Shear-induced Particle Migration Research Articles

Characteristics of slurry transport regimes: Insights from experiments and interface-resolved Direct Numerical Simulations

A common way to transport solids in large quantities is by using a carrier fluid to transport the solids as a concentrated solid/liquid mixture or slurry through a pipeline. Typical examples are found in dredging, mining and drilling applications. Dependent on the slurry properties and flow conditions, horizontal slurry pipe flow is either in the fixed-bed, sliding-bed or fully-suspended regime. In terms of non-dimensional numbers, the flow is fully characterized by the bulk liquid Reynolds number (Re), the Galileo number (Ga, a measure for the tendency of particles to settle under gravity), the solid bulk concentration (ϕb), the particle/fluid density ratio (ρp/ρf), the particle/pipe diameter ratio (Dp/Dpipe), and parameters related to direct particle interactions such as the Coulomb coefficient of sliding friction (μc). To further our fundamental understanding of the flow dynamics, we performed experiments and interface-resolved Direct Numerical Simulations (DNS) of slurry flow in a horizontal pipe. The experiments were performed in a transparent flow loop with Dpipe=4 cm. We measured the pressure drop along the pipeline, the spatial solid concentration distribution in the cross-flow plane through Electrical Resistance Tomography (ERT), and used a high-speed camera for flow visualization. The slurry consisted of polystyrene beads in water with Dp=2mm, ρp/ρf=1.02, Ga between 40–45 and ϕb between 0.26–0.33. The different flow regimes were studied by varying the flow rate, with Re varying from 3272 till 13830. The simulations were performed for the same flow parameters as in the experiments. Taking the experimental uncertainty into account, the results from the DNS and the experiments are in reasonably good agreement. The results for the pressure drop agree also fairly well with popular empirical models from literature. In addition, we performed a parametric DNS study in which we solely varied Re and Ga. In all flow regimes, a secondary flow of Prandtl’s second kind is present, ascribed to the presence of internal flow corners and a ridge of densely packed particles at the pipe bottom during transition towards the fully-suspended regime. In the bulk of the turbulent flow above the bed, secondary flow transport of streamwise momentum dominates over turbulent diffusion in regions where the secondary flow is strong and vice versa where it is weak. The transition between flow regimes appears to be governed by the competition between the net gravity force on the particles and shear-induced particle migration from particle–particle interactions. This competition can be expressed by the Shields number, θ. For θ≲0.75, gravity is dominant and the flow is in the fixed-bed regime. For θ≳0.75, shear-induced migration becomes progressively more important for increasing θ. Low-concentration zones flanking the sliding bed start to form at the top corners of the bed, and gradually expand downwards along the pipe wall till the pipe bottom is reached. For θ≳1.5, shear-induced migration is responsible for lifting the particle bed away from the wall, associated with the onset of the suspended regime. For θ≫1, gravity is of minor importance and the mean flow eventually reaches axi-symmetry with a high-concentration particle core at the pipe center and negligible secondary flow.

Four-way CFD-DEM coupling to simulate concrete pipe flow: Mechanism of formation of lubrication layer

This study introduced a four-way CFD-DEM coupling approach to simulate the shear-induced particle migration (SIPM) mechanism leading to formation of the lubrication layer (LL) during concrete pumping. The CFD-DEM simulations considered the coupled effect of concentration (10 %–40 %) and wide size distribution (1–17 mm) of aggregate and rheology of the mortar for forces between the suspending matrix and the particles (and vice versa), as well as force transmission directly between particles (and the pipe wall). The formation of the LL was successfully simulated through a more realistic understanding the SIPM mechanism and rheological evaluation across the pipe with comparable calculation times compared to the one-way coupled DEM approach, especially for high concentrations. The simulated LL thicknesses of 0.8–2.7 mm compared well with experimental values. The flow rate and rheological heterogeneity of pumped concrete, and rheology of the LL, were found mostly controlled by the granular-skeleton characteristics rather than the suspending-matrix rheology.

Shear-induced particle migration in a cement slurry under oscillatory pipe flow

Shear-induced particle migration in a cement slurry under oscillatory pipe flow

Physics-informed neural networks for understanding shear migration of particles in viscous flow

We harness the physics-informed neural network (PINN) approach to extend the utility of phenomenological models for particle migration in shear flow. Specifically, we propose to constrain the neural network training via a model for the physics of shear-induced particle migration in suspensions. Then, we train the PINN against experimental data from the literature, showing that this approach provides both better fidelity to the experiments, and a novel understanding of the relative roles of the hypothesized migration fluxes. We first verify the PINN approach for solving the inverse problem of radial particle migration in a non-Brownian suspension in an annular Couette flow. In this classical case, the PINN yields the same value (as reported in the literature) for the ratio of the two parameters of the empirical model. Next, we apply the PINN approach to analyze experiments on particle migration in both non-Brownian and Brownian suspensions in Poiseuille slot flow, for which a definitive calibration of the phenomenological migration model has been lacking. Using the PINN approach, we identify the unknown/empirical parameters in the physical model through the inverse solver capability of PINNs. Specifically, the values are significantly different from those for the Couette cell, highlighting an inconsistency in the literature that uses the latter value for Poiseuille flow. Importantly, the PINN results also show that the inferred values of the empirical model’s parameters vary with the shear Péclet number and the particle bulk volume fraction of the suspension, instead of being constant as assumed in some previous literature.

