The design and flow dynamics of non brownian suspension.

Les suspensions denses de particules non colloïdales présentent des caractéristiques originales. Dans un écoulement de cisaillement non homogène, on observe que les particules migrent des région de cisaillement élevé vers des régions de faible taux de cisaillement. Ce phénomène s'appelle migration induite par cisaillement (SIM). Le modèle de suspension (SBM) de Nott et Brady (1994) est une approche de la SIM où des flux diffusifs apparaissent naturellement à partir de gradients de contraintes de la phase particulaire. Cependant, il existe encore des débats sur la nature des contraintes particulaire dans les suspensions denses. Des expériences récentes montrent que le SBM échoue non seulement à prédire la distribution stationnaire des particules, mais aussi à rendre compte de sa cinétique. Nous avons conçu et construit une configuration originale pour revoir les modèles disponibles en effectuant des expériences bien résolues dans le temps et l'espace. Nous présentons nos résultats expérimentaux ainsi qu'une détermination préliminaires des paramètres du modèle tenant compte des inhomogénéités de fraction volumique et du taux de cisaillement.Dense suspensions of noncolloidal particles exhibit novel features. In a non-homogeneous shear flow, it is observed that particles migrate from the high shear rate region to the low shear rate region. This phenomenon is called Shear-Induced Migration (SIM). The Suspension Balance Model (SBM) of Nott and Brady (1994) has been taken as an approach to model SIM. Where the SIM is attributed to the diffusive fluxes that arise naturally from gradients in the particle phase stresses. However, there are still unanswered questions and an ongoing debate on the nature of particle stress in the dense suspensions. Recent experiments show that the SBM not only fails in predicting the steady-state distribution of particle phase in the flow of dense suspensions but also fails in predicting the kinetics of the SIM. In addition, recent theoretical works question the derivation of the SBM and the simple supposition of drag closures in inhomogeneous flows of dense suspensions. We have designed and built an original setup to revisit available drag closures via performing well-resolved experiments. We present our preliminary results of a drag closure taking into account inhomogeneities of the solid phase volume fraction and the shear rate

La dynamique de conception et de flux de la suspension non brownienne.

Dense suspensions of noncolloidal particles exhibit novel features. In a non-homogeneous shear flow, it is observed that particles migrate from the high shear rate region to the low shear rate region. This phenomenon is called Shear-Induced Migration (SIM). The Suspension Balance Model (SBM) of Nott and Brady (1994) has been taken as an approach to model SIM. Where the SIM is attributed to the diffusive fluxes that arise naturally from gradients in the particle phase stresses. However, there are still unanswered questions and an ongoing debate on the nature of particle stress in the dense suspensions. Recent experiments show that the SBM not only fails in predicting the steady-state distribution of particle phase in the flow of dense suspensions but also fails in predicting the kinetics of the SIM. In addition, recent theoretical works question the derivation of the SBM and the simple supposition of drag closures in inhomogeneous flows of dense suspensions. We have designed and built an original setup to revisit available drag closures via performing well-resolved experiments. We present our preliminary results of a drag closure taking into account inhomogeneities of the solid phase volume fraction and the shear rate.Les suspensions denses de particules non colloïdales présentent des caractéristiques originales. Dans un écoulement de cisaillement non homogène, on observe que les particules migrent des région de cisaillement élevé vers des régions de faible taux de cisaillement. Ce phénomène s'appelle migration induite par cisaillement (SIM). Le modèle de suspension (SBM) de Nott et Brady (1994) est une approche de la SIM où des flux diffusifs apparaissent naturellement à partir de gradients de contraintes de la phase particulaire. Cependant, il existe encore des débats sur la nature des contraintes particulaire dans les suspensions denses. Des expériences récentes montrent que le SBM échoue non seulement à prédire la distribution stationnaire des particules, mais aussi à rendre compte de sa cinétique. Nous avons conçu et construit une configuration originale pour revoir les modèles disponibles en effectuant des expériences bien résolues dans le temps et l'espace. Nous présentons nos résultats expérimentaux ainsi qu'une détermination préliminaires des paramètres du modèle tenant compte des inhomogénéités de fraction volumique et du taux de cisaillement

Time-resolved 2D concentration maps in flowing suspensions using X-ray

International audienceIn this paper, we introduce a new technique based on X-ray radiography with high temporal (O(0.1 s)) and spatial (O(10 μm)) resolutions to study fast suspension flows regardless of optical access. We benefit from the axial symmetry of our flow configuration, a wide gap Couette setup, to extract a 3D solid volume fraction field from a single X-ray projection image. We propose a mathematical algorithm based on the inversion of Abel transform in conjunction with H1 regularization and data denoising to measure the solid volume fraction field in suspensions in a fraction of a second. We show that the results are in excellent agreement with those obtained from micro Computed Tomography (CT scan) in one hour. This significant reduction in the data acquisition time opens a new avenue in the field of suspensions. As a proof of concept, we study the kinetics of shear-induced migration for suspensions of particles in both Newtonian and yield stress suspending fluids. The latter experiments include two different conditions: With and without a plug region. In both cases, we are able to capture in detail the kinetics of migration. In the presence of a plug region, we manage to accurately describe the particle accumulation at the interface between the sheared and the static regions. Remarkably, even in the absence of sedimentation, the concentration profiles show a complex 2D structure, with no z-invariant region, which illustrates the strong impact of top and bottom boundary effects on migration. We also show the importance of boundary effects on the shear induced migration of particles in a Newtonian suspending fluid. This further shows the necessity of developing techniques that give access to the full spatial concentration field, as the one we present here