arXiv subjects

M. Ebrahim Foulaadvand

Publications and source records attributed to M. Ebrahim Foulaadvand.

17 recordsLinked to original sources

arXiv2017

Energetics of a driven Brownian harmonic oscillator

We provide insights into energetics of a Brownian oscillator in contact with a heat bath and driven by an external unbiased time-periodic force that takes the system out of thermodynamic equilibrium. Solving the corresponding Langevin equation, we compute average kinetic and potential energies in the long-time stationary state. We also derive the energy balance equation and study the energy flow in the system. In particular, we identify the energy delivered by the external force, the energy dissipated by a thermal bath and the energy provided by thermal equilibrium fluctuations. Next, we illustrate Jarzynski work-fluctuation relation and consider the stationary state fluctuation theorem for the total work done on the system by the external force. Finally, by determining time scales in the system, we analyze the strong damping regime and discuss the problem of overdamped dynamics when inertial effects can be neglected.

cond-mat.stat-mech↗

arXiv2017

Signal optimization in urban transport: A totally asymmetric simple exclusion process with traffic lights

We consider the exclusion process on a ring with time-dependent defective bonds at which the hoping rate periodically switches between zero and one. This system models main roads in city traffics, intersecting with perpendicular streets. We explore basic properties of the system, in particular dependence of the vehicular flow on the parameters of signalization as well as the system size and the car density. We investigate various types of the spatial distribution of the vehicular density, and show existence of a shock profile. We also measure waiting time behind traffic lights, and examine its relationship with the traffic flow.

physics.soc-ph↗

arXiv2017

Thermal ratchet effect in confining geometries

Stochastic model of the Feynman-Smoluchowski ratchet is proposed and solved using generalization of the Fick-Jacobs theory. The theory fully captures nonlinear response of the ratchet to the difference of heat bath temperatures. The ratchet performance is discussed using the mean velocity, the average heat flow between the two heat reservoirs and the figure of merit, which quantifies energetic cost for attaining a certain mean velocity. Limits of the theory are tested comparing its predictions to numerics. We also demonstrate connection between the ratchet effect emerging in the model and rotations of the probability current and explain direction of the mean velocity using simple discrete analogue of the model.

cond-mat.stat-mech↗

arXiv2016

Transport coefficients for a confined Brownian ratchet operating between two heat reservoirs

We discuss two-dimensional diffusion of a Brownian particle confined to a periodic asymmetric channel with soft walls modeled by a parabolic potential. In the channel, the particle experiences different noise intensities, or temperatures, in the transversal and longitudinal directions. The model is inspired by the famous Feynman's ratchet and pawl. Using ideas of the Fick-Jacobs approximation we derive the mean velocity of the particle, the effective diffusion coefficient, and the stationary probability density in the potential unit cell. The derived results are exact for small channel width. Yet, we check by exact numerical calculation that they qualitatively describe the ratchet effect observed for an arbitrary width of the channel.

cond-mat.stat-mech↗

arXiv2014

Structural and elastic properties of a confined 2D colloidal solid: a molecular dynamics study

We implement molecular dynamics simulations in canonical ensemble to study the effect of confinement on a $2d$ crystal of point particles interacting with an inverse power law potential proportional to $r^{-12}$ in a narrow channel. This system can describe colloidal particles at the air-water interface. It is shown that the system characteristics depend sensitively on the boundary conditions at the two {\it walls} providing the confinement. The walls exert perpendicular forces on their adjacent particles. The potential between walls and particles varies as the inverse power of ten. Structural quantities such as density profile, structure factor and orientational order parameter are computed. It is shown that orientational order persists near the walls even at temperatures where the system in the bulk is in fluid state. The dependence of elastic constants, stress tensor elements, shear and bulk modulii on density as well as the channel width is discussed. Moreover, the effect of channel incommensurability with the triangular lattice structure is discussed. It is shown that incommensurability notably affects the system properties. We compare our findings to those obtained by Monte Carlo simulations and also to the case with the periodic boundary condition along the channel width. .

cond-mat.soft↗

arXiv2014

Thermal rectification of a single-wall carbon nanotube: a molecular dynamics study

