Agent-based modeling of a multi-room multi-floor building emergency evacuation

Panic during emergency building evacuation can cause crowd stampede, resulting in serious injuries and casualties. Agent-based methods have been successfully employed to investigate the collective human behavior during emergency evacuation in cases where the configurational space is extremely simple–usually one rectangular room–but not in evacuations of multi-room or multi-floor buildings. This implies that the effect of the complexity of building architecture on the collective behavior of the agents during evacuation has not been fully investigated. Here, we employ a system of self-moving particles whose motion is governed by the social-force model to investigate the effect of complex building architecture on the uncoordinated crowd motion during urgent evacuation. In particular, we study how the room door size, the size of the main exit, the desired speed and the friction coefficient affect the evacuation time and under what circumstances the evacuation efficiency improves.     

Mathematical modeling of intracellular signaling pathways

Dynamic modeling and simulation of signal transduction pathways is an important topic in systems biology and is obtaining growing attention from researchers with experimental or theoretical background. Here we review attempts to analyze and model specific signaling systems. We review the structure of recurrent building blocks of signaling pathways and their integration into more comprehensive models, which enables the understanding of complex cellular processes. The variety of mechanisms found and modeling techniques used are illustrated with models of different signaling pathways. Focusing on the close interplay between experimental investigation of pathways and the mathematical representations of cellular dynamics, we discuss challenges and perspectives that emerge in studies of signaling systems.     

Numerical modeling of transport barrier formation

In diverse media the characteristics of mass and heat transfer may undergo spontaneous and abrupt changes in time and space. This can lead to the formation of regions with strongly reduced transport, so called transport barriers (TB). The presence of interfaces between regions with qualitatively and quantitatively different transport characteristics impose severe requirements to methods and numerical schemes used by solving of transport equations. In particular the assumptions made in standard methods about the solution behavior by representing its derivatives fail in points where the transport changes abruptly. The situation is complicated further by the fact that neither the formation time nor the positions of interfaces are known a priori. A numerical approach, operating reliably under such conditions, is proposed. It is based on the introduction of a new dependent variable related to the variation after one time step of the original one integrated over the volume. In the vicinity of any grid knot the resulting differential equation is approximated by a second order ordinary differential equation with constant coefficients. Exact analytical solutions of these equations are conjugated between knots by demanding the continuity of the total solution and its first derivative. As an example the heat transfer in media with heat conductivity decreasing abruptly when the temperature e-folding length exceeds a critical value is considered. The formation of TB both at a heating power above the critical level and caused with radiation energy losses non-linearly dependent on the temperature is modeled.     

Protein motors induced enhanced diffusion in intracellular transport

Diffusion of transported particles in the intracellular medium is described by means of a generalized diffusion equation containing forces due to the cytoskeleton network and to the protein motors. We find that the enhanced diffusion observed in experiments depends on the nature of the force exerted by the protein motors and on parameters characterizing the intracellular medium which is described in terms of a generalized Debye spectrum for the noise density of states.     

Computer modeling of carrier transport in PbSnSe photodiodes

A numerical technique has been used to solve the carrier transport equations for n⁺−p Pb_0.935Sn_0.065Se photodiode configuration. The model computes the spatial distribution of the electric field, electron and hole concentrations and the generation-recombination mechanisms. Also the effect of doping profiles on the photodiode parameters (R₀A product, quantum efficiency) is analysed. Results of calculations indicate the potential possibilities of constructing higher quality PbSnSe photodiodes. The R₀A product of experimentally measured PbSnSe photodiodes is controlled by Shockley-Read generation-recombination mechanisms.     

Public transport networks: empirical analysis and modeling

Public transport networks of fourteen cities of so far unexplored network size are analyzed in standardized graph representations: the simple graph of the network map, the bipartite graph of routes and stations, and both one mode projections of the latter. Special attention is paid to the inter-relations and spatial embedding of transport routes. This systematic approach reveals rich behavior beyond that of the ubiquitous scale-free complex network. We find strong evidence for structures in PTNs that are counter-intuitive and need to be explained, among these a pronounced diversity in the expression of typical network characteristics within the present sample of cities, a surprising geometrical behavior with respect to the two-dimensional geographical embedding and an unexpected attraction between transport routes. A simple model based on these observations reproduces many of the identified PTN properties by growing networks of attractive self-avoiding walks.     

