**1. Introduction**

Research on the topic of particle motion is ongoing. Many pollutants that are present in the environment form a dispersed multiphase system, in which the continuous phase is the medium of the environment (gas or liquid) and the dispersed phase is made of pollutant particles. Plastic particles with diameters between 1 and 10 mm, termed mesoplastic particles or mesoplastics, represent a ubiquitous and concerning pollutant [1,2]. However, there is still a lack of consensus with respect to the definition of plastic debris size classification [1–3]. In the literature, plastic particles with diameters of less than 5 mm are often classified as microplastic particles or microplastics [3–5].

One source of pollution is the wastewater generated by various industries [6], for example, the textile industry [7]. Owing to the negative impact of plastic particles on the environment, it is of particular interest and importance to research how they spread in the environment [8–13]. The propagation of mesoplastics occurs in rivers and oceans; therefore, the topic of the motion of particles suspended in water should be the focus of further research [14,15]. Depending on their properties, mesoplastics can settle and propagate as sediment, flow freely suspended with water or float on the free surface of the water. In rivers, natural and man-made obstacles exist, which produce different flow features and regimes that can be replicated in a laboratory-scale physical model to investigate various scientific and engineering problems with respect to different modes of mesoplastic propagation.

The transport of particles on a laboratory scale has been studied in open-channel flows in the past. Cook et al. [16] experimentally studied the longitudinal dispersion of microplastic particles and suggested the use of a fluorescent tracer to study the movement of plastic particles in the environment. Yu et al. proposed an empirical formula to predict the incipient motion of microplastic particles with diameters up to 5 mm [17]. The general importance and applicability of studying particle settling was highlighted in an article

**Citation:** Kevorkijan, L.; Žic, E.; Lešnik, L.; Biluš, I. Settling of Mesoplastics in an Open-Channel Flow. *Energies* **2022**, *15*, 8786. https://doi.org/10.3390/en15238786

Academic Editors: Artur Bartosik and Dariusz Asendrych

Received: 20 October 2022 Accepted: 17 November 2022 Published: 22 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

by Yi and colleagues, who studied the settling of sturgeon eggs in an open channel [18]. In addition to experimental work, different numerical simulation approaches have been used to study the propagation of plastic particles. The dispersal of microplastic particles in coastal waters was studied by Fatahi and colleagues [15] using computational fluid dynamics (CFD); a discrete phase model (DPM) was used to track the motion of particles. Roy et al. [19] conducted a parametric study of plastic particle propagation using a Lagrangian tracking model in a lid-driven cavity flow. On a larger scale, an Eulerian modelling approach is sometimes adopted, as in a study of the transport and deposition of microplastic particles in the Baltic sea conducted by Schernewski et al. [14], who concluded that microplastic emissions resulted mostly from wastewater, as well as sewer and stormwater overflow. Another approach to Lagrangian tracking is the dense discrete phase model (DDPM), which is often used to simulate particle-laden flows in pipelines [20]. Recent attempts to model the motion of microplastic or mesoplastic particles include works by Holjevi´c et al. [21] and Travaš et al. [22], who modelled the transport of different microplastic (or mesoplastic) particles in a laminar open-channel flow.

In this work, the settling of mesoplastic particles is investigated in a 3D turbulent open-channel flow using numerical simulations and experiments.
