Experimental study and analysis of velocity correlation and intermittency of very slow and dense pebble flow in a silo bed

The very slow and dense pebble flow in a silo bed is analogous with a working fluid through the bed. Thus, herein it is studied and analyzed by the particle tracking method from macroscopic and microscopic perspectives. The velocity profile and fluctuation characteristics at different heights of the silo bed are analyzed. By analyzing the velocity evolution of individual particles, intermittent and distinct bulk motion events of pebbles are observed. The time correlations of particle velocities have periodic and oscillatory tails which are different from the rapid granular flow with the exponentially decaying tails. Two parameters, namely the correlation time τc and the intermittency index C2, are proposed. The effect of these parameters on the flow regime is analyzed for selected streamlines. The autocorrelation time τc seems to follow power-law regressions (τc∼Hα) with the reduction of heights on different streamlines. The relationships of autocorrelation, intermittency and shear functions are discussed in the whole flow field. Finally, a further discussion on the features of extremely slow flow as well as the characterization of flow patterns is performed based on the parameter analysis, including the correlation time, intermittency index, and shear rates. The results are important for improving the understanding of the physics of pebble flows and application of pebble beds in nuclear engineering.