensity.

scholarly journals Small-scale convective turbulence constrains microbial patchiness

2021 ◽  
Author(s):  
Alexander Christensen ◽  
Matthew Piggott ◽  
Erik van Sebille ◽  
Maarten van Reeuwijk ◽  
Samraat Pawar
Keyword(s):  
Aquatic Ecosystems ◽  
Spatial Dynamics ◽  
Biogeochemical Cycles ◽  
Trophic Levels ◽  
Small Scale ◽  
Primary Role ◽  
Biological Productivity ◽  
Convective Turbulence ◽  
Scale Turbulence ◽  
Microbial Model

Abstract Microbes play a primary role in aquatic ecosystems and biogeochemical cycles. Patchiness is a critical component of these activities, influencing biological productivity, nutrient cycling and dynamics across trophic levels. Incorporating spatial dynamics into microbial models is a long-standing challenge, particularly where small-scale turbulence is involved. Here, we combine a realistic simulation of turbulence with an individual-based microbial model to test the key hypothesis that the coupling of motility and turbulence drives intense microscale patchiness. We find that such patchiness is depth-structured and requires high motility: Near the fluid surface, strong convective turbulence overpowers motility, homogenising motile and non-motile microbes equally. In deeper, thermocline-like conditions, highly motile microbes are up to 1.6-fold more patch-concentrated than non-motile microbes. Our results demonstrate that the delicate balance of turbulence and motility that triggers micro-scale patchiness is not a ubiquitous consequence of motility, and that the intensity of such patchiness in real-world conditions is modest.

scholarly journals Formation Mechanism of Dust Devil–Like Vortices in Idealized Convective Mixed Layers

2013 ◽  
Vol 70 (4) ◽  
pp. 1173-1186 ◽  
Author(s):  
Junshi Ito ◽  
Hiroshi Niino ◽  
Mikio Nakanishi
Keyword(s):  
Formation Mechanism ◽  
Mixed Layer ◽  
Weather Conditions ◽  
Small Scale ◽  
Material Surface ◽  
Dust Devil ◽  
Dust Devils ◽  
Eddy Simulation ◽  
Convective Mixed Layer ◽  
Mixed Layers

Abstract Dust devils are small-scale vertical vortices often observed over deserts or bare land during the daytime under fair weather conditions. Previous numerical studies have demonstrated that dust devil–like vertical vortices can be simulated in idealized convective mixed layers in the absence of background winds or environmental shear. Their formation mechanism, however, has not been completely clarified. In this paper, the authors attempt to clarify the vorticity source of a dust devil–like vortex by means of a large-eddy simulation, in which a material surface initially placed in the vortex is tracked backward and the circulation on the material surface is examined. The material surface is found to originate from downdrafts, which already have sufficient circulation. As the material surface converges toward the vortex, the vorticity is increased because of conservation of circulation. It is shown that a convective mixed layer is inherently accompanied by circulation, which is scaled by a product of the convective velocity scale and the depth of the convective mixed layer. This circulation is considered to be originally generated by tilting of baroclinically generated horizontal vorticity principally at middepths of the convective mixed layer.

Numerical Approaches for Modeling Gas–Solid Fluidized Bed Reactors: Comparison of Models and Application to Different Technical Problems

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Peter Ostermeier ◽  
Annelies Vandersickel ◽  
Stephan Gleis ◽  
Hartmut Spliethoff
Keyword(s):  
Calcium Carbonate ◽  
Size Distribution ◽  
Fluidized Bed ◽  
Fluid Model ◽  
Industrial Applications ◽  
Small Scale ◽  
Fluidized Bed Reactors ◽  
Discrete Phase Model ◽  
Technical Problems ◽  
Numerical Approaches

