Granular jet impingement on a fixed target

Granular flow-solid wall interaction: investigation of the teapot effect

The evolution of granular flows generally involves solid boundaries, which add complexity to their dynamics and pose challenges to understand relevant natural and industrial phenomena. While an interesting "teapot effect" has been observed for liquid flowing over the solid surface of a teapot's spout, a similar phenomenon for discrete particles receives far less attention. In this work, we experimentally investigated the interactions between granular flows and a wedge-shaped solid edge (spout), showing that the trailing edge of the solid boundary plays a key role in causing velocity non-uniformity and splitting the flow into "dispersed" and "uniform" regions. Tuning the parameters (inclination angle, particle diameter, radii and surface roughness of the trailing edge) of the granular flow, a dimensionless number was summarized and successfully predicted the dispersion of the granular flows. Moreover, we also proved that introducing stronger cohesive forces between particles could harness the granular flows from heterogenous structures to grain clusters, which can be employed to switch between different flow regimes and regulate the dispersion behavior of particle flows. This study reveals the interaction of granular flow over complex solid boundaries, potentially offering new insights into particle-dominated flow dynamics.

Morphodynamics of a dense particulate medium under radial explosion

In this paper, we investigate the initiation and growth of instability patterns arising from the shock loaded internal surfaces of granular rings confined in a Hele-Shaw cell using both experimental and numerical approaches. A variety of patterns are formed in granular media consisting of grains with varying morphologies. When the particle shape becomes increasingly irregular, and/or the gap in the Hele-Shaw cell becomes narrower, it is increasingly hard for confined particles to fluidize. Consequently the emergent pattern transitions from a smooth circle with trivial undulation which grows in a self-similar manner to an unstable finger-like structure with significant tip-splitting. The distinct growth mode of the well-defined instability pattern is closely associated with its inception phase alongside the transmission of the compaction front. The runaway growth of the incipient perturbations gives rise to the unstable growth of the late-time finger-like instabilities. Conversely the minimal growth of the perturbations in the inception phase guarantees the ensuing self-similar growth of the instability patterns featuring insignificant corrugation. The grain-scale simulations reveal the fundamental role played by the heterogeneous non-linear force network inherent to granular media in the stable-to-unstable transition of the instability pattern. The present work reveals the correlation between the grain-scale physics underpinning the formation of surface instability upon shock loading granular media and the nature of the resulting macro-scale instability patterns. The macroscopic flowability of particles through the confined space is found to be the foremost indicator of the nature of the shock induced granular instability pattern.

Subharmonic instability of a self-organized granular jet

Downhill flows of granular matter colliding in the lowest point of a valley, may induce a self-organized jet. By means of a quasi two-dimensional experiment where fine grained sand flows in a vertically sinusoidally agitated cylinder, we show that the emergent jet, that is, a sheet of ejecta, does not follow the frequency of agitation but reveals subharmonic response. The order of the subharmonics is a complex function of the parameters of driving.

Impact dynamics of granular jets with noncircular cross sections

Using high-speed photography, we investigate two distinct regimes of the impact dynamics of granular jets with noncircular cross sections. In the steady-state regime, we observe the formation of thin granular sheets with anisotropic shapes and show that the degree of anisotropy increases with the aspect ratio of the jet's cross section. Our results illustrate the liquidlike behavior of granular materials during impact and demonstrate that a collective hydrodynamic flow emerges from strongly interacting discrete particles. We discuss the analogy between our experiments and those from the Relativistic Heavy Ion Collider, where similar anisotropic ejecta from a quark-gluon plasma have been observed in heavy-ion impact.

Simulation of granular jets: is granular flow really a perfect fluid?

We perform three-dimensional simulations of the impact of a granular jet for both frictional and frictionless grains. Small shear stress observed in the experiment [X. Cheng et al., Phys. Rev. Lett. 99, 188001 (2007)] is reproduced through our simulation. However, the fluid state after the impact is far from a perfect fluid, and thus the similarity between granular jets and quark gluon plasma is superficial because the observed viscosity is finite and its value is consistent with the prediction of the kinetic theory.

Microscopic dissipation in a cohesionless granular jet impact

Sufficiently fine granular systems appear to exhibit continuum properties, though the precise continuum limit obtained can be vastly different depending on the particular system. In the present paper the continuum limit of an unconfined, dense granular flow is investigated. To do this a two-dimensional dense cohesionless granular jet impinging upon a target is used as a test system. This is simulated via a time-step-driven hard-sphere method and apply a mean-field theoretical approach to connect the macroscopic flow with the microscopic material parameters of the grains. It is observed that the flow separates into a cone with an interior cone angle determined by the conservation of momentum and the dissipation of energy. From the cone angle a dimensionless quantity A-B that characterizes the flow is extracted. This quantity is found to depend both on whether or not a dead zone, i.e., a stationary region near the target, is present and on the value of the coefficient of dynamic friction. A theory is presented for the scaling of A-B with the coefficient of friction that suggests that dissipation is primarily a perturbative effect in this flow rather than the source of qualitatively different behavior.

Bubbling in vibrated granular films

With the help of experiments, computer simulations, and a theoretical investigation, a general model is developed of the flow dynamics of dense granular media immersed in air in an intermediate regime where both collisional and frictional interactions may affect the flow behavior. The model is tested using the example of a system in which bubbles and solid structures are produced in granular films shaken vertically. Both experiments and large-scale, three-dimensional simulations of this system are performed. The experimental results are compared with the results of the simulation to verify the validity of the model. The data indicate evidence of formation of bubbles when peak acceleration relative to gravity exceeds a critical value Γ(b). The air-grain interfaces of bubblelike structures are found to exhibit fractal structure with dimension D=1.7±0.05.