Flow of wet granular materials: a numerical study

We simulate dense assemblies of frictional spherical grains in steady shear flow under controlled normal stress $P$ in the presence of a small amount of an interstitial liquid, which gives rise to capillary menisci, assumed isolated (pendular regime), and to attractive forces. The system behavior depends on two dimensionless control parameters: inertial number $I$ and reduced pressure $P^*=aP/(\pi\Gamma)$, comparing confining forces $\sim a^2P$ to meniscus tensile strength $F_0=\pi\Gamma a$, for grains of diameter $a$ joined by menisci with surface tension $\Gamma$. We pay special attention to the quasi-static limit of slow flow and observe systematic, enduring strain localization in some of the cohesion-dominated ($P^*\sim 0.1$) systems. Homogeneous steady flows are characterized by the dependence of internal friction coefficient $\mu^*$ and solid fraction $\Phi$ on $I$ and $P^*$. We record fairly small but not negligible normal stress differences and the moderate sensitivity of the system to saturation within the pendular regime. Capillary forces have a significant effect on the macroscopic behavior of the system, up to $P^*$ values of several units. The concept of effective pressure may be used to predict an order of magnitude for the strong increase of $\mu^*$ as $P^*$ decreases but such a crude approach is unable to account for the complex structural changes induced by capillary cohesion. Likewise, the Mohr-Coulomb criterion for pressure-dependent critical states is, at best, an approximation valid within a restricted range of pressures, with $P^*\ge 1$. At small enough $P^*$, large clusters of interacting grains form in slow flows, in which liquid bonds survive shear strains of several units. This affects the anisotropies associated to different interactions, and the shape of function $\mu^*(I)$, which departs more slowly from its quasistatic limit than in cohesionless systems.Comment: 20 pages, 29 figures with 39 subfigure

Numerical simulation of the flow of wet granular materials

We use a DEM method to simulate the shear flow of dense assemblies of 3D monosized frictional spherical grains under a fixed normal stress P in steady-state, either dry or in the presence of a small amount of an interstitial liquid, which gives rise to capillary menisci and attractive forces. Dry grain assemblies are used as a reference system for which the rheological properties - in particular the approach to the quasistatic critical state – are rather well known and can be measured with good accuracy. The system behavior is characterized by the dependence of internal friction coefficient, solid fraction, normal stress differences and internal state parameters on two dimensionless control parameters: the inertial number, I and the reduced pressure, P*, comparing confining forces to contact tensile strength. Capillary forces have a significant effect on the macroscopic behavior of the system, up to P* values of several unities, especially for longer force ranges associated with larger menisci. We relate this effect to fabric anisotropy parameters of contact and distant interactions. The sensitivity of the results to the chosen model for liquid bridge formation and volume distribution for bonding and debonding pairs has also been studied

The impact of waste management promotion program in Razi Hospital in Qazvin, Iran (2013-2018)

زمینه و اهداف : پسماند مراقبت‌های بهداشتی (15 تا 25 درصد پسماند خطرناک) دومین پسماند خطرناک در جهان است. بیمارستان‌ها و مراکز درمانی مهم ترین مراکز تولید پسماندهای بیمارستانی هستند. هدف از این مطالعه بررسی تاثیر برنامه ارتقاء مدیریت پسماند بیمارستان رازی قزوین درسال‌های1392 تا 1397 (6 سال) است. مواد و روش‌ها : این پژوهش از نوع مداخله‌ای طی 6 سال (1397- 1392)، پسماندهای بیمارستان در چهار دسته اصلی از نظر کمی آنالیز و داده‌های مربوطه گردآوری گردید. برای تحلیل داده‌ها از آمار توصیفی درصد و میانگین نرم افزار Excel استفاده شد. ملاحظات اخلاقی در تمام مراحل اجرای مطالعه رعایت شد. یافته‌ها : نتایج نشان داد در سال1392 (سال پایه) میزان پسماندهای خطرناک7/46 درصد و پسماندهای عادی 3/53 درصد بود. با اجرای برنامه ارتقاء مدیریت پسماند در سال 1393 و 1394 میزان پسماندهای خطرناک 35 درصد و پسماندهای عادی 65 درصد، در سال 1395میزان پسماندهای خطرناک 7/32 درصد و پسماندهای عادی 3/67 درصد، درسال 1396میزان پسماندهای خطرناک 4/33 درصد و پسماندهای عادی 6/66 درصد و درسال 1397میزان پسماندهای خطرناک 3/30 درصد و پسماندهای عادی 7/69 درصد شد. نتیجه‌گیری : با توجه به یافته‌های پژوهش حاضر و مقایسه آن با مطالعات ذکر شده، برنامه‌های ارتقاء مدیریت پسماند در بیمارستان رازی موثر بوده است. اگرچه با استاندارد WHO در زمینه میزان تولید پسماند خطرناک همچنان فاصله داشته و نیازمند تداوم برنامه‌ها و بهبود مستمر آن ها وجود دارد.Background: Health care centers wastes (15-25% hazardous waste) are the second most dangerous waste in the world.Hospitals and medical centers are the major sources of health-care waste generation. The aim of this study was to investigate the effect of waste management promotion program of Razi hospital in Qazvin, Iran during 2013-2018 (6 years). Materials and methods: This interventional study was conducted during 6 years (2013-2018), in which hospital wastes were collected in four main categories quantitatively and the relevant data were collected. Descriptive statistics of percentage and mean were conducted for data analysis using Excel software. Ethical considerations were observed at all stages of the study. Results: The results showed that the rate of general and hazardous waste generation were 53.3% and 46.7%, respectively. With the implementation of waste management promotion program in 2014 and 2015, hazardous and ordinary wastes percentage reached to 35% and 65%. Similarly, the contribution of hazardous wastes fell down to 32.7% versus to 67.3% of general waste in 2016. Likewise, hazardous wastes were 33.4% and normal wastes were 66.6% in 2017; and in the following year (2018), the contribution of hazardous and general wastes were 30.3% and 69.7%, respectively. Conclusion:  Based on the findings of the present study and comparing them with the mentioned studies, waste management promotion programs in Razi Hospital have been effective. However, it remains far from WHO standards in the field of hazardous waste generation and necessitates continuation of programs and continuous improvement of them