Large-scale effects on meso-scale modeling for scalar transport

The transport of scalar quantities passively advected by velocity fields with a component at small scale ℓ can be modeled at scales larger than ℓ by means of an effective drift and an effective diffusivity, which can be determined by means of multiple-scale techniques. We show that the presence of a weak flow at large scales Lℓ induces interesting effects on the scalar transport at the meso-scales (i.e. at scales intermediate between ℓ and L). In particular, it gives rise to non-isotropic and non-homogeneous corrections to the meso-scale drift and diffusivity. We discuss an approximation that allows us to retain the second-order effects caused by the large-scale flow. This provides a rather accurate meso-scale modeling for both asymptotic and pre-asymptotic scalar transport properties. Numerical simulations in model flows are used to illustrate the importance of such large-scale effects.     

Impact of non-ideal transport modeling on supercritical flow simulation

The simulation of a supercritical fluid flow requires sophisticated models for real gas thermodynamic and non-ideal phenomena. They both are presently addressed through the simulation of a non-reacting and reacting high pressure H₂/O₂ splitter-plate configuration. In particular, the diffusion velocity of species is evaluated through the gradient of chemical potential (${\mathbf{d}}_l^{\text{NI}}=X_l{\left(?{\mu }_l\right)}_T$) expressed with the Peng–Robinson equation of state, or with the classical low-pressure approach ${\mathbf{d}}_l^{\mathrm{I}}=?X_l,$ which only uses the gradient of the lth species molar fraction, X_l. In addition, the high pressure binary diffusion coefficients are estimated by the correction of Kurochkin et al. or with the Takahashi approach. The results for the non-reaction case are consistent with the literature for mean and rms values using ${\mathbf{d}}_l^{\mathrm{I}}$. The use of ${\mathbf{d}}_l^{\text{NI}}$ has a limited impact but the temperature profiles become steeper. In the reactive case, the two approaches lead to a difference of 50 K on the average temperature just downstream of the injector and about 100 K further downstream. A non-ideal transport is then required for the modeling of supercritical flow simulation.     

Contemplating charge transport by modeling of DNA nucleobases based nano structures

Electrical charge transport through two basic strands Thymine and Adenine of DNA has been analyzed using jellium model approach. The FFT-2D computations have been performed for semi empirical Extended Huckel Theory using Atomistix Tool kit to contemplate the charge transport metrics like current and conductance. We have scrutinized the behavior of the devices in the range of -2 V–2 V for a step size of 0.2 V. A prominent observation is the drop in HLGs of Adenine and Thymine, when working as device as compared to their intrinsic values and this is comparative more visible in case of Adenine. The current in the thymine based device exhibit linear increase with voltage in spite of having low conductance. Further the broader transmission peaks represent the strong coupling of electrodes to the scattering molecule (Thymine). The NDR effect of Adenine based device for higher bias can be utilized in various future electronics applications.     

Determination of eddy transport coefficients in thermo-lattice Boltzmann modeling of two-dimensional turbulence

Thermal Lattice Boltzmann (TLBE) techniques are used to consider the time evolution of free-decaying two dimensional (2D) turbulence induced by a double velocity shear layer. In particular, we consider the effect of this turbulence at a Reynolds number of 2555 on a strong temperature gradient. Since all structures are resolved on the 1024×1024 grid, the Smagorinsky model is employed to compute directly the eddy viscosity and eddy diffusivity. These transport coefficients play an integral part in large eddy simulations at very high Reynolds numbers where a direct simulation cannot resolve all excited scales. TLBE codes have the virtue of being readily extended to 3D, can readily handle nonperiodic geometries, and are ideally suited for multi-parallel computer architectures.This work was supported by a joint US-Czech DoE Grant #93066. Computations were performed under the auspices of the SPP (Special Parallel Processing) on the C90 at NERSC.     

Statistical modeling of nonstationary transport of resonance radiation in a low-temperature plasma

A method is suggested for solving the nonstationary kinetic problem in a two-level system for the spatially inhomogeneous case with account of trapped and induced radiation. The error of this method is discussed in detail, and the solution obtained of the decay of the excited state is compared with data available in the literature.Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vyssikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 17–21, June 1994.     

