Role of Carrier Phase Scales in the Initiation of Particle Motion

Understanding the mechanisms by which carrier phase structures mobilize large and heavy particles is critical to processes such as sediment transport, erosion and particulate handling. This study investigates the unsteady mobilization of such particles from a particle bed driven by a turbulent boundary layer. An experimental framework was developed to isolate the effects of carrier phase scales, specifically their size and energy, on the mobilization process. Simultaneous measurements were conducted of the turbulent carrier phase (air) velocity field and the mobilization activity of the particle phase (soda-lime particles). Upon exceeding the threshold shear stress, incipient motion was found to be primarily driven by the turbulent structures in the carrier phase. Among these, large-scale structures played the dominant role, with greater energy content at a given scale resulting in increased particle mobilization. In particular, a strong positive correlation was observed between mobilization activity and large-scale streamwise velocity fluctuations in the near-wall region, while the correlation with wall-normal fluctuations was almost negligible. A scale-wise correlation analysis showed that streamwise structures with lengths on the order of 30δ were the most significant contributors to incipient particle motion.