Clustering is critical to understanding the multiscale behavior of fluidization. However, its time-resolved evolution at the particle level is seldom touched. Here, we explore both the time-averaged and time-resolved dynamics of clusters in a quasi-2D fluidized bed. Particle tracking velocimetry is adopted, and then clusters are identified by using Voronoi analysis. The time-averaged results show that the number distribution of the cluster size follows a power law ($ n À2:2 c ) except for large clusters (n c > 100). Time-resolved analysis demonstrates that the cluster coalescence can be simplified as a collision between two inelastic clusters, during which the mean speed, kinetic energy, and total Voronoi cell area of particles decrease until the formation of the big cluster. Then, a model is proposed to predict its energy loss, which gives ΔE ~t3/2 . Moreover, the breakup of the cluster is linked to increasing dimensionless torque on the particles.
The free path of a vibro-fluidized two-dimensional (2D) inelastic granular gas confined in a rectangular box is investigated by 2D event-driven molecular simulation. By tracking particles in the simulation, we analyze the local free path. The probability distribution of the free path shows a high tail deviating from the exponential prediction. The anisotropy of the free path is found when we separate the free path to x and y components. The probability distribution of y component is exponential, while x component has a high tail. The probability distribution of angle between the relative velocity and the unit vector joined two particle centers deviates from the distribution of two random vectors, indicating the existence of the dynamic heterogeneities in our system. We explain these results by resorting to the kinetic theory with two-peak velocity distribution. The kinetic theory agrees well with the simulation result.
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