Electrophoretic mobility of silica particles in a mixture of toluene and ethanol at different particle concentrations

International audienceIn this paper we present measurements of the electrophoretic mobility of colloidal particles by using heterodyne detection of light scattering. The measurements have been done up to concentrations of 5.4 % of silica nanoparticles, with a diameter of the order of 80 nm, in a mixture of 70 % toluene and 30 % ethanol. In order to make possible the measurements at these concentrations the liquid mixture is chosen as to match the index of refraction of the particles, thus resulting in a transparent suspension

Numerical simulations of particles in a shear flow

In this paper, we present an immersed domain approach coupled with a viscous contact model for studying the rheological behaviour of dense suspensions in a shear flow. We here demonstrate the importance of contact modelling as well as the choice of the boundary conditions on the macroscopic properties of the suspension

Problèmes de contact pour des particules en écoulement cisaillé

National audienceSee http://hal.archives-ouvertes.fr/docs/00/59/28/28/ANNEX/r_X8K316T9.pd

Rheology of sheared suspensions of rough frictional particles

International audienceThis paper presents three-dimensional numerical simulations of non-Brownian concen- trated suspensions in a Couette flow at zero Reynolds number using a fictitious domain method. Contacts between particles are modelled using a DEM-like approach (Discrete Element Method), which allows for a more physical description, including roughness and friction. This work emphasizes the effect of friction between particles and its role on rheological properties, especially on normal stress differences. Friction is shown to no- tably increase viscosity and second normal stress difference |N2| and decrease |N1|, in better agreement with experiments. The hydrodynamic and contact contributions to the overall particle stress are particularly investigated. This shows that the effect of friction is mostly due to the additional contact stress since the hydrodynamic stress remains unaffected by friction. Simulation results are also compared with experiments, such as normal stresses or effective friction coefficient μ(Iv), and the agreement is improved when friction is accounted for. This suggests that friction is operative in actual suspensions

Rheology of non-Brownian suspensions of rough frictional particles under shear reversal: A numerical study

International audienceWe perform particle scale simulations of suspensions submitted to shear reversal. The simulations are based on the Force Coupling Method, adapted to account for short range lubrication interactions together with direct contact forces between particles, including surface roughness, contact elasticity and solid friction. After shear reversal, three consecutive steps are identified in the viscosity transient: an instantaneous variation, followed by a rapid contact force relaxation, and finally a long time evolution. The separated contributions of hydrodynamics and contact forces to the viscosity are investigated during the transient, allowing a qualitative understanding of each step. In addition, the influence of the contact law parameters (surface roughness height and friction coefficient) on the transient are evaluated. Concerning the long time transient, the difference between the steady viscosity and minimum viscosity is shown to be proportional to the contact contribution to the steady viscosity, allowing in principle easy determination of the latter in experiments. The short time evolution is studied as well. After the shear reversal, the contact forces vanish over a strain that is very short compared to the typical strain of the long time transient, allowing to define an apparent step between the viscosity before shear reversal and after contact force relaxation. This step is shown to be an increasing function of the friction coefficient between particles. Two regimes are identified as a function of the volume fraction. At low volume fraction, the step is small compared to the steady contact viscosity, in agreement with a particle pair model. As the volume fraction increases, the value of the viscosity step increases faster than the steady contact viscosity, and, depending on the friction coefficient, may approach it

Différences de contraintes normales dans les suspensions non-browniennes concentrées

Nous nous intéressons aux deux différences de contraintes normales qui apparaissent lorsqu'on cisaille une suspension de sphères non-browniennes suffisamment concentrée. Nous mesurons simultanément les contraintes normales totales dans la suspension et la pression dans le liquide suspendant afin d'évaluer les contraintes normales d'origine particulaire. Les deux différence de contraintes normales sont mesurées en utilisant deux géométries d'écoulement distinctes : deux plans parallèles rotatifs et une géométrie de poiseuille annulaire

Deep learning for classification of noisy QR codes

We wish to define the limits of a classical classification model based on deep learning when applied to abstract images, which do not represent visually identifiable objects.QR codes (Quick Response codes) fall into this category of abstract images: one bit corresponding to one encoded character, QR codes were not designed to be decoded manually. To understand the limitations of a deep learning-based model for abstract image classification, we train an image classification model on QR codes generated from information obtained when reading a health pass. We compare a classification model with a classical (deterministic) decoding method in the presence of noise. This study allows us to conclude that a model based on deep learning can be relevant for the understanding of abstract images.Comment: in French language. RFIAP 2022 - Reconnaissance des Formes, Image, Apprentissage et Perception, Jul 2022, Vannes (Bretagne), Franc

Contraintes normales particulaires dans les suspensions non-browniennes concentrées

Nous étudions l'écoulement d'une suspension concentrée de particules non-browniennes entre deux disques parallèles en rotation. Le disque inférieur est immobile et il est percé de trous recouverts de grilles qui retiennent les particules. La pression du liquide derrière les grilles, appelée pression de grille, est mesurée. Par ailleurs le disque immobile est équipé de capteurs de pression qui affleurent sa surface. Ces derniers permettent de mesurer le profil radial de contrainte normale dans la direction du gradient de vitesse. Nous montrerons que l'on peut déduire de ce profil, les deux différences de contraintes normales. Par ailleurs, en adoptant une description diphasique de la suspension, la contrainte particulaire dans la direction du gradient de vitesse pourra être obtenue en soustrayant la pression de grille (identifiée à la pression dans le liquide) à la contrainte normale totale mesurée grâce aux capteurs directs. Connaissant la contrainte normale particulaire dans la direction du gradient de vitesse et les deux différences de contraintes normales, nous serons en mesure de présenter les trois composantes de contrainte normale. La connaissance des contraintes normales particulaires présente un intérêt certain car ce sont elles qui sont censées contrôler la migration de particules sous écoulement. Nous verrons cependant que l'interprétation des mesures présentées ici est extrêmement délicate et qu'il n'est pas certain que les grandeurs que nous mesurons soient celles qui interviennent dans la migration

High Performance Pre-Computing: Prototype Application to a Coastal Flooding Decision Tool

International audienceAfter defining the High Performance Pre- Computing --referred as HPPC-- concept, the aim of the present study is to develop a prototype whether to approve or not the benefits of this concept. Our application case tries to answer the geophysical issue of coastal flooding. This is an example of an alert system based on the HPPC architecture, thus on pre-computed scenarios. The prototype provides the scientists with an ergonomic and on-demand tool allowing the run of scenarios of any implemented numerical models. These runs are available through a web application which submits the corresponding jobs on the remote french public cluster of HPC@LR. In this study we simulate the waves propagation over a Mediterranean grid using the wave model WaveWatch III⃝R . A reference simulation using usual conditions is approximated using the k-NN algorithm over 12, 98 and then 980 pre-computed scenarios. This simple experiment demonstrates how useful the pre-computing of scenarios is for alert systems as far as enough and relevant scenarios are pre-computed. This is the reason why searches continue in each critical points of the HPPC architecture such as the design of experiment, the approximation of the results by meta-models and the research of the closest scenarios in this big data context