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Debashish Chowdhury

Publications and source records attributed to Debashish Chowdhury.

102 records · Page 6Linked to original sources
arXiv1999

Vehicular Traffic: A System of Interacting Particles Driven Far From Equilibrium

In recent years statistical physicists have developed {\it discrete} "particle-hopping" models of vehicular traffic, usually formulated in terms of {\it cellular automata}, which are similar to the microscopic models of interacting charged particles in the presence of an external electric field. Concepts and techniques of non-equilibrium statistical mechanics are being used to understand the nature of the steady states and fluctuations in these so-called "microscopic" models. In this brief review we explain, primarily to the nonexperts, these models and the physical implications of the results.

cond-mat.stat-mech↗
arXiv1999

Periodicity-dependent stiffness of periodic hydrophilic-hydrophobic hetero-polymers

From extensive Monte Carlo simulations of a Larson model of perfectly periodic heteropolymers (PHP) in water a striking stiffening is observed as the period of the alternating hydrophobic and hydrophilic blocks is shortened. At short period and low temperature needle-like conformations are the stable conformation. As temperature is increased thermal fluctuations induce kinks and bends. At large periods compact oligomeric globules are observed. From the generalized Larson prescription, originally developed for modelling surfactant molecules in aqueous solutions, we find that the shorter is the period the more stretched is the PHP. This novel effect is expected to stimulate polymer synthesis and trigger research on the rheology of aqueous periodic heteropolymer solutions.

cond-mat.stat-mech↗
arXiv1999

Spatio-temporal organization of vehicles in a cellular automata model of traffic with 'slow-to-start' rule

The spatio-temporal organizations of vehicular traffic in cellular-automata models with "slow-to-start" rules are qualitatively different from those in the Nagel-Schreckenberg (NaSch) model of highway traffic. Here we study the effects of such a slow-to-start rule, introduced by Benjamin, Johnson and Hui (BJH), on the the distributions of the distance-headways, time-headways, jam sizes and sizes of the gaps between successive jams by a combination of approximate analytical calculations and extensive computer simulations. We compare these results for the BJH model with the corresponding results for the NaSch model and interpret the qualitative differences in the nature of the spatio-temporal organizations of traffic in these two models in terms of a phase separation of the traffic caused by the slow-to-start rule in the BJH model.

cond-mat.stat-mech↗
arXiv1999

Self-organization of traffic jams in cities: effects of stochastic dynamics and signal periods

We propose a cellular automata model for vehicular traffic in cities by combining (and appropriately modifying) ideas borrowed from the Biham-Middleton-Levine (BML) model of city traffic and the Nagel-Schreckenberg (NS) model of highway traffic. We demonstrate a phase transition from the "free-flowing" dynamical phase to the completely "jammed" phase at a vehicle density which depends on the time periods of the synchronized signals and the separation between them. The intrinsic stochasticity of the dynamics, which triggers the onset of jamming, is similar to that in the NS model, while the phenomenon of complete jamming through self-organization as well as the final jammed configurations are similar to those in the BML model. Using our new model, we have made an investigation of the time-dependence of the average speeds of the cars in the "free-flowing" phase as well as the dependence of flux and jamming on the time period of the signals.

cond-mat.stat-mech↗
arXiv1998

Distributions of Time- and Distance-Headways in the Nagel-Schreckenberg Model of Vehicular Traffic: Effects of Hindrances

In the Nagel-Schreckenberg model of vehicular traffic on single-lane highways vehicles are modelled as particles which hop forward from one site to another on a one dimensional lattice and the inter-particle interactions mimic the manner in which the real vehicles influence each other's motion. In this model the number of empty lattice sites in front of a particle is taken to be a measure of the corresponding distance-headway(DH). The time-headway(TH) is defined as the time interval between the departures (or arrivals) of two successive particles recorded by a detector placed at a fixed position on the model highway. We investigate the effects of spatial inhomogeneities of the highway (static hindrances) on the DH and TH distributions in the steady-state of this model.

cond-mat.stat-mech↗
arXiv1998

A generalized spin model of financial markets

We reformulate the Cont-Bouchaud model of financial markets in terms of classical "super-spins" where the spin value is a measure of the number of individual traders represented by a portfolio manager of an investment agency. We then extend this simplified model by switching on interactions among the super-spins to model the tendency of agencies getting influenced by the opinion of other managers. We also introduce a fictitious temperature (to model other random influences), and time-dependent local fields to model slowly changing optimistic or pessimistic bias of traders. We point out close similarities between the price variations in our model with $N$ super-spins and total displacements in an $N$-step Levy flight. We demonstrate the phenomena of natural and artificially created bubbles and subsequent crashes as well as the occurrence of "fat tails" in the distributions of stock price variations.

