Session R33: Granular Flows: General

Experimental Investigation of Avalanching Behavior in Magnetic Granular Materials

Psyche 16 is an M-type asteroid located in the main asteroid belt between Mars and Jupiter. Measurements of Psyche’s UV spectrum indicate that it is comprised mostly of silicate and iron minerals, and it is possible Psyche is comprised of a former planetary core and therefore have its own magnetic field. If Psyche does have a sufficiently strong magnetic field, we hypothesize that the granular material at the asteroid’s surface would form characteristically different structures compared to granular material observed on most other asteroids due to non-negligible attraction forces between the magnetic particles due to the relative strength of the magnetic field compared to gravity. To construct a phase map of avalanching behavior given variable strength of cohesion forces to gravitational forces, we perform experiments using steel particles of different sizes subjected to an external magnetic field of variable strength during an avalanching event. To quantify changes in the equilibrium structure of the pile following the avalanche event, we quantify the angle of repose and the runout distance of the particles.    

Experimental study on the ceramic granular flow in a rotary drum

In the present study, we experimentally investigate the granular flow of zirconium dioxide particles in a rotating drum (at the speed of 1-30 rpm), focusing on understanding the forces between particle (diameter in order of 100 m). The diameter of the drum is 80 mm and two different heights of 1 and 8 mm are considered. While varying the drum height and rotating speed, we measured the trajectory, velocity and acceleration of each particle using the particle tracking velocimetry technique. It is found that the mechanisms of inducing stresses between particles are disparate each other depending on the location of the particle, i.e., stress received by particle in active layer is mainly generated from the particle-particle collision while the stress received by particles in the plug flow region is mostly attributed to the translational stress. Further discussion about the transition condition of the granular bed motion, from rolling to cascading, of which is associated with both the fill fraction of the rotary drum and diameter of the particle relative to the rotary drum diameter, is to be reported.   

Lift Force on an Intruder in Granular Shear Flows

In order to understand particle-level forces in flowing granular materials, discrete element method (DEM) simulations are conducted to investigate the lift force on a single spherical intruder in uniform shear flows of smaller spheres in the absence of gravity. In the simulations, a constant slip velocity is imposed on the intruder in the streamwise direction, while the intruder’s vertical motion is constrained. The net vertical force on the intruder due to contacts with bed particles is measured as the lift force. Our results show that with increased slip velocity, the vertical lift force rises to a peak before decreasing and eventually changing sign. Analysis of the packing fraction, pressure field, and vertical contact force on the intruder provide insights into the origin of the non-monotonic dependence of the lift force on the slip velocity as well as the influence of confining pressure, and intruder-to-bed-particle-diameter ratio.  Funded by NSF grant CBET-1929265.   

A Novel Discrete Element Model for Collisions of Multiple Wet Particles

The collision of wet granular particles occurs in natural phenomena such as avalanches, erosion, and mudslides, as well as in industrial processes like coated pharmaceutical and fertilizer production. Simulations involving multiple wet particles require a model that captures the kinematics and hydrodynamics without being excessively computationally intensive. A novel discrete element model algorithm is presented here, where the relative motion between each pair of neighboring particles is resolved in a rotating polar coordinate system, considering interactions with other nearby particles. During collision, the thin viscous liquid layer between particles exerts lubrication and capillary forces, which are then used in the dynamical equations for the particles to determine their translational and rotational velocities. Initial results for a system of three wetted spheres exhibit a wide variety of collision outcomes, including agglomeration of all three spheres, separation of all three spheres, and separation of one sphere from the remaining pair. A key parameter is the Stokes number, representing the ratio of particle inertia to viscous forces, with separation versus agglomeration occurring at higher Stokes numbers. A generalized framework of equations for a system of an arbitrary number of spheres has been developed, and additional results will be presented for systems of multiple wet spheres.   

Distinguishing strength and stability in a jammed granular column as a function of fluid saturation

A wide variety of geotechnical, engineering, and physics studies have focused on the granular materials of different particle sizes and shapes for a variety of fluid saturations. These studies can be used to describe on a case by case basis the physical characteristics, such as shear wave and p-wave velocities, that are dominated by either the response of the granular material or the interstitial fluid matrix. What is lacking is a more universal description of the granular+fluid system, here in its jammed state, that can predict global characteristics such as strength and stability. We present a low-dimensional "phase space" model that outlines the contributions to relative strength and stability of a jammed granular column at different fractional fluid saturations as determined from prior bulk studies. While the model does not include higher order effects (spatial or temporal dependence of the parameters), it allows a granular temperature to be defined which allows the strength and stability of the column to be distinguished for a full range of fluid saturations. The model's success can be shown for two drastically different granular media: fine, nearly monodisperse, smaller grains and coarse, more polydisperse, larger grains.   

Visco-frictional trapping of particles in a submerged porous medium

Particle filtration through a porous medium is a much-studied field due to its numerous industrial or natural applications, particularly for deep bed and cake filtration regimes. The percolation regime has received relatively less attention and corresponds to the continuous motion of particles in a porous environment under the principal effect of gravity, and generally without any interaction other than contact with the porous matrix. In the present case, both the moving particles and the porous medium are composed of spherical particles of diameters d and D, respectively. Dry percolation in such conditions has been analysed and exhibits lateral diffusion as well as a constant mean transit velocity which scales with  [1]. The behavior is significantly different in the case studied here, where the porous stack is fully saturated. A previous study experimentally investigated the transition from deep filtration to percolation regime under laminar flow conditions by matching the refractive index of the liquid to that of the particles of the porous system [2]. Here, based on the same experimental technique, we focus on the particular case where there is no fluid flow, for which we observe a new type of particle trapping, by contrast with either the dry condition or the flow situation. We show that this capture comes from a combination of solid friction and viscous effects, which strongly reduce the velocity of the moving particles by lubrication and can also suppress any rebound.  This visco-frictionnal trapping is highly metastable and the captured particles can easily be moved again. We will also present a statistical analysis of the trajectories followed in the porous material, including transit velocity, penetration depth and radial dispersion.

[1] Ippolito, I, Samson, L., Bourlès, S., Hulin, J.-P., Diffusion of a single particle in a 3D random packing of spheres, Eur. Phys. J. E 3, 227–236 (2000).

[2] Ghidaglia, C., de Arcangelis, L., Hinch, J., Guazzelli, E., Transition in particle capture in deep bed filtration, Phys. Rev. E 53, R3028(R) (1996).