Large-scale simulation of steady and time-dependent active suspensions with the force-coupling method

We present a new development of the force-coupling method (FCM) to address the accurate simulation of a large number of interacting micro-swimmers. Our approach is based on the squirmer model, which we adapt to the FCM framework, resulting in a method that is suitable for simulating semi-dilute squirmer suspensions. Other effects, such as steric interactions, are considered with our model. We test our method by comparing the velocity field around a single squirmer and the pairwise interactions between two squirmers with exact solutions to the Stokes equations and results given by other numerical methods. We also illustrate our method's ability to describe spheroidal swimmer shapes and biologically-relevant time-dependent swimming gaits. We detail the numerical algorithm used to compute the hydrodynamic coupling between a large collection ($10^4-10 ^5$) of micro-swimmers. Using this methodology, we investigate the emergence of polar order in a suspension of squirmers and show that for large domains, both the steady-state polar order parameter and the growth rate of instability are independent of system size. These results demonstrate the effectiveness of our approach to achieve near continuum-level results, allowing for better comparison with experimental measurements while complementing and informing continuum models.Comment: 37 pages, 21 figure

A general formulation of Bead Models applied to flexible fibers and active filaments at low Reynolds number

This contribution provides a general framework to use Lagrange multipliers for the simulation of low Reynolds number fiber dynamics based on Bead Models (BM). This formalism provides an efficient method to account for kinematic constraints. We illustrate, with several examples, to which extent the proposed formulation offers a flexible and versatile framework for the quantitative modeling of flexible fibers deformation and rotation in shear flow, the dynamics of actuated filaments and the propulsion of active swimmers. Furthermore, a new contact model called Gears Model is proposed and successfully tested. It avoids the use of numerical artifices such as repulsive forces between adjacent beads, a source of numerical difficulties in the temporal integration of previous Bead Models.Comment: 41 pages, 15 figure

Turbulent fluid acceleration generates clusters of gyrotactic microorganisms

The motility of microorganisms is often biased by gradients in physical and chemical properties of their environment, with myriad implications on their ecology. Here we show that fluid acceleration reorients gyrotactic plankton, triggering small-scale clustering. We experimentally demonstrate this phenomenon by studying the distribution of the phytoplankton Chlamydomonas augustae within a rotating tank and find it to be in good agreement with a new, generalized model of gyrotaxis. When this model is implemented in a direct numerical simulation of turbulent flow, we find that fluid acceleration generates multi-fractal plankton clustering, with faster and more stable cells producing stronger clustering. By producing accumulations in high-vorticity regions, this process is fundamen- tally different from clustering by gravitational acceleration, expanding the range of mechanisms by which turbulent flows can impact the spatial distribution of active suspensions.Comment: 5 pages, 4 figure

Large-scale simulation of steady and time-dependent active suspensions with the force-coupling method

We present a new development of the force-coupling method (FCM) to address the accurate simulation of a large number of interacting micro-swimmers. Our approach is based on the squirmer model, which we adapt to the FCM framework, resulting in a method that is suitable for simulating semi-dilute squirmer suspensions. Other effects, such as steric interactions, are considered with our model. We test our method by comparing the velocity field around a single squirmer and the pairwise interactions between two squirmers with exact solutions to the Stokes equations and results given by other numerical methods. We also illustrate our method’s ability to describe spheroidal swimmer shapes and biologically-relevant time-dependent swimming gaits. We detail the numerical algorithm used to compute the hydrodynamic coupling between a large collection (10^4–10^5) of micro-swimmers. Using this methodology, we investigate the emergence of polar order in a suspension of squirmers and show that for large domains, both the steady-state polar order parameter and the growth rate of instability are independent of system size. These results demonstrate the effectiveness of our approach to achieve near continuum-level results, allowing for better comparison with experimental measurements while complementing and informing continuum models

Micro/Meso simulations of a fluidized bed with heat transfer

Particulate flows encountered in fluidized beds are frequently used in industrial applications (refining, energy, chemicals/petrochemicals, pharmaceutics). The wide range of spatial scales and interactions between the phases in such systems yields a complex flow often coupled with mass and/or heat transfer. These transfer have been widely investigated in the past in an experimental and numerical manners for dilute (1, 2) and dense suspensions (3, 4). Multi-scale modeling is a numerical approach developed to understand these phenomena from the particle scale (microscale) to process unit (macroscale). An intermediate scale (mesoscale) ranging from 104 to 108 particles is also introduced. Fluid phase is resolved using an Eulerian description while particles may be followed in a Lagrangian or an Eulerian manner. Meso/macroscale require closure laws for momentum, heat/mass transfer that can be derived either from experiments or microscale Particle-Resolved simulations (PRS). In this work, numerical simulations of gas-solid fluidization with heat transfer are performed at the microscale (DLM/FD) and the mesoscale (Euler/Lagrange) with our massively parallel code PeliGRIFF (5). A soft-sphere model combined with a Discrete Element Method (DEM) to track particles trajectory and contacts is used at both scales and interphase drag/heat transfer closure laws derived from our own PRS are used at the mesoscale. We select a system that comprises a few thousands of particles and extract statistically averaged local and global heat transfer. We carry out a direct comparison of the predictions obtained at both scales and suggest how the mesoscale modeling might be improved to provide more accurate solutions. REFERENCES W.E. Ranz and W.R. Marshall. Evaporation from drops, Part I and I. Chemical Engineering Science, 48:141-146;173-180, 1952. Z.G. Feng and E.E. Michaelides. Heat transfer in particulate flows with Direct Numerical Simulation (DNS). International Journal of Heat and Mass Transfer, 52:777-786, 2009. D.J. Gunn. Transfer of heat or mass to particles in fixed and fluidized beds, International Journal of Heat and Mass Transfer, 21:467-476,1978. N.G. Deen and E.A.J.F. Peters, J.T. Padding and J.A.M. Kuipers. Review of direct numerical simulation of fluid-particle mass, momentum and heat transfer in dense gas-solid flows, Chemical Engineering Science, 116:710-724, 2014. A. Wachs, A. Hammouti, G. Vinay, and M. Rahmani. Accuracy of finite Volume/Staggered grid Distributed Lagrange Multipliers/Fictitious Domain simulations of particulate Flows. Computers & Fluids 115, 154–172, 2015

