Shock-induced interfacial instabilities of granular media

This paper investigates the shock-induced instability of the interfaces between gases and dense granular media with finite length via the coarse-grained compressible computational fluid dynamics–discrete parcel method. Despite generating a typical spike-bubble structure reminiscent of the Richtmyer–Meshkov instability (RMI), the shock-driven granular instability (SDGI) is governed by fundamentally different mechanisms. Unlike the RMI arising from baroclinic vorticity deposition on the interface, the SDGI is closely associated with the interfacial and bulk granular dynamics, which evolve with the transient coupling between particles and gases. Consequently, the SDGI follows a growth law distinctly different from that of the RMI, namely a semilinear slow regime followed by an exponentially expedited regime and a quadratic asymptotic regime. We further establish the instability criteria of the SDGI for granular media with infinite and finite lengths, which do not exist in the RMI. A scaling growth law of the SDGI for dense granular media with finite length is derived by normalizing the time with the rarefaction propagation time, which successfully collapses the data from cases with varying shock strength, particle column length and particle volume fraction and ought to hold for granular media with varying particle parameters. The effect of the initial perturbation magnitude can be properly considered in the scaling growth law by incorporating it into the length normalization.

Chrono::GPU: An Open-Source Simulation Package for Granular Dynamics Using the Discrete Element Method

We report on an open-source, publicly available C++ software module called Chrono::GPU, which uses the Discrete Element Method (DEM) to simulate large granular systems on Graphics Processing Unit (GPU) cards. The solver supports the integration of granular material with geometries defined by triangle meshes, as well as co-simulation with the multi-physics simulation engine Chrono. Chrono::GPU adopts a smooth contact formulation and implements various common contact force models, such as the Hertzian model for normal force and the Mindlin friction force model, which takes into account the history of tangential displacement, rolling frictional torques, and cohesion. We report on the code structure and highlight its use of mixed data types for reducing the memory footprint and increasing simulation speed. We discuss several validation tests (wave propagation, rotating drum, direct shear test, crater test) that compare the simulation results against experimental data or results reported in the literature. In another benchmark test, we demonstrate linear scaling with a problem size up to the GPU memory capacity; specifically, for systems with 130 million DEM elements. The simulation infrastructure is demonstrated in conjunction with simulations of the NASA Curiosity rover, which is currently active on Mars.

Velocity and Volume Fraction Measurements of Granular Flows in a Steep Flume

ABSTRACT Laboratory experiments on granular flows remain essential tools for gaining insight into several aspects of granular dynamics that are inaccessible from field-scale investigations. Here, we report an experimental campaign on steady dry granular flows in a flume with inclination of 35°. Different flow rates are investigated by adjusting an inflow gate, while various kinematic boundary conditions are observed by varying the basal roughness. The flume is instrumented with high-speed cameras and a no-flicker LED lamp to get reliable particle image velocimetry measurements in terms of both time averages and second-order statistics (i.e., granular temperature). The same measuring instruments are also used to obtain concurrent estimations of the solid volume fraction at the sidewall by employing the stochastic-optical method (SOM). This innovative approach uses a measurable quantity, called two-dimensional volume fraction, which is correlated with the near-wall volume fraction and is obtainable from digital images under controlled illumination conditions. The knowledge of this quantity allows the indirect measurement of the near-wall volume fraction thanks to a stochastic transfer function previously obtained from numerical simulations of distributions of randomly dispersed spheres. The combined measurements of velocity and volume fraction allow a better understanding of the flow dynamics and reveal the superposition of different flow regimes along the flow depth, where frictional and collisional mechanisms exhibit varying relative magnitudes.

How universal is the vibration-velocity controlled granular convection?

