publications

@article{8fda6705603a42dc816f892c2a400490,

title = {Topological structure and dynamics of three-dimensional active nematics},

author = {Guillaume Duclos and Raymond Adkins and Debarghya Banerjee and {Matthew S E } Peterson and Minu Varghese and Itamar Kolvin and Arvind Baskaran and {Robert A } Pelcovits and {Thomas R } Powers and Aparna Baskaran and Federico Toschi and {Michael F } Hagan and {Sebastian J } Streichan and Vincenzo Vitelli and {Daniel A } Beller and Zvonimir Dogic},

doi = {10.1126/science.aaz4547},

issn = {0036-8075},

year  = {2020},

date = {2020-01-01},

journal = {Science},

volume = {367},

number = {6482},

pages = {1120--1124},

publisher = {American Association for the Advancement of Science (AAAS)},

abstract = {Topological structures are effective descriptors of the nonequilibrium dynamics of diverse many-body systems. For example, motile, point-like topological defects capture the salient features of two-dimensional active liquid crystals composed of energy-consuming anisotropic units. We dispersed force-generating microtubule bundles in a passive colloidal liquid crystal to form a three-dimensional active nematic. Light-sheet microscopy revealed the temporal evolution of the millimeter-scale structure of these active nematics with single-bundle resolution. The primary topological excitations are extended, charge-neutral disclination loops that undergo complex dynamics and recombination events. Our work suggests a framework for analyzing the nonequilibrium dynamics of bulk anisotropic systems as diverse as driven complex fluids, active metamaterials, biological tissues, and collections of robots or organisms.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{efabf45ce9844b1d8f94506184714688,

title = {Statistical properties of turbulence in the presence of a smart small-scale control},

author = {Michele Buzzicotti and Luca Biferale and Federico Toschi},

doi = {10.1103/PhysRevLett.124.084504},

issn = {0031-9007},

year  = {2020},

date = {2020-01-01},

journal = {Physical Review Letters},

volume = {124},

number = {8},

publisher = {American Physical Society},

abstract = {By means of high-resolution numerical simulations, we compare the statistical properties of homogeneous and isotropic turbulence to those of the Navier-Stokes equation where small-scale vortex filaments are strongly depleted, thanks to a nonlinear extra viscosity acting preferentially on high vorticity regions. We show that the presence of such smart small-scale drag can strongly reduce intermittency and non-Gaussian fluctuations. Our results pave the way towards a deeper understanding on the fundamental role of degrees of freedom in turbulence as well as on the impact of (pseudo)coherent structures on the statistical small-scale properties. Our work can be seen as a first attempt to develop smart-Lagrangian forcing or drag mechanisms to control turbulence.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{b37a36cda08840a4a249c0973eca6e25,

title = {On interevent time distributions of avalanche dynamics},

author = {Pinaki Kumar and Evangelos Korkolis and Roberto Benzi and Dmitry Denisov and Andr é and Peter Schall and Federico Toschi and Jeannot Trampert},

doi = {10.1038/s41598-019-56764-6},

issn = {2045-2322},

year  = {2020},

date = {2020-01-01},

journal = {Scientific Reports},

volume = {10},

number = {1},

publisher = {Nature Publishing Group},

abstract = {Physical systems characterized by stick-slip dynamics often display avalanches. Regardless of the diversity of their microscopic structure, these systems are governed by a power-law distribution of avalanche size and duration. Here we focus on the interevent times between avalanches and show that, unlike their distributions of size and duration, the interevent time distributions are able to distinguish different mechanical states of the system. We use experiments on granular systems and numerical simulations of emulsions to show that systems having the same probability distribution for avalanche size and duration can have different interevent time distributions. Remarkably, these interevent time distributions look similar to those for earthquakes and, if different from an exponential, are indirect evidence of non trivial space-time correlations among avalanches. Our results therefore indicate that interevent time statistics are essential to characterise the dynamics of avalanches.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{0c6a159182504960bcab1be66495fb3c,

title = {Pedestrian orientation dynamics from high-fidelity measurements},

author = {Joris Willems and Alessandro Corbetta and Vlado Menkovski and Federico Toschi},

year  = {2020},

date = {2020-01-01},

journal = {arXiv},

publisher = {Cornell University Library},

abstract = {We investigate in real-life conditions and with very high accuracy the dynamics of body rotation, or yawing, of walking pedestrians - an highly complex task due to the wide variety in shapes, postures and walking gestures. We propose a novel measurement method based on a deep neural architecture that we train on the basis of generic physical properties of the motion of pedestrians. Specifically, we leverage on the strong statistical correlation between individual velocity and body orientation: the velocity direction is typically orthogonal with respect to the shoulder line. We make the reasonable assumption that this approximation, although instantaneously slightly imperfect, is correct on average. This enables us to use velocity data as training labels for a highly-accurate point-estimator of individual orientation, that we can train with no dedicated annotation labor. We discuss the measurement accuracy and show the error scaling, both on synthetic and real-life data: we show that our method is capable of estimating orientation with an error as low as 7.5 degrees. This tool opens up new possibilities in the studies of human crowd dynamics where orientation is key. By analyzing the dynamics of body rotation in real-life conditions, we show that the instantaneous velocity direction can be described by the combination of orientation and a random delay, where randomness is provided by an Ornstein-Uhlenbeck process centered on an average delay of 100ms. Quantifying these dynamics could have only been possible thanks to a tool as precise as that proposed.},