Microbiota Variation Across Life Stages of European Field-Caught Anopheles atroparvus and During Laboratory Colonization: New Insights for Malaria Research

The potential use of bacteria for developing novel vector control approaches has awakened new interests in the study of the microbiota associated with vector species. To set a baseline for future malaria research, a high-throughput sequencing of the bacterial 16S ribosomal gene V3-V4 region was used to profile the microbiota associated with late-instar larvae, newly emerged females, and wild-caught females of a sylvan Anopheles atroparvus population from a former malaria transmission area of Spain. Field-acquired microbiota was then assessed in non-blood-fed laboratory-reared females from the second, sixth, and 10th generations. Diversity analyses revealed that bacterial communities varied and clustered differently according to origin with sylvan larvae and newly emerged females distributing closer to laboratory-reared females than to their field counterparts. Inter-sample variation was mostly observed throughout the different developmental stages in the sylvan population. Larvae harbored the most diverse bacterial communities; wild-caught females, the poorest. In the transition from the sylvan environment to the first time point of laboratory breeding, a significant increase in diversity was observed, although this did decline under laboratory conditions. Despite diversity differences between wild-caught and laboratory-reared females, a substantial fraction of the bacterial communities was transferred through transstadial transmission and these persisted over 10 laboratory generations. Differentially abundant bacteria were mostly identified between breeding water and late-instar larvae, and in the transition from wild-caught to laboratory-reared females from the second generation. Our findings confirmed the key role of the breeding environment in shaping the microbiota of An. atroparvus. Gram-negative bacteria governed the microbiota of An. atroparvus with the prevalence of proteobacteria. Pantoea, Thorsellia, Serratia, Asaia, and Pseudomonas dominating the microbiota associated with wild-caught females, with the latter two governing the communities of laboratory-reared females. A core microbiota was identified with Pseudomonas and Serratia being the most abundant core genera shared by all sylvan and laboratory specimens. Overall, understanding the microbiota composition of An. atroparvus and how this varies throughout the mosquito life cycle and laboratory colonization paves the way when selecting potential bacterial candidates for use in microbiota-based intervention strategies against mosquito vectors, thereby improving our knowledge of laboratory-reared An. atroparvus mosquitoes for research purposes.info:eu-repo/semantics/publishedVersio

Experimental and numerical investigation on mixing and axial dispersion in Taylor-Couette flow patterns

Taylor-Couette flows between two concentric cylinders have great potential applications in chemical engineering. They are particularly convenient for two-phase small scale devices enabling solvent extraction operations. An experimental device was designed with this idea in mind. It consists of two concentric cylinders with the inner one rotating and the outer one fixed. Moreover, a pressure driven axial flow can be superimposed. Taylor-Couette flow is known to evolve towards turbulence through a sequence of successive hydrodynamic instabilities. Mixing characterized by an axial dispersion coefficient is extremely sensitive to these flow bifurcations, which may lead to flawed modelling of the coupling between flow and mass transfer. This particular point has been studied using experimental and numerical approaches. Direct numerical simulations (DNS) of the flow have been carried out. The effective diffusion coefficient was estimated using particles tracking in the different Taylor-Couette regimes. Simulation results have been compared with literature data and also with our own experimental results. The experimental study first consists in visualizing the vortices with a small amount of particles (Kalliroscope) added to the fluid. Tracer residence time distribution (RTD) is used to determine dispersion coefficients. Both numerical and experimental results show a significant effect of the flow structure on the axial dispersion

El conocimiento especializado del profesor de matemáticas detectado en la resolución del problema de las cuerdas

En este documento mostramos un análisis sobre el conocimiento que evidencia un profesor de matemáticas de secundaria al resolver el problema de las cuerdas (se colocan n puntos sobre una circunferencia, ¿es posible determinar el número de todas las cuerdas que pueden trazarse?), usando el modelo analítico de conocimiento profesional mathematics teacher’s specialised knowledge (MTSK). Los resultados muestran la potencialidad del modelo como herramienta de análisis para profundizar en la comprensión y caracterización del conocimiento del profesor de matemáticas, en particular del conocimiento de los temas

Una perspectiva del conocimiento matemático para la enseñanza del profesor centrada en su especialización: El modelo MTSK

Esta comunicación toma como eje de discusión la especificidad del conocimiento del profesor de matemáticas plasmada en modelos analíticos. Fruto de esta reflexión y de la búsqueda de instrumentos teóricos que nos permitieran analizar dicho conocimiento, elaboramos un modelo al que denominamos Mathematics Teacher’s Specialised Knowledge (MTSK) que considera la especialización en el conocimiento del profesor como característica definitoria. Ayudados de ejemplos, discutiremos cómo visualizamos esta especificidad y presentaremos brevemente el MTSK como un resultado del trabajo teórico y empírico que ha llevado a cabo el grupo de investigación de didáctica de la matemática en la Universidad de Huelva, España