cond-mat.stat-mech↗
arXiv1998

Immune Network: An Example of Complex Adaptive Systems

The phenomenon of immunological memory has been known for a long time. But, the underlying mechanism is poorly understood. According to the theory of clonal selection the response to a specific invading antigen (e.g., bacteria) is offered by a specific clone of the cells. Some of the lymphocytes activated during the primary response remain dormant and keep circulating in the immune system for a long time carrying the memory of the encounter and, therefore, these long-lived cells are called memory cells. Proponents of the alternative network theory maintain that the immune response is offered by a "network" of clones in a collective manner. In recent years several possible scenarios of the "structure" and function of the immune network have been considered. We have developed mathematical models for describing the population dynamics of the immunocompetent cells in a unified manner. We have incorporated intra-clonal as well as inter-clonal interactions in a discrete formulation and also studied a continuum version of this model.

cond-mat.stat-mech↗
arXiv1997

A Microscopic Model of Gemini Surfactants: Self-assemblies in Water and at Air-Water Interface

We report the results of large scale Monte Carlo (MC) simulations of novel microscopic models of gemini surfactants to elucidate (i) their spontaneous aggregation in bulk water and (ii) their spatial organization in a system where water is separated from the air above it by a sharp well defined interface. We study the variation of the critical micellar concentration (CMC) with the variation of the (a) length of the spacer, (b) length of the hydrophobic tail and (c) the bending rigidity of the hydrocarbon chains forming the spacer and the tail; some of the trends of variation are counter-intuitive but are in excellent agreement with the available experimental results. Our simulations elucidate the effects of the geometrical shape, size and density of the surfactant molecules, the ionic nature of the heads and hydrophobicity/hydrophilicity of the spacer not only on the shapes of the micellar aggregates and the magnitude of the CMC, but also on their conformations close to the air-water interface.

cond-mat.soft↗
arXiv1997

Distributions of Time Headways in Particle-hopping Models of Vehicular Traffic

We report the first analytical calculation of the distribution of the time headways in some special cases of a particle-hopping model of vehicular traffic on idealized single-lane highways and compare with the corresponding results of our computer simulation. We also present numerical results for the time-headway distribution in more general situations in this model. We compare our results with the empirical data available in the literature.

cond-mat.stat-mech↗
arXiv1997

Particle-hopping Models of Vehicular Traffic: Distributions of Distance Headways and Distance Between Jams

We calculate the distribution of the distance headways (i.e., the instantaneous gap between successive vehicles) as well as the distribution of instantaneous distance between successive jams in the Nagel-Schreckenberg (NS) model of vehicular traffic. When the maximum allowed speed, $V_{max}$, of the vehicles is larger than unity, over an intermediate range of densities of vehicles, our Monte Carlo (MC) data for the distance headway distribution exhibit two peaks, which indicate the coexistence of "free-flowing" traffic and traffic jams. Our analytical arguments clearly rule out the possibility of occurrence of more than one peak in the distribution of distance headways in the NS model when $V_{max} = 1$ as well as in the asymmetric simple exclusion process. Modifying and extending an earlier analytical approach for the NS model with $V_{max} = 1$, and introducing a novel transfer matrix technique, we also calculate the exact analytical expression for the distribution of distance between the jams in this model; the corresponding distributions for $V_{max} > 1$ have been computed numerically through MC simulation.

cond-mat.stat-mech↗
arXiv1997

Micellar Aggregates of Gemini Surfactants: Monte Carlo Simulation of a Microscopic Model

We propose a "microscopic" model of gemini surfactants in aqueous solution. Carrying out extensive Monte Carlo simulations, we study the variation of the critical micellar concentration (CMC) of these model gemini surfactants with the variation of the (a) length of the spacer connecting the two hydrophilic heads, (b) length of the hydrophobic tail and (c) the bending rigidity of the hydrocarbon chains forming the spacer and the tail; some of the trends of variation are counter-intuitive but are in excellent agreement with the available experimental results. Our simulations also elucidate the dependence of the shapes of the micellar aggregates and the magnitude of the CMC on the geometrical shape and size of the surfactant molecules and the electrical charge on the hydrophilic heads.

cond-mat.soft↗