| 1. |
- Adrian, Juliane, et al.
(författare)
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A Glossary for Research on Human Crowd Dynamics
- 2019
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Ingår i: Collective Dynamics. - 2366-8539. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- This article presents a glossary of terms that are frequently used in research onhuman 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 allowseveral 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.
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| 2. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Breakup of small aggregates in bounded and unbounded turbulent flows
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- Breakup of small tracer-like aggregates is studied by means of numerical simulations in four different flows, namely homogeneous isotropic turbulence, smooth stochastic flow, turbulent channel flow, and developing boundary layer flow. Aggregate breakup occurs when the local hydrodynamic stress σ ∼ ε1/2, where ε is the local energy dissipation, overcomes a given threshold value σcr [or equivalently εcr ∼ σcr2 ] characteristic for a given type of aggregates. Following the aggregate trajectory upon release and detecting the first occurrence of local energy dissipation exceeding the predefined threshold allows for estimating the breakup rate as a function of εcr. Results show that the breakup rate decreases with increasing threshold. For small values of the threshold, this decrease assumes consistent scaling among the different flows which is explained by universal small scale flow properties.
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| 3. |
- Bäbler, Matthäus, et al.
(författare)
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Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 766
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Tidskriftsartikel (refereegranskat)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. The simplest criterion for breakup is adopted, whereby aggregate breakup occurs when the local hydrodynamic stress sigma similar to epsilon(1/2), with epsilon being the energy dissipation at the position of the aggregate, overcomes a given threshold sigma(cr), which is characteristic for a given type of aggregate. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a scaling behaviour among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, the results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.
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| 4. |
- Corbetta, Alessandro, et al.
(författare)
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Fluctuations around mean walking behaviors in diluted pedestrian flows
- 2017
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Ingår i: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 95:3
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Tidskriftsartikel (refereegranskat)abstract
- Understanding and modeling the dynamics of pedestrian crowds can help with designing and increasing the safety of civil facilities. A key feature of a crowd is its intrinsic stochasticity, appearing even under very diluted conditions, due to the variability in individual behaviors. Individual stochasticity becomes even more important under densely crowded conditions, since it can be nonlinearly magnified and may lead to potentially dangerous collective behaviors. 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 yearlong 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 behaviors 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 directions. 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 model for the dynamics of undisturbed pedestrians, based on stochastic differential equations, that provides a good agreement with our field observations, including the occurrence of rare events.
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| 5. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Asymmetric invasion in anisotropic porous media
- 2021
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Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 104:4
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Tidskriftsartikel (refereegranskat)abstract
- We report and discuss, by means of pore-scale numerical simulations, the possibility of achieving a directional-dependent two-phase flow behavior during the process of invasion of a viscous fluid into anisotropic porous media with controlled design. By customising the pore-scale morphology and heterogeneities with the adoption of anisotropic triangular pillars distributed with quenched disorder, we observe a substantially different invasion dynamics according to the direction of fluid injection relative to the medium orientation, that is depending if the triangular pillars have their apex oriented (flow aligned) or opposed (flow opposing) to the main flow direction. Three flow regimes can be observed: (i) for low values of the ratio between the macroscopic pressure drop and the characteristic pore-scale capillary threshold, i.e., for Δp0/pc≤1, the fluid invasion dynamics is strongly impeded and the viscous fluid is unable to reach the outlet of the medium, irrespective of the direction of injection; (ii) for intermediate values, 1<Δp0/pc≤2, the viscous fluid reaches the outlet only when the triangular pillars are flow-opposing oriented; (iii) for larger values, i.e., for Δp0/pc>2, the outlet is again reached irrespective of the direction of injection. The porous medium anisotropy induces a lower effective resistance when the pillars are flow-opposing oriented, suppressing front roughening and capillary fingering. We thus argue that the invasion process occurs as long as the pressure drop is larger then the macroscopic capillary pressure determined by the front roughness, which in the case of flow-opposing pillars is halved. We present a simple approximated model, based on Darcy's assumptions, that links the macroscopic effective permeability with the directional-dependent front roughening, to predict the asymmetric invasion dynamics. This peculiar behavior opens up the possibility of fabrication of porous capillary valves to control the flow along certain specific directions.
