Time persistency of floating particle clusters in free-surface turbulence

We study the dispersion of light particles floating on a flat shear-free surface of an open channel in which the flow is turbulent. This configuration mimics the motion of buoyant matter (e.g. phytoplankton, pollutants or nutrients) in water bodies when surface waves and ripples are smooth or absent. We perform direct numerical simulation of turbulence coupled with Lagrangian particle tracking, considering different values of the shear Reynolds number (Re{\tau} = 171 and 509) and of the Stokes number (0.06 < St < 1 in viscous units). Results show that particle buoyancy induces clusters that evolve towards a long-term fractal distribution in a time much longer than the Lagrangian integral fluid time scale, indicating that such clusters over-live the surface turbulent structures which produced them. We quantify cluster dynamics, crucial when modeling dispersion in free-surface flow turbulence, via the time evolution of the cluster correlation dimension

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

Thermal stratification hinders gyrotactic micro-organism rising in free-surface turbulence

Thermal stratification in water bodies influences the exchange of heat, momentum, and chemical species across the air-water interface by modifying the sub-surface turbulence characteristics. Turbulence modifications may in turn prevent small motile algae (phytoplankton, in particular) from reaching the heated surface. We examine how different regimes of stable thermal stratification affect the motion of these microscopic organisms (modelled as gyrotactic self-propelling cells) in a freesurface turbulent channel flow. This archetypal setup mimics an environmentally plausible situation that can be found in lakes and oceans. Results from direct numerical simulations of turbulence coupled with Lagrangian tracking reveal that rising of bottom-heavy self-propelling cells depends strongly on the strength of stratification, especially near the thermocline where high temperature and velocity gradients occur: Here hydrodynamic shear may disrupt directional cell motility and hamper nearsurface accumulation. For all gyrotactic re-orientation times considered in this study (spanning two orders of magnitude), we observe a reduction of the cell rising speed and temporary confinement under the thermocline: If re-orientation is fast, cells eventually trespass the thermocline within the simulated time span; if re-orientation is slow, confinement lasts much longer because cells align in the streamwise direction and their vertical swimming is practically annihilated

Wind effect on gyrotactic micro-organism surfacing in free-surface turbulence

We examine the effect of wind-induced shear on the orientation and distribution of motile micro-swimmers in free-surface turbulence. Winds blowing above the air-water interface can influence the distribution and productivity of motile organisms via the shear generated just below the surface. Swimmer dynamics depend not only on the advection of the fluid but also on external stimuli like nutrient concentration, light, gravity, which are in turn coupled to and influenced by the distribution of the swimmers. Here we focus on gyrotaxis, resulting from the gravitational torque generated by an asymmetric mass distribution within the organism. The combination of such torque with the viscous torque due to shear can reorient swimmers, reducing their vertical migration and causing entrapment in horizontal fluid layers. Through DNS-based Euler-Lagrangian simulations we investigate the effect of wind-induced shear on the motion of gyrotactic swimmers in turbulent open channel flow. We consider different wind forcing and swimmers with different reorientation time (reflecting the ability to react to turbulent fluctuations). We show that only stable (high-gyrotaxis) swimmers may reach the surface and form densely concentrated filaments, the topology of which depends on the wind direction. Otherwise swimmers exhibit weaker vertical fluxes and loose segregation at the surface. \ua9 2017 Elsevier Ltd

Inchiesta su Gramsci

Antonio Gramsci è oggetto non soltanto di studi e ricerche, ma anche di polemiche che negli ultimi anni hanno conosciuto una notevole recrudescenza, segnale in fondo di una inesausta vitalità del pensiero di uno degli autori italiani più studiati e tradotti a livello internazionale. Ma le polemiche testimoniano altresì che esistono momenti e vicende della biografia gramsciana, su cui ancora necessitano scavi documentari, ipotesi di lavoro, o semplici riletture di testi e documenti già noti. Il libro nasce, da una parte, dalla registrazione delle polemiche tuttora in corso, ma dall’altro, dalla consapevolezza che anche le contese apparentemente stolte e superflue, possono aiutare a sciogliere nodi, o quanto meno a porre nuove domande: purché si sia animati da autentica volontà di sapere, come la rivista «Historia Magistra», dal cui seno il libro è in qualche modo scaturito. Fondato su un Questionario, il volume ha raccolto le risposte di 26 studiosi e studiose, coordinati da Angelo d’Orsi, autore anche della robusta Introduzione. In appendice un testo del nipote di Gramsci, Antonio junior