Internal stresses and breakup of rigid isostatic aggregates in homogeneous and isotropic turbulence

By characterising the hydrodynamic stresses generated by statistically homogeneous and isotropic turbulence in rigid aggregates, we estimate theoretically the rate of turbulent breakup of colloidal aggregates and the size distribution of the formed fragments. The adopted method combines Direct Numerical Simulation of the turbulent field with a Discrete Element Method based on Stokesian dynamics. In this way, not only the mechanics of the aggregate is modelled in detail, but the internal stresses are evaluated while the aggregate is moving in the turbulent flow. We examine doublets and cluster-cluster isostatic aggregates, where the failure of a single contact leads to the rupture of the aggregate and breakup occurs when the tensile force at a contact exceeds the cohesive strength of the bond. Due to the different role of the internal stresses, the functional relationship between breakup frequency and turbulence dissipation rate is very different in the two cases. In the limit of very small and very large values, the frequency of breakup scales exponentially with the turbulence dissipation rate for doublets, while it follows a power law for cluster-cluster aggregates. For the case of large isostatic aggregates it is confirmed that the proper scaling length for maximum stress and breakup is the radius of gyration. The cumulative fragment distribution function is nearly independent of the mean turbulence dissipation rate and can be approximated by the sum of a small erosive component and a term that is quadratic with respect to fragment size.Comment: 31 pages, 19 figure

Simulation of coalescence, break up and mass transfer in bubble columns by using the Conditional Quadrature Method of Moments in OpenFOAM

The evaluation of the mass transfer rates and the fluid-dynamics aspects of bubble columns are strongly affected by the intrinsic poly-dispersity of the gas phase, namely the different dispersed bubbles are usually distributed over a certain range of size and chemical composition values. In our previous work, gas-liquid systems were investigated by coupling Computational Fluid Dynamics with mono-variate population balance models (PBM) solved by using the quadrature method of moments (QMOM). Since mass transfer rates depend not only on bubble size, but also on bubble composition, the problem was subsequently extended to the solution of multi-variate PBM (Buffo et al. 2013). In this work, the conditional quadrature method of moments (CQMOM) is implemented in the open-source code OpenFOAM for describing bubble coalescence, breakage and mass transfer of a realistic partially aerated rectangular bubble column, experimentally investigated by Diaz et al.(2008). Eventually, the obtained results are here compared with the experimental data availabl

Dynamics of a shear-induced aggregation process by a combined Monte Carlo-Stokesian Dynamics approach

In the present work we investigated the collision efficiency of colloidal aggregates suspended in a shear flow. A Discrete Element Method (DEM), built in the framework of Stokesian Dynamics, was developed to model hydrodynamic and colloidal interactions acting on each primary particle composing the aggregates. Aggregates with complex geometries were generated by means of a combined DEM-Monte Carlo algorithm able to reproduce a shear-induced aggregation process occurring in a dilute colloidal suspension. Simulations, involving pairs of aggregates, were conducted according to a grid-based technique, in order to evaluate collision efficiencies. Size disproportion between aggregates and morphology shape anisotropy emerged as the principal causes affecting collision efficiencies. This work constitutes a first attempt to extend the traditional Von Smoluchowski’s theory of shear-induced coagulation of spherical particles to the case of randomly-structured aggregates

Dynamics of shear induced aggregation through a combined Monte Carlo-Stokesian dynamics approach

