Dynamics of passive and active particles at the surface of a stratified/unstratified turbulent open channel flow

In this thesis the behaviour of passive and active particles in a turbulent open channel flow has been investigated. The surface of the turbulent open channel has been treated as a flat free-slip surface that bounds a three- dimensional volume in which the flow is turbulent. This configuration mimics the motion of active/passive ocean surfactants (e.g. phytoplank- ton, floaters or drifters) when surface waves and ripples are absent. The investigation include the study for stable stratified open channel flow. The nature of the surface turbulence is crucial for the dynamics of parti- cles which float in the upper layers. Surface turbulence has been analysed in terms of energy transfer among the scales and the role of surface compress- ibility has been included in this analysis. An extensive campaign of Direct Numerical Simulations (DNS) coupled with Lagrangian Particle Tracking (LPT) is used to study these phenomena. The governing equations are solved using a pseudo-spectral method for the specific case of turbulent water flow in a channel. Results show that free-surface is characterised by an inverse energy cas- cade which becomes persistent at higher Reynolds number. Surface is forced by means of upwellings which appear as two-dimensional sources for the surface-parallel fluid velocity and alternate to sinks associated with down- drafts of fluid from the surface to the bulk. Consequently, surface compress- ibility is increased. Passive buoyant particles reach the surface by means of upwellings and form highly concentrated filaments in downwelling regions. They cluster at large scales and persist for long time. In case of stratification, the surfacing is influenced by the presence of internal gravity waves and the clustering at the surface is destroyed. Finally, the case of self propelled active particles which mimics the behaviour of gyrotactic phytoplankton, has been exam- ined. The main preliminary result is that the presence of stratification is able to make the vertical migration more unstable and to delay the surfacing of the swimming cell