Refractive-index and density matching in concentrated particle suspensions: a review

Optical measurement techniques such as particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are now routinely used in experimental fluid mechanics to investigate pure fluids or dilute suspensions. For highly concentrated particle suspensions, material turbidity has long been a substantial impediment to these techniques, which explains why they have been scarcely used so far. A renewed interest has emerged with the development of specific methods combining the use of iso-index suspensions and imaging techniques. This review paper gives a broad overview of recent advances in visualization techniques suited to concentrated particle suspensions. In particular, we show how classic methods such as PIV, LDV, particle tracking velocimetry, and laser induced fluorescence can be adapted to deal with concentrated particle suspension

The dam-break problem for concentrated suspensions of neutrally buoyant particles

This paper addresses the dam-break problem for particle suspensions, that is, the flow of a finite volume of suspension released suddenly down an inclined flume. We were concerned with concentrated suspensions made up of neutrally-buoyant non-colloidal particles within a Newtonian fluid. Experiments were conducted over wide ranges of slope, concentration, and mass. The major contributions of our experimental study are the simultaneous measurement of local flow properties far from the sidewalls (velocity profile and, with lower accuracy, particle concentration) and macroscopic features (front position, flow depth profile). To that end, the refractive index of the fluid was adapted to closely match that of the particles, enabling data acquisition up to particle volume fractions of 60%. Particle migration resulted in the blunting of the velocity profile, in contrast to the parabolic profile observed in homogeneous Newtonian fluids. The experimental results were compared with predictions from lubrication theory and particle migration theory. For solids fractions as large as 45%, the flow behaviour did not differ much from that of a homogeneous Newtonian fluid. More specifically, we observed that the velocity profiles were closely approximated by a parabolic form and there was little evidence of particle migration throughout the depth. For particle concentrations in the 52%–56% range, the flow depth and front position were fairly well predicted by lubrication theory, but taking a closer look at the velocity profiles revealed that particle migration had noticeable effects on the shape of the velocity profile (blunting), but had little impact on its strength, which explained why lubrication theory performed well. Particle migration theories (such as the shear-induced diffusion model) successfully captured the slow evolution of the velocity profiles. For particle concentrations in excess of 56%, the macroscopic flow features were grossly predicted by lubrication theory (to within 20% for the flow depth, 50% for the front position). The flows seemed to reach a steady state, i.e., the shape of the velocity profile showed little time dependence

Granular suspension avalanches. I. Macro-viscous behavior

We experimentally studied the flow behavior of a fixed volume of granular suspension, initially contained in a reservoir and released down an inclined flume. Here “granular suspension” refers to a suspension of non-Brownian particles in a viscous fluid. Depending on the solids fraction, density mismatch, and particle size distribution, a wealth of behaviors can be observed. Here we report and interpret results obtained with granular suspensions, which consisted of neutrally buoyant particles with a solids fraction (ϕ = 0.575–0.595) close to the maximum random packing fraction (estimated at ϕm = 0.625). The particles had the same refractive index as the fluid, which made it possible to measure the velocity profiles inside the moving bulk and far from the sidewalls. Additional information such as the front position and the flow depth was also recorded. Three regimes were observed. At early times, the flow features were reminiscent of homogeneous Newtonian fluids (e.g., the same dependence of the front position on time). At later times, the free surface became more and more bumpy as fractures developed within the bulk. This fracture process ultimately gave rise to a stick-slip regime, in which the suspension moved intermittently. In this paper, we focus on the first regime referred to as the macro-viscous regime. Although the bulk flow properties looked like those of Newtonian fluids, the internal dynamics were much richer

Granular suspension avalanches. II. Plastic regime

We present flume experiments showing plastic behavior for perfectly density-matched suspensions of non-Brownian particles within a Newtonian fluid. In contrast with most earlier experimental investigations (carried out using coaxial cylinder rheometers), we obtained our rheological information by studying thin films of suspension flowing down an inclined flume. Using particles with the same refractive index as the interstitial fluid made it possible to measure the velocity field far from the wall using a laser-optical system. At long times, a stick-slip regime occurred as soon as the fluid pressure dropped sufficiently for the particle pressure to become compressive. Our explanation was that the drop in fluid pressure combined with the surface tension caused the flow to come to rest by significantly increasing flow resistance. However, the reason why the fluid pressure diffused through the pores during the stick phases escaped our understanding of suspension rheology

