# Física

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Artículo de Publicación Periódica Active particles with desired orientation flowing through a bottleneck(2018-12) Parisi, Daniel; Cruz Hidalgo, Raúl; Zuriguel, IkerMás... "We report extensive numerical simulations of the flow of anisotropic self-propelled particles through a constriction. In particular, we explore the role of the particles’ desired orientation with respect to the moving direction on the system flowability. We observe that when particles propel along the direction of their long axis (longitudinal orientation) the flow-rate notably reduces compared with the case of propulsion along the short axis (transversal orientation). And this is so even when the effective section (measured as the number of particles that are necessary to span the whole outlet) is larger for the case of longitudinal propulsion. This counterintuitive result is explained in terms of the formation of clogging structures at the outlet, which are revealed to have higher stability when the particles align along the long axis. This generic result might be applied to many different systems flowing through bottlenecks such as microbial populations or different kind of cells. Indeed, it has already a straightforward connection with recent results of pedestrian (which self-propel transversally oriented) and mice or sheep (which self-propel longitudinally oriented)."Más... Artículo de Publicación Periódica Analytical study of coherence in seeded modulation instability(2016-09) Bonetti, Juan I.; Hernández, Santiago M.; Fierens, Pablo Ignacio; Grosz, DiegoMás... "We derive analytical expressions for the coherence in the onset of modulation instability, in excellent agreement with thorough numerical simulations. As usual, we start by a linear perturbation analysis, where broadband noise is added to a cw pump; then, we investigate the effect of adding a deterministic seed to the cw pump, a case of singular interest as it is commonly encountered in parametric amplification schemes. Results for the dependence of coherence on parameters such as fiber type, pump power, propagated distance, and seed signal-to-noise ratio are presented. Finally, we show the importance of including higher-order linear and nonlinear dispersion when looking at longer-wavelength regions"Más... Artículo de Publicación Periódica Anti-stokes Raman gain enabled by modulation instability in mid-IR waveguides(2018-11) Sánchez, Alfredo D.; Fierens, Pablo Ignacio; Hernández, Santiago M.; Bonetti, Juan I.; Brambilla, Gilberto; Grosz, DiegoMás... "The inclusion of self-steepening in the linear stability analysis of modulation instability (MI) leads to a power cutoff above which the MI gain vanishes. Under these conditions, MI in mid-IR waveguides is shown to give rise to the usual double-sideband spectrum but with Raman-shaped sidelobes. This results from the energy transfer of a CW laser simultaneously to both stokes and anti-stokes bands in pseudo-parametric fashion. As such, the anti-stokes gain matches completely the stokes profile over the entire gain bandwidth. This remarkable behavior, not expected from an unexcited medium, is shown not to follow from a conventional four-wave mixing interaction between the pump and the Stokes band. We believe this observation to be of relevance in the area of Raman-based sensors, which, in several instances, rely on monitoring small power variations of the anti-stokes spectral component."Más... Artículo de Publicación Periódica Characteristics and stability of soliton crystals in optical fibres for the purpose of optical frequency comb generation(2017-06) Zajnulina, Marina; Böhm, M.; Bodenmüller, D.; Blow, K.; Chavez Boggioa, J. M.; Rieznik, Andrés; Roth, M. M.Más... "We study the properties of a soliton crystal, a bound state of several optical pulses that propagate with a fixed temporal separation through the optical fibres of the proposed approach for generation of optical frequency combs (OFC) for astronomical spectrograph calibration. This approach - also being suitable for subpicosecond pulse generation for other applications - consists of a conventional single-mode fibre and a suitably pumped Erbium-doped fibre. Two continuous-wave lasers are used as light source. The soliton crystal arises out of the initial deeply modulated laser field at low input powers; for higher input powers, it dissolves into free solitons. We study the soliton crystal build-up in the first fibre stage with respect to different fibre parameters (group-velocity dispersion, nonlinearity, and optical losses) and to the light source characteristics (laser frequency separation and intensity difference). We show that the soliton crystal can be described by two quantities, its fundamental frequency and the laser power-threshold at which the crystal dissolves into free solitons. The soliton crystal exhibits features of a linear and nonlinear optical pattern at the same time and is insensitive to the initial laser power fluctuations. We perform our studies using the numerical technique called Soliton Radiation Beat Analysis."Más... Artículo de Publicación Periódica Characterization of autoregressive processes using entropic quantifiers(2018-01) Traversaro Varela, Francisco; Redelico, FranciscoMás... "The aim of the contribution is to introduce a novel information plane, the causal-amplitude informational plane. As previous works seems to indicate, Bandt