Proceedings of the 11 th International Conference on Fluid Flow, Heat and Mass Transfer (FFHMT 2024) Chestnut Conference Centre - University of Toronto, Toronto, Canada – June 16-18, 2024 Paper No. 028 DOI: 10.11159/ffhmt24.028 028-1 Terminal Settling Velocity of Cylindrical Rods with Various Geometries Applicable to Atmospheric Microplastics Amirhossein Hamidi 1 , Daniel Daramsing 1 , Mark D. Gordon 2 , Liisa M. Jantunen 3 , Ronald E. Hanson 1 1 Department of Mechanical Engineering, York University 4700 Keele St, Toronto, M3J1P0, Ontario, Canada. Emails: ahamidi7@yorku.ca; danielvd@my.yorku.ca; hansonre@yorku.ca; 2 Earth and Space Science and Engineering, York University, 4700 Keele St, Toronto, M3J1P0, Ontario, Canada. Email: mgordon@yorku.ca 3 Air Quality Processes Research Section, Environment and Climate Change Canada, 6248 8th Line, Egbert, L0L1N0, Ontario, Canada. Email: Liisa.Jantunen@ec.gc.ca; Abstract – In this research, a set of straight, curved, V-shaped, and U-shaped cylindrical rods are dropped in a chamber filled with a quiescent 90% glycerin mixture to approximate the settling of microplastic fibres in the atmosphere. The 3D fall trajectory and terminal velocity of the rods are determined using images captured from two cameras facing the two perpendicular sides of the chamber. The results show that the terminal velocities of the curved and V-shaped rods are both greater than that of the straight rods with the same diameter and aspect ratio, with a maximum difference of 17% and 57% relative to the straight rods for the curved and V-shaped rods respectively within the ranges studied in this research. U-shaped rods always exhibit a greater terminal velocity than straight rods with the same dimensions, with a maximum difference of 39% calculated in this research. As the aspect ratio of a U-shaped rod increases, the terminal velocity initially increases, reaches a peak value, and then decreases, reflecting the interplay between the length of the rod arms and the inclination angle. Keywords: Microplastics; Microfibres; Settling Velocity; Particle; Environment. 1. Introduction Microplastics (MPs) are a subset of plastics with sizes ranging between 1 µm and 5 mm [1-3]. MP pollution is known to be spread through the environment, spans the globe from urban to remote areas [4-7], and can pose various risks to human health and ecosystems. For instance, inhaling MPs can lead to respiratory illnesses [8-10]. These particles may also contain harmful components like monomers, chemicals, colors, and additives, which can further endanger human health when inhaled or ingested [11, 12]. Their presence in ecosystems, including food sources like fish and table salt, raises concerns about food safety and potential health issues such as reproductive problems and cancer [13]. The discovery of MPs in Arctic glaciers has ecological implications as they can absorb sunlight, reducing the ice surface albedo. This contributes to accelerated ice melting and disrupts climate patterns [14, 15]. Of reported atmospheric deposition samples, fibres are a prevalent shape, and often the most abundant [2, 5, 16, 17]. Past efforts to model the atmospheric transport of microplastic fibres (MFs) have often relied on simplified representations of shapes such as spheres or a single straight fibre to estimate transport properties such as terminal velocity [6, 18, 19]. In the present research, the settling of rigid cylindrical rods of various finite lengths and shapes are studied to better understand how length and shape can affect microfibre behaviour at the low Reynolds numbers expected in local or global atmospheric transport and to provide input for simulations of atmospheric transport. Numerous aerodynamic models to estimate the terminal velocity of general non-spherical particles are reviewed by Michaelides and Feng [20]. At the core of modelling predictions is the particle size or shape representation. For example, Ganser [21] modelled the drag coefficient of isometric and nonisometric particles using a generalized Reynolds number, a function of Stoke’s and Newton’s shape factors, to predict the drag coefficient. In the study conducted by Song et al. [22], drop tests were performed using spheres, cubes, and cylinders of varying sizes and aspect ratios in glycerin mixtures with different viscosities. They introduced sphericity, indicating the ratio of the area of a volume-equivalent sphere to the particle