Particles and fibers in fluid flows for environmental applications

The thesis is divided in two parts showing applications of active and passive models, in which the Lagrangian tracking of rigid fibers and slender swimmers can be employed to access to flow properties, and to study a turbulent puff in order to access information on spreading of virus-containing droplets. The first part of the thesis addresses the measure of flow properties by means of slender objects, i.e. rigid fibers. Two different strategies have been employed to model the fiber: first an active model, a fully-coupled fiber described in terms of an immersed-boundary method, and, second, a passive model, a one-way coupling where the fiber is described by the slender body theory. After the characterization of the fiber inertia in terms of rotational Stokes Number, the motion of the fiber is investigated for different classes of closed streamline flows, steady or time dependent, two-dimensional or three dimensional. For sufficiently small Stokes, the fiber turns out to be a proxy of the underlying flow by considering the velocity difference between the fiber end points and the same difference relative to the underlying fluid velocity, both projected along the direction normal to the fiber. Moreover, by composing an assembly of rigid fiber, the whole flow gradient tensor can be accurately reconstructed by simply tracking the fiber assembly and measuring suitable fiber velocity differences evaluated at the fiber ends. Furthermore, it has been investigated the possibility to measure two points flow properties by means of slender swimmers. A swimmer model, describing both pusher and puller swimmers, has been proposed and validated in the Stokes limit finding excellent agreement. The slender swimmer model has been exploited in a chaotic flow field up to a flow Reynolds number of 10, a swimming number ranging between zero and one and different swimmer inertia measured in terms of a suitable definition of the swimmer Stokes number. The following results have been obtained: (i) pusher and puller reach different swimming velocities for the same, given, propulsive force, due to a different distribution of the vorticity within the wake. (ii) for a wide range of flow/swimmer Reynolds numbers, both pusher and puller swimmers are able to sense hydrodynamic signals with good accuracy. The second part of the thesis is devoted to understand the role of the turbulence on the fate of virus-containing droplets expelled during a human cough, modeled as a turbulent puff, under realistic conditions. To this aim, high resolution DNS have been performed for the fluid flow and humidity field, complemented by a passive Lagrangian solver for the droplet dynamics including a dynamical equation for the evolution of the droplet radii modeling the evaporation-condensation process. After having validated the turbulent puff against theoretical predictions, the results show how a full account of turbulence is crucial to determine the fate of virus-containing droplets. Then, the dependence of the results on the droplets initial size distribution and different ambient humidity is investigated. As a further step, it is analyzed the dependence of results on the airborne virus spreading on gender. Finally, the effectiveness of the barriers as protection devices within indoor environment is investigated. This study is clearly motivated by the recent pandemic situation due to COVID-19 infection, although it is valid for all the infections where the main route of transmission is via airborne virus-containing droplets, by contributing to select optimal strategies of protection and mitigation of the airborne infection transmission, within indoor and outdoor environments.