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In this paper we report experimental results on the granular temperature and velocity profile of glass beads in steady-state flow inside a flat-bottomed silo. We found that the dynamics of the granular system during the discharge is stable on the mesoscopic scale. In particular, the ordered directional movement of particles can occur near the orifice. In addition, the correlation between the granular temperature distribution and velocity vector field is opposite in the radial and axial directions. According to the kinetic model, the main cause for the reversal of spatial correlation is that the intensity of the particle diffusion decrease with the decrease of silo height. This highlights a surprising and unrecognized role that the diffusion motion of particles plays in energy dissipation for hopper flow.

There is a paucity of studies on the relationship between the control parameters and kinetic parameters in binary-size granular segregation models. In this study, the machine vision method and particle image velocimetry (PIV) were used to measure the shape index of segregation pattern and streamwise velocity, respectively. Two scaled parameters, control parameters and kinetic parameters, were calculated based on the continuous flow model and segregation model. The control parameters include volume flow rate, and the kinetic parameters include the ratio of segregation effects to diffusion effects. By analyzing the relationship between the two parameters under the core and streak patterns, the configuration of segregation pattern at steady-state changes with the control parameters wherein the ratio of segregation effects to diffusion effects decreases approximately linearly as the volume flow rate decreases. These experimental results aid in revealing the mechanism of segregation and providing the reference data for improving the theoretical models.

Previous study of our group showed that the compaction of sand particles is observed before while dilatancy occurs during avalanches in the active layer (es. Chen et al., Sci Sin Phys Mech Astron 49(6), 2019). In current works, we investigated the dynamic of irregular particles in the passive layer by speckle visibility spectroscopy, discovering two types of cycle rearrangements. We determine that one kind of rearrangement corresponds to the compaction of the pre-avalanche granular bed, and the other rearrangement is triggered by granular avalanches occurring in the active layer. The rearrangement caused by avalanche has a greater dissipation energy by comparing the other rearrangement. We describe simple mechanisms which account for the non-uniformity of dissipated energy. We further found that the propagation speeds of the rearrangements in the passive layer are an order of magnitude faster than that in the active layer and the propagation directions for both rearrangements are opposite to each other.

Considerable amount of experimental work has been performed to elucidate the behavior of granular flow inside a rotating drum but yet it is not completely understood. Furthermore, most of these investigations have involved spherical or nearly spherical particles. The extent of the experiments involving non-spherical particles previously carried out was limited by the experimental techniques available for the studies. In this work, a simple imaging method and a speckle visibility spectroscopy (SVS) technique were used to measure the volume fraction and granular temperature of irregular glass sand particles under the slumping regime within a rotating drum. By comparing the temporal variation of the volume fraction of the granular bed with SVS measurement, we found the compaction and dilatancy of the granular flow during a slumping cycle can be easily characterized by granular temperature temporal curves. At different rotating speeds, we found the lower repose angle increases linearly with the rotational speed and the compaction duration is inversely proportional to the critical angle of compaction. We find, in addition, the compaction induced relative motion of particles decreases with the increase of rotational speed, while the avalanche induced motion of particles is independent of rotational speed. Finally, using the mathematical model of avalanche duration and angle of repose proposed by Mellmann to calculate the particle avalanche duration, we found that the compaction of the particle system shortens the avalanche duration.

A synchronized speckle visibility spectroscopy (SVS) has been used to simultaneously measure the granular temperatures, δv2, of the active and passive layers. It has been found that two-particle rearrangements are present in the passive layer. The first rearrangement, P1, occurred before the avalanche, caused by the sliding of the upper portion of the particles. The second rearrangement, P2, occurred slightly later before the surface flow, triggered by the avalanche of active layer. Comparative analysis of maximum values of two pulses in one cycle led us to conclude that the characteristics of particle motion in passive layer were mainly affected by filling degree. We believe that the particle system may have been in a transitional state at filling degree of 15 and 30%. When the filling degree is greater than 30% in a particle system, the maximum values of each period of the P2 pulses confirmed the power-law distribution.

Due to the irregular shapes, the dynamics of irregular particles in a rotary drum are more complicated than those of regular particles. In this study, speckle visibility spectroscopy was used in conjunction with a simple imaging technique to investigate the behavior of irregular sand in a rotating drum under the slumping regime. We found that particle rearrangement originates from the upstream side of the granular bed after the avalanche flow. Furthermore, the energy dissipation of the rearrangement is high closer to the upstream side of the active layer. The greater the degree of particle filling, the greater is the amount of energy dissipated by the rearrangement. Finally, we proposed a model to predict the rearrangement after avalanche in a rotating drum.