Rhéophysique des matériaux granulaires en présence d'un fluide interstitiel : simulations numériques

We numerically simulate the shear flow of dense assemblies of 3D frictional spherical grains under a fixed normal stress P in steady-state, either in the presence of a small amount of an interstitial liquid, which gives rise to capillary menisci and attractive forces, or in the fully saturated state, when the mechanical properties of suspensions in Stokes flow are controlled by hydrodynamic and contact forces. Dry grain assemblies are used as a reference system for which the rheological properties - in particular the approach to the critical state – are rather well known and can be measured with good accuracy. A non-saturating wetting fluid creates capillary attractive intergranular forces, the effects of which on the rheology are investigated in the pendular state, with shear rates ranging from quasistatic to inertial regimes. The system behavior is characterized by the dependence of internal friction coefficient, solid fraction, normal stress differences and internal state parameters on two dimensionless control parameters: the inertial number, I and the reduced pressure, P*, comparing confining forces to contact tensile strength. We focus on steady homogeneous flows, excluding localized flow patterns which we observe to occur for low P* (of order 0.1). The apparent internal friction coefficient increases to 0.9 in the quasistatic limit for P*=0.4, from its dry value 0.35, while solid fraction decreases from 0.59 to 0.52. We relate the significant effect of capillary forces on the macroscopic behavior of the system, up to P* values of several unities, to fabric anisotropy parameters of contact and distant interactions. As P* decreases, many cohesive contacts are observed to survive the tumbling motion associated to the shear flow, and their average age exceeds the reciprocal shear rate. Corresponding clusters of grains with enduring capillary bonds gather a large proportion of grains and percolate through the sample. The results are shown to be moderately sensitive to saturation within the pendular range, yet rather strongly affected by the hysteretic nature of liquid bridges. In the presence of viscous forces, assuming lubrication effects to dominate the hydrodynamic interactions, we adopt a simplified version of the (overdamped) Stokesian dynamics approach, in which hydrodynamic interactions only couple close neighbours. Rheological properties are strongly influenced by direct intergranular contacts and friction, which are permitted due to a very small distance lubrication cutoff modeling surface asperities. The same critical state as in the dry case is approached in the quasistatic limit. We discuss expressions of rheological laws involving the viscous number instead of the inertial number, and the divergence of effective viscosities in steady flow and in isotropic random suspensions as either the critical state or the random close packing solid fraction are approachedNous étudions la rhéologie d'un ensemble de grains sphériques et frottants, par la simulation numérique, à l'échelle des grains, d'écoulements de cisaillement sous une contrainte normale P contrôlée, en présence d'un liquide interstitiel. En faible teneur, ce liquide se présente sous forme de ménisques intergranulaires qui transmettent des forces capillaires attractives ; s'il sature l'espace intergranulaire, on s'intéresse alors à l'écoulement de Stokes de la suspension dense ainsi constituée, où dominent les forces visqueuses. Les assemblages de grains secs constituent un système de référence aux propriétés mécaniques bien connues, en particulier l'approche de l'état critique de la mécanique des sols dans la limite quasi-statique. L'effet des ménisques capillaires qui joignent les grains en présence d'un liquide en faible saturation (régime pendulaire) est étudié pour les taux de cisaillement allant du régime quasi statique au régime inertiel. La rhéologie est caractérisée par le frottement interne apparent, la compacité de l'assemblage, les différences de contraintes normales et diverses variables internes, fonctions de deux paramètres de contrôle adimensionnés : le nombre inertiel I et la pression réduite P*, qui compare les forces de confinement à l'adhésion dans les contacts. Notre étude concerne les états homogènes, ce qui exclut les états de cisaillement localisés observés à faible P*, de l'ordre de 0,1. Le coefficient de frottement interne augmente de 0.35 (cas sec) à 0.9 environ pour P*=0.4, tandis que la compacité décroît de 0.59 à 0.52. L'important effet des forces capillaires sur la rhéologie, sensible pour des P* de plusieurs unités, est relié à la texture anisotrope des contacts et des ponts liquides. Lorsque P* décroît, nombre de contacts cohésifs sont maintenus pour des intervalles de déformation de plusieurs unités, survivant aux effets de rotation et de cisaillement de l'écoulement, et forment des amas percolants dans le système entier. Les résultats sont modérément sensibles à la saturation dans le régime pendulaire, mais fortement affectés par l'hystérèse de la conformation des ménisques. En présence de forces visqueuses et non plus capillaires, une version simplifiée de la dynamique stokésienne est adoptée dans laquelle les forces de lubrification entre proches voisins, supposées dominantes, sont les seules interactions hydrodynamiques. La rhéologie est fortement influencée par les contacts intergranulaires directes, qu'autorise la coupure à courte distance de la singulérité de lubrification du fait de la rugosité de surface des particules. Le même état critique que celui des grains secs est approché dans la limite quasi-statique. Nous discutons de lois rhéologiques exprimées en fonction du nombre visqueux qui remplace alors le nombre inertiel, et de la divergence de la viscosité effective à l'approche de la compacité critique en écoulement permanent, ou de la compacité maximale des assemblages aléatoires pour les configurations isotropes désordonnée