and Pompe methodology for estimating entropy does not allow to distinguish between probability distributions which could be fundamental for simulation or for probability analysis purposes. Once a time series is identified as stochastic by the causal complexity-entropy informational plane, the novel causal-amplitude gives a deeper understanding of the time series, quantifying both, the autocorrelation strength and the probability distribution of the data extracted from the generating processes. Two examples are presented, one from climate change model and the other from financial markets"Más... Artículo de Publicación Periódica Clogging transition of vibration-driven vehicles passing through constrictions(2017-12) Patterson, Germán; Fierens, Pablo Ignacio; Sangiuliano Jimka, Federico; König, Pablo; Garcimartín, Ángel; Zuriguel, Iker; Pugnaloni, Luis Ariel; Parisi, DanielMás... "We report experimental results on the competitive passage of elongated self-propelled vehicles rushing through a constriction. For the chosen experimental conditions, we observe the emergence of intermittencies similar to those reported previously for active matter passing through narrow doors. Noteworthy, we find that, when the number of individuals crowding in front of the bottleneck increases, there is a transition from an unclogged to a clogged state characterized by a lack of convergence of the mean clog duration as the measuring time increases. It is demonstrated that this transition—which was reported previously only for externally vibrated systems such as colloids or granulars—appears also for self-propelled agents. This suggests that the transition should also occur for the flow through constrictions of living agents (e.g., humans and sheep), an issue that has been elusive so far in experiments due to safety risks."Más... Artículo de Publicación Periódica Continuous-space automaton model for pedestrian dynamics(2011-05-11) Baglietto, Gabriel; Parisi, DanielMás... An off-lattice automaton for modeling pedestrian dynamics is presented. Pedestrians are represented by disks with variable radius that evolve following predefined rules. The key feature of our approach is that although positions and velocities are continuous, forces do not need to be calculated. This has the advantage that it allows using a larger time step than in force-based models. The room evacuation problem and circular racetrack simulations quantitatively reproduce the available experimental data, both for the specific flow rate and for the fundamental diagram of pedestrian traffic with an outstanding performance. In this last case, the variation of two free parameters (r min and rmax) of the model accounts for the great variety of experimental fundamental diagrams reported in the literature. Moreover, this variety can be interpreted in terms of these model parameters.Más... Artículo de Publicación Periódica Effect of physical distancing on the speed-density relation in pedestrian dynamics(2021-04) Echeverría Huarte, Iñaki; Garcimartín, Ángel; Parisi, Daniel; Martín-Gómez, CésarMás... "We report experimental results of the speed-density relation emerging in pedestrian dynamics when individuals keep a prescribed safety distance among them. To this end, we characterize the movement of a group of people roaming inside an enclosure varying different experimental parameters: (i) global density, (ii) prescribed walking speed, and (iii) suggested safety distance. Then, by means of the Voronoi diagram we are able to compute the local density associated to each pedestrian, which is afterward correlated with its corresponding velocity at each time. In this way, we discover a strong dependence of the speed-density relation on the experimental conditions, especially with the (prescribed) free speed. We also observe that when pedestrians walk slowly, the speed-density relation depends on the global macroscopic density of the system, and not only on the local one. Finally, we demonstrate that for the same experiment, each pedestrian follows a distinct behavior, thus giving rise to multiple speed-density curves."Más... Artículo de Publicación Periódica Effect of physical distancing on the speed–density relation in pedestrian dynamics(2021-04) Echeverría Huarte, Iñaki; Garcimartín, Ángel; Parisi, Daniel; Cruz Hidalgo, R.; Martín-Gómez, César; Zuriguel, IkerMás... "We report experimental results of the speed-density relation emerging in pedestrian dynamics when individuals keep a prescribed safety distance among them. To this end, we characterize the movement of a group of people roaming inside an enclosure varying different experimental parameters: (i) global density, (ii) prescribed walking speed, and (iii) suggested safety distance. Then, by means of the Voronoi diagram we are able to compute the local density associated to each pedestrian, which is afterward correlated with its corresponding velocity at each time. In this way, we discover a strong dependence of the speed-density relation on the experimental conditions, especially with the (prescribed) free speed. We also observe that when pedestrians walk slowly, the speed-density relation depends on the global macroscopic density of the system, and not only on the local one. Finally, we demonstrate that for the same experiment, each pedestrian follows a distinct behavior, thus giving rise to multiple speed-density curves."Más... Artículo de Publicación Periódica Evaluation of the