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| 6. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Asymmetric Two-phase Flows Resistance in Homogeneous and Heterogeneous Anisotropic Porous Microstructure
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Two-phase flows in porous media exhibit anomalous behaviours at low capillary numbers due to the complex mechanism of interaction between flow spatial configuration and topology of the microstructure. In this study, we investigate the asymmetrical nature of the two-phase flow resistance induced by the anisotropic features of the porous microstructure. We perform pore-scale direct numerical simulations of two-phase flows in porous media composed of solid particles with different shapes and orientations, using the Lattice-Boltzmann method. The results indicate that the infiltration of a fluid into a single pore is regulated by the topological traits of the pore, including its anisotropy. These traits determine a geometrical characteristic length of the pore ℓp quantifying the flow resistance, which is directional-dependent: if the capillary length ℓγ=γ/pc (i.e. the ratio between surface tension and capillary pressure) falls below the characteristic pore length ℓγ<ℓp, pore infiltration occurs, otherwise the fluid remains trapped. We extend the analysis to heterogeneous anisotropic microstructure in order to investigate the effect of the spatial configuration of the pores on the global flow resistance. *This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 790744.
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| 7. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Directional-dependent invasion dynamics in anisotropic porous media with customised disorder
- 2021
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We show possibility of achieving a directional-dependent two-phase flow behaviour during the process of invasion of a viscous fluid into anisotropic porous media with customised pore-scale morphology and heterogeneity. Via pore-scale numerical simulations, we observe a substantially different invasion dynamics according to the medium orientation relative to the direction of fluid injection, i.e. with flow-aligned or flow-opposing oriented pillars. The porous medium anisotropy induces a lower effective resistance when the pillars are flow-opposing oriented, suppressing front roughening and capillary fingering, while promoting transverse invasion with respect to the direction of fluid injection. We argue that fluid infiltration occurs as long as the pressure drop is larger then the macroscopic capillary pressure determined by the front roughness. We present a simple approximated model, based on Darcy's assumptions, that links the macroscopic effective permeability with the directional-dependent front roughening. The model correctly predicts an intermediate flow regime, defined by a specific range of values of the ratio between the macroscopic pressure drop and the medium characteristic pore-scale capillary threshold, within which the injected viscous fluid reaches the outlet only whith flow-opposing oriented pillars. The prediction of the observed directional-dependent fluid conductance is important for e.g. the fabrication of porous materials that act as capillary valves to control the flow along certain specific directions. This work is supported by the Horizon 2020 research and innovation programme, Grant agreement No 790744, and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), Grant Numbers 2019-01261. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at C3SE and HPC2N partially funded by the Swedish Research Council through Grant agreement no. 2018-05973.
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| 8. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Pore trapping mechanisms in two-phase flows through fuel cells porous media
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The peak performances in fuel cells are strongly limited by flooding of the electrodes. Flooding occurs as a consequence of the intense electrochemical reaction in the cell, when, at high current density, a high amount of water is produced and trapped in the gas diffusion layers (GDLs). The trapped water blocks the pores of the fibrous GDLs and impedes the uniform diffusion of fuel gas along the cell area. Therefore, a fast removal of water from the porous layer is demanded. How to enhance such a mechanism in order to improve the water management remains still an open question. In the present study we address this problem with an innovative bottom-up approach: by means of Lattice-Boltzmann simulations, we investigate the effects of the microstructure at the pore-scale on the two-phase flows dynamics, in order to optimise the GDLs design and obtain an efficient water management at the macroscale. Results show that, during imbibition, the wetting phase can be trapped at the pore throat, impeding liquid removal. The trapping mechanism is primarily governed by the thermodynamic energy barriers induced by the pore expansions, in a complementary way to Haines jump during drainage. This undesired phenomenon is exacerbated in hydrophobic media during imbibition. These findings suggest a possible new route for innovative design of gas diffusion layers in fuel cells applications.
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| 9. |
- Ronchi, Enrico, et al.
(författare)
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New approaches to evacuation modelling for fire safety engineering applications
- 2019
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Ingår i: Fire Safety Journal. - : Elsevier BV. - 0379-7112. ; 106, s. 197-209
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Tidskriftsartikel (refereegranskat)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.
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| 10. |
- Xue, Xiao, 1989, et al.
(författare)
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A lattice Boltzmann study of particle settling in a fluctuating multicomponent fluid under confinement
- 2021
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Ingår i: European Physical Journal E. - : Springer Science and Business Media LLC. - 1292-8941 .- 1292-895X. ; 44:11
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Tidskriftsartikel (refereegranskat)abstract
- We present mesoscale numerical simulations based on the coupling of the fluctuating lattice Boltzmann method for multicomponent systems with a wetted finite-size particle model. This newly coupled methodologies are used to study the motion of a spherical particle driven by a constant body 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. After some validations steps in the absence of 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 settling velocity is quantitatively assessed, especially in comparison with unconfined situations. Results show that the expected effects of confinement are very well captured by the numerical simulations, wherein the 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 proposed methodology in highlighting the effects of confinement on the motion of particles in the presence of thermal fluctuations.
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