keywords = {cs.LG, physics.soc-ph},

pubstate = {published},

tppubtype = {article}

}

@article{1e8ee0c6d37d40c886799ab8582ff9ee,

title = {Unified theoretical and experimental view on transient shear banding},

author = {Roberto Benzi and Thibaut Divoux and Catherine Barentin and S é and Mauro Sbragaglia and Federico Toschi},

doi = {10.1103/PhysRevLett.123.248001},

issn = {0031-9007},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review Letters},

volume = {123},

number = {24},

publisher = {American Physical Society},

abstract = {Dense emulsions, colloidal gels, microgels, and foams all display a solidlike behavior at rest characterized by a yield stress, above which the material flows like a liquid. Such a fluidization transition often consists of long-lasting transient flows that involve shear-banded velocity profiles. The characteristic time for full fluidization τf has been reported to decay as a power law of the shear rate γ and of the shear stress σ with respective exponents α and β. Strikingly, the ratio of these exponents was empirically observed to coincide with the exponent of the Herschel-Bulkley law that describes the steady-state flow behavior of these complex fluids. Here we introduce a continuum model, based on the minimization of a "free energy," that captures quantitatively all the salient features associated with such transient shear banding. More generally, our results provide a unified theoretical framework for describing the yielding transition and the steady-state flow properties of yield stress fluids.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{313daeaee9854121ab26a4b18b0ea997,

title = {Discrete Eulerian model for population genetics and dynamics under flow},

author = {Giorgia Guccione and Roberto Benzi and Abigail Plummer and Federico Toschi},

doi = {10.1103/PhysRevE.100.062105},

issn = {2470-0045},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review E},

volume = {100},

number = {6},

publisher = {American Physical Society},

abstract = {Marine species reproduce and compete while being advected by turbulent flows. It is largely unknown, both theoretically and experimentally, how population dynamics and genetics are changed by the presence of fluid flows. Discrete agent-based simulations in continuous space allow for accurate treatment of advection and number fluctuations, but can be computationally expensive for even modest organism densities. In this report, we propose an algorithm to overcome some of these challenges. We first provide a thorough validation of the algorithm in one and two dimensions without flow. Next, we focus on the case of weakly compressible flows in two dimensions. This models organisms such as phytoplankton living at a specific depth in the three-dimensional, incompressible ocean experiencing upwelling and/or downwelling events. We show that organisms born at sources in a two-dimensional time-independent flow experience an increase in fixation probability.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{046d24a1bab542e983a477781595c64f,

title = {Deep learning velocity signals allows to quantify turbulence intensity},

author = {Alessandro Corbetta and Vlado Menkovski and Roberto Benzi and Federico Toschi},

year  = {2019},

date = {2019-01-01},

journal = {arXiv},

publisher = {Cornell University Library},

abstract = {Turbulence, the ubiquitous and chaotic state of fluid motions, is characterized by strong and statistically non-trivial fluctuations of the velocity field, over a wide range of length- and time-scales, and it can be quantitatively described only in terms of statistical averages. Strong non-stationarities hinder the possibility to achieve statistical convergence, making it impossible to define the turbulence intensity and, in particular, its basic dimensionless estimator, the Reynolds number. Here we show that by employing Deep Neural Networks (DNN) we can accurately estimate the Reynolds number within $15%$ accuracy, from a statistical sample as small as two large-scale eddy-turnover times. In contrast, physics-based statistical estimators are limited by the rate of convergence of the central limit theorem, and provide, for the same statistical sample, an error at least $100$ times larger. Our findings open up new perspectives in the possibility to quantitatively define and, therefore, study highly non-stationary turbulent flows as ordinarily found in nature as well as in industrial processes.},

keywords = {cond-mat.stat-mech, cs.AI, cs.LG, physics.flu-dyn},

pubstate = {published},

tppubtype = {article}

}

@article{e86486974c294689ad18cb25f012fe2e,

title = {Topological structure and dynamics of three dimensional active nematics},

author = {Guillaume Duclos and Raymond Adkins and Debarghya Banerjee and {Matthew S E } Peterson and Minu Varghese and Itamar Kolvin and Arvind Baskaran and {Robert A } Pelcovits and {Thomas R } Powers and Aparna Baskaran and Federico Toschi and {Michael F } Hagan and {Sebastian J } Streichan and Vincenzo Vitelli and {Daniel A } Beller and Zvonimir Dogic},

year  = {2019},

date = {2019-01-01},

journal = {arXiv},

publisher = {Cornell University Library},

abstract = {Point-like motile topological defects control the universal dynamics of diverse two-dimensional active nematics ranging from shaken granular rods to cellular monolayers. A comparable understanding in higher dimensions has yet to emerge. We report the creation of three-dimensional active nematics by dispersing extensile microtubule bundles in a passive colloidal liquid crystal. Light-sheet microscopy reveals the millimeter-scale structure of active nematics with a single bundle resolution and the temporal evolution of the associated nematic director field. The dominant excitations of three-dimensional active nematics are extended charge-neutral disclination loops that undergo complex dynamics and recombination events. These studies introduce a new class of non-equilibrium systems whose turbulent-like dynamics arises from the interplay between internally generated active stresses, the chaotic flows and the topological structure of the constituent defects.},

