Settling processes of cylindrical microplastics in quiescent water: A fully resolved numerical simulation study

Area of Science:

• Environmental science
• Fluid dynamics
• Materials science

Background:

• Marine microplastics (MPs) pose environmental risks, with their transport and fate influenced by settling processes.
• Understanding the settling behavior of MP fibers, characterized by cylindrical geometry, is crucial for predicting their movement in aquatic environments.

Purpose of the Study:

• To develop and validate a three-dimensional numerical model for simulating the settling of marine microplastics with complex shapes in still water.
• To investigate the influence of aspect ratio, angular displacement, and density on the settling dynamics of cylindrical microplastic fibers.

Main Methods:

• Utilized the lattice Boltzmann method (LBM) and the immersed boundary method (IBM) to construct a fully resolved three-dimensional numerical model.
• Performed simulations of cylindrical microplastic fiber settling and compared results with established semi-empirical settling velocity formulas.

Main Results:

• The numerical model accurately reproduced the settling velocity of cylindrical MPs, showing good agreement with theoretical formulas.
• A critical aspect ratio for cylindrical MPs was identified to be between 0.93 and 0.94, associated with a reduction in drag force.
• Angular displacement and aspect ratio were found to influence the horizontal movement of MPs, while density primarily affected vertical settling velocity.

Conclusions:

• The study provides a validated numerical framework for simulating microplastic settling, crucial for environmental transport models.
• The identified critical aspect ratio offers insights into the hydrodynamic behavior of microplastic fibers, impacting their environmental fate.
• Microplastic shape and density are key factors governing their movement and distribution in marine ecosystems.