Direct numerical simulation of the distribution of floating microplastic particles in an open channel flow

Microplastic fragments in the aquatic environment constitute a major threat for the health and fitness of organisms. However, our quantitative understanding in the microplastic load in typical natural river systems is severely limited due to the large uncertainties associated with the sources and the pathways of the microplastic contamination. To address this knowledge gap, we performed direct numerical simulations of the dynamics and distribution of microplastic particles in turbulent open channel flow at moderate Reynolds numbers. The particle dynamics is characterised by four nondimensional parameters, namely: Reynolds number of the open channel flow ((Formula presented.)), nondimensional particle diameter ((Formula presented.)) and Galileo ((Formula presented.)) and Stokes ((Formula presented.)) numbers of the particles of which the latter two include the particle-fluid density ratio ((Formula presented.)). To limit our scope to the most relevant configuration, we focused on the distribution of weakly buoyant microplastic particles at (Formula presented.), whereas the remaining parameters were adjusted to cover the orders of magnitude that can be found in a typical laboratory facility, as well as a natural river. Our simulation results show that the steady-state microplastic distribution in the turbulent flow is influenced by the Stokes and the Galileo numbers significantly, which ranges from the complete accumulation on the free surface to the homogeneous distribution, and somewhere in between. Moreover, the Galileo number, alongside the flow Reynolds number, were also shown to influence the temporal scaling of the transient behaviour of the gradual accumulation of the microplastics towards the free surface. Both of our findings highlight the complex nature of the particle–turbulence interactions, and motivate further investigations in this approach.