| dc.description.abstract | Turbulence and intermittency are manifestations of non-linear dynamics that have central influence in transport mechanisms in fluids. These phenomena have been mainly studied in inertial flows, but this situation is recently changing due to a growing number of investigations conducted in driven and active granular systems exhibiting signs of such dynamics. Despite all the research developed in these areas there are still open questions regarding the validity or the accuracy of any analogy between the turbulent behaviors in inertial and granular flows. Addressing these issues, in this thesis we develop experimental and theoretical research on active and driven granular media, confined in Hele-Shaw cells and exhibiting coherent structures under different dynamical regimes. Our aim is to characterize quantitatively complex dynamics ruled by long range correlations and provide evidence on the connection between inertial turbulence and chaotic-like behaviours in granular flows.
For the active granular case we analyze a synthetic system made of polystyrene beads in a leaky dielectric fluid under the Quincke effect. We study two regimes: i) a steady state where we find, for the first time, a transition from an apparently chaotic dynamics to large scales coherent structures, solely by changing the external electric field and ii), a non-stationary regime where the same transition occurs evolving with time under a fixed value of the field strength. We find that the spatio-temporal disordered phase resembles the behavior reported in bacterial colonies. Moreover, the velocity statistics and the energy spectrum display quantitative tendencies analogous to those found in some inertial turbulent flows. In ii), we obtain that the characteristic vortex diameter grows with the square root of time, a result consistent with studies of vortex growing in two-dimensional inertial turbulence. Using a particle simulation model, this transient state can be reproduced by a competition between short-range alignment interactions and hydrodynamic dipolar interactions coupled with self-orientation dynamics.
For the driven granular case we characterize a confined heap of grains also in two regimes: I) the heap growth displaying a transition from a continuous to an intermittent dynamics, and II) a stationary state in the surface flow of this confined pile. In I), the functional dependencies with the input flux and feeding height of pile size at which the intermittent regime start is experimentally determined. These results are described by a model that considers finite size effects resulting from long-rage correlations between grains. In II), the velocity profile at the free surface of the heap, considering granular turbulent-like eddies in the context of the Prandtl mixing length approach, is modeled. We obtain for first time an analytical expression for this profile, that accurately reproduce several experimental data. | en_US |