Discrete-element modeling of shear-induced particle migration during concrete pipe flow: Effect of size distribution and concentration of aggregate on formation of lubrication layer

The paper seeks to better understand the particulate mechanics giving rise to the lubrication layer (LL) in flows with wide particle-size distributions (PSD) typical of concrete pumping applications. The study uses a soft-sphere discrete element method (DEM) to simulate the shear-induced particle migration (SIPM) mechanism of formation of the LL. To provide realistic understanding of SIPM and rheological heterogeneity of concrete, three wide PSDs (fine, medium, and coarse) and three different concentrations (10 %–40 %) of five spherical-particle subclasses (1–17 mm diameter) were investigated. The radial evolution of concentration and particle distribution was simulated over time and the LL formation was successfully simulated. The predicted LL thicknesses compared well with experimental values. The coupled effect of PSD, concentration, and mean diameter of particles on wall effect, SIPM, and rheological heterogeneities across the pipe was evaluated. Higher rheological heterogeneity across the pipe was obtained for higher concentration and coarser particle size distributions.

Frame-invariant modeling for non-Brownian suspension flows

In this paper, we present a continuum modeling for three-dimensional flows of non-colloidal, non-Brownian suspensions of particles immersed in a Newtonian liquid. Such suspensions exhibit complex behaviors such as jamming, anisotropic normal stresses and shear-induced particle migration. These non-Newtonian effects arise from the solid contact forces between the particles when the suspension is sufficiently concentrated. The modeling consists of a macroscopic one-phase model close to the ”Suspension Balance Model” (Nott and Brady, 1994; Mills and Snabre, 1995; Morris and Boulay, 1999; Nott et al., 2011). The particles migration flux is governed by the divergence of the contact stresses tensor. In order to describe the flow in general 3D geometries, a frame-invariant constitutive law for the stresses in flowing suspensions is developed. It is similar to the second-order fluid modeling, which is well-known in polymer rheology, and allows for the presence of anisotropic normal stresses. The material functions are deduced from discrete simulation data from the literature. The behavior of the model in shear and extensional flows is discussed, as well as its limitations when used for a more general flow. To assess the modeling, numerical computations are performed using a finite volume method from the OpenFOAM suite. The implementation of the modeling is first validated by studying particles migration in some classical rheometric flows and then by studying the complex flow of a suspension in a tube through an abrupt expansion. • Continuum modeling of three-dimensional suspension flows is presented. • The particle migration is governed by the divergence of the contact stresses tensor. • Normal stresses modeling is inspired by the second-order fluid in polymer rheology. • The numerical implementation is performed by the finite volume method. • The model and its implementation are validated on several migration case studies.

Flow of bidisperse suspensions under the effect of standing bulk acoustic waves

Microchannel flow of dense bidispersed suspensions containing particles of two different sizes, both in high concentrations, can lead to selective enrichment of larger particles along the channel center due to shear-induced particle migration. Here, we show experimentally that subjecting such a flow to an acoustic standing wave field hastens the particle migration and segregation process, and also design a numerical model that explains the observations.

Mechanistic origins of concrete pumping: a comprehensive outlook and way forward

The construction of high-rise buildings and tunnels necessitates pumping concrete over long distances. Concrete pumping is a complex physico-chemical process that results in the formation of different layers with distinct characteristics inside the pipe. A thin cement paste/mortar layer formed at the concrete–pipe interface, called the lubrication layer (LL), is vital for facilitating concrete pumping. Engineering the pumping process involves understanding the origin and controlling the onset, magnitude and velocity of the LL. However, the formation, evolution and characteristics of the LL are still largely unexplored. This paper presents a comprehensive review of the mechanistic origins of the LL, its influencing parameters and measured data on LL thickness using various techniques to date. An attempt is made to find an overall link between different theories (shear-induced particle migration, wall depletion theory etc.) in terms of concrete rheology. The paper also provides a fundamental understanding of the different mechanisms involved in LL formation, with the help of different theories put forth by researchers in the field of particle rheology. Based on the insights derived from the discussions, recommendations are made for future research directions in concrete pumping.