Recently, a number of articles have reported that granular convection induced by continuous vibration is controlled by vibration velocity, in contrast with some previous studies. We have reported such an example for the Brazil nut effect when the vibration is given discontinuously, using a one-layer granular bed in a cell with down-facing side walls. Here, we report the effect of vibration phase and wall friction using the same experimental system, to confirm rising motion of an intruder induced by granular convection is again governed by vibration velocity. We compare two different cases of vibration phase for giving intermittent vibration cycles, and found one, in which granular packing is well established before grains start to lose contacts due to vibration, provides distinctly high reproducibility. We further control the side wall friction using a microfabrication technique, and found that significantly high reproducibility is attained in a cell with vertical side walls when a millimeter texture is introduced on the side walls. Our results indicate that the granular convection is universally controlled by vibration velocity. The present study opens a way to conduct highly reproducible experiments on granular dynamics, which is indispensable for deep physical understanding of granular flow and segregation.

Embedded inertial sensor for tracking projectile impact on granular media

Due to the opacity of most granular materials, it is often desirable to have three dimensional (3D) particle tracking techniques beyond optical imaging to explore granular dynamics. Using inertial measurement units (IMU) embedded in a projectile, we obtain the trajectory of projectile impacting on a granular medium under microgravity using tri-axial acceleration and angular velocity data. In addition to the standard algorithm for reconstruction, we emphasize solutions to various sources of error to determine projectile trajectory accurately.

Using a proper orthogonal decomposition to elucidate features in granular flows

We apply proper orthogonal decomposition (POD) technique to analyze granular rheology in a laboratory-scale pulsed-fluidized bed. POD allows us to describe the inherent dynamics and energy budget in the dominant spatio-temporal modes in addition to identifying spatial coherence. This enables us to elucidate non-linear interactions between the different mechanisms which has been a shortcoming of conventional statistics-based approaches. The bubbling pattern is a result of interplay between the harmonic and sub-harmonic components. The mesoscopic flow features which contribute to the pattern are dependent on the modal energy budget which change with the pulsing frequency. It is also observed that the granular dynamics can be sufficiently reconstructed by the leading POD modes despite the presence of bubbles which represent kinematic shocks contributing to higher-order modes. In short, we highlight the utility of POD while analyzing fluidized granular flows, and pave the way for future analyses. Graphic abstract

For granular geography

This article makes the case for developing granular geographies as an intervention into materialist geography. It does so by exploring sand extraction, which has so far been little explored within human geography, and how the granular dynamics of force chains, friction, and phase transitions make the discrete geographies of sand’s commodification porous to one another. By framing sand’s materiality through granular relations, I examine the ubiquity of sand in contemporary urbanization and its transformation into a resource. This is illustrated through a focus on Singapore’s excessive appetite for importing sand to fuel construction and its extensive land reclamation project, mapping sand’s trajectory as a resource from cheap nature to sovereign territory.

Multibody Dynamics Versus Fluid Dynamics: Two Perspectives on the Dynamics of Granular Flows

Considering that granular material is second only to water in how often it is handled in practical applications, characterizing its dynamics represents a ubiquitous problem. However, studying the motion of granular material poses stiff computational challenges. The underlying question in this contribution is whether a continuum representation of the granular material, established in the framework of the smoothed particle hydrodynamics (SPH) method, can provide a good proxy for the fully resolved granular dynamics solution. To this end, two approaches described herein were implemented to run on graphics processing unit (GPU) cards to solve the three-dimensional (3D) dynamics of the granular material via two solution methods: a discrete one, and a continuum one. The study concentrates on the case when the granular material is packed but shows fluid-like behavior under large strains. On the one hand, we solve the Newton–Euler equations of motion to fully resolve the motion of the granular system. On the other hand, we solve the Navier–Stokes equations to describe the evolution of the granular material when treated as a homogenized continuum. To demonstrate the similarities and differences between the multibody and fluid dynamics, we consider three representative problems: (i) a compressibility test (highlighting a static case); (ii) the classical dam break problem (highlighting high transients); and (iii) the dam break simulation with an obstacle (highlighting impact). These experiments provide insights into conditions under which one can expect similar macroscale behavior from multibody and fluid dynamics systems governed by manifestly different equations of motion and solved by vastly different numerical solution methods. The models and simulation platform used are publicly available and part of an open source code called Chrono. Timing results are reported to gauge the efficiency gains associated with treating the granular material as a continuum.