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Mohammad Khorrami

Publications and source records attributed to Mohammad Khorrami.

At least 19 recordsLinked to original sources
arXiv2023

Slipping and rolling on a rough accelerating surface

The two-dimensional motion of an object on a moving rough horizontal plane is investigated. Two cases are studied: the plane having a translational acceleration, and a rotating plane. For the first case, the motions of a point particle and a sphere are studied, and it is shown that the solution to the latter problem can be expressed in terms of the solution to the former one. Examples of constant acceleration and periodic acceleration along a fixed line, and specifically sinusoidal acceleration along a fixed line, are studied in more detail. Also a situation is investigated where the friction is anisotropic, that is the friction coefficient depends on the direction of the velocity. In this situation, there may be stick-slip motions, and these are investigated in detail. For the second case, the motion of a point particle on a rough turntable is investigated.

physics.class-ph↗
arXiv2022

Comparison of two artificial neural networks trained for the surrogate modeling of stress in materially heterogeneous elastoplastic solids

The purpose of this work is the systematic comparison of the application of two artificial neural networks (ANNs) to the surrogate modeling of the stress field in materially heterogeneous periodic polycrystalline microstructures. The first ANN is a UNet-based convolutional neural network (CNN) for periodic data, and the second is based on Fourier neural operators (FNO). Both of these were trained, validated, and tested with results from the numerical solution of the boundary-value problem (BVP) for quasi-static mechanical equilibrium in periodic grain microstructures with square domains. More specifically, these ANNs were trained to correlate the spatial distribution of material properties with the equilibrium stress field under uniaxial tensile loading. The resulting trained ANNs (tANNs) calculate the stress field for a given microstructure on the order of 1000 (UNet) to 2500 (FNO) times faster than the numerical solution of the corresponding BVP. For microstructures in the test dataset, the FNO-based tANN, or simply FNO, is more accurate than its UNet-based counterpart; the normalized mean absolute error of different stress components for the former is 0.25-0.40% as compared to 1.41-2.15% for the latter. Errors in FNO are restricted to grain boundary regions, whereas the error in U-Net also comes from within the grain. In comparison to U-Net, errors in FNO are more robust to large variations in spatial resolution as well as small variations in grain density. On other hand, errors in U-Net are robust to variations in boundary box aspect ratio, whereas errors in FNO increase as the domain becomes rectangular. Both tANNs are however unable to reproduce strong stress gradients, especially around regions of stress concentration.

cond-mat.mtrl-sci↗
arXiv2021

On the higher-order pseudo-continuum characterization of discrete kinematic results from experimental measurement or discrete simulation

The purpose of this work is the development and determination of higher-order continuum-like kinematic measures which characterize discrete kinematic data obtained from experimental measurement (e.g., digital image correlation) or kinematic results from discrete modeling and simulation (e.g., molecular statics, molecular dynamics, or quantum DFT). From a continuum point of view, such data or results are in general non-affine and incompatible, due for example to shear banding, material defects, or microstructure. To characterize such information in a (pseudo-) continuum fashion, the concept of discrete local deformation is introduced and exploited. The corresponding measures are determined in a purely discrete fashion independent of any relation to continuum fields. Demonstration and verification of the approach is carried out with the help of example applications based on non-affine and incompatible displacement information. In particular, for the latter case, molecular statics results for the displacement of atoms in and around a dislocation core in fcc Au are employed. The corresponding characterization of lattice distortion in and around the core in terms of higher-order discrete local deformation measures clearly shows that, even in the simplest case of planar cores, such distortion is only partly characterized by the Nye tensor.

cond-mat.mtrl-sci↗
arXiv2018

Modelling of Dictyostelium Discoideum Movement in Linear Gradient of Chemoattractant

Chemotaxis is a ubiquitous biological phenomenon in which cells detect a spatial gradient of chemoattractant, and then move towards the source. Here we present a position-dependent advection-diffusion model that quantitatively describes the statistical features of the chemotactic motion of the social amoeba {\it Dictyostelium discoideum} in a linear gradient of cAMP (cyclic adenosine monophosphate). We fit the model to experimental trajectories that are recorded in a microfluidic setup with stationary cAMP gradients and extract the diffusion and drift coefficients in the gradient direction. Our analysis shows that for the majority of gradients, both coefficients decrease in time and become negative as the cells crawl up the gradient. The extracted model parameters also show that besides the expected drift in the direction of chemoattractant gradient, we observe a nonlinear dependency of the corresponding variance in time, which can be explained by the model. Furthermore, the results of the model show that the non-linear term in the mean squared displacement of the cell trajectories can dominate the linear term on large time scales.