note = {13 pages, 5 figures, plus Supplementary Information of 12 pages, 4 figures},

keywords = {cond-mat.soft},

pubstate = {published},

tppubtype = {article}

}

@article{4008646d8f584ce7a26171d228d6b022,

title = {Statistical properties of thermally expandable particles in soft-turbulence Rayleigh-Bénard convection},

author = {Kim Alards and Rudie Kunnen and Herman Clercx and Federico Toschi},

doi = {10.1140/epje/i2019-11882-y},

issn = {1292-8941},

year  = {2019},

date = {2019-01-01},

journal = {European Physical Journal E : Soft Matter},

volume = {42},

number = {9},

publisher = {Springer},

abstract = {Abstract.: The dynamics of inertial particles in Rayleigh-Bénard convection, where both particles and fluid exhibit thermal expansion, is studied using direct numerical simulations (DNS) in the soft-turbulence regime. We consider the effect of particles with a thermal expansion coefficient larger than that of the fluid, causing particles to become lighter than the fluid near the hot bottom plate and heavier than the fluid near the cold top plate. Because of the opposite directions of the net Archimedes’ force on particles and fluid, particles deposited at the plate now experience a relative force towards the bulk. The characteristic time for this motion towards the bulk to happen, quantified as the time particles spend inside the thermal boundary layers (BLs) at the plates, is shown to depend on the thermal response time, τ T, and the thermal expansion coefficient of particles relative to that of the flui},

keywords = {Problems and Applications, Topical issue: Flowing Matter},

pubstate = {published},

tppubtype = {article}

}

@article{d03238a0027f4132ad45a1d7327e35d7,

title = {Particle settling in a fluctuating multicomponent fluid under confinement},

author = {Xiao Xue and Luca Biferale and Mauro Sbragaglia and Federico Toschi},

year  = {2019},

date = {2019-01-01},

journal = {arXiv},

publisher = {Cornell University Library},

abstract = {We study the motion of a spherical particle driven by a constant volume force in a confined channel with a fixed square cross-section. The channel is filled with a mixture of two liquids under the effect of thermal fluctuations. We use the lattice Boltzmann method to simulate a fluctuating multicomponent fluid in the mixed-phase, and particle-fluid interactions are tuned to reproduce different wetting properties at the particle surface. The numerical set-up is first validated in the absence of thermal fluctuations; to this aim, we quantitatively compute the drift velocity at changing the particle radius and compare it with previous experimental and numerical data. In the presence of thermal fluctuations, we study the fluctuations in the particle's velocity at changing thermal energy, applied force, particle size, and particle wettability. The importance of fluctuations with respect to the mean drift velocity is quantitatively assessed, especially in comparison to unconfined situations. Results show that confinement strongly enhances the importance of velocity fluctuations, which can be one order of magnitude larger than what expected in unconfined domains. The observed findings underscore the versatility of the lattice Boltzmann simulations in concrete applications involving the motion of colloidal particles in a highly confined environment in the presence of thermal fluctuations.},

note = {12pages, 7 figures},

keywords = {cond-mat.soft, physics.comp-ph, physics.flu-dyn},

pubstate = {published},

tppubtype = {article}

}

@article{67228bed5d894c6688f967ac3899f499,

title = {Unified theoretical and experimental view on transient shear banding},

author = {Roberto Benzi and Thibaut Divoux and Catherine Barentin and S é and Mauro Sbragaglia and Federico Toschi},

year  = {2019},

date = {2019-01-01},

journal = {arXiv},

publisher = {Cornell University Library},

abstract = {Dense emulsions, colloidal gels, microgels, and foams all display a solid-like behavior at rest characterized by a yield stress, above which the material flows like a liquid. Such a fluidization transition often consists of long-lasting transient flows that involve shear-banded velocity profiles. The characteristic time for full fluidization, $tau_textf$, has been reported to decay as a power-law of the shear rate $dot gamma$ and of the shear stress $sigma$ with respective exponents $alpha$ and $beta$. Strikingly, the ratio of these exponents was empirically observed to coincide with the exponent of the Herschel-Bulkley law that describes the steady-state flow behavior of these complex fluids. Here we introduce a continuum model based on the minimization of an out-of-equilibrium free energy that captures quantitatively all the salient features associated with such textittransient shear-banding. More generally, our results provide a unified theoretical framework for describing the yielding transition and the steady-state flow properties of yield stress fluids.},

note = {5 pages, 4 figures - supplemental 5 pages, 4 figures},

keywords = {cond-mat.soft, cond-mat.stat-mech, physics.flu-dyn},

pubstate = {published},

tppubtype = {article}

}

@article{709efdea7c844697ad1b12ad5a26718c,

title = {Sharp transitions in rotating turbulent convection: Lagrangian acceleration statistics reveal a second critical Rossby number},

author = {Kim Alards and Rudie Kunnen and R J A M Stevens and Detlef Lohse and Federico Toschi and Herman Clercx},

