ORCID Profile
0000-0003-4840-5050
Current Organisation
Imperial College London
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Publisher: Public Library of Science (PLoS)
Date: 27-07-2022
DOI: 10.1371/JOURNAL.PCBI.1010291
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 in idual-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 ex le 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 lify 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.
Publisher: IWA Publishing
Date: 24-11-2011
Abstract: This study discusses the effect of combined first-order, bulk and wall reactions on the overall intensity of mass withdrawal in smooth pipes. A one-dimensional model for the transport of high Schmidt number compounds (where the Schmidt number is the ratio of the fluid viscosity to the solute diffusivity) is extended with the effect of a first-order bulk reaction. Profiles of velocity and eddy diffusivity are obtained with a standard high Reynolds number k–∊ closure in combination with a modified Van Driest wall function. By comparing the results of the 1D model to an analytical asymptotic high Sc approximation, it is shown that the interaction between bulk and wall reactions is very weak. In terms of the decay coefficient, a maximum deviation of 4% is observed for very high bulk demand due to the attenuation of the streamwise solute mass flux. The parameter range for which the concentration profiles can be safely assumed to be uniform, both in the viscous sublayer and in the bulk, is established.
Publisher: Elsevier BV
Date: 11-2012
Publisher: The Royal Society
Date: 22-02-2012
Abstract: We present closed-form solutions for high Schmidt number mass transfer in a hydrodynamically fully developed turbulent flow. Governing equations for the near- and far-field are developed for a large class of boundary conditions (BCs) for which the mass flux is a function of the concentration at the wall. We show that for this class of BCs, which includes nonlinear wall reactions, the mass transfer coefficient is independent of the BC and the Sherwood correlation is therefore universal. For Dirichlet, Neumann and Robin BCs, the far-field solutions are in good correspondence with the method of separating variables and near-field solutions are in good agreement with numerical simulations. However, in contrast with the far-field solutions, the Sherwood correlation in the near-field depends on the specific BC. As an ex le of nonlinear BCs, solutions for a second-order wall reaction are derived which are compared with numerical simulations and found to be in excellent agreement.
Publisher: Springer Science and Business Media LLC
Date: 07-05-2013
Publisher: Cambridge University Press (CUP)
Date: 09-05-2017
DOI: 10.1017/JFM.2017.245
Abstract: It was previously observed by Krug et al. ( J. Fluid Mech. , vol. 765, 2015, pp. 303–324) that the surface area $A_{\\unicode[STIX]{x1D702}}$ of the turbulent/non-turbulent interface (TNTI) in gravity currents decreases with increasing stratification, significantly reducing the entrainment rate. Here, we consider the multiscale properties of this effect using direct numerical simulations of temporal gravity currents with different gradient Richardson numbers $Ri_{g}$ . Our results indicate that the reduction of $A_{\\unicode[STIX]{x1D702}}$ is caused by a decrease of the fractal scaling exponent $\\unicode[STIX]{x1D6FD}$ , while the scaling range remains largely unaffected. We further find that convolutions of the TNTI are characterized by different length scales in the streamwise and wall-normal directions, namely the integral scale $h$ and the shear scale $l_{Sk}=k^{1/2}/S$ (formed using the mean shear $S$ and the turbulent kinetic energy $k$ ) respectively. By recognizing that the anisotropy implied by the different scaling relations increases with increasing $Ri_{g}$ , we are able to model the $Ri_{g}$ dependence of $\\unicode[STIX]{x1D6FD}$ in good agreement with the data.
Publisher: Springer Science and Business Media LLC
Date: 28-06-2013
Publisher: American Society of Civil Engineers (ASCE)
Date: 09-2010
Publisher: Begellhouse
Date: 2012
DOI: 10.1615/ICHMT.2012.PROCSEVINTSYMPTURBHEATTRANSFPAL.1740
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Maarten van Reeuwijk.