Gas–solid fluidized bed reactors play an important role in many industrial applications. Nevertheless, there is a lack of knowledge of the processes occurring inside the bed, which impedes proper design and upscaling. In this work, numerical approaches in the Eulerian and the Lagrangian framework are compared and applied in order to investigate internal fluidized bed phenomena. The considered system uses steam/air/nitrogen as fluidization gas, entering the three-dimensional geometry through a Tuyere nozzle distributor, and calcium oxide/corundum/calcium carbonate as solid bed material. In the two-fluid model (TFM) and the multifluid model (MFM), both gas and powder are modeled as Eulerian phases. The size distribution of the particles is approximated by one or more granular phases with corresponding mean diameters and a sphericity factor accounting for their nonspherical shape. The solid–solid and fluid–solid interactions are considered by incorporating the kinetic theory of granular flow (KTGF) and a drag model, which is modified by the aforementioned sphericity factor. The dense discrete phase model (DDPM) can be interpreted as a hybrid model, where the interactions are also modeled using the KTGF; however, the particles are clustered to parcels and tracked in a Lagrangian way, resulting in a more accurate and computational affordable resolution of the size distribution. In the computational fluid dynamics–discrete element method (CFD–DEM) approach, particle collisions are calculated using the DEM. Thereby, more detailed interparticulate phenomena (e.g., cohesion) can be assessed. The three approaches (TFM, DDPM, CFD–DEM) are evaluated in terms of grid- and time-independency as well as computational demand. The TFM and CFD–DEM models show qualitative accordance and are therefore applied for further investigations. The MFM (as a variation of the TFM) is applied in order to simulate hydrodynamics and heat transfer to immersed objects in a small-scale experimental test rig because the MFM can handle the required small computational cells. Corundum is used as a nearly monodisperse powder, being more suitable for Eulerian models, and air is used as fluidization gas. Simulation results are compared to experimental data in order to validate the approach. The CFD–DEM model is applied in order to predict mixing behavior and cohesion effects of a polydisperse calcium carbonate powder in a larger scale energy storage reactor.

scholarly journals Biodiversity and trophic ecology of hydrothermal vent fauna associated with tubeworm assemblages on the Juan de Fuca Ridge

2018 ◽  
Vol 15 (9) ◽  
pp. 2629-2647 ◽  
Author(s):  
Yann Lelièvre ◽  
Jozée Sarrazin ◽  
Julien Marticorena ◽  
Gauthier Schaal ◽  
Thomas Day ◽  
...  
Keyword(s):  
Food Webs ◽  
Hydrothermal Vent ◽  
Trophic Structure ◽  
Juan De Fuca Ridge ◽  
Small Scale ◽  
Ecological Niches ◽  
Primary Role ◽  
Fuca Ridge ◽  
Juan De Fuca ◽  
Successional Stages

Abstract. Hydrothermal vent sites along the Juan de Fuca Ridge in the north-east Pacific host dense populations of Ridgeia piscesae tubeworms that promote habitat heterogeneity and local diversity. A detailed description of the biodiversity and community structure is needed to help understand the ecological processes that underlie the distribution and dynamics of deep-sea vent communities. Here, we assessed the composition, abundance, diversity and trophic structure of six tubeworm samples, corresponding to different successional stages, collected on the Grotto hydrothermal edifice (Main Endeavour Field, Juan de Fuca Ridge) at 2196 m depth. Including R. piscesae, a total of 36 macrofaunal taxa were identified to the species level. Although polychaetes made up the most diverse taxon, faunal densities were dominated by gastropods. Most tubeworm aggregations were numerically dominated by the gastropods Lepetodrilus fucensis and Depressigyra globulus and polychaete Amphisamytha carldarei. The highest diversities were found in tubeworm aggregations characterised by the longest tubes (18.5 ± 3.3 cm). The high biomass of grazers and high resource partitioning at a small scale illustrates the importance of the diversity of free-living microbial communities in the maintenance of food webs. Although symbiont-bearing invertebrates R. piscesae represented a large part of the total biomass, the low number of specialised predators on this potential food source suggests that its primary role lies in community structuring. Vent food webs did not appear to be organised through predator–prey relationships. For example, although trophic structure complexity increased with ecological successional stages, showing a higher number of predators in the last stages, the food web structure itself did not change across assemblages. We suggest that environmental gradients provided by the biogenic structure of tubeworm bushes generate a multitude of ecological niches and contribute to the partitioning of nutritional resources, releasing communities from competition pressure for resources and thus allowing species to coexist.