doi = {10.1103/PhysRevFluids.4.074601},

issn = {2469-990X},

year  = {2019},

date = {2019-01-01},

journal = {Physical Review Fluids},

volume = {4},

number = {7},

pages = {1--19},

publisher = {American Physical Society},

abstract = {In Rayleigh–Bénard convection (RBC) for fluids with Prandtl number Pr≳1, rotation beyond a critical (small) rotation rate is known to cause a sudden enhancement of heat transfer, which can be explained by a change in the character of the boundary layer (BL) dynamics near the top and bottom plates of the convection cell. Namely, with increasing rotation rate, the BL signature suddenly changes from Prandtl–Blasius type to Ekman type. The transition from a constant heat transfer to an almost linearly increasing heat transfer with increasing rotation rate is known to be sharp and the critical Rossby number Roc occurs typically in the range 2.3≲Roc≲2.9 (for Rayleigh number Ra=1.3×10},

note = {16 pages, 9 Figures},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{3bcafd866cbd49059b6ca109743cb862,

title = {New approaches to evacuation modelling for fire safety engineering applications},

author = {Enrico Ronchi and Alessandro Corbetta and {Edwin R } Galea and Max Kinateder and Erica Kuligowski and Denise Mcgrath and Adam Pel and Youssef Shiban and Peter Thompson and Federico Toschi},

doi = {10.1016/j.firesaf.2019.05.002},

issn = {0379-7112},

year  = {2019},

date = {2019-01-01},

journal = {Fire Safety Journal},

volume = {106},

pages = {197--209},

publisher = {Elsevier},

abstract = {This paper presents the findings of the workshop “New approaches to evacuation modelling”, which took place on the 11th of June 2017 in Lund (Sweden)within the Symposium of the International Association for Fire Safety Science (IAFSS). The workshop gathered international experts in the field of fire evacuation modelling from 19 different countries and was designed to build a dialogue between the fire evacuation modelling world and experts in areas outside of fire safety engineering. The contribution to fire evacuation modelling of five topics within research disciplines outside fire safety engineering (FSE)have been discussed during the workshop, namely 1)Psychology/Human Factors, 2)Sociology, 3)Applied Mathematics, 4)Transportation, 5)Dynamic Simulation and Biomechanics. The benefits of exchanging information between these two groups are highlighted here in light of the topic areas discussed and the feedback received by the evacuation modelling community during the workshop. This included the feasibility of development/application of modelling methods based on fields other than FSE as well as a discussion on their implementation strengths and limitations. Each subject area is here briefly presented and its links to fire evacuation modelling are discussed. The feedback received during the workshop is discussed through a set of insights which might be useful for the future developments of evacuation models for fire safety engineering.},

keywords = {Egress, Emergency, Evacuation modelling, Exit choice, Fire safety, Human behaviour, Pedestrian dynamics, Pre-evacuation, Smoke},

pubstate = {published},

tppubtype = {article}

}

@article{6cd0288cc1244a8a9cf47d6884ae3f0b,

title = {Topical issue on fluids and structures: multi-scale coupling and modeling},

author = {Luca Biferale and Stefano Guido and Andrea Scagliarini and Federico Toschi},

doi = {10.1140/epje/i2019-11808-9},

issn = {1292-8941},

year  = {2019},

date = {2019-01-01},

journal = {European Physical Journal E : Soft Matter},

volume = {42},

number = {3},

pages = {28},

publisher = {Springer},

keywords = {Topical issue: Fluids and Structures: Multi-scale coupling and modeling},

pubstate = {published},

tppubtype = {article}

}

@article{adf67c42d6074f0fb2ad603537f6f3d1,

title = {Fixation probabilities in weakly compressible fluid flows},

author = {Abigail Plummer and Roberto Benzi and {David R } Nelson and Federico Toschi},

doi = {10.1073/pnas.1812829116},

issn = {0027-8424},

year  = {2019},

date = {2019-01-01},

journal = {Proceedings of the National Academy of Sciences of the United States of America (PNAS)},

volume = {116},

number = {2},

pages = {373--378},

abstract = {Competition between biological species in marine environments is affected by the motion of the surrounding fluid. An effective 2D compressibility can arise, for example, from the convergence and divergence of water masses at the depth at which passively traveling photosynthetic organisms are restricted to live. In this report, we seek to quantitatively study genetics under flow. To this end, we couple an off-lattice agent-based simulation of two populations in 1D to a weakly compressible velocity field—first a sine wave and then a shell model of turbulence. We find for both cases that even in a regime where the overall population structure is approximately unaltered, the flow can significantly diminish the effect of a selective advantage on fixation probabilities. We understand this effect in terms of the enhanced survival of organisms born at sources in the flow and the influence of Fisher genetic waves.},

keywords = {Fisher wave, Population genetics, Selective advantage, Stochastic processes, Turbulence},

pubstate = {published},

tppubtype = {article}

}

@article{88739111b7b14cfcb36893c00262541e,

title = {A glossary for research on human crowd dynamics},

author = {Juliane Adrian and Nikolai Bode and Martyn Amos and Mitra Baratchi and Mira Beermann and Maik Boltes and Alessandro Corbetta and Guillaume Dezecache and John Drury and Zhijian Fu and Roland Geraerts and Steve Gwynne and Gesine Hofinger and Aoife Hunt and Tinus Kanters and Angelika Kneidl and Krisztina Konya and Gerta K ö and Mira K ü and Georgios Michalareas and Fergus Neville and Evangelos Ntontis and Stephen Reicher and Enrico Ronchi and Andreas Schadschneider and Armin Seyfried and Alastair Shipman and Anna Sieben and Michael Spearpoint and {Gavin Brent} Sullivan and Anne Templeton and Federico Toschi and Zeynep Y ü and Francesco Zanlungo and Iker Zuriguel and Natalie {van der Wal} and Frank {van Schadewijk} and Cornelia {von Krüchten} and Nanda Wijermans},