We have investigated the thermal rectification phenomenon in a single-wall mass graded carbon nanotube by molecular dynamics simulation. Second generation Brenner potential has been used to model the inter atomic carbon interaction. Fixed boundary condition has been taken into account. We compare our findings to a previous study by Alaghemandi et al which has been done with a different potential and boundary condition. The dependence of the rectification factor $R$ on temperature, nanotube diameter and length as well as mass gradient are obtained. It is shown that by increasing the temperature, the rectification decreases whereas by increasing the other parameters namely the mass gradient, diameter and the tube length it increases.

cond-mat.mtrl-sci↗

arXiv2014

One dimensional Brownian motion in hard rods: adiabatic piston problem

We have investigated the motion characteristics of a movable piston immersed in a one dimensional gas of hard rods by event-oriented molecular dynamics in the absence of thermal noise. Periodic and reflecting boundary conditions are explored. It is shown that the piston undergoes systematic oscillations with decaying amplitudes in short times before it comes to global thermodynamic equilibrium. Moreover, the diffusion of the piston is explored and analytical expressions for its equilibrium mean-squared displacement is obtained. It is shown that MSD of the piston does not differ much from the normal rods despite its mass and length are significantly larger.

cond-mat.soft↗

arXiv2011

Temporal and structural characteristics of a two dimensional gas of hard needles

We have simulated the dynamics of a 2D gas of hard needles by event-oriented molecular dynamics. Various quantities namely translational and rotational diffusion constants and intermediate self scattering function have been explored and their dependence on density is obtained. Despite absence of positional ordering, the rotational degree of freedom behaves nontrivially. Slowing down is observed in the angular part of the motion. It is shown that above a certain density the rotational mean squared displacement exhibits a three stage regime including a plateau.

cond-mat.soft↗

arXiv2011

Vehicular traffic flow at an intersection with the possibility of turning

We have developed a Nagel-Schreckenberg cellular automata model for describing of vehicular traffic flow at a single intersection. A set of traffic lights operating in fixed-time scheme controls the traffic flow. Open boundary condition is applied to the streets each of which conduct a uni-directional flow. Streets are single-lane and cars can turn upon reaching to the intersection with prescribed probabilities. Extensive Monte Carlo simulations are carried out to find the model flow characteristics. In particular, we investigate the flows dependence on the signalisation parameters, turning probabilities and input rates. It is shown that for each set of parameters, there exist a plateau region inside which the total outflow from the intersection remains almost constant. We also compute total waiting time of vehicles per cycle behind red lights for various control parameters.

physics.soc-ph↗

arXiv2010

Computational modelling of the collective stochastic motion of Kinesin nano motors

We have developed a two dimensional stochastic molecular dynamics model for the description of intra cellular collective motion of bio motors, in particular Kinesins, on a microtubular track. The model is capable or reproducing the hand-over-hand mechanism of the directed motion along the microtubule. The model gives the average directed velocity and the current of Kinesins along the microtubule. It is shown that beyond a certain density of Kinesins, the average velocity and current undergo notable decrease which is due to formation of traffic jams in the system.

cond-mat.soft↗

arXiv2010

Mechanical filtering in forced-oscillation of two coupled pendulums

Forced oscillation of a system composed of two pendulums coupled by a spring in the presence of damping is investigated. In the steady state and within the small angle approximation we solve the system equations of motion and obtain the amplitudes and phases of in terms of the frequency of the sinusoidal driving force. The resonance frequencies are obtained and the amplitude ratio is discussed in details. Contrary to a single oscillator, in this two-degree of freedom system four resonant frequencies, which are close to mode frequencies, appear. Within the pass-band interval the system is shown to exhibit a rich and complicated behaviour. It is shown that damping crucially affects the system properties. Under certain circumstances, the amplitude of the oscillator which is directly connected to the driving force becomes smaller than the one far from it. Particularly we show the existence of a driving frequency at which the connected oscillator's amplitude goes zero.

