Uniform Granular Flow Research Articles

Implementing moving object capability in a two-phase Eulerian model for sediment transport applications

This article presents the validation of a modified 6DoF solver which includes the contribution of particle forces on rigid objects. The solver is an extension of sedFoam, employed for a wide range of sediment transport applications. The first validation consists in a simple case of a sphere freely falling in a pure fluid. Then, the same sphere is submerged in a fluid with suspended particles. The next step involves placing a granular bed under the sphere to study its arrest due to particle-particle interactions. Lastly, the trajectory of a cylinder immersed in a uniform granular flow is studied to emphasize the lift and drag forces exerted by the particle phase on the solid object. Results show that overSedDyMFoam accurately reproduces experimental/numerical measurements successfully capturing the behavior of solid objects subjected to granular and fluid forces. This implementation is a significant step towards understanding complex interactions between fluid, particles, and structures.

Acoustic Emissions of Nearly Steady and Uniform Granular Flows: A Proxy for Flow Dynamics and Velocity Fluctuations

AbstractThe seismic waves emitted during granular flows are generated by different sources: high frequencies by interparticle collisions and low frequencies by global motion and large scale deformation. To unravel these different mechanisms, an experimental study has been performed on the seismic waves emitted by dry, dense, quasi‐steady granular flows. The emitted seismic waves were recorded using shock accelerometers and the flow dynamics were captured with a fast camera. The mechanical characteristics of the particle collisions were analyzed, along with the intervals between collisions and the correlations in particles' motion. The high‐frequency seismic waves (1–50 kHz) were found to originate from particle collisions and waves trapped in the flowing layer. The low‐frequency waves (20–60 Hz) were generated by particles' oscillations along their trajectories, that is, from cycles of dilation/compression during coherent shear. The profiles of granular temperature (i.e., the mean squared value of particle velocity fluctuations) and average velocity were measured and related to each other, then used in a simple steady granular flow model, in which the seismic signal consists of the variously attenuated contributions of shear‐induced Hertzian collisions throughout the flow, to predict the rate at which seismic energy was emitted. Agreement with the measured seismic power was reasonable, and scaling laws relating the seismic power, the shear strain rate and the inertial number were derived. In particular, the emitted seismic power was observed to be approximately proportional to the root mean square velocity fluctuation to the power 3.1 ± 0.9, with the latter related to the mean flow velocity.

Granular stresses in granular flows subjected to different obstacles

Although the application and development of continuum approaches demonstrate immense promise to model granular flows, the assumption of isotropic stress state in a constitutive law needs to be carefully examined, especially for accurately modeling the kinematics and kinetics of obstructed granular flows. This paper investigates the effect of obstacles on granular stresses and stress ratios in 3D granular silo flows by using discrete element method (DEM). The adopted obstacles include a conical insert, disk insert, BINSERT, hollow cylinder insert and hollow silo insert, as well as the absence of any insert for comparison. A novel (simple but robust) method with ring measurement cells was proposed to determine average stress tensors for granular flows. An innovative slice method with null friction boundaries was also proposed to significantly reduce DEM computational time. The validity of the proposed DEM model was justified by a repose angle experiment and two numerical verification examples. Results reveal that the blocking effect from the obstacles and the hopper wall of the silos led to a significant increase in normal and shear stresses. The normal stresses exhibited slight anisotropy in steady and uniform granular flows, and resulted in a stress ratio of approximately 1.0, indicating an approximately isotropic stress state. Unlike the normal stresses, the shear stresses however showed strong anisotropy even in the steady and uniform regime. The obstructed granular silo flows exhibited strong anisotropy not only in normal stresses, but also in shear stresses. The stress ratio in the obstructed granular flows can appreciably deviate from unity, varying between 0.27 and 2.24. Moreover, the vertical stresses dominated the contact force transmission in the granular silo flows, and showed the formation of dynamic granular arching.

Volume fraction and velocity fields of nearly uniform granular flows in a narrow channel geometry with smooth bed

The combined knowledge of the velocity and volume fraction fields is crucial for investigating the dynamics of granular flows, especially in the dense-collisional regime where both frictional and collisional dissipation mechanisms are significant. A laboratory investigation on steady dry granular flows in a straight channel is reported, where slip conditions are allowed at the basal surface and side walls. The stochastic-optical method (SOM), proposed by Sarno et al. (2016) for estimating the volume fraction in granular mixtures, is applied for the first time to granular flows. The velocity at the free surface and at the flume sidewall is measured by using a multi-pass particle image velocimetry (PIV) approach. The measurements of the velocity and volume fraction reveal a superimposition of different dynamic structures, which can be distinguished by means of a volume fraction threshold. Additionally, the profiles of measured volume fraction are exploited to estimate the pressure distribution, so as to numerically describe the velocity profiles by using the μ(I) rheology. It is found that the employment of the experimental volume fraction is superior in describing the flow dynamics, especially near the free surface.