scholarly journals Energetic Submesoscales Maintain Strong Mixed Layer Stratification during an Autumn Storm

2017 ◽  
Vol 47 (10) ◽  
pp. 2419-2427 ◽  
Author(s):  
Daniel B. Whitt ◽  
John R. Taylor
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
Small Scale ◽  
Upper Ocean ◽  
Ocean Mixed Layer ◽  
Ocean Stratification ◽  
The North ◽  
Large Eddy ◽  
The Mean ◽  
Scale Turbulence

AbstractAtmospheric storms are an important driver of changes in upper-ocean stratification and small-scale (1–100 m) turbulence. Yet, the modifying effects of submesoscale (0.1–10 km) motions in the ocean mixed layer on stratification and small-scale turbulence during a storm are not well understood. Here, large-eddy simulations are used to study the coupled response of submesoscale and small-scale turbulence to the passage of an idealized autumn storm, with a wind stress representative of a storm observed in the North Atlantic above the Porcupine Abyssal Plain. Because of a relatively shallow mixed layer and a strong downfront wind, existing scaling theory predicts that submesoscales should be unable to restratify the mixed layer during the storm. In contrast, the simulations reveal a persistent and strong mean stratification in the mixed layer both during and after the storm. In addition, the mean dissipation rate remains elevated throughout the mixed layer during the storm, despite the strong mean stratification. These results are attributed to strong spatial variability in stratification and small-scale turbulence at the submesoscale and have important implications for sampling and modeling submesoscales and their effects on stratification and turbulence in the upper ocean.

scholarly journals Mixing in the Transition Layer during Two Storm Events

2011 ◽  
Vol 41 (1) ◽  
pp. 42-66 ◽  
Author(s):  
Kathleen Dohan ◽  
Russ E. Davis
Keyword(s):  
Mixed Layer ◽  
Transition Layer ◽  
Wavelet Transforms ◽  
Ocean Dynamics ◽  
Small Scale ◽  
Storm Events ◽  
Inertial Waves ◽  
Space Time Structure ◽  
Inertial Energy

Abstract Upper-ocean dynamics analyzed from mooring-array observations are contrasted between two storms of comparable magnitude. Particular emphasis is put on the role of the transition layer, the strongly stratified layer between the well-mixed upper layer, and the deeper more weakly stratified region. The midlatitude autumn storms occurred within 20 days of each other and were measured at five moorings. In the first storm, the mixed layer follows a classical slab-layer response, with a steady deepening during the course of the storm and little mixing of the thermocline beneath. In the second storm, rather than deepening, the mixed layer shoals while intense near-inertial waves are resonantly excited within the mixed layer. These create a large shear throughout the transition layer, generating turbulence that broadens the transition layer. Details of the space–time structure of the frequencies in both short waves and near-inertial waves are presented. Small-scale waves are excited within the transition layer. Their frequencies change with time and there are no clear peaks at harmonics of inertial or tidal frequencies. Wavelet transforms of the inertial oscillations show the evolution as a spreading in frequency, a deepening of the core into the transition layer, and a shift off the inertial frequency. A second near-inertial energy core appears below the transition layer at all moorings coincident with a rapid decay of mixed layer currents. An overall result is that direct wind-generated motions extend to the depth of the transition layer. The transition layer is a location of enhanced wave activity and enhanced shear-driven mixing.

An Observational Study of Wind Profiles in the Baroclinic Convective Mixed Layer

1999 ◽  
Vol 90 (1) ◽  
pp. 47-82 ◽  
Author(s):  
Margaret a. Lemone ◽  
Mingyu Zhou ◽  
Chin-Hoh Moeng ◽  
Donald H. Lenschow ◽  
L. Jay Miller ◽  
...  
Keyword(s):  
Observational Study ◽  
Mixed Layer ◽  
Convective Mixed Layer ◽  
Wind Profiles
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