physics.bio-ph↗
arXiv2017

Chaotic behavior in Casimir oscillators: A case study for phase change materials

Casimir forces between material surfaces at close proximity of less than 200 nm can lead to increased chaotic behavior of actuating devices depending on the strength of the Casimir interaction. We investigate these phenomena for phase change materials in torsional oscillators, where the amorphous to crystalline phase transitions lead to transitions between high and low Casimir force and torque states respectively, without material compositions. For a conservative system bifurcation curve and Poincare maps analysis show the absence of chaotic behavior but with the crystalline phase (high force/torque state) favoring more unstable behavior and stiction. However, for a non-conservative system chaotic behavior can occur introducing significant risk for stiction, which is again more pronounced for the crystalline phase. The latter illustrates the more general scenario that stronger Casimir forces and torques increase the possibility for chaotic behavior. The latter is making impossible to predict whether stiction or stable actuation will occur on a long term basis, and it is setting limitations in the design of micro/nano devices operating at short range nanoscale separations.

physics.app-ph↗
arXiv2016

Decoherence in quantum systems in a static gravitational field

A small quantum system is studied which is a superposition of states localized in different positions in a static gravitational field. The time evolution of the correlation between different positions is investigated, and it is seen that there are two time scales for such an evolution (decoherence). Both time scales are inversely proportional to the red shift difference between the two points. These time scales correspond to decoherences which are linear and quadratic, respectively, in time.

quant-ph↗
arXiv2014

Autonomous models on a Cayley tree

The most general single species autonomous reaction-diffusion model on a Cayley tree with nearest-neighbor interactions is introduced. The stationary solutions of such models, as well as their dynamics, are discussed. To study dynamics of the system, directionally-symmetric Green function for evolution equation of average number density is obtained. In some limiting cases the Green function is studied. Some examples are worked out in more detail.

math-ph↗
arXiv2014

Closedness of orbits in a space with SU(2) Poisson structure

The closedness of orbits of central forces is addressed in a three dimensional space in which the Poisson bracket among the coordinates is that of the SU(2) Lie algebra. In particular it is shown that among problems with spherically symmetric potential energies, it is only the Kepler problem for which all of the bounded orbits are closed. In analogy with the case of the ordinary space, a conserved vector (apart from the angular momentum) is explicitly constructed, which is responsible for the orbits being closed. This is the analog of the Laplace-Runge-Lenz vector. The algebra of the constants of the motion is also worked out.

physics.class-ph↗
arXiv2014

The similarity of attractive and repulsive forces on a lattice

On a lattice, as the momentum space is compact, the kinetic energy is bounded not only from below but also from above. It is shown that this, somehow removes the distinction between repulsive and attractive forces. In particular, it is seen that a region with attractive force would appear forbidden for states with energies higher than a certain value, while repulsive forces could develop bound-states. An explicit transformation is introduced which transforms the spectrum of a system corresponding to a repulsive force, to that of a similar system corresponding to an attractive force. Explicit numerical examples are presented for discrete energies of bound-states of a particle experiencing repulsive force by a piecewise constant potential. Finally, the parameters of a specific one dimensional translationally invariant system on continuum are tuned so that the energy of the system resembles the kinetic energy of a system on a one dimensional lattice. In particular, it is shown that the parameters could be tuned so that while the width of the first energy band and its position are kept finite, the energy gap between the first energy band and the next energy band go to infinity, so that effectively only the first energy band is relevant.

quant-ph↗
arXiv2014

Phase transitions in a reaction-diffusion model on a line with boundaries

A one-dimensional model on a line of the length L is investigated, which involves particle diffusion as well as single particle annihilation. There are also creation and annihilation at the boundaries. The static and dynamical behaviors of the system are studied. It is seen that the system could exhibit a dynamical phase transition. For small drift velocities, the relaxation time does not depend on the absorbtion rates at the boundaries. This is the fast phase. For large velocities, the smaller of the absorbtion rates at boundaries enter the relaxation rate and makes it longer. This is the slow phase. Finally, the effect of a random particle creation in the bulk is also investigated.

