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 cells