Several methods have been proposed to investigate the dynamics of processes including aggregation and breakup of colloidal particles. Most approaches resort to Population Balance Equations, often solved in a stochastic way (Monte Carlo methods). This method has a relatively low computational cost, but is not completely predictive, in that it needs models for the rates of aggregation and breakup and the morphology of the aggregates. On the contrary, highly accurate and fully predictive description of single aggregation or breakup events can be obtained by Discrete Element Methods (DEMs), where the motion of each primary particle of an aggregate is tracked by solving its equation of motion. However, so far the high computational cost of DEMs has restricted their use to the simulation of short sequences of events, thus preventing their application to representative samples of a population of particles. The present work aims to investigate the mechanism of flow-induced coagulation of a large population of particles suspended in an aqueous medium and subject to uniform shear flow. The developed method combines a Monte Carlo approach to determine, on the basis of probabilistic considerations, the sequence of aggregation and breakup events and the clusters involved, and a Discrete Element Method, built in the framework of Stokesian Dynamics, to accurately reproduce the event; the DEM model is able, in fact, to evaluate the fluid-dynamic stresses acting on each monomer and to model properly the inter-particle interactions: besides Van der Waals attraction and contact forces between monomers, an elastic spring-like model, to give tangential and torsional resistance to the bonded monomers, has been implemented; this model has been proven to reproduce accurately the resistance to relative motion that monomers exhibit at the contact area. Simulations were performed to predict the dynamic behavior of the suspension with particular regard to the determination of size distribution, morphologies of aggregates and their temporal evolution, as determined by the simultaneous aggregation and breakage processes. Results highlight that collision efficiencies, breakage phenomena and, consequently, size distributions are significantly influenced by the flow field intensity. Results are encouraging and demonstrate that a Monte Carlo approach combined with a DEM model can be a powerful tool for the study of the dynamics of a colloidal particle population

ON THE IMPLEMENTATION OF MOMENT TRANSPORT EQUATIONS IN OPENFOAM TO PRESERVE CONSERVATION, BOUNDEDNESS AND REALIZABILITY

Different industrial scale multiphase systems can be successfully described by considering their polydispersity (e.g. particle/droplet/bubble size and velocity distributions) and phase coupling issues are properly overcome only by considering the evolution in space and time of such distributions, dictated by the so-called Generalized Population Balance Equation (GPBE). A computationally efficient approach for solving the GPBE is represented by the quadrature-based moment methods, where the evolution of the entire particle/droplet/bubble population is recovered by tracking some specific moments of the distribution and the quadrature approximation is used to solve the "closure problem" typical of moment-based methods. In this contribution some crucial computational and numerical details concerning the implementation of these methods into the opensource Computational Fluid Dynamics (CFD) code OpenFOAM are discussed. These aspects are in fact very often overlooked, resulting in implementations that do not satisfy the properties of conservation, realizability and boundedness. These constraints have to be satisfied in a consistent way, with respect to what done with the other conserved transported variables (e.g. volume fraction of the disperse phase) also when higher-order discretization schemes are used. These issues are illustrated on examples taken on our work on the simulation of fluid-fluid multiphase system

Les figures de rhétorique dans les interactions académiques en français

Questo studio é fondato sull’ipotesi secondo la quale la figura non è uno scarto rispetto al linguaggio ordinario, ma una configurazione che gli appartiene fondamentalmente e che si ritrova dunque nel discorso scientifico o a pretesa scientifica, come nel caso del discorso accademico. Si è verificata, in un corpus di interazioni universitarie trascritte, la presenza e il funzionamento di figure retoriche appartenenti a quattro classi: le figure di parola, che sfruttano la materia sonora della lingua (ritmo, suono), sono formate dal significante di una o più parole, da gruppi sillabici, e hanno uno stretto legame con i luoghi morfologici; le figure di senso o tropi, che la retorica antica chiama ‹figurae verborum›, hanno il ruolo di cancellare la differenza fra due concetti e affermare un’identità fittizia; le figure di costruzione interessano la sintassi e la costruzione del discorso, sono prodotte dall’alterazione della morfosintassi e talune servono a strutturare passaggi del discorso; le figure di pensiero o ‹figurae sententiarum› non dipendono dal suono, dal senso e dall’ordine delle parole, ma riguardano la relazione fra le idee