doi = {10.17815/CD.2019.19},

issn = {2366-8539},

year  = {2019},

date = {2019-01-01},

journal = {Collective Dynamics},

volume = {4},

pages = {1--13},

publisher = {Forschungszentrum Jülich},

abstract = {This article presents a glossary of terms that are frequently used in research on human crowds. This topic is inherently multidisciplinary as it includes work in and across computer science, engineering, mathematics, physics, psychology and social science, for example. We do not view the glossary presented here as a collection of finalised and formal definitions. Instead, we suggest it is a snapshot of current views and the starting point of an ongoing process that we hope will be useful in providing some guidance on the use of terminology to develop a mutual understanding across disciplines. The glossary was developed collaboratively during a multidisciplinary meeting. We deliberately allow several definitions of terms, to reflect the confluence of disciplines in the field. This also reflects the fact not all contributors necessarily agree with all definitions in this glossary.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{3ff9675dcc7a455a86c8e7c4decb19f6,

title = {Physics-based modeling and data representation of pairwise interactions among pedestrians},

author = {A Corbetta and J A Meeusen and C Lee and Roberto Benzi and F Toschi},

doi = {10.1103/PhysRevE.98.062310},

issn = {2470-0045},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review E},

volume = {98},

number = {6},

publisher = {American Physical Society},

abstract = {In this work we study pedestrian-pedestrian interactions from observational experimental data in diluted pedestrian crowds. While in motion, pedestrians continuously adapt their walking paths trying to preserve mutual comfort distances and to avoid collisions. Leveraging on a high-quality, high-statistics data set, composed of several few millions real-life trajectories acquired from state-of-the-art observational experiments (about 6 months of high-resolution pedestrian tracks acquired in a train station), we develop a quantitative model capable of addressing interactions in the case of binary collision avoidance. We model interactions in terms of both long-range (sight based) and short-range (hard-contact avoidance) forces, which we superimpose on our Langevin model for noninteracting pedestrian motion [Corbetta, Phys. Rev. E 95, 032316 (2017)2470-004510.1103/PhysRevE.95.032316] (here further tested and extended). The model that we propose here features a Langevin dynamics with fast random velocity fluctuations that are superimposed on the slow dynamics of a hidden model variable: the intended walking path. In the case of interactions, social forces may act both on the intended path and on the actual walked path. The model is capable of reproducing quantitatively relevant statistics of the collision avoidance motion, such as the statistics of the side displacement and of the passing speed. Rare occurrences of actual bumping events are also recovered. Furthermore, comparing with large data sets of real-life tracks involves an additional computational challenge so far neglected: identifying automatically, within a database containing very heterogeneous conditions, only the relevant events corresponding to binary avoidance interactions. In order to tackle this challenge, we propose a general approach based on a graph representation of pedestrian trajectories, which allows us to effectively operate complexity reduction for efficient data classification and selection.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{e5b00c30eb0a4b9296c91a4696acba91,

title = {Effect of particle shape on fluid statistics and particle dynamics in turbulent pipe flow},

author = {A Gupta and {H J H } Clercx and F Toschi},

doi = {10.1140/epje/i2018-11724-6},

issn = {1292-8941},

year  = {2018},

date = {2018-01-01},

journal = {European Physical Journal E : Soft Matter},

volume = {41},

number = {10},

publisher = {Springer},

abstract = {Anisotropic particles are present in many natural and industrial flows. Here we perform direct numerical simulation (DNS) of turbulent pipe flows with dispersed finite-size prolate spheroids simulated by means of the lattice Boltzmann method (LBM). We consider three different particle shapes: spheroidal (aspect ratio 2 and 3) and spherical. These three simulations are complemented with a reference simulation of a single-phase flow. For the sake of comparison, all simulations, laden or unladen have the same energy input. The flow geometry used is a straight pipe with length eight times its radius where the fluid is randomly seeded with 256 finite-size particles. The volume fraction of particles in the flow has been kept fixed at 0.48% by varying the major and minor axis of each particle such that their volume remains the same. We studied the effect of different particle shapes on particle dynamics and orientation, as well as on the flow modulation. We show that the local accumulation of spheres close to the wall decreases for spheroids with increasing aspect ratio. These spheroidal particles rotate slower than spheres near to the wall and tend to stay with their major axes aligned to the flow streamwise direction. Despite the lower rotation rates, a higher intermittency in the rotational rates was observed for spheroids and this increase at increasing the aspect ratio. The drag reduction observed for particles with higher aspect ratio have also been investigated using the one-dimensional energy and dissipation spectra. These results point to the relevance of particle shapes on their dynamics and their influence on the turbulent flow.},

keywords = {Problems and Applications, Topical issue: Flowing Matter},

pubstate = {published},

tppubtype = {article}

}

@article{d3f9ebd509d54075b1c4f2049e7689c4,

title = {Evolution of populations expanding on curved surfaces},

author = {{Daniel A } Beller and {Kim M J } Alards and Francesca Tesser and {Ricardo A } Mosna and Federico Toschi and Wolfram M ö},