Refractive-index and density matching in concentrated particle suspensions: a review

Optical measurement techniques such as particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are now routinely used in experimental fluid mechanics to investigate pure fluids or dilute suspensions. For highly concentrated particle suspensions, material turbidity has long been a substantial impediment to these techniques, which explains why they have been scarcely used so far. A renewed interest has emerged with the development of specific methods combining the use of iso-index suspensions and imaging techniques. This review paper gives a broad overview of recent advances in visualization techniques suited to concentrated particle suspensions. In particular, we show how classic methods such as PIV, LDV, particle tracking velocimetry, and laser induced fluorescence can be adapted to deal with concentrated particle suspensions

Viscoplastic dambreak waves: review of simple computational approaches and comparison with experiments

We investigated the dam-break problem for Herschel-Bulkley fluids: a fixed volume of a viscoplastic material (a polymeric gel called Carbopol ultrez 10) was released and flowed down an inclined flume. Using Particle Image Velocimetry techniques, we measured the velocity profiles far from the sidewalls, the front position as a function of time, and the flow depth evolution at a given place. The experimental data were compared to three models of increasing complexity: the kinematic wave model, an advection diffusion model (lubrication theory), and the one-layer Saint-Venant equations. Surprisingly, the best agreement was obtained with the simplest model (kinematic wave model) even though it could not capture the details of the head profile (regarded as a shock wave, i.e., a discontinuity). Lubrication theory (the advection diffusion model) performed well from a qualitative viewpoint. Computed velocity profiles and depth evolution were in reasonably good agreement with data, but this model overestimated initial acceleration, which resulted in a systematic difference between theoretical and experimental curves of the front position over time. This shortcoming was not fixed when using a more elaborate model (Saint-Venant equations), rather it was exacerbated. The relatively modest performance of the more elaborate models was intriguing (for Newtonian liquids, the best agreement was obtained with the most sophisticated model)

Asymmetry in Size-Segregation Rates.

We show experimentally and by comparison with theory that during particle size-segregation in a sheared granular flow large particles rise slower in regions of many small particles and small particles sink faster in regions of many large particles. Binary mixtures with increasing amount of small particles take longer to fully segregate and vice versa. In addition, the saturation of the bottom of the flow with small particles is faster than saturation of large particles at the top of the flow. Our results, therefore, show that the segregation rates of the large and small particles have an asymmetric dependency on the local particle concentration. This has important repercussions for the modeling of size segregation which has up till now not taken into account this effect and considered symmetric dependency of segregation rates on concentration. This discovery draws parallels between the dynamics of size segregation and the processes of traffic flow, sedimentation and particle diffusion, which also exhibit asymmetric behavior

Étude sur modele physique et numerique des evacuateurs de crue et des fosses d’erosion du barrage de Koman en Albanie

Le barrage de Koman, de 115 m de hauteur, est situé dans la partie nord de l’Albanie et fait partie des paliers hydroélectriques de la rivière Drin. L’aménagement dispose de deux évacuateurs de crue en tunnel avec des sauts de ski à leurs extrémités. Deux grandes fosses d’érosion se sont créées dans les alluvions avals. Le comportement hydraulique des deux évacuateurs et le processus d’affouillement ont été étudiés sur un modèle réduit et par modélisation numérique avec Flow-3D. L’écoulement dans les tunnels et sur les sauts de ski, ainsi que les trajectoires des jets et les zones d’impact ont pu être déterminés. L’évolution des fosses d’érosion et leurs étendues ont été étudiées. Les essais physiques ainsi que la modélisation numérique ont montré que, pour les deux évacuateurs, les sauts de ski ne peuvent guider les jets convenablement que jusqu’à 50 % de leur capacité nominale. Au-delà, les sauts de ski ne peuvent plus dévier l’écoulement conformément aux prévisions théoriques, et la longueur du jet diminue, approchant la zone d’impact du pied du barrage. Les résultats de la modélisation physique et numérique pour l’état actuel ont pu être corroborés par les observations in situ. Le modèle physique et la simulation numérique permettent de reproduire fidèlement la création d’une fosse d’érosion dans les alluvions due à l’impact du jet plongeant