Prediction of lubrication layer properties of pumped concrete based on flow induced particle migration

Flow induced particle migration facilitates the formation of lubrication layer, which dominates the pumping performance of concrete. However, the properties of lubrication layer, such as thickness, composition, and rheological properties, are dynamic during the prompting process. In this study, a numerical model is proposed to model the formation of this lubrication layer from a new perspective of particle migration with consideration of wall effects which was often ignored in the past. To predict the properties of lubrication layer, diffusive flux models (DFM) of particle migration in non-Newtonian suspensions are extended to account for the wall effects. The proposed model is validated by ultrasonic velocity profile (UVP) experiments. The results show that, the wall effect induced particle migration makes a major contribution to the formation of lubrication layer, as well as the shear induced particle migration. The lubrication layer is found to be barren of aggregate but enriches in sand. In addition, the material and rheological properties of lubrication layer are inhomogeneous and not necessarily equivalent to that of mortar.

Discrete-Element Modeling of Shear-Induced Particle Migration During Concrete Pipe Flow: Effect of Size Distribution and Concentration of Aggregate on Formation of Lubrication Layer

Discrete-Element Modeling of Shear-Induced Particle Migration During Concrete Pipe Flow: Effect of Size Distribution and Concentration of Aggregate on Formation of Lubrication Layer

Two-fluid modeling of heat transfer in flows of dense suspensions

We develop a two-fluid model (TFM) for heat transfer in dense non-Brownian suspensions. Specifically, we propose closure relations for the inter-phase heat transfer coefficient and the thermal diffusivity of the particle phase based on calibration against experimental data. The model is then employed to simulate non-isothermal flow in an annular Couette cell. We find that, when the shear rate is controlled by the rotation of the inner cylinder, both the shear and thermal gradients are responsible for particle migration. Within the TFM framework, we identify the origin and functional form of a “thermo-rheological” migration force that rationalizes our observations. Furthermore, we apply our model to flow in eccentric Couette cells. Our simulations reveal that the system’s heat transfer coefficient is affected by both the classic shear-induced migration of particles and the newly identified thermo-rheological migration effect.

Self-similarity in particle accumulation on the advancing meniscus

When a mixture of viscous oil and non-colloidal particles displaces air between two parallel plates, the shear-induced migration of particles leads to the gradual accumulation of particles on the advancing oil–air interface. This particle accumulation results in the fingering of an otherwise stable fluid–fluid interface. While previous works have focused on the resultant instability, one unexplored yet striking feature of the experiments is the self-similarity in the concentration profile of the accumulating particles. In this paper, we rationalise this self-similar behaviour by deriving a depth-averaged particle transport equation based on the suspension balance model, following the theoretical framework of Ramachandran (J. Fluid Mech., vol. 734, 2013, pp. 219–252). The solutions to the particle transport equation are shown to be self-similar with slight deviations, and in excellent agreement with experimental observations. Our results demonstrate that the combination of the shear-induced migration, the advancing fluid–fluid interface and Taylor dispersion yield the self-similar and gradual accumulation of particles.

A first-order physical model for the prediction of shear-induced particle migration and lubricating layer formation during concrete pumping

For the first time, a first-order physical correlation between pumping parameters (pressure and flow rate), granular skeleton properties (particle diameter and packing properties) and LL properties (thickness and viscosity) is proposed. This correlation is derived from the analysis of the equations driving shear-induced particle migration in dense suspensions. The model predictions in terms of lubricating layer apparent viscosity are compared to experimental measurements in the case of model concretes prepared with glass beads of various diameters, conventional-vibrated concretes and self-compacting concretes. This comparison is carried out for two flow typologies, namely the Sliper, known for its ability to mimic concrete pumping, and a co-axial tribometer. In all cases, the proposed model is shown to be able to capture qualitatively the observed main features and their evolutions despite the absence of any fitting parameters.

Onset of thermal convection in non-colloidal suspensions

Abstract

Roles of solid effective stress and fluid-particle interaction force in modeling shear-induced particle migration in non-Brownian suspensions

The applicability of some constitutive equations for the solid stress tensor used in the mixture model to describe the shear-induced migration of neutrally buoyant particles in Newtonian fluids is investigated. It is shown that in moderately dense suspensions, where direct particle contacts and interparticle forces are negligible, migration must be due to the lubrication forces between the particles. Results highlight that the tensor accounting for these forces is part of the stress tensor of the fluid phase, not of the solid phase and, in particular, that it coincides with the part of the particle-presence stress tensor related to the lubrication forces.

Puzzling Bubble Rise Speed Increase in Dense Granular Suspensions.

