FIELDTURB

Particles and Fields in Turbulence and in Complex Flows

Bari:

The research of the node in Bari mainly concerned non-equilibrium statistical mechanics,  dynamics of complex fluids, and active matter. Systems of active particles have been studied in the context of Langevin approach. Diffusion, velocity fluctuations and effective temperature for a system of active dumbbells have been analyzed in [BA04] while the phase diagram of active particle systems in relation with the Kosterlitz-Thouless-Halperin-Nelson "scenario" for hard brownian disks has been studied in [BA01]. Lattice Boltzmann Methods have been applied to study the morphology and the flow patterns of active emulsions [BA03, BA06, BA08]. The behavior of a liquid droplet of active  cholesteric liquid crystal show self-propulsive behavior [BA09], see also  [BA05] for the motion of an active droplet without ordering chemical potentials. A review of LBM approach to the numerical study of active matter is done in [BA07], and improvements of LBM algorithms have been proposed in [BA02]. Fluctuations and bursts in a model for supercoiling-dependent DNA transcription have been studied in [BA10]. 

Genova:

During the last year we continued to work on:  i) data assimilation in turbulence and ii) energy harvesting in laminar flows. In collaboration with the Sezione of Roma2, in i)  we applied an important data assimilation technique (known as nudging), where partial field measurements are used to control the evolution of a dynamical system, to the canonical problem of fluid dynamics: three-dimensional homogeneous and isotropic turbulence. By doing numerical experiments we performed a systematic assessment of how well the technique reconstructs large- and small-scale features of the flow with respect to the quantity and the quality or type of data supplied to it.  Our study fixed the standard requirements for future applications of nudging to complex turbulent flows. Results are currently in press on PRX. As far as point ii) is concerned, we analysed how a network of elastic objects interacts with a laminar flow field. Due to nonlinear mutual interactions, the resulting dynamics is generally different from that of single devices and the setup optimisation (to maximise the energy extraction) turned out to be nontrivial. A parametric exploration was performed by varying the mutual distance between the devices. For the in-line arrangement, a recovery in for downstream devices was achieved by tuning their elasticity. Moreover, cooperative effects in the side-by-side arrangement are found to be substantially beneficial in terms of resulting power, which increases (i.e. constructive interference) up to 100% with respect to the single-device configuration. These results were also confirmed by wind-tunnel experiment as shown in [GE11].

Lecce:

FieldTurb activity in Lecce is about three main directions. The first is about Statistical mechanics for AI and biological applications, for which we have focused on different problems related to statistical mechanics appoarch to AI and the wide family of learning machines, with applications on the problem of collective behavior of cells in LabOnChip experiments. The second focuses on the behaviour of particles in complex flows, and the problem of fragmentation of inertial agggregates due to turbulent hydrodynamical stresses. The third is about quantum dynamics in superfluids, and in particular the study of supersolidity in Bose-Eintein condensates, and the turbulent-like behaviours in exciton-polariton systems, and the analogy with its classical counterpart.

Roma 2:

We have worked on ML application to fluids, including problems connected to optimal navigation in complex flows, turbulent data classification and data assimilation. We have performed a systematic investigation of turbulence under rotation. We have implemented a systematic investigation of Nudging for data synchronization and further explored numerical studies of rheological properties of foams and emulsions under confinement and their connections with avalanches statistics. We have started to investigate nano-droplet properties and active-matter multi-phase micro flows.

Torino:

In collaboration with the Section of Roma 2 we have studied the statistics of non-spherical particles transported by a turbulent flow and in particular the relative alignment with the local flow. The results obtained from state-of-the-art numerical simulations are explained by a statistical model solved by perturbative technique [TO35]. We extended the classical studies on the formation of a large-scale condensate in two-dimensional turbulence by considering an active fluid (i.e. forced by suspended microorganisms) [TO53] and a quasi-two-dimensional flow [TO37].  In both cases we show that it is possible to produce a large scale condensate in presence of an inverse cascade and we study the range of parameters in which this is observed. We have studied the problem of Rayleigh-Taylor (RT) turbulent convection in the presence of time-periodic acceleration [TO56]. This work is motivated by applications of RT instability to inertial confinement fusion where alternate phases of unstable and stable density stratification are observed. We show that, by controlling the period of the acceleration modulation, it is possible to arrest the growth of the RT instability. We have also studied the stabilizing effect of rotation on turbulent convection [TO40]. A major achievement in the study of nonlinear wave dynamics has been the use of the concept of instanton in the understanding of the problem of rogue waves. Indeed, in [TO44], a  statistical theory of rogue waves is proposed and tested against experimental data collected in a long water tank where random waves with different degrees of nonlinearity are mechanically generated and free to propagate along the flume. Strong evidence is given that the rogue waves observed in the tank are hydrodynamic instantons, that is, saddle point configurations of the action associated with the stochastic model of the wave system. The results indicate that the instantons describe equally well rogue waves created by simple linear superposition (in weakly nonlinear conditions) or by nonlinear focusing (in strongly nonlinear conditions), paving the way for the development of a unified explanation to rogue wave formation. We have investigated the problem of transport of mass, energy and charge in 1-dimensional classical and quantum systems. In particular we have analyzed the steady state fluctuations of the corresponding observables, identifying mechanisms that induce anomalous transport, as well as a kind of universality class to which light-front dominated transport belongs. This allows us to use exactly solvable models to compute the corresponding transport exponents [TO41].