On the dynamics of aligned inertial particles settling in a quiescent, stratified two-layer medium

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We explored the settling dynamics of vertically aligned particles in a quiescent, stratified two-layer fluid using particle tracking velocimetry. Glass spheres of d = 4mm diameter were released at frequencies of 4, 6 and 8Hz near the free surface, traversing through an upper ethanol layer (H1), where H is height or layer thickess, varying from 10d to 40d and a lower oil layer. Results reveal pronounced lateral particle motion in the ethanol layer, attributed to a higher Galileo number (Ga = 976, ratio of buoyancy-gravity to viscous effects), compared with the less active behaviour in the oil layer (Ga = 16). The ensemble vertical velocity of particles exhibited a minimum just past the density interface, becoming more pronounced with increasing H1, and suggesting that enhanced entrainment from ethanol to oil resulted in an additional buoyancy force. This produced distinct patterns of particle acceleration near the density interface, which were marked by significant deceleration, indicating substantial resistance to particle motion. An increased drag coefficient occurred for H1/d = 40 compared with a single particle settling in oil; drag reduced as the particle-release frequency ( fp) increased, likely due to enhanced particle interactions at closer proximity. Particle pair dispersions, lateral (R2 L) and vertical (R2z ), were modulated by H1, initial separation r0 and fp. The R2 L dispersion displayed ballistic scaling initially, Taylor scaling for r0 < H1 and Richardson scaling for r0 > H1. In contrast, R2z followed a R2z ∼ t5.5 scaling under r0 < H1. Both R2 L and R2z plateaued at a distance from the interface, depending on H1 and fp.

Original languageEnglish (US)
Article numberA26
JournalJournal of Fluid Mechanics
StatePublished - Jun 10 2024


  • particle/fluid flow

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering
  • Applied Mathematics


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