Dynamics of laminar pressure-driven channel flows laden with neutrally buoyant finite-size particles.

Since the pioneering work of Reynolds (1883), much effort has been allocated on the topic of laminar-turbulent transition regime in a single-phase flow, with special focusing on the unstable and intermittent natures of this regime (Mullin, 2011). The transition regime of dispersed flows carried less attention even though dispersed flows are used in many industrial processes. As for suspensions of neutrally buoyant particles, Matas et al. (2003) observed changes in the values of the critical Reynolds numbers depending on both the solid volume fraction and the particle-to-pipe sizeratio. Typically, the transition occurs at lower Reynolds numbers when the flow carries macro-sized particles at dilute to moderate concentrations (up to 25%). On the contrary, the critical Reynolds numbers of the onset of transition is shifted towards greater values when particles are micro-sized and their concentration is higher. In this work, we aim at understanding the mechanisms lying behind the shift of the laminar-turbulent transition regime down to lower critical Reynolds numbers in suspension flows of macro-sized particles. Fully-coupled numerical simulations are used to investigate the interactions between neutrally-buoyant finite-size particles and a transitional channel flow. To our knowledge, other than the simulations of Shao et al. (2012) and Garcia-Villalba et al. (2012) performed in turbulent channel flows, there are no direct numerical simulations performed on fluctuating suspension flows in channels or pipes with finite-size particles. The numerical method chosen for this work is the Force-Coupling Method (FCM) (Maxey and Patel, 2001, Lomholt and Maxey 2003). It is fully-resolved in the sense that the fluid equations are solved at a length-scale smaller than the particle radius. In a first step, the laminarization process of a single-phase flow initially turbulent at Re=6000 is statistically characterized (Re is based on the average flow velocity, the channel height and the kinematic viscosity). In a second step, particles are randomly added to the fluctuating channel flow at a solid volume fraction of 5%, the size ratio of particle diameter to channel height being 1/16. The starting point of the calculation of the suspension flow is a snapshot taken from the single-phase flow case at Re=1625 (the smallest Reynolds number at which the flow does not relaminarize)

Inertia-driven particle migration and mixing in a wall-bounded laminar suspension flow

Laminar pressure-driven suspensionflows are studied in the situation of neutrally buoyant particles at finite Reynolds number. The numerical method is validated for homogeneous particle distribution (no lateral migration across the channel): the increase of particle slip velocities and particle stress with inertia and concentration is in agreement with former works in the literature. In the case of a two-phase channel flow with freely moving particles, migration towards the channel walls due to the Segré-Silberberg effect is observed, leading to the development of a non-uniform concentration profile in the wall-normal direction (the concentration peaks in the wall region and tends towards zero in the channel core). The particle accumulation in the region of highest shear favors the shear-induced particle interactions and agitation, the profile of which appears to be correlated to the concentration profile. A 1D model predicting particle agitation, based on the kinetic theory of granular flows in the quenched state regime when Stokes number St = O(1) and from numerical simulations when St < 1, fails to reproduce the agitation profile in the wall normal direction. Instead, the existence of secondary flows is clearly evidenced by long time simulations. These are composed of a succession of contra-rotating structures, correlated with the development of concentration waves in the transverse direction. The mechanism proposed to explain the onset of this transverse instability is based on the development of a lift force induced by spanwise gradient of the axial velocity fluctuations. The establishment of the concentration profile in the wall-normal direction therefore results from the combination of the mean flow Segré-Silberberg induced migration, which tends to stratify the suspension and secondary flows which tend to mix the particles over the channel cross section

The effect of neutrally buoyant finite-size particles on channel flows in the laminar-turbulent transition regime

The presence of finite-size particles in a channel flow close to the laminar-turbulent transition is simulated with the Force Coupling Method which allows two-way coupling with the flow dynamics. Spherical particles with channel height-to-particle diameter ratio of 16 are initially randomly seeded in a fluctuating flow above the critical Reynolds number corresponding to single phase flow relaminarization. When steady-state is reached, the particle volume fraction is homogeneously distributed in the channel cross-section (φ=5%) except in the near-wall region where it is larger due to inertia-driven migration. Turbulence statistics (intensity of velocity fluctuations, small-scale vortical structures, wall shear stress) calculated in the fully coupled two-phase flow simulations are compared to single-phase flow data in the transition regime. It is observed that particles increase the transverse r.m.s. flow velocity fluctuations and they break down the flow coherent structures into smaller, more numerous and sustained eddies, preventing the flow to relaminarize at the single-phase critical Reynolds number. When the Reynolds number is further decreased and the suspension flow becomes laminar, the wall friction coefficient recovers the evolution of the laminar single-phase law provided that the suspension viscosity is used in the Reynolds number definition. The residual velocity fluctuations in the suspension correspond to a regime of particulate shear-induced agitatio