Dynamic modeling of stepwise internal transport barrier formation in DIII-D negative-central-shear discharges

The GLF23 transport model is used to dynamically follow bifurcations in the energy and toroidal momentum confinement in DIII-D discharges with an internal transport barrier. The temperatures and toroidal velocity profiles are evolved while self-consistently computing the effects of E x B shear stabilization during the formation and expansion of internal transport barriers. The barrier is predicted to form in a stepwise fashion through a series of sudden jumps in the core-electron and ion temperatures and toroidal rotation velocity. These results are consistent with experimental observations. In the simulations, the step transitions are a direct result of local E x B driven transport bifurcations.     

1-D modeling of ion transport in rectangular nanofluidic channels

Based on the continuity hypothesis of fluid, 1-D mathematical models of ions’ transport in the rectangular nanofluidic channels are established by using the Poisson-Boltzmann (PB) equation and the modified Navier-Stokes (N-S) equations. The deduced equations are solved with MATLAB software. The results show that the distribution of the electric potential and the flow field could be predicted by the parameters, such as conductivity, surface charge density, solution concentration and channel height. The relationships between the parameters and the flow characteristics of the solution are also discussed. The research will help to the accurate manipulation of the solution in the nanofluidic channels.     

Reduced modeling of Flame-Wall-Interactions of premixed isooctane-air systems including detailed transport and surface reactions

In this work, the Reaction-Diffusion Manifold (REDIM) method, a method for model reduction, is applied to a premixed isooctane-air system with Flame-Wall-Interactions (FWI). In order to provide a highly accurate reduced kinetic model, a detailed model for the diffusive processes is applied and complex boundary conditions that account for heterogeneous wall reactions are implemented.The REDIM is constructed and validated by comparing results of detailed and reduced kinetics in the system state space. The results of the reduced computations are compared with those of the detailed computations. It is shown that the reduced kinetics reproduce the results of the FWI very accurately. In particular, the difference between detailed kinetics with and without wall reactions is larger than the difference between detailed and reduced kinetics with heterogeneous wall reactions, which demonstrates the quality of the model reduction.     

Agent-based approach for generation of a money-centered star network

The history of trade is a progression from a pure barter system. A medium of exchange emerges autonomously in the market, a position currently occupied by money. We investigate an agent-based computational economics model consisting of interacting agents considering distinguishable properties of commodities which represent salability. We also analyze the properties of the commodity network using a spanning tree. We find that the “storage fee” is more crucial than “demand” in determining which commodity is used as a medium of exchange.     

Study and modeling of the transport mechanism in a semi insulating GaAs Schottky diode

The current through a metal–semiconductor junction is mainly due to the majority carriers. Three distinctly different mechanisms exist in a Schottky diode: diffusion of carriers from the semiconductor into the metal, thermionic emission–diffusion (TED) of carriers across the Schottky barrier and quantum–mechanical tunneling through the barrier. The insulating layer converts the MS device in an MIS device and has a strong influence on its current–voltage (I–V) and the parameters of a Schottky barrier from 3.7 to 15 eV. There are several possible reasons for the error that causes a deviation of the ideal behavior of Schottky diodes with and without an interfacial insulator layer. These include the particular distribution of interface states, the series resistance, bias voltage and temperature. The GaAs and its large concentration values of trap centers will participate in an increase of the process of thermionic electrons and holes, which will in turn the IV characteristic of the diode, and an overflow maximum value [NT = 3 × 10²⁰] is obtained. The I–V characteristics of Schottky diodes are in the hypothesis of a parabolic summit.     

Scaling theory put into practice: first-principles modeling of transport in doped silicon nanowires

We combine the ideas of scaling theory and universal conductance fluctuations with density-functional theory to analyze the conductance properties of doped silicon nanowires. Specifically, we study the crossover from ballistic to diffusive transport in boron or phosphorus doped Si nanowires by computing the mean free path, sample-averaged conductance G, and sample-to-sample variations std(G) as a function of energy, doping density, wire length, and the radial dopant profile. Our main findings are (i) the main trends can be predicted quantitatively based on the scattering properties of single dopants, (ii) the sample-to-sample fluctuations depend on energy but not on doping density, thereby displaying a degree of universality, and (iii) in the diffusive regime the analytical predictions of the Dorokhov-Mello-Pereyra-Kumar theory are in good agreement with our ab initio calculations.