Reply to "Comment on 'Flow of wet granular materials: A numerical study' "

International audienceIn his Comment on our 2015 paper[1], Bruno Chareyre criticizes, as inaccurate, the simple approach we adopted to explain the strong enhancement of the quasistatic shear strength of the material caused by capillary cohesion. He also observes that a similar form of the " effective stress " approach, accounting for the capillary shear stress, which we neglected, results in a quantitatively correct prediction of this yield stress. We agree with these remarks, which we deem quite relevant and valuable. We nevertheless point out that the initial approximation, despite ∼ 25% errors on shear strength in the worst cases, provides a convenient estimate of the Mohr-Coulomb cohesion of the material, which is directly related to the coordination number. We argue that the effective stress assumption, despite its surprising success in the range of states explored in [1], is bound to fail in strongly cohesion-dominated material states

Influence of shape and cohesion on rheophysics of granular materials

International audienceWe describe the results of two separate studies of the influence of shape and cohesion on therheophysics of granular materials, both based on discrete numerical simulations. The fist onecompares dense flows of dry frictional spheres and polyhedra down a rough inclined plane.The flow diagrams and constitutive laws previously established for spheres are extended topolyhedra : in both cases, solid fraction and effective friction variations with inertial numbercan be described as linear over a certain range. In the case of polyhedra, there is a significantincrease of the effective friction, while the wall roughness tends to jam the flow in a bottomlayer, a few diameters thick. The second study investigates steady - state, pressure - controlledsimple shear flows of frictional cohesive disks or spheres. Interparticle cohesion may resultfrom van der Waals or capillary forces, and introduces a dimensionless cohesion numbercomparing confining forces to contact tensile strength. It is shown that the rheophysics is welldescribed by simple dependencies of solid fraction and effective friction on both inertial andcohesion numbers. Moreover, both studies provide microstructural insight on the origin of theconstitutive laws : role of face-to-face contacts in polyhedral assemblies, characterisation ofdensity inhomogeneities and sensitivity to attractive force range in cohesive flow

Basic Mechanical Properties of Wet Granular Materials: A DEM Study

International audienceNumerical simulations, by the discrete element method (DEM), of a model granular assembly, made of spherical balls, are used to investigate the influence of a small amount of an interstitial wetting liquid, forming capillary bridges between adjacent particles, on two basic aspects of granular material rheology: (1) the plastic response in isotropic compression, and (2) the critical state under monotonic shear strain, and its generalization to steady, inertial flow. Tensile strength F0=πΓa, in contacts between beads of diameter aa joined by a small meniscus of a liquid with surface tension Γ, introduces a new force scale and a new dimensionless control parameter, P∗=a2P/F0, for grains of diameter a under confining stress P. Under low P*, as cohesion dominates, capillary cohesion may stabilize very loose structures. Upon increasing pressure P in isotropic compression, such structures gradually collapse. The resulting irreversible compaction is well described by the classical linear relation between logP* and void ratio in some range, until a dense structure forms that retains its stability without cohesion as confinement dominates for large P*. In steady shear flow, with uniform velocity gradient γ˙=∂v1/∂x2γ˙=∂v1/∂x2 under normal stress P=σ22, the apparent internal friction coefficient, which is defined as μ∗=σ12/σ22, depends on P* and inertial number (reduced shear rate) I=γ˙√m/aP, and so does solid fraction Φ. The material exhibits, as P* decreases, a strongly enhanced resistance to shear (larger μ*). In the quasistatic limit, for I→0, it is roughly predicted by a simple effective pressure assumption by which the capillary forces are deemed equivalent to an isotropic pressure increase applied to the dry material as long as P*≥1, while the yield criterion approximately assumes the Mohr-Coulomb form. At lower P*, such models tend to break down as liquid bonding, causing connected clusters to survive over significant strain intervals, strongly influences the microstructure. Systematic shear banding is observed at very small P