Laminar-turbulent transition of channel flows: the effect of neutrally buoyant finite-size particles

Numerical simulations were performed on channel flows laden with resolved finite-size neutrally buoyant particles at moderate volumetric concentration. In the case of fluctuating flows close to laminar-turbulent transition, the particle volume fraction is homogeneously distributed in the channel except an accumulation layer in the near-wall region (particle migration is driven by inertia). Particles increase the level of perturbations close to the wall leading to significant enhancement of both the velocity fluctuations and the wall friction coefficient. Additionally, particles break down the large-scale flow structures into smaller, more numerous and sustained eddies. When the flow Reynolds number is decreased, flow relaminarization occurs at critical Reynolds number RecS (based on the effective suspension viscosity) significantly below the critical Reynolds number Rec of single-phase flow transition. In the case of laminar flows, the suspension segregates into pure fluid and particle laden wall layers due to cross-stream migration. An instability is observed characterized by the formation of dune-like patterns at the separation between pure fluid and concentrated suspension. Increasing the Reynolds number yields transition to turbulence at a threshold above RecS

Transition à la turbulence des écoulements de suspension (simulations numériques et analyse physique)

Le travail de cette thèse aborde le sujet de l influence des particules non-pesantes et de taille macroscopique sur les écoulements en canal dans des conditions proches du seuil de la transition laminaire-turbulent. Les suspensions sont faiblement concentrées (fraction volumique = 5%). Le couplage hydrodynamique existant entre la phase dispersée et la phase continue est résolu numériquement par la Force-Coupling Method, et les particules sont suivies d une façon lagrangienne. Dans un écoulement laminaire de Couette ou de Poiseuille plan, nous montrons que les contraintes induites par la phase solide augmentent avec l inertie, et l influence de la concentration est plus faible qu en régime de Stokes. Les particules avancent avec un retard dans la direction de l écoulement et migrent à travers les lignes de courant (effet Segré-Silberberg en Poiseuille). Les vitesses de migration et de glissement s amplifient avec l inertie et sont du même ordre de grandeur quand Rep = O(1). Quand les particules sont lâchées librement dans un écoulement de Poiseuille plan en-deça du seuil critique de transition à la turbulence, la suspension initiale- ment homogène ( = 5%) devient stratifiée, après un temps d écoulement de plusieurs dizaines d unités de temps (rapport de la hauteur du canal sur la vitesse moyenne de l écoulement). Après une centaine d unités de temps, nous observons le développement d une instabilité à l interface entre la zone chargée en particules et la zone de fluide pur. Des motifs dunaires prennent place dans la direction de la vorticité. Ces motifs sont soutenus par des écoulements secondaires d intensités faibles mais non-nulles. Dans un écoulement au-dessus du seuil de transition, nous avons étudié les profils des phases continues et dispersées et réalisé des visualisations 3D afin de comprendre pourquoi les particules macroscopiques diminuent le nombre de Reynolds critique de relaminarisation de l écoulement. Nous observons que les particules provoquent une augmentation significative des fluctuations de vitesses dans les directions transverses et qu elles modifient les structures rotationnelles de l écoulement, qui deviennent plus petites, plus nombreuses et plus énergétiques (plus grandes vitesses de rotation). Le coefficient de frottement pariétal de l écoulement de suspension en régime de transition est supérieur à celui de l écoulement monophasique. Quand le nombre de Reynolds est diminué et que l écoulement devient finalement laminaire, le coefficient de frottement pariétal rejoint la loi laminaire d un écoulement monophasique, à condition de substituer la viscosité effective de la suspension à la viscosité du fluide dans l expression du nombre de Reynolds. D après nos résultats, la turbulence de l écoulement de suspension est conservée jusqu à des nombres de Reynolds bien inférieurs à celui de l écoulement monophasique en canal, en accord avec les observations ex- périmentales de Matas, Morris et Guazzelli (PRL, 2003) pour une géométrie cylindrique. Par ailleurs, nous montrons que selon le sens de la transition, laminaire -> turbulent ou turbulent -> laminaire, le nombre de Reynolds critique de transition d un régime à l autre n est pas le même. Nous n avons pas observé d influence significative de la concentration en ce qui concerne la valeur du nombre de Reynolds critique de relaminarisation pour les deux concentrations étudiées ( = 2.5% et 5%).This PhD addresses the influence of macroscopic and neutrally buoyant particles on the channel flows close to the laminar-turbulent transition regime. The suspension flow is moderately concentrated (solid volumetric concentration _ = 5%). The hydrodynamic coupling between the dispersed and carrier fluid is numerically resolved using the Force-Coupling Method approach. Particle trajectories are obtained by lagrangian tracking. In laminar wall-bounded flows, we show that the stress induced by the solid phase increases with inertia, and that the effect of the concentration is weaker than in the Stokes regime. The particles lag the flow and they migrate across the streamlines (Segré-Silberberg effect in Poiseuille flow). The migration and slip velocities are of the same order of magnitude for Rep = O(1). When the particles are freely suspended in a Poiseuille flow below the transition threshold, the initially homogeneous suspension (_ = 5%) becomes stratified after several ten time units (channel height/average flow velocity). After a hundred time units, the different rheological properties of the suspension segregated parts induce an instability yielding the formation of dune-like patterns, sustained by weak but finite secondary flows. In the fluctuating flow regime, we studied the profiles of the continuous and dispersed phase and realized 3D visualizations in order to understand why finite size particles delay the relaminarization threshold. The particles induce a significant increase of the velocity fluctuations in the transverse directions and they modify the rotational flow structures, which become smaller, more numerous and more energetic (larger rotation velocity). The wall-friction coefficient of the suspension flow in the transition regime is larger than the single-phase flow case. When the Reynolds number is decreased and the flow becomes laminar, the friction coefficient recovers the laminar law of a single phase flow provided that the fluid viscosity is replaced by the effective suspension viscosity in the Reynolds number definition. Our results clearly show that the two-phase channel flow turbulence is conserved down to a threshold well below the single phase flow limit, in agreement with the observations of Matas, Morris et Guazzelli (PRL, 2003) for a cylindrical geometry. In addition, we show that according to the transition direction, i.e. laminar 7! turbulent or turbulent 7! laminar, the switch from a regime to another does not occur at the same critical Reynolds number. Finally, in the limit of moderately concentrated (_ = 2.5 5%) suspension flow in a channel, the concentration has no significant influence on the critical Reynolds number.TOULOUSE-INP (315552154) / SudocSudocFranceF

