Spatially and temporally resolved measurements of bead resuspension and saltation in a turbulent water channel flow

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Abstract

Abstract Resuspension and saltation of nearly neutrally buoyant polystyrene beads dp = 583 ± 14.4μm , ρ9 = 1050 kg m-3 in a turbulent boundary layer were studied using time-resolved particle image velocimetry and particle tracking velocimetry in a horizontal water channel facility Reh = 7353. The time difference between frames was Δ t+ = 0.297, comparable to the particle Stokes number, τp+ = 0.267. Near-wall coherent structures were visualized using spatial distributions of vorticity and swirling strength in combination with those of the instantaneous u1u2 correlations and u1. Two case studies, the first on resuspension and the second on saltation, showed that in all cases lift-off coincided with the passage of a vortex core, creating an ejection-sweep cycle ('burst') responsible for lift-off. In all cases beads left the wall when immersed in near-wall ejections and exposed to positive shear. As a consequence, a high shear-induced lift force coincided with bead lift-off, while the Magnus force due to bead rotation and translation-induced lift were negligible. The wall-normal component of the drag force mostly opposed lift-off, causing the bead's deceleration. The difference between resuspension and saltation was governed by the type of coherent flow structures encountered by the beads when lifted out of the viscous sublayer. Resuspension was observed when beads were carried upwards by the combined action of a strong, spatially coherent upstream fast moving u 1>0 flow structure and a downstream ejection. On the other hand, saltation was accompanied by similar but weaker and spatially less coherent near-wall turbulence structures.

Original languageEnglish
Pages (from-to)389-423
Number of pages35
JournalJournal of Fluid Mechanics
Volume715
DOIs
StatePublished - 25 Jan 2013

Keywords

  • particle/fluid flow
  • sediment transport
  • turbulent boundary layers

All Science Journal Classification (ASJC) codes

  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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