nlin.CD↗

arXiv2010

Optimisation of quantum Monte Carlo wave function: steepest descent method

We have employed the steepest descent method to optimise the variational ground state quantum Monte Carlo wave function for He, Li, Be, B and C atoms. We have used both the direct energy minimisation and the variance minimisation approaches. Our calculations show that in spite of receiving insufficient attention, the steepest descent method can successfully minimise the wave function. All the derivatives of the trial wave function respect to spatial coordinates and variational parameters have been computed analytically. Our ground state energies are in a very good agreement with those obtained with diffusion quantum Monte Carlo method (DMC) and the exact results.

physics.comp-ph↗

arXiv2009

Asymmetric Exclusion Processes with Disorder: Effect of Correlations

Multi-particle dynamics in one-dimensional asymmetric exclusion processes with disorder is investigated theoretically by computational and analytical methods. It is argued that the general phase diagram consists of three non-equilibrium phases that are determined by the dynamic behavior at the entrance, at the exit and at the slowest defect bond in the bulk of the system. Specifically, we consider dynamics of asymmetric exclusion process with two identical defect bonds as a function of distance between them. Two approximate theoretical methods, that treat the system as a sequence of segments with exact description of dynamics inside the segments and neglect correlations between them, are presented. In addition, a numerical iterative procedure for calculating dynamic properties of asymmetric exclusion systems is developed. Our theoretical predictions are compared with extensive Monte Carlo computer simulations. It is shown that correlations play an important role in the particle dynamics. When two defect bonds are far away from each other the strongest correlations are found at these bonds. However, bringing defect bonds closer leads to the shift of correlations to the region between them.

cond-mat.stat-mech↗

arXiv2008

Simulation of traffic flow at a signalised intersection

We have developed a Nagel-Schreckenberg cellular automata model for describing of vehicular traffic flow at a single intersection. A set of traffic lights operating either in fixed-time or traffic adaptive scheme controls the traffic flow. Closed boundary condition is applied to the streets each of which conduct a uni-directional flow. Extensive Monte Carlo simulations are carried out to find the model characteristics. In particular, we investigate the dependence of the flows on the signalisation parameters.

nlin.CG↗

arXiv2008

Asymmetric simple exclusion process describing conflicting traffic flows

We use the asymmetric simple exclusion process for describing vehicular traffic flow at the intersection of two streets. No traffic lights control the traffic flow. The approaching cars to the intersection point yield to each other to avoid collision. This yielding dynamics is model by implementing exclusion process to the intersection point of the two streets. Closed boundary condition is applied to the streets. We utilize both mean-field approach and extensive simulations to find the model characteristics. In particular, we obtain the fundamental diagrams and show that the effect of interaction between chains can be regarded as a dynamic impurity at the intersection point.

physics.soc-ph↗

arXiv2008

Characteristics of the asymmetric simple exclusion process in the presence of quenched spatial disorder

We investigate the effect of quenched spatial disordered hopping rates on the characteristics of the asymmetric simple exclusion process (ASEP) with open boundaries both numerically and by extensive simulations. Disorder averages of the bulk density and current are obtained in terms of various input and output rates. We study the binary and uniform distributions of disorder. It is verified that the effect of spatial inhomogeneity is generically to enlarge the size of the maximal current phase. This is in accordance with the mean field results obtained by Harris and Stinchcombe. Furthermore, we obtain the dependence of the current and the bulk density on the characteristics of the disorder distribution function. It is shown that the impact of disorder crucially depends on the particle input and out rates. In some situations, disorder can constructively enhance the current.

cond-mat.stat-mech↗

arXiv2007

Vehicular traffic flow at a non-signalised intersection

We have developed a modified Nagel-Schreckenberg cellular automata model for describing a conflicting vehicular traffic flow at the intersection of two streets. No traffic lights control the traffic flow. The approaching cars to the intersection yield to each other to avoid collision. Closed boundary condition is applied to the streets. Extensive Monte Carlo simulations is taken into account to find the model characteristics. In particular, we obtain the fundamental diagrams and show that the effect of interaction of two streets can be regarded as a dynamic impurity located at the intersection point. Our results suggest that yielding mechanism gives rise to a high total flow throughout the intersection especially in the low density regime. In some ranges of densities, yielding mechanism even improves and regulates the flow in comparison to the absence of perpendicular flow.

physics.soc-ph↗