Effects of shape on microplastic particle-fluid-wall interaction and transport in a turbulent boundary layer
Abstract
Microplastic particles—plastic pieces less than 5 mm in size, commonly spherical, elongated, or flat in shape—are found everywhere on Earth's surface, no matter how remote the location, transported far and wide by wind and water. However, the grain-scale dynamics of microplastic particles in turbulent boundary layer flows are not fully understood. To address this gap, the dynamics of millimetric sphere-, rod- and disk-shaped plastic particles in a saltation-suspension regime are studied in a turbulent boundary layer. Simultaneous, time-resolved flow fields, particle trajectories, and particle orientations are obtained experimentally using particle image velocity and particle tracking velocimetry in a paddlewheel-driven water channel. Statistics of particle velocity, particle acceleration, and fluid velocity interpolated at particle locations are computed to investigate particle interaction with the fluid turbulence and with the wall. All three types of particle interact with the wall by touching down, remaining in contact with the wall while sliding, rolling or tumbling, then lifting off. Particle lift-off is strongly associated with regions where vf' > 0 and uf' < 0 at the particle location (ejections), while touchdown is only weakly associated with regions where vf' < 0 and uf' > 0 (sweeps). Particle orientations are compared between the disk and rod particles and are found to have opposite behaviors. Rod particles preferentially orient their axis of rotational symmetry in the streamwise direction and not in the vertical, whereas disks preferentially orient their symmetry axis in the vertical direction and not in the streamwise. In both cases the symmetry axis is rarely oriented spanwise, i.e., rarely aligned with the mean vorticity. These preferences are strongest in the near-wall region. Tumbling behavior is observed for both types of non-spherical particles, especially near the wall, and strongly affects the particle-fluid-wall interplay compared to spherical particles of similar inertia. In addition, the particle diffusivities differ significantly from the fluid momentum diffusivity. These results indicate that microplastic particle transport may not be well described by models developed for spherical particles or natural sediment transport.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP0130005B
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY;
- 4558 Sediment transport;
- OCEANOGRAPHY: PHYSICAL