doi = {10.1209/0295-5075/123/58005},

issn = {0295-5075},

year  = {2018},

date = {2018-01-01},

journal = {EPL},

volume = {123},

number = {5},

publisher = {Institute of Physics},

abstract = {The expansion of a population into new habitat is a transient process that leaves its footprints in the genetic composition of the expanding population. How the structure of the environment shapes the population front and the evolutionary dynamics during such a range expansion is little understood. Here, we investigate the evolutionary dynamics of populations consisting of many selectively neutral genotypes expanding on curved surfaces. Using a combination of individual-based off-lattice simulations, geometrical arguments, and lattice-based stepping-stone simulations, we characterise the effect of individual bumps on an otherwise flat surface. Compared to the case of a range expansion on a flat surface, we observe a transient relative increase, followed by a decrease, in neutral genetic diversity at the population front. In addition, we find that individuals at the sides of the bump have a dramatically increased expected number of descendants, while their neighbours closer to the bump's centre are far less lucky. Both observations can be explained using an analytical description of straight paths (geodesics) on the curved surface. Complementing previous studies of heterogeneous flat environments, the findings here build our understanding of how complex environments shape the evolutionary dynamics of expanding populations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{181b79e7a9204995b79fc0c169fde21f,

title = {A numerical tool for the study of the hydrodynamic recovery of the Lattice Boltzmann Method},

author = {Guillaume Tauzin and Luca Biferale and Mauro Sbragaglia and Abhineet Gupta and Federico Toschi and Andreas Bartel and Matthias Ehrhardt},

doi = {10.1016/j.compfluid.2018.05.031},

issn = {0045-7930},

year  = {2018},

date = {2018-01-01},

journal = {Computers & Fluids},

volume = {172},

pages = {241--250},

publisher = {Elsevier},

abstract = {We investigate the hydrodynamic recovery of Lattice Boltzmann Method (LBM) by analyzing exact balance relations for energy and enstrophy derived from averaging the equations of motion on sub-volumes of different sizes. In the context of 2D isotropic homogeneous turbulence, we first validate this approach on decaying turbulence by comparing the hydrodynamic recovery of an ensemble of LBM simulations against the one of an ensemble of Pseudo-Spectral (PS) simulations. We then conduct a benchmark of LBM simulations of forced turbulence with increasing Reynolds number by varying the input relaxation times of LBM. This approach can be extended to the study of implicit subgrid-scale (SGS) models, thus offering a promising route to quantify the implicit SGS models implied by existing stabilization techniques within the LBM framework.},

keywords = {Hydrodynamics, Lattice Boltzmann Method, Turbulence modeling},

pubstate = {published},

tppubtype = {article}

}

@article{c9750f5f458f48c999fa8f07e9f76c72,

title = {Hybrid lattice Boltzmann-direct simulation Monte Carlo approach for flows in three-dimensional geometries},

author = {G {Di Staso} and S Srivastava and E Arlemark and {H J H } Clercx and F Toschi},

doi = {10.1016/j.compfluid.2018.03.043},

issn = {0045-7930},

year  = {2018},

date = {2018-01-01},

journal = {Computers & Fluids},

volume = {172},

pages = {492--509},

publisher = {Elsevier},

abstract = {We present the results of a comparative study performed with three numerical methods applied to a flow in a three-dimensional geometry characterized by weak compressibility and large rarefaction effects. The employed methods, all based on the kinetic theory of gases, are the Lattice Boltzmann Method (LBM) in a regularized formulation, the Direct Simulation Monte Carlo (DSMC) approach and a hybrid method coupling the LBM and the DSMC recently developed by Di Staso et al., in this contribution extended to the case of simulations involving many particles and three-dimensional geometries. Owing to the common kinetic nature shared by the employed methods and to their implementation in a single code infrastructure, a detailed comparison of the results can be performed on a quantitative ground. The numerical results permit to determine, for the studied flow problem, the range of applicability in terms of a geometry-based Knudsen number for the present LBM formulation. The need to employ the hybrid method is justified by the very large computational cost of the DSMC simulation. Limitations of the current hybrid method formulation in treating thermal and large compressibility effects are underlined and possible strategies to overcome them are delineated. Finally, good scalability properties of the parallel algorithms, as well as the large computational cost reduction guaranteed by the hybrid method, while providing an accurate solution, are demonstrated.},

keywords = {Direct simulation Monte Carlo, Hybrid method, Kinetic theory, Lattice Boltzmann Method, Rarefied gas flows},

pubstate = {published},

tppubtype = {article}

}

@article{2091e326523f46f9b40375248ab42774,

title = {Effects of thermal fluctuations in the fragmentation of a nanoligament},

author = {X Xue and M Sbragaglia and L Biferale and F Toschi},

doi = {10.1103/PhysRevE.98.012802},

issn = {2470-0045},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review E},

volume = {98},

number = {1},

publisher = {American Physical Society},

abstract = {We study the effects of thermally induced capillary waves in the fragmentation of a liquid ligament into multiple nanodroplets. Our numerical implementation is based on a fluctuating lattice Boltzmann (LB) model for nonideal multicomponent fluids, including nonequilibrium stochastic fluxes mimicking the effects of molecular forces at the nanoscales. We quantitatively analyze the statistical distribution of the breakup times and the droplet volumes after the fragmentation process at changing the two relevant length scales of the problem, i.e., the thermal length scale and the ligament size. The robustness of the observed findings is also corroborated by quantitative comparisons with the predictions of sharp interface hydrodynamics. Beyond the practical importance of our findings for nanofluidic engineering devices, our study also explores a novel application of LB in the realm of nanofluidic phenomena.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259191-202,