status of rotary machines by time causal information theory quantifiers(2017-03) Redelico, Francisco; Traversaro Varela, Francisco; Oyarzábal, Nicolás Andrés; Vilaboa, Iván; Rosso, Osvaldo A.Más... "In this paper several causal Information Theory quantifiers, i.e. Shannon entropy, statistical complexity and Fisher information using the Bandt and Pompe permutation probability distribution, measure are applied to describe the behavior of a rotating machine. An experiment was conducted where a rotating machine runs balanced and then, after a misalignment, runs unbalanced. All the causal Information Theory quantifiers applied are capable to distinguish between both states and grasp the corresponding transition between them. "Más... Artículo de Publicación Periódica The evolutionary minority game with local coordination(2003) Burgos, Enrique; Ceva, Horacio; Perazzo, Roberto P. J.Más... "We discuss a modification of the evolutionary minority game (EMG) in which agents are placed in the nodes of a regular or a random graph. A neighborhood for each agent can thus be defined and a modification of the usual relaxation dynamics can be made in which each agent updates her decision depending upon her neighborhood. We report numerical results for the topologies of a ring, a torus and a random graph changing the size of the neighborhood. We find the surprising result that in the EMG a better coordination (a lower frustration) can be achieved if agents base their actions on local information disregarding the global trend in the self-segregation process."Más... Artículo de Publicación Periódica Experimental investigation of noise-assisted information transmission and storage via stochastic resonance(2010-05) Patterson, Germán; Goya, Andrés F.; Fierens, Pablo Ignacio; Ibáñez, Santiago Agustín; Grosz, DiegoMás... "We present experimental results on the information transmission and storage via stochastic resonance in circuits designed and built around Schmitt triggers (STs). First, we investigate the performance of a transmission line comprised of five STs and show it to exhibit stochastic resonance. Each ST in the line is fed with white Gaussian noise, and the first ST is driven by a non-return-to-zero pseudo-random bit sequence with sub-threshold amplitude. Parameters such as bit error rate (Q-factor) are measured (calculated) and shown to exhibit a minimum (maximum) for an optimum amount of noise. Interestingly, we find that system performance degrades with the number of STs as if the system were linear and impaired only by additive Gaussian noise. We then propose and build a 1-bit storage device based on two STs in a loop configuration. We demonstrate that such a system is capable of storing one bit of information only in the presence of noise, and that there is a regime where the efficiency of such a device increases with increasing noise. Our results point to the feasibility of building 'blocks' that can transmit, store and eventually process information, whose performance is not only robust against noise, but can actually benefit from it. © 2010 Elsevier B.V. All rights reserved."Más... Artículo de Publicación Periódica Flow of pedestrians through narrow doors with dierent competitiveness(2016) Garcimartín, Ángel; Parisi, Daniel; Pastor, José Martín; Martín-Gómez, César; Zuriguel, IkerMás... "We report a thorough analysis of the intermittent flow of pedestrians through a narrow door. The observations include five diferent sets of evacuation drills with which we have investigated the efect of door size and competitiveness on the flow dynamics. Although the outcomes are in general compatible with the existence of the faster-is-slower efect, the temporal evolution of the instantaneous flow rate provides evidence of new features. These stress the crucial role of the number of people performing the tests, which has an influence on the obtained results. Once the transients at the beginning and end of the evacuation are removed, we have found that the time lapses between the passage of two consecutive pedestrians display heavy-tailed distributions in all the scenarios studied. Meanwhile, the distribution of burst sizes decays exponentially; this can be linked to a constant probability of finding a long-lasting clog during the evacuation process. Based on these results, a discussion is presented on the caution that should be exercised when measuring or describing the intermittent flow of pedestrians through narrow doors."Más... Artículo de Publicación Periódica A generic model for the study of supercontinuum generation in graphene-covered nanowires(2022-01) Linale, N.; Fierens, Pablo Ignacio; Vermeulen, N.; Grosz, DiegoMás... "We study supercontinuum (SC) generation in graphene-covered nanowires based on a generic model that correctly accounts for the evolution of the photon number under Kerr and two-photon absorption processes, and the influence of graphene is treated within the framework of saturable photoexcited-carrier refraction. We discuss the role of the various effects on the generation of SC by a thorough analysis of short-pulse propagation in two different kinds of graphene-covered nanowires, one made of silicon nitride and the other made of silicon. Finally, we discuss the effect of stacking graphene layers as a means to enhance SC generation with pulse powers compatible with those in integrated optical devices."Más... Artículo de Publicación Periódica A geometrical view of