We present an anomalous experimental observation on the rising speed of air bubbles in a Hele-Shaw cell containing a suspension of spherical, neutrally buoyant, non-Brownian particles. Strikingly, bubbles rise faster in suspensions as compared to particle-less liquids of the same effective viscosity. By carefully measuring this bubble speed increase at various particle volume fraction and via velocity field imaging, we demonstrate that this strange bubble dynamics is linked to a reduction in the bulk dissipation rate. A good match between our experimental data and computations based on a Suspension Balance Model (SBM) illustrates that the underlying mechanism for this dissipation-rate deficit is related to a nonuniform particle distribution in the direction perpendicular to the channel walls due to shear-induced particle migration.

Modelling of shear bands during solidification

Major developments have been accomplished in the understanding of the physics involved in granular media and suspensions when subjected to shearing stresses. However, although partially solidified alloys have been reported to exhibit similar characteristics as granular material under certain conditions, such findings have never been fully addressed by the solidification community. Here, the modelling approaches adopted in granular mechanics for the description shear-induced particle migration have been included into a solidification model. It is shown that shear-induced particle migration has a considerable effect on the solid fraction distribution and macrosegregation formation in casting processes where a continuous and significant shear rate exists in a conduit with relatively small geometrical aspect ratio.

Effect of Different Shear Rates on Particle Microstructure of Cementitious Materials in a Wide Gap Vane-in-cup Rheometer.

Rheological properties of cementitious suspensions are affected not only by their mixture composition but also by process-related factors such as shear history. To enable a model-based description, investigations were carried out on the effect of shear history (shear rate variation over time) on the cement paste agglomeration state. Therefore, a Focused Beam Reflectance Measurement (FBRM) system and a wide gap rheometer were coupled to study the relation between shear history and in-situ chord length distribution simultaneously, indicating particle agglomeration. Hence, the effect of average shear rates (resulting from the applied shear profile), as well as shear rate distribution within the gap (local shear rates) on the particle agglomeration state have been investigated. The rheological properties of cement paste were evaluated with the Reiner–Riwlin approach. Furthermore, the agglomeration state of the particles was compared for different average shear rates and local shear rates at various positions of the FBRM probe. The results show that the median chord length increases in all positions when the average shear rate is decreased, indicating increasing particle agglomeration. Moreover, due to variable local shear rates at different FBRM probe positions, different agglomeration states are observed, resulting from two factors, shear rate dependent particle agglomeration and shear-induced particle migration.

Investigating the migration law of aggregates during concrete flowing in pipe

During pumping fresh concrete pipes, aggregate migration is occurred under the action of various forces. In order to study the migration characteristic of aggregates during concrete flowing in pipes, in this paper, we built the test system based on particle tracking technology. Transparent concrete was prepared. The particle capture function of high-speed camera was used to track particle motion. Results showed that the radial displacement of particles depended on the initial location and aggregate size. Small aggregates near pipe wall or center axis had fewer motion. For the particles with larger diameter, the migration direction almost pointed to the axis of pipe. The migration displacement of nature aggregates was smaller than that of ground grinding ball. The migration displacement firstly increased and then decreased with increasing the diameter of particle. Moreover, the effect of particle shape on migration was explored. Finally, the wall effect, gravity induced particle migration and shear induced particle migration were compared based on the phenomenon of flow induced particle migration, results showed that the phenomenon of shear induced particle migration played the main role in the formation of lubrication layer. This study was of great significance to reveal the transport law of dense particle flow in the pipeline.

Minimization of the Influence of Shear-Induced Particle Migration in Determining the Rheological Characteristics of Self-Compacting Mortars and Concretes.

Determining the exact rheological properties of cementitious materials in fundamental units is a crucial step in concrete science. It is undisputed that before measuring rheological properties in concrete rheometers, it is necessary to pre-shear the fresh mortar or concrete. Due to the migration of the coarse particles into areas with lower shear stress, however, segregation takes place. An experimental set-up was developed to determine the effects on the measured values of the concrete rheometer ICAR. This allows the active homogenization (pre-shearing) of the material before each change of speed. In the tests higher raw values (macroscopic data) could be measured. This clearly influences the calculated rheological Bingham parameters and modified Bingham parameters for a self-compacting concrete (SCC) with a maximum grain size of 16 mm. Therefore, the homogeneity of the material, a main hypothesis of rheological measurements, does not seem to be fulfilled with the coaxial rheometer used. The process of the indispensable pre-shearing therefore requires more attention in the future so that measurement errors can be minimized. Especially in numerical simulation, suitable rheological models and the realistic determination of parameters are crucial. Since the shear-induced particle migration is largely dependent on the maximum grain size, an ultra-high performance concrete (UHPC) with a maximum particle size of only 0.5 mm was also investigated in the laboratory mixer KNIELE KKM-RT. The integrated rheometer enables also the active homogenization of the fresh concrete during pre-shearing but without the danger of over-mixing, as it is the case for the experimental ICAR setup. This article proves that relevant differences can also be identified for such a material.