publications

@book{de2f1b1359cc43c9bae3edfd39e67e7a,

title = {Asymmetric pedestrian dynamics on a staircase landing from continuous measurements},

author = {A Corbetta and C M Lee and A Muntean and F Toschi},

year  = {2015},

date = {2015-01-01},

publisher = {Technische Universiteit Eindhoven},

series = {CASA-report},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{db4bac3642af4db7b0e7b01cd2d0265f,

title = {Cooperativity flows and Shear-Bandings : a statistical field theory approach},

author = {R Benzi and M Sbragaglia and M Bernaschi and S Succi and F Toschi},

year  = {2015},

date = {2015-01-01},

publisher = {s.n.},

series = {arXiv},

abstract = {Cooperativity effects have been proposed to explain the non-local rheology in the dynamics of soft jammed systems. Based on the analysis of the double well free-energy model proposed by L. Bocquet, A. Colin & A. Ajdari (em Phys. Rev. Lett. bf 103, 036001 (2009)), we show that cooperativity effects resulting from the non-local nature of the fluidity (inverse viscosity), are intimately related to the emergence of shear-banding configurations. This connection materializes through the onset of inhomogeneous compact solutions (compactons), wherein the fluidity is confined to finite-support subregions of the flow and strictly zero elsewhere. Compactons coexistence with regions of zero fluidity ("non-flowing vacuum") is shown to be stabilized by the presence of mechanical noise, which ultimately shapes up the equilibrium distribution of the fluidity field, acting as an order parameter for the flow-noflow transitions occurring in the material.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{c65f1a454b5f4ae4bf80b3a8c6dd46bf,

title = {Inertial particle acceleration in strained turbulence},

author = {Chung-min Lee and A Gylfason and P Perlekar and F Toschi},

year  = {2015},

date = {2015-01-01},

publisher = {s.n.},

series = {arXiv},

abstract = {The dynamics of inertial particles in turbulence is modelled and investigated by means of direct numerical simulation of an axisymmetrically expanding homogeneous turbulent strained flow. This flow can mimic the dynamics of particles close to stagnation points. The influence of mean straining flow is explored by varying the dimensionless strain rate parameter $Sk_0/epsilon_0$ from 0.2 to 20. We report results relative to the acceleration variances and probability density functions for both passive and inertial particles. A high mean strain is found to have a significant effect on the acceleration variance both directly, through an increase in wave number magnitude, and indirectly, through the coupling of the fluctuating velocity and the mean flow field. The influence of the strain on normalized particle acceleration pdfs is more subtle. For the case of passive particle we can approximate the acceleration variance with the aid of rapid distortion theory and obtain good agreement with simulation data. For the case of inertial particles we can write a formal expressions for the accelerations. The magnitude changes in the inertial particle acceleration variance and the effect on the probability density function are then discussed in a wider context for comparable flows, where the effects of the mean flow geometry and of the anisotropy at the small scales are present.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{ed461d970ca440ba899078018b0b6aad,

title = {On the Peterlin approximation for turbulent flows of polymer solutions},

author = {D Vincenzi and P Perlekar and L Biferale and F Toschi},

year  = {2015},

date = {2015-01-01},

publisher = {s.n.},

series = {arXiv},

abstract = {We study the impact of the Peterlin approximation on the statistics of the end-to-end separation of poly- mers in a turbulent flow. The FENE and FENE-P models are numerically integrated along a large number of Lagrangian trajectories resulting from a direct numerical simulation of three-dimensional homogeneous isotropic turbulence. Although the FENE-P model yields results in qualitative agreement with those of the FENE model, quantitative differences emerge. The steady-state probability of large extensions is overesti- mated by the FENE-P model. The alignment of polymers with the eigenvectors of the rate-of-strain tensor and with the direction of vorticity is weaker when the Peterlin approximation is used. At large Weissenberg numbers, both the correlation times of the extension and of the orientation of polymers are underestimated by the FENE-P model.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{d4d12c5487b14c65912693b2c31457ab,

