Mechanisms for deposition and resuspension of heavy particles in turbulent flow over wavy interfaces

Abstract

It has been long recognized that turbulent flow over steep waves can produce coherent flow structures of different temporal and spatial scales. In particular, quasistreamwise vortices grow up on the upslope side of the wave and interact with geometry-dependent vortical structures, aligned spanwise and located within the recirculation bubble in the wave trough, thus creating the conditions for the development of a three-dimensional highly turbulent flow field. In this work, we have analyzed the trajectories of O(105) small dense particles (either in solid form or in the form of liquid droplets) released into a turbulent air flow over waves precisely to clarify the role of coherent vortical structures in controlling particle deposition and resuspension. The three-dimensional time-dependent flow field at Reτ=170 is calculated using large-eddy simulation, and the dynamics of individual different-sized particles is described using a Lagrangian approach. Drag, gravity, and lift are used in the equation of motion for particles that have no influence on the flow field. Particle-wall interaction is fully elastic. Our findings show that different-sized particles interact selectively with vortical flow structures, producing different distribution patterns and dispersion rates qualitatively depending on the particle-to-fluid time-scale ratio. Specifically, we find that quasistreamwise vortices on the upslope side of the wave control particle dispersion and eventual segregation in the flow separation region downstream the wave crest as well as in the shear layer forming behind the wave, just above the separation region. These vortices generate momentum mixing events that also entrain and move particles towards and away from the wavy wall. This process is similar to the sweep/ejection cycle occurring in turbulent flow over a flat boundary layer.