math-ph↗
arXiv2014

On the relation between the spin and the magnetic moment of the proton

In the context of the quark model of hadrons the spin and the magnetic moment of proton can not be taken proportional. This is in contradiction with the widely used relation between these two properties of the proton. This apparent difficulty is addressed by the most elementary notions of the relevant physics. In particular it is emphasized that the widely used relation is only valid in the lowest orders of perturbation, in which transitions between different baryons do not occur. For other processes where such transitions do occur, such as inelastic scattering off the protons, the quark model relation for the magnetic moment is used to give an estimation for the amplitude of transition between states with different total spins.

hep-ph↗
arXiv2014

A suspended rope wrapped around a cylinder

The problem of a suspended rope wrapped around a fixed cylinder is studied. If the suspension force is larger than a certain threshold (which is larger than the weight of the rope), the rope would remain tightly wrapped around the cylinder. For suspension forces smaller than that threshold but larger than another threshold, the rope becomes loose (loses contact with the cylinder) at some points, but still remains at rest. Theses thresholds are obtained when there is no friction. The system is then analyzed also with friction, and the threshold of tight-wrapping is obtained for that case as well.

physics.class-ph↗
arXiv2013

Entropy as a measure of diffusion

The time variation of entropy, as an alternative to the variance, is proposed as a measure of the diffusion rate. It is shown that for linear and time-translationally invariant systems having a large-time limit for the density, at large times the entropy tends exponentially to a constant. For systems with no stationary density, at large times the entropy is logarithmic with a coefficient specifying the speed of the diffusion. As an example, the large time behaviors of the entropy and the variance are compared for various types of fractional-derivative diffusions.

cond-mat.stat-mech↗
arXiv2012

The spectrum and the phase transition of models solvable through the full interval method

The most general exclusion single species reaction-diffusion models with nearest-neighbor interactions one a one dimensional lattice are investigated, for which the evolution of full intervals are closed. Using a generating function method, the probability that n consecutive sites be full is investigated. The stationary values of these probabilities, as well as the spectrum of the time translation generator (Hamiltonian) of these are obtained. It is shown that depending on the reaction rates the model could exhibit a dynamical phase transition.

cond-mat.stat-mech↗
arXiv2012

Autonomous models solvable through the full interval method

The most general exclusion single species one dimensional reaction-diffusion models with nearest-neighbor interactions which are both autonomous and can be solved exactly through full interval method are introduced. Using a generating function method, the general solution for, $F_n$, the probability that $n$ consecutive sites be full, is obtained. Some other correlation functions of number operators at nonadjacent sites are also explicitly obtained. It is shown that for a special choice of initial conditions some correlation functions of number operators called full intervals remain uncorrelated.

cond-mat.stat-mech↗
arXiv2012

Externally driven one-dimensional Ising model

A one dimensional kinetic Ising model at a finite temperature on a semi-infinite lattice with time varying boundary spins is considered. Exact expressions for the expectation values of the spin at each site are obtained, in terms of the time dependent boundary condition and the initial conditions. The solution consists of a transient part which is due to the initial condition, and a part driven by the boundary. The latter is an evanescent wave when the boundary spin is oscillating harmonically. Low- and high-frequency limits are investigated with greater detail. The total magnetization of the lattice is also obtained. It is seen that for any arbitrary rapidly varying boundary conditions, this total magnetization is equal to the the boundary spin itself, plus essentially the time integral of the boundary spin. A nonuniform model is also investigated.

cond-mat.stat-mech↗
arXiv2011

Nonuniform autonomous one-dimensional exclusion nearest neighbor reaction diffusion models, II

In a recent article the most general non-uniform reaction-diffusion models on a one-dimensional lattice with boundaries were considered, for which the time evolution equations of correlation functions are closed and the stationary profile can be obtained using a transfer-matrix method. Here models are investigated for which also the equation of relaxation towards the stationary profile could be solved through a similar transfer-matrix method. A classification is given, and dynamical phase transitions are studied.

cond-mat.stat-mech↗