title = {Turbulence on a fractal fourier set},

author = {A S Lanotte and R Benzi and L Biferale and S K Malapaka and F Toschi},

year  = {2015},

date = {2015-01-01},

publisher = {s.n.},

series = {arXiv},

abstract = {The dynamical effects of mode reduction in Fourier space for three dimensional turbulent flows is studied. We present fully resolved numerical simulations of the Navier-Stokes equations with Fourier modes constrained to live on a fractal set of dimension D. The robustness of the energy cascade and vortex stretching mechanisms are tested at changing D, from the standard three dimensional case to a strongly decimated case for D = 2.5, where only about $3%$ of the Fourier modes interact. While the direct energy cascade persist, deviations from the Kolmogorov scaling are observed in the kinetic energy spectra. A model in terms of a correction with a linear dependency on the co-dimension of the fractal set, $E(k)sim k^- 5/3 + 3 -D $, explains the results. At small scales, the intermittent behaviour due to the vorticity production is strongly modified by the fractal decimation, leading to an almost Gaussian statistics already at D ~ 2.98. These effects are connected to a genuine modification in the triad-to-triad nonlinear energy transfer mechanism.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{19e7217c779d4d36af5cb6f060b6c795,

title = {Collective dynamics from bacteria to crowds : an excursion through modeling, analysis and simulation},

editor = {A Muntean and F Toschi},

doi = {10.1007/978-3-7091-1785-9},

isbn = {978-3-7091-1784-2},

year  = {2014},

date = {2014-01-01},

publisher = {Springer},

address = {Germany},

edition = {1.},

series = {CISM courses and lectures},

abstract = {Multiscale models in social applications combine mean-field and kinetic equations with either microscopic or macroscopic level descriptions. In this book the reader will find not only a wide spectrum of multiscale analysis results (like convergence proofs), but also practically important information such as derivations of mean-field equations, methods to handle hard contacts numerically, to model group behavior, to quantitative estimate microscopic/macroscopic segregation of competing species, to quantitative understand the limits of validity of mass-action kinetics for simple reactions.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{731586fb4ea64f259a1ad7a5d52a3586,

title = {Direct evidence of plastic events and dynamic heterogeneities in soft-glasses},

author = {R Benzi and M Sbragaglia and P Perlekar and M Bernaschi and S Succi and F Toschi},

year  = {2014},

date = {2014-01-01},

publisher = {s.n.},

series = {arXiv},

abstract = {By using fluid-kinetic simulations of confined and concentrated emulsion droplets, we investigate the nature of space non-homogeneity in soft-glassy dynamics and provide quantitative measurements of the statistical features of plastic events in the proximity of the yield-stress threshold. Above the yield stress, our results show the existence of a finite stress correlation scale, which can be mapped directly onto the cooperativity scale, recently introduced in the literature to capture non-local effects in the soft-glassy dynamics. In this regime, the emergence of a separate boundary (wall) rheology with higher fluidity than the bulk, is highlighted in terms of near-wall spontaneous segregation of plastic events. Near the yield stress, where the cooperative scale cannot be estimated with sufficient accuracy, the system shows a clear increase of the stress correlation scale, whereas plastic events exhibit intermittent clustering in time, with no preferential spatial location. A quantitative measurement of the space-time correlation associated with the motion of the interface of the droplets is key to spot the long-range amorphous order at the yield stress threshold.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{e4c06cebfd644e1daac764878d5f7fba,

title = {Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows},

author = {M U Babler and L Biferale and L Brandt and U Feudel and K Guseva and A S Lanotte and C Marchioli and F Picano and G Sardina and A Soldati and F Toschi},

year  = {2014},

date = {2014-01-01},

publisher = {s.n.},

series = {arXiv},

abstract = {Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer, and homogeneous isotropic turbulence. Aggregate breakup occurs when the local hydrodynamic stress $sigmasim varepsilon^1/2$, where $varepsilon$ is the energy dissipation at the position of the aggregate, overcomes a given threshold $sigma_mathrmcr$, characteristic for a given type of aggregates. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a universal scaling among the different flows. For high thresholds, the breakup rates show strong differences among the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss limitations and applicability of a set of independent proxies.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{6923ef3321364a6281b3feba732304c0,

title = {On clustering of vertically constrained passive particles in homogeneous, isotropic turbulence},

author = {M {De Pietro} and M A T {Hinsberg van} and L Biferale and H J H Clercx and P Perlekar and F Toschi},