Bibliothèque publique, un lieu de travail (La)

Les troubles musculo-squelettiques sont fréquents chez les personnels de bibliothèques, en raison des postures statiques prolongées, des mouvements répétés, de l\u27utilisation d\u27outils informatiques ou de tâches de manutention. Ce rapport fait état de travaux qui ont mené à l\u27élaboration d\u27un modèle d\u27intervention ergonomique pour contribuer aux projets d\u27aménagement et concevoir des situations de travail qui ne soient pas source d\u27atteintes à la santé pour le personnel. Ces travaux leur ont permis d\u27établir un lien entre les activités du travail en bibliothèque et de nombreux facteurs de risques de troubles musculo-squelettiques. Les auteurs ont documenté l\u27influence de différents choix faits pendant deux projets d\u27aménagement en bibliothèque sur l\u27apparition de telles lésions. Les résultats de leur étude ont servi de base à la rédaction de publications pratiques qui permettent aux gestionnaires et au personnel des bibliothèques, aux ergonomes et aux spécialistes municipaux en santé et en sécurité du travail d\u27effectuer des choix d\u27équipements et d\u27aménagement qui tiennent compte de la prévention des troubles musculo-squelettiques

Particle resolved direct numerical simulation of a liquid–solid fluidized bed: Comparison with experimental data

Particle-resolved direct numerical simulations of a 3-D liquid–solid fluidized bed experimentally investigated by Aguilar-Corona (2008) have been performed at different fluidization velocities (corresponding to a range of bed solid volume fraction between 0.1 and 0.4), using Implicit Tensorial Penalty Method. Particle Reynolds number and Stokes number are O(100) and O(10), respectively. In this paper, we compare the statistical quantities computed from numerical results with the experimental data obtained with 3-D trajectography and High Frequency PIV. Fluidization law predicted by the numerical simulations is in very good agreement with the experimental curve and the main features of trajectories and Lagrangian velocity signal of the particles are well reproduced by the simulations. The evolution of particle and flow velocity variances as a function of bed solid volume fraction is also well captured by the simulations. In particular, the numerical simulations predict the right level of anisotropy of the dispersed phase fluctuations and its independence of bed solid volume fraction. They also confirm the high value of the ratio between the fluid and the particle phase fluctuating kinetic energy. A quick analysis suggests that the fluid velocity fluctuations are mainly driven by fluid–particle wake interactions (pseudo-turbulence) whereas the particle velocity fluctuations derive essentially from the large scale flow motion (recirculation). Lagrangian autocorrelation function of particle fluctuating velocity exhibits large-scale oscillations, which are not observed in the corresponding experimental curves, a difference probably due to a statistical averaging effect. Evolution as a function of the bed solid volume fraction and the collision frequency based upon transverse component of particle kinetic energy correctly matches the experimental trend and is well fitted by a theoretical expression derived from Kinetic Theory of Granular Flows