scalar modulation instability in optical fibers(2017) Hernández, Santiago M.; Fierens, Pablo Ignacio; Sánchez, Alfredo D.; Grosz, Diego; Bonetti, Juan I.Más... "Full models of scalar modulation instability (MI) in optical fibers available in the literature usually involve complex formulations. In this paper, we present a novel approach to the analysis of MI in optical fibers by means of a simple geometrical description in the power versus frequency plane. This formulation allows us to relate the shape of the MI gain to any arbitrary dispersion profile of the medium, thus providing a simple insight. As a result, we derive a straightforward explanation of the nontrivial dependence of the cutoff power on high-order dispersion and explicitly derive the power that maximizes the gain. Our approach puts forth a tool to synthesize a desired MI gain with the potential application to a number of parametric-amplification and supercontinuum-generation devices whose initialstage dynamics rely upon MI."Más... Artículo de Publicación Periódica A higher-order perturbation analysis of the nonlinear Schrödinger equation(2019-06) Bonetti, Juan I.; Hernández, Santiago M.; Fierens, Pablo Ignacio; Grosz, DiegoMás... "A well-known and thoroughly studied phenomenon in nonlinear wave propagation is that of modulation instability (MI). MI is usually approached as a perturbation to a pump, and its analysis is based on preserving only terms which are linear on the perturbation, discarding those of higher order. In this sense, the linear MI analysis is relevant to the understanding of the onset of many other nonlinear phenomena, such as supercontinuum generation, but it has limitations as it can only be applied to the propagation of the perturbation over short distances. In this work, we propose approximations to the propagation of a perturbation, consisting of additive white noise, that go beyond the linear modulation instability analysis, and show them to be in excellent agreement with numerical simulations and experimental measurements."Más... Artículo de Publicación Periódica Impact of DCF properties on system design(2006) Essiambre, René-Jean; Winzer, Peter J.; Grosz, DiegoMás... "The dispersion-compensating ﬁber is an important optical element of current and future optical networks. In this paper, we review the impact that various properties of dispersion-compensating ﬁbers has on the performance of optical communication systems."Más... Artículo de Publicación Periódica Influence of bottleneck lengths and position on simulated pedestrian egress(2017-01) Parisi, Daniel; Patterson, GermánMás... "In this paper, the problem of pedestrian egress under di erent geometries is studied by means of two numerical models. The length of the bottleneck after the exit and the distance of the exit to the lateral wall of a squared room are investigated. Both models show that an increase in the bottleneck length increases the evacuation time by more than 20%, for any exit position. Hence, a bottleneck length tending to zero is the best choice. On the contrary, the results of moving the exit closer to the lateral wall are different in both models and, thus, its convenience cannot be stated. To unveil whether this layout modi cation is favorable, experimental data are required. Moreover, the discrepancy between models indicates that they should be validated considering several scenarios."Más... Artículo de Publicación Periódica Joint position and clock tracking of wireless nodes under mixed LOS-NLOS conditions(2022-10) Grisales Campeón, Juan Pablo; Fierenz, Pablo IgnacioMás... We propose an algorithm for the simultaneous position and clock tracking of a wireless mobile node by a set of reference nodes. Based on a protocol similar to that of two-way ranging, our algorithm efficiently estimates the position and velocity of the mobile, and the skew and offset of its clock. We take into account that the propagation conditions between each reference node and the mobile change as the latter moves. In particular, changes between line-of-sight (LOS) and several non-line-of-sight (NLOS) scenarios are considered. We study the performance of our algorithm and compare it to other relevant proposals in the literature by means of simulations, showing that our proposed method improves localization accuracy.Más... Artículo de Publicación Periódica Libor at crossroads: stochastic switching detection using information theory quantifiers(2016-07) Fernández Bariviera, Aurelio; Guercio, M. Belén; Martinez, Lisana B.; Rosso, Osvaldo A.Más... "This paper studies the 28 time series of Libor rates, classified in seven maturities and four currencies, during the last 14 years. The analysis was performed using a novel technique in financial economics: the Complexity-Entropy Causality Plane. This planar representation allows the discrimination of different stochastic and chaotic regimes. Using a temporal analysis based on moving windows, this paper unveils an abnormal movement of Libor time series around the period of the 2007 financial crisis. This alteration in the stochastic dynamics of Libor is contemporary of what press called "Libor scandal", i.e. the manipulation of interest rates carried out by several prime banks. We argue that our methodology is suitable as a market watch mechanism, as it makes visible the temporal redution in informational efficiency of the market."Más...

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