year  = {2014},

date = {2014-01-01},

publisher = {s.n.},

series = {arXiv.org},

abstract = {We analyze the dynamics of small particles vertically confined, by means of a linear restoring force, to move within a horizontal fluid slab in a three-dimensional (3D) homogeneous isotropic turbulent velocity field. The model that we introduce and study is possibly the simplest description for the dynamics of small aquatic organisms that, due to swimming, active regulation of their buoyancy or any other mechanism, maintain themselves in a shallow horizontal layer below the free surface of oceans or lakes. By varying the strength of the restoring force, we are able to control the thickness of the fluid slab in which the particles can move. This allows us to analyze the statistical features of the system over a wide range of conditions going from a fully 3D incompressible flow (corresponding to the case of no confinement) to the extremely confined case corresponding to a 2D slice. The background 3D turbulent velocity field is evolved by means of fully resolved direct numerical simulations. Whenever some level of vertical confinement is present, the particle trajectories deviate from that of fluid tracers and the particles experience an effectively compressible velocity field. Here, we have quantified the compressibility, the preferential concentration of the particles and the correlation dimension by changing the strength of the restoring force. The main result is that it exists a particular value of the force constant, corresponding to a mean slab depth approximately equal to a few times the Kolmogorov length scale eta, that maximizes the clustering of the particles.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{f1f99502bb3147368d190628f9f3206d,

title = {Parameter estimation of social forces in crowd dynamics models via a probabilistic method},

author = {A Corbetta and A Muntean and F Toschi and K Vafayi},

year  = {2014},

date = {2014-01-01},

publisher = {Technische Universiteit Eindhoven},

series = {CASA-report},

abstract = {Focusing on a specific crowd dynamics situation, including real life experiments and measurements, our paper targets a twofold aim: (1) we present a Bayesian probabilistic method to estimate the value and the uncertainty (in the form of a probability density function) of parameters in crowd dynamic models from the experimental data; and (2) we introduce a fitness measure for the models to classify a couple of model structures (forces) according to their fitness to the experimental data, preparing the stage for a more general model-selection and validation strategy inspired by probabilistic data analysis. Finally, we review the essential aspects of our experimental setup and measurement technique. Keywords: Crowd dynamics, parameter estimation, Bayes theorem, models classification, data analysis.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{925b3e82aea141c09dabc35574d8725e,

title = {Stochastic resonance in soft-glassy materials},

author = {R Benzi and M Bernaschi and P Perlekar and M Sbragaglia and S Succi and F Toschi},

year  = {2014},

date = {2014-01-01},

publisher = {s.n.},

series = {arXiv.org},

abstract = {Flow in soft-glasses occurs via a sequence of reversible elastic deformations and local irreversible plastic rearrangements. Yield events in the material cause kicks adding up to an effectively thermal noise, an intuition that has inspired the development of phenomenological models aiming at explaining the main features of soft-glassy rheology. In this letter, we provide a specific scenario for such mechanical activation, based on a general paradigm of non-equilibrium statistical mechanics, namely it stochastic resonance. By using mesoscopic simulations of emulsion droplets subject to an oscillatory strain, we characterize the response of the system and highlight a resonance-like behavior in the plastic rearrangements. This confirms that the synchronization of the system response to an external time-dependent load is triggered by the mechanical noise resulting from disordered configurations (polydispersity).},

keywords = {},

pubstate = {published},

tppubtype = {book}

}

@book{ceb7f7f8b03b484ca07ebd1db713bf04,

title = {Heat flux scaling in turbulent Rayleigh-Bénard convection with an imposed longitudinal wind},

author = {Andrea Scagliarini and A Gylfason and F Toschi},

year  = {2013},

date = {2013-01-01},

publisher = {s.n.},

series = {arXiv.org},

abstract = {We present a numerical study of Rayleigh-Bénard convection disturbed by a longitudinal wind. Our results show that under the action of the wind, the vertical heat flux through the cell initially decreases, due to the mechanism of plumes-sweeping, and then increases again when turbulent forced convection dominates over the buoyancy. As a result, the Nusselt number is a non-monotonic function of the shear Reynolds number. We provide a simple model that captures with good accuracy all the dynamical regimes observed. We expect that our findings can lead the way to a more fundamental understanding of the of the complex interplay between mean-wind and plumes ejection in the Rayleigh-Bénard phenomenology.},

keywords = {},

pubstate = {published},

tppubtype = {book}

}