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Total 107 records

#### Three-dimensional forced Burgers turbulence supplemented with a continuity equation

, Article Physical Review E - Statistical, Nonlinear, and Soft Matter Physics ; Volume 63, Issue 5 II , 2001 , Pages 563081-5630813 ; 15393755 (ISSN) ; Masoudi, A. A ; Rahimi Tabar, M. R ; Rastegar, A. R ; Shahbazi, F ; Sharif University of Technology
2001

Abstract

The turbulent limit of three dimensional Burger equation was studied. An adjusting parameter emerged through the calculations, based on mean field analysis, that was the scaling exponent of conditional density-density correlations. Two kinds of solutions were also shown for velocity-increment probability density function (PDF)

#### Reaction-diffusion models describing a two-lane traffic flow

, Article Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics ; Volume 62, Issue 5 , 2000 , Pages 5940-5947 ; 1063651X (ISSN) ; Sharif University of Technology
2000

Abstract

A unidirectional two-lane road is approximated by a set of two parallel closed one-dimensional chains. Two types of cars, i.e., slow and fast ones are considered in the system. Based on the Nagel-Schreckenberg model of traffic flow [K. Nagel and M. Schreckenberg, J. Phys. 2, 2221 (1992)], a set of reaction-diffusion processes is introduced to simulate the behavior of the cars. Fast cars can pass the slow ones using the passing lane. We write and solve the mean-field rate equations for the density of slow and fast cars, respectively. We also investigate the properties of the model through computer simulations and obtain the fundamental diagrams. A comparison between our results and the...

#### An analytical solution for inverse determination of residual stress field

, Article Journal of Solid Mechanics ; Volume 4, Issue 2 , 2012 , Pages 114-127 ; 20083505 (ISSN) ; Farrahi, G. H ; Smith, D. J ; Sharif University of Technology
2012

Abstract

An analytical solution is presented that reconstructs residual stress field from limited and incomplete data. The inverse problