The influence of surface roughness on the rheology of immersed and dry frictional spheres

Pressure-imposed rheometry is used to examine the influence of surface roughness on the rheology of immersed and dry frictional spheres in the dense regime. The quasi-static value of the effective friction coefficient is not significantly affected by particle roughness while the critical volume fraction at jamming decreases with increasing roughness. These values are found to be similar in immersed and dry conditions. Rescaling the volume fraction by the maximum volume fraction leads to collapses of rheological data on master curves. The asymptotic behaviors are examined close to the jamming transition

Non-Poisson statistics of settling spheres

International audienceDirect tracking of the particle positions in a sedimenting suspension indicates that the particles are not simply randomly distributed. The initial mixing of the suspension leads to a microstructure which consists of regions devoid of particles surrounded by regions where particles have an excess of close neighbors and which is maintained during sedimentation

Pinch-off of a viscous suspension thread

International audienceThe pinch-off of a capillary thread is studied at large Ohnesorge number for non-Brownian, neutrally buoyant, mono-disperse, rigid, spherical particles suspended in a Newtonian liquid with viscosity η 0 and surface tension σ. Reproducible pinch-off dynamics is obtained by letting a drop coalesce with a bath. The bridge shape and time evolution of the neck diameter, h min , are studied for varied particle size d, volume fraction φ and liquid contact angle θ. Two successive regimes are identified: (i) a first effective-viscous-fluid regime which only depends upon φ and (ii) a subsequent discrete regime, depending both on d and φ, in which the thinning localises at the neck and accelerates continuously. In the first regime, the suspension behaves as an effective viscous fluid and the dynamics is solely characterised by the effective viscosity of the suspension, η e ∼ −σ / ˙ h min , which agrees closely with the steady shear viscosity measured in a conventional rheometer and diverges as (φ c − φ) −2 at the same critical particle volume fraction, φ c. For φ 35 %, the thinning rate is found to increase by a factor of order one when the flow becomes purely extensional, suggesting non-Newtonian effects. The discrete regime is observed from a transition neck diameter, h min ≡ h * ∼ d (φ c − φ) −1/3 , down to h min ≈ d, where the thinning rate recovers the value obtained for the pure interstitial fluid, σ /η 0 , and lasts t * ∼ η e h * /σ

Charriage de particules dans un écoulement cisaillé

Colloque avec actes et comité de lecture. Internationale.National audienceUn lit de particules soumis à un écoulement de fluide, par exemple le lit d'une rivière, se met en mouvement quand les forces hydrodynamiques deviennent supérieures à une fraction du poids apparent des particules. Nous étudions expérimentalement le transport de particules dans un tube à section rectangulaire. Nous comparons les résultats aux prédictions d'un modèle continu à deux phases, dans lequel nous utilisons une rhéologie granulaire pour la contrainte solide

Spreading fronts and fluctuations in sedimentation

International audienceA diffuse interface or ''front'' at the top of the suspension is investigated experimentally and numerically. The width of the front is found to grow linearly in time, mainly due to a polydispersity of particle size in the very dilute experiments, and due only to fluctuations in particle density in the simulations. Away from the front, the fluctuations in the particle velocities are found not to decay

Migration induite par cisaillement dans les suspensions

Les suspensions de particules non-colloïdales plongées dans des fluides newtoniens opposent une résistance visqueuse à un cisaillement. Par ailleurs, ces matériaux développent des différences contraintes normales non nulles [1], proportionnelles à la contrainte cisaillement. Celles-ci ont pour origine une microstructure anisotrope due à la formation de contacts directs durant leurs écoulements. Une conséquence importante de ce comportement non-newtonien est le développement progressif d’hétérogénéités de concentration sous cisaillement. Ce phénomène de migration semble pouvoir être modélisé grâce à une description diphasique des suspensions [2] où les changements de densité sont essentiellement induits par les gradients des contraintes normales exercées sur la phase particulaire. Nous présentons ici une étude expérimentale détaillée de la migration sous cisaillement en géométrie de Couette, dans l’objectif de tester cette description diphasique et d’évaluer les contraintes particulaires. La suspension étudiée est celle de Boyer et al.[1], dont la loi de comportement (viscosité et contraintes normales) est donc connue a priori. Nous couvrons une large gamme de concentrations, depuis le régime dilué jusqu’au régime très concentré, à proximité de la fraction volumique de divergence de la viscosité. L’étude est effectuée dans une géométrie à cylindres coaxiaux insérée dans in Imageur à Résonance Magnétique qui donne accès aux champs de vitesse et de concentration dans le matériau [3]. La suspension est initialement homogène ; nous mesurons l’évolution temporelle des champs de vitesse et de concentration lorsque qu’une vitesse de rotation constante est imposée au cylindre interne. Nous montrons tout d’abord que la viscosité stationnaire du matériau inhomogène est en accord avec [1]. L’amplitude des inhomogénéités est en revanche plus faible et la cinétique plus rapide que ce qui se déduit des modèles standards basés sur contraintes normales totales. Nous proposons une expression des contraintes particulaires qui permet de rendre compte de nos observations. [1] F. Boyer, O. Pouliquen, E. Guazzelli, J Fluid Mech 686, 5-25 (2011); Phys. Rev. Lett. 107, 188301 (2011) [2] P. Nott, E. Guazzelli, O. Pouliquen, Phys Fluids 23, 043304 (2011) [3] G. Ovarlez, F. Bertrand, S. Rodts, J Rheol 50, 259-292 (2006

On stratification control of the velocity fluctuations in sedimentation

International audienceWe have tested whether stratification can govern local velocity fluctuations in suspensions of sedimenting spheres. Comparison of the proposed scaling for local control of fluctuations by stratification to experimental data demonstrates that this mechanism cannot account for the reduction of the observed velocity fluctuations

Bedload Transport. Part 1 : Two-Phase Model and 3D Numerical Implementation.

GENCI- IDRIS (Grant 2010- 96212)International audienceWe have developed a three dimensional numerical model (Chauchat and Médale, 2010) based on the two-phase modeling having a Newtonian rheology for the fluid phase and Coulombtype friction for the particulate phase which has been developed by Ouriemi et al. (2009a) to study bedload transport in pipe flows. The governing equations are discretized by a finite element scheme and a penalisation method is introduced to cope with the incompressibility constraint. A regularisation technique is used to deal with the visco-plastic behaviour of the granular phase. We have performed three-dimensional computations for bedload transport in rectangular cross-section duct when the bed interface remains fixed. This numerical model which captures the complex coupling between the granular media and the fluid should enable a better understanding of the sediment transport mechanisms in these duct flows (Ouriemi et al., 2009b). The geometry corresponds to the one used in Pailha et al. (2011) where the authors investigate experimentally the behaviour of the flowing granular layer. The presented model and numerical results will be used for a detailed comparison in a near future

Investigation of the mobile granular layer in bedload transport by laminar shearing flows

International audienceThe mobile layer of a granular bed composed of spherical particles is experimentally investigated in a laminar rectangular channel flow. Both particle and fluid velocity profiles are obtained using particle image velocimetry for different index-matched combinations of particles and fluid and for a wide range of fluid flow rates above incipient motion. A full three-dimensional investigation of the flow field inside the mobile layer is also provided. These experimental observations are compared to the predictions of a two-phase continuum model having a frictional rheology to describe particle-particle interactions. Different rheological constitutive laws having increasing degrees of sophistication are tested and discussed

Fluctuations and stratification in sedimentation of dilute suspensions of spheres

International audienceWe have tested in experiments and simulations whether stratification can control velocity fluctuations in suspensions of sedimenting spheres. The initial value and early decay of the velocity fluctuations are not affected by stratification. On the other hand, in the descending front where the stratification is strong and well defined, the velocity fluctuations are inhibited according to a previously proposed scaling. In between, after the initial decay and before the arrival of the front, the local value of the stratification does not always play a role