Stopping dynamics of a steady uniform granular flow over a rough incline

Granular material flowing on complex topographies are ubiquitous in industrial and geophysical situations. Even model granular flows are difficult to understand and predict. Recently, the frictional rheology μ (I ) -describing the ratio of the shear stress to the normal stress as a function of the inertial number I , that compares inertial and confinement effects- allows unifying different configurations of granular flows. However it does not succeed in describing some phenomenologies, such as creep flow, deposit height, … Is it attributable to the rheology, to non-local effects, ...? Here, we consider a thin layer of grains flowing steadily and uniformly on a rough incline, when the input mass flow rate is suddenly stopped. We focus on the arrest dynamics by using both experimental and numerical approaches. We measure the height and surface velocities of the granular layer during the long-time stopping dynamics and we compare our experimental results with computations of depthaveraged equations for a fluid of rheology μ (I ).

A SPH model for the simulation of free surface granular flows in a dense regime

Smoothed Particle Hydrodynamics (SPH) is a particle-based Lagrangian numerical method, particularly suitable for modeling free surface flows of fluids with different constitutive equations. In this work, a SPH model is developed to solve flow problems of granular matter flowing in a dense regime, featuring a constitutive equation proposed by Pouliquen et al. (2006) [17] and Jop et al. (2006) [18].The problem of resolving the flow close to static conditions has been solved by introducing a weak compressibility for the grains: this leads to a third order algebraic equations to be solved for computing granular material density. A new viscosity regularisation method, suitable for any visco-plastic fluid with varying yield stress has been developed, in order to reproduce stopping flow and static conditions.Validation of the model has been performed by solving the simple uniform granular flow problem, for which the solution is known analytically and by reproducing a set of experimental data on granular columns collapses by Lube et al. (2004) [19].

Control of the fluctuation in the uniform granular flow by a random force field

A normal distribution random force field is introduced into the study of granular flow, and its effect is observed by computer simulations. The results show that the random force almost does not cause changes in average density and velocity in a uniform granular flow, and affects little the fluctuation of the density. The main effect of the random force field is that it increases the fluctuation of the granular velocity and maintains the granular flow at a certain dispersed kinetic energy by competing with the dissipation of the granular system. It is also shown that the dispersed kinetic energy obtained by the random force field is not equally distributed in each degree of freedom, and that the equipartition of energy is difficult to realize in granular system because of its dissipation property.

On dense granular flows.

The behaviour of dense assemblies of dry grains submitted to continuous shear deformation has been the subject of many experiments and discrete particle simulations. This paper is a collective work carried out among the French research group Groupement de Recherche Milieux Divisés (GDR MiDi). It proceeds from the collection of results on steady uniform granular flows obtained by different groups in six different geometries both in experiments and numerical works. The goal is to achieve a coherent presentation of the relevant quantities to be measured i.e. flowing thresholds, kinematic profiles, effective friction, etc. First, a quantitative comparison between data coming from different experiments in the same geometry identifies the robust features in each case. Second, a transverse analysis of the data across the different configurations, allows us to identify the relevant dimensionless parameters, the different flow regimes and to propose simple interpretations. The present work, more than a simple juxtaposition of results, demonstrates the richness of granular flows and underlines the open problem of defining a single rheology.

Dense granular flow around an immersed cylinder

The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. Experiments are performed in a tall vertical chute producing a quasi two-dimensional granular flow. A storage bin at the top of the chute feeds glass particles into the channel while the mean velocity of the flow is controlled by varying the exit width of a hopper located at the channel bottom. Measurements of the drag force acting on a fixed cylinder are made using a strain gauge force measurement system. The flow velocity field is measured through a transparent wall using a particle image velocimetry analysis of high speed video recordings of the flow. Experiments are performed for a range of upstream particle velocities, cylinder diameters, and two sizes of glass particles. For the range of velocities studied, the mean drag force acting on the cylinder is independent of the mean flow velocity, contrary to what is expected from any ordinary fluid. The drag force increases with cylinder diameter and decreases with particle diameter. The drag force scales with the asymptotic static stress state in a tall granular bed. The drag coefficient, defined in terms of a dynamic pressure and an effective cylinder diameter, scales with the flow Froude number based on the hydraulic diameter of the channel. This analysis indicates that the drag acting on the cylinder is strongly affected by the surrounding channel geometry. Although the drag force on the cylinder does not change with the upstream flow velocity, the flow streamlines do change with velocity. A large stagnation zone forms at the leading edge of the cylinder while at the trailing edge an empty wake is observed. The wake size increases with flow velocity. Measurements of the flow vorticity and granular temperature are also presented and discussed.