Epigenetic memory in response to environmental stressors

Exposure to environmental stressors, toxicants, and nutrient deficiencies can affect DNA in several ways. Some exposures cause damage and alter the structure of DNA, but there is increasing evidence that the same or other environmental exposures, including those that occur during fetal development in utero, can cause epigenetic effects that modulate DNA function and gene expression. Some epigenetic changes to DNA that affect gene transcription are at least partially reversible (i.e., they can be enzymatically reversed after cessation of exposure to environmental agents), but some epigenetic modifications seem to persist, even for decades. To explain the effects of early life experiences (such as famine and exposures to other stressors) on the long-term persistence of specific patterns of epigenetic modifications, such as DNA methylation, we propose an analogy with immune memory. We propose that an epigenetic memory can be established and maintained in self-renewing stem cell compartments. We suggest that the observations on early life effects on adult diseases and the persistence of methylation changes in smokers support our hypothesis, for which a mechanistic basis, however, needs to be further clarified. We outline a new model based on methylation changes. Although these changes seem to be mainly adaptive, they are also implicated in the pathogenesis and onset of diseases, depending on individual genotypic background and types of subsequent exposures. Elucidating the relationships between the adaptive and maladaptive consequences of the epigenetic modifications that result from complex environmental exposures is a major challenge for current and future research in epigenetics.-Vineis, P., Chatziioannou, A., Cunliffe, V. T., Flanagan, J. M., Hanson, M., Kirsch-Volders, M., Kyrtopoulos, S. Epigenetic memory in response to environmental stressors

Contribuer à un projet d’aménagement par l’analyse ergonomique du travail : le cas d’une bibliothèque publique

L’ergonomie est de plus en plus intégrée à la conception de situations de travail, que ce soit dans le cadre de projets industriels ou de projets architecturaux. Dans ce dernier cas, en participant à la définition des espaces, les ergonomes contribuent également à influencer, en partie, l’organisation du travail future. Lors d’une intervention dans un projet de construction d’une nouvelle bibliothèque, des ergonomes chercheurs ont accompagné le déroulement du projet, aux phases concours d’architecture, études de concepts et études préliminaires. C’est en s’appuyant essentiellement sur l’analyse ergonomique du travail effectuée dans la bibliothèque existante, de même que dans des sites de référence que les ergonomes ont pu analyser les propositions architecturales et élaborer, avec un groupe de travail, des suggestions de modifications au cadre futur de travail. Cet article rend compte des activités mises en œuvre par les ergonomes au cours de leur intervention, notamment l’analyse ergonomique du travail, et de la manière dont l’intervention a pu influencer l’espace et l’organisation du travail. La trajectoire de la conception de deux des trois étages du bâtiment de la nouvelle bibliothèque illustrera ce propos et nous amènera à discuter de la portée et des limites de l’intervention ergonomique d’accompagnement et des conditions qui en rendent possible une issue positive pour le travail futur.Ergonomic considerations are increasingly being factored into work situation design, in both industrial and architectural projects. In the latter case, by participating in the definition of spaces, ergonomists also have an impact, in part, on future work organisation. Ergonomic researchers accompanied the construction of a new library in the architectural competition, design study and preliminary study phases. Basing themselves primarily on the ergonomic work analysis of the existing library and reference sites, the ergonomists analyzed the architectural proposals and then developed, with a working group, suggestions for modifying the future work framework. This article discusses the intervention activities implemented by the ergonomists during their intervention, namely ergonomic work analysis, and how the intervention was able to affect the work space and work organization. The design trajectory of two of the new library’s three floors will illustrate this and lead to a discussion of the scope and limitations of the accompanying ergonomic intervention and the conditions necessary for a positive outcome for future work.Se integra cada vez más la ergonomía a la concepción de situaciones de trabajo, que sea como parte de proyectos industriales o de proyectos arquitectónicos. En este caso, al participar a la definición de los espacios, los ergonomistas también contribuyen a influir en parte sobre la organización del trabajo venidero. Mientras intervinieron en un proyecto de construcción de una nueva biblioteca, los investigadores ergonomistas acompañaron el desarrollo del proyecto hasta las fases del concurso de arquitectura, estudios de conceptos y estudios preliminares. Los ergonimistas pudieron analizar las proposiciones arquitectónicas y elaborar, con un grupo de trabajo, sugerencias de modificaciones en el contexto futuro del trabajo, basándose esencialmente sobre el análisis ergonómico del trabajo efectuado en la biblioteca existente así como en los sitios de referencia. Este artículo informa sobre las actividades puestas en práctica por los ergonomistas a lo largo de su intervención como el análisis ergonómico del trabajo, y sobre la manera con la cual la intervención pudo influir sobre el espacio y la organización del trabajo. La trayectoria del diseño de dos de los tres pisos del edificio de la nueva biblioteca ilustrará este estudio y nos llevará a debatir el alacance y los límites de la intervención ergonómica de apoyo y las condiciones que hacen posible una solución positiva para el trabajo venidero