title = {Computational study of radial particle migration and stresslet distributions in particle-laden turbulent pipe flow},

author = {A Gupta and H J H Clercx and F Toschi},

year  = {2018},

date = {2018-01-01},

journal = {European Physical Journal E},

volume = {41},

number = {3},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259191-291,

title = {Directional change of tracer trajectories in rotating Rayleigh-Benard convection},

author = {K M J Alards and H Rajaei and R P J Kunnen and F Toschi and H J H Clercx},

year  = {2018},

date = {2018-01-01},

journal = {Physical Review E},

volume = {97},

number = {6},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259191-183,

title = {Lattice Boltzmann simulations of droplet dynamics in time-dependent flows},

author = {F Milan and M Sbragaglia and L Biferale and F Toschi},

year  = {2018},

date = {2018-01-01},

journal = {European Physical Journal E},

volume = {41},

number = {1},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{05a494845f7c4dcbbca95ca054bffb5b,

title = {Modelling sea ice and melt ponds evolution: sensitivity to microscale heat transfer mechanisms},

author = {A Scagliarini and E Calzavarini and Daniela Mansutti and F Toschi},

year  = {2018},

date = {2018-01-01},

journal = {arXiv},

volume = {2018},

publisher = {Cornell University Library},

abstract = {We present a mathematical model describing the evolution of sea ice and meltwater during summer. The system is described by two coupled partial differential equations for the ice thickness h and pond depth w fields. We test the sensitivity of the model to variations of parameters controlling fluid-dynamic processes at the pond level, namely the variation of turbulent heat flux with pond depth and the lateral melting of ice enclosing a pond. We observe that different heat flux scalings determine different rates of total surface ablations, while the system is relatively robust for what concerns the statistical distributions of pond surface areas, which are extracted by means of cluster analysis. Finally, we study pond morphology in terms of fractal dimensions, showing that the role of lateral melting is minor, whereas there is evidence of an impact from the initial sea ice topography.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259191-257,

title = {Simulation of Finite-Size Particles in Turbulent Flows Using the Lattice Boltzmann Method},

author = {A Gupta and H J H Clercx and F Toschi},

year  = {2018},

date = {2018-01-01},

journal = {Communications in Computational Physics},

volume = {23},

number = {3},

pages = {665-684},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{c9d76a1b63fb406b81daaf598f6126a6,

title = {Frame vs. trajectory analyses of pedestrian dynamics asymmetries in a staircase landing},

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

doi = {10.17815/CD.2017.10},

issn = {2366-8539},

year  = {2017},

date = {2017-01-01},

journal = {Collective Dynamics},

volume = {1},

number = {A10},

pages = {1--26},

publisher = {Forschungszentrum Jülich},

abstract = {Real-life, out-of-laboratory, measurements of pedestrian walking dynamics allow extensive and fully-resolved statistical analyses. However, data acquisition in real-life is subjected to the randomness and heterogeneity that characterizes crowd flows over time. In a typical real-life location, disparate flow conditions follow one another in random order: for instance, a low density pedestrian co-flow dynamics may suddenly turn into a high density counter-flow scenario and then back again. Isolating occurrences of similar flow conditions within the acquired data is a paramount first step in the analyses in order to avoid spurious statistics and to enable qualitative comparisons.In this paper we extend our previous investigation on the asymmetric pedestrian dynamics on a staircase landing, where we collected a large statistical database of measurements from ad hoc continuous recordings. This contribution has a two-fold aim: first, method-wise, we discuss an analysis workflow to consider large-scale experimental measurements, suggesting two querying approaches to automatically extract occurrences of similar flow scenarios out of datasets. These pursue aggregation of similar scenarios on either a frame or a trajectory basis. Second, we employ these two different perspectives to further explore asymmetries in the pedestrian dynamics in our measurement site. We report cross-comparisons of statistics of pedestrian positions, velocities and accelerations vs. flow conditions as well as vs. querying approach.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259191-299,

title = {Copepods encounter rates from a model of escape jump behaviour in turbulence},

author = {H Ardeshiri and F G Schmitt and S Souissi and F Toschi and E Calzavarini},

year  = {2017},

date = {2017-01-01},

journal = {Journal of Plankton Research},

volume = {39},

number = {6},

pages = {878-890},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259191-171,

title = {Enhanced settling of nonheavy inertial particles in homogeneous isotropic turbulence: The role of the pressure gradient and the Basset history force},

author = {M A T van Hinsberg and H J H Clercx and F Toschi},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review E},

volume = {95},

number = {2},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259191-155,

title = {Finite-size effects on bacterial population expansion under controlled flow conditions},

author = {F Tesser and J C H Zeegers and H J H Clercx and L Brunsveld and F Toschi},

year  = {2017},

date = {2017-01-01},

journal = {Scientific Reports},

volume = {7},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259192-336,

title = {Flowing Matter 2017},

author = {L Campo-Deano and N Araujo and I P Mora and F Toschi},

year  = {2017},

date = {2017-01-01},

journal = {Applied Rheology},

volume = {27},

number = {3},

pages = {47-49},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259192-292,

title = {Fluctuations around mean walking behaviors in diluted pedestrian flows},

