Particle dispersion in turbulent flows is fundamental to science as well as to technology. Examples of open scientific issues include emissions reduction in combustion, rheological characterization of fibrous particle suspension, plankton population dynamics, transport and mixing of pollutants, to name a few. The simplest numerical framework to study the dynamical and statistical features of turbulent particle dispersion is based on the assumption that particles can be modeled as point-like spheres brought about by the flow. In spite of its simplicity, this framework has led to significant advancements in the study of particles-turbulence interactions, allowing the precise identification of the coherent structures responsible for particle sedimentation and reentrainment in turbulent boundary layers [1]. In this talk we examine one important source of bias for particle dispersion that arises when particles are non-spherical (elongated) and may actively move within the fluid (motile). In particular, we show how elongation and motility add to particle inertia to modulate particle motion, preferential concentration and accumulation in turbulent flow [2-4]. We also discuss the additional modeling complexities associated to non-sphericity and selfpropulsion. To these aims, we focus on the simple foundational problem of Kolmogorov-scale axisymmetric ellipsoids at low concentrations in channel flow; and we use a Eulerian-Lagrangian approach based on direct numerical simulation of turbulence to describe particle dynamics. Results relevant for particles suspended in wall-bounded and environmental turbulence are presented to give insights into the complex interactions between anisotropic particles and turbulence, which are relevant in practical applications such as industrial process optimization and oceanic carbon cycling. Two different situations will be considered: inertial fibers in turbulent channel flow and selfpropelling micro-swimmers in free-surface turbulence. Depending on the application, inertial fibers are treated either as rigid bodies, which translate and rotate as rigid bodies relative to the surrounding fluid, or as flexible bodies, modeled as chains of rods connected through ball-and-socket joints that enable bending and twisting.

Self-propelling micro-swimmers are modeled as inertia-less cells biased by gyrotaxis (defined as directed locomotion resulting from a combination of gravitational and viscous torques in a flow) and can be considered as archetypal systems for characterizing the locomotion behavior of more complex biological organisms like swimming bacteria or motile cells.

Cristian Marchioli is Associate Professor of Fluid Mechanics at the University of Udine and Editor of Acta Mechanica. In 2011-2015 he served as chairman of the COST Action on “Fiber suspension flow modeling” and, in 2016, as scientific secretary of the 9th Int. Conference on Multiphase Flow (ICMF 2016). Currently, Prof. Marchioli is member of the scientific council of the International Center of Mechanical Sciences, where he coordinated the 2013 and 2014 advanced schools on particle dynamics in turbulence. His research interests involve multiphase flow modeling, from small-scale particle-turbulence interactions to large-scale modeling of gas-solid/gas-liquid flows. He has edited the book "Collective dynamics of particles: From viscous to turbulent flows" (Springer), and has contributed to ERCOFTAC’s "Best practice guidelines for computational fluid dynamics of dispersed multiphase flows". Prof. Marchioli has published 50+ papers and 100+ conference proceedings on multiphase flow modeling and simulation. He has also delivered several invited and keynote lectures (among others: 20th AIMETA Conf. in 2011, 13th Int. Conf. Multiphase Flows in Industrial Plants and Nordita Conf. on Dynamics of Particles in Flows in 2014, 3rd Int. Conf. Numerical Methods in Multiphase Flow in 2017).