SIMULATION OF A REACTIVE GAS-LIQUID SYSTEM WITH QUADRATURE-BASED MOMENTS METHOD

The description of the interaction between fluid dynamics and fast chemical reactions in gas-liquid systems is complicated by the fact that the gas phase is poly-dispersed, namely it is constituted by bubbles characterized by a distribution of velocity, size and composition values. Phase coupling can be successfully described only if the modeling approach acknowledges the existence of this distribution, whose evolution in space and time is governed by the so-called Generalized Population Balance Equation (GPBE). A computationally efficient approach for solving the GPBE is represented by the Quadrature-Based Moment Methods (QBMM), where the evolution of the entire bubble population is recovered by tracking some specific moments of the distribution. In the present work, one of these methods, the Conditional Quadrature Method of Moments (CQMOM) has been implemented in the OpenFOAM two-fluid solver compressibleTwoPhaseEulerFoam , to simulate a chemically reacting gas-liquid system. To reduce the computational time and increase stability, a second-order operator-splitting technique for the solution of the chemically reacting species was also implemented, allowing to solve the different processes involved with their own time-scale. This modeling approach is here validated by comparing predictions with experiments, for the chemical absorption of CO 2 in NaOH solution, performed in a rectangular bubble column

Analysis of the Catecholaminergic Phenotype in Human SH-SY5Y and BE(2)-M17 Neuroblastoma Cell Lines upon Differentiation

Human cell lines are often used to investigate cellular pathways relevant for physiological or pathological processes or to evaluate cell toxicity or protection induced by different compounds, including potential drugs. In this study, we analyzed and compared the differentiating activities of three agents (retinoic acid, staurosporine and 12-O-tetradecanoylphorbol-13-acetate) on the human neuroblastoma SH-SY5Y and BE(2)-M17 cell lines; the first cell line is largely used in the field of neuroscience, while the second is still poorly characterized. After evaluating their effects in terms of cell proliferation and morphology, we investigated their catecholaminergic properties by assessing the expression profiles of the major genes involved in catecholamine synthesis and storage and the cellular concentrations of the neurotransmitters dopamine and noradrenaline. Our results demonstrate that the two cell lines possess similar abilities to differentiate and acquire a neuron-like morphology. The most evident effects in SH-SY5Y cells were observed in the presence of staurosporine, while in BE(2)-M17 cells, retinoic acid induced the strongest effects. Undifferentiated SH-SY5Y and BE(2)-M17 cells are characterized by the production of both NA and DA, but their levels are considerably higher in BE(2)-M17 cells. Moreover, the NAergic phenotype appears to be more pronounced in SH-SY5Y cells, while BE(2)-M17 cells have a more prominent DAergic phenotype. Finally, the catecholamine concentration strongly increases upon differentiation induced by staurosporine in both cell lines. In conclusion, in this work the catecholaminergic phenotype of the human BE(2)-M17 cell line upon differentiation was characterized for the first time. Our data suggest that SH-SY5Y and BE(2)-M17 represent two alternative cell models for the neuroscience field

From text saliency to linguistic objects: learning linguistic interpretable markers with a multi-channels convolutional architecture

A lot of effort is currently made to provide methods to analyze and understand deep neural network impressive performances for tasks such as image or text classification. These methods are mainly based on visualizing the important input features taken into account by the network to build a decision. However these techniques, let us cite LIME, SHAP, Grad-CAM, or TDS, require extra effort to interpret the visualization with respect to expert knowledge. In this paper, we propose a novel approach to inspect the hidden layers of a fitted CNN in order to extract interpretable linguistic objects from texts exploiting classification process. In particular, we detail a weighted extension of the Text Deconvolution Saliency (wTDS) measure which can be used to highlight the relevant features used by the CNN to perform the classification task. We empirically demonstrate the efficiency of our approach on corpora from two different languages: English and French. On all datasets, wTDS automatically encodes complex linguistic objects based on co-occurrences and possibly on grammatical and syntax analysis.Comment: 7 pages, 22 figure