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

year  = {2017},

date = {2017-01-01},

journal = {Physical Review E},

volume = {95},

number = {3},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259192-249,

title = {Geometry of tracer trajectories in rotating turbulent flows},

author = {K M J Alards and H Rajaei and L Del Castello and R P J Kunnen and F Toschi and H J H Clercx},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review Fluids},

volume = {2},

number = {4},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RID:0801180259192-264,

title = {Multiscale anisotropic fluctuations in sheared turbulence with multiple states},

author = {K P Iyer and F Bonaccorso and L Biferale and F Toschi},

year  = {2017},

date = {2017-01-01},

journal = {Physical Review Fluids},

volume = {2},

number = {5},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{ba1dbdb0ddd24f03bd2313cde67ca761,

title = {Fluctuations around mean walking behaviours in diluted pedestrian flows},

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

year  = {2016},

date = {2016-01-01},

journal = {arXiv.org, e-Print Archive, Physics},

number = {1610.07429v1},

pages = {1--10},

abstract = {Understanding and modeling the dynamics of pedestrian crowds can help with designing and increasing the safety of civil facilities. A key feature of crowds is its intrinsic stochasticity, appearing even under very diluted conditions, due to the variability in individual behaviours. Individual stochasticity becomes even more important under densely crowded conditions, since it can be nonlinearly magnified and may lead to potentially dangerous collective behaviours. To understand quantitatively crowd stochasticity, we study the real-life dynamics of a large ensemble of pedestrians walking undisturbed, and we perform a statistical analysis of the fully-resolved pedestrian trajectories obtained by a year-long high-resolution measurement campaign. Our measurements have been carried out in a corridor of the Eindhoven University of Technology via a combination of Microsoft Kinect 3D-range sensor and automatic head-tracking algorithms. The temporal homogeneity of our large database of trajectories allows us to robustly define and separate average walking behaviours from fluctuations parallel and orthogonal with respect to the average walking path. Fluctuations include rare events when individuals suddenly change their minds and invert their walking direction. Such tendency to invert direction has been poorly studied so far even if it may have important implications on the functioning and safety of facilities. We propose a novel model for the dynamics of undisturbed pedestrians, based on stochastic differential equations, that provides a good agreement with our experimental observations, including the occurrence of rare events.},

keywords = {physics.data-an, physics.soc-ph},

pubstate = {published},

tppubtype = {article}

}

@article{68dcea056c30443eaa8e4cafce542089,

title = {Continuous measurements of real-life bidirectional pedestrian flows on a wide walkway},

author = {A Corbetta and J A Meeusen and C -M Lee and F Toschi},

year  = {2016},

date = {2016-01-01},

journal = {arXiv},

pages = {1--9},

publisher = {Cornell University Library},

abstract = {Employing partially overlapping overhead kinectTMS sensors and automatic pedestrian tracking algorithms we recorded the crowd traffic in a rectilinear section of the main walkway of Eindhoven train station on a 24/7 basis. Beside giving access to the train platforms (it passes underneath the railways), the walkway plays an important connection role in the city. Several crowding scenarios occur during the day, including high- and low-density dynamics in uni- and bi-directional regimes. In this paper we discuss our recording technique and we illustrate preliminary data analyses. Via fundamental diagrams-like representations we report pedestrian velocities and fluxes vs. pedestrian density. Considering the density range $0$ - $1.1,$ped/m$^2$, we find that at densities lower than $0.8,$ped/m$^2$ pedestrians in unidirectional flows walk faster than in bidirectional regimes. On the opposite, velocities and fluxes for even bidirectional flows are higher above $0.8,$ped/m$^2$.},

note = {9 pages, 7 figures},

keywords = {physics.data-an, physics.soc-ph},

pubstate = {published},

tppubtype = {article}

}

@article{05679cb42abe4f178b839c71697fa9f8,

title = {Preferential rotation of chiral dipoles in isotropic turbulence},

author = {S Kramel and G A Voth and S Tympel and F Toschi},

year  = {2016},

date = {2016-01-01},

journal = {arXiv.org, e-Print Archive, Physics},

abstract = {Particles in the shape of chiral dipoles show a preferential rotation in three dimensional homogeneous isotropic turbulence. A chiral dipole consists of a rod with two helices of opposite handedness, one at each end. We can use 3d printing to fabricate these particles with length in the inertial range and track their rotations in a turbulent flow between oscillating grids. High aspect ratio chiral dipoles will align with the extensional eigenvectors of the strain rate tensor and the helical ends will respond to the strain field by spinning around its long axis. The mean of the measured spinning rate is non-zero and reflects the average stretching the particles experience. We use Stokesian dynamics simulations of chiral dipoles in pure strain flow to quantify the dependence of spinning on particle shape. Based on the known response to pure strain, we build a model that gives the spinning rate of small chiral dipoles using Lagrangian velocity gradients from high resolution direct numerical simulations. The statistics of chiral dipole spinning determined with this model show surprisingly good agreement with the measured spinning of much larger chiral dipoles in the experiments.},

keywords = {physics.flu-dyn},

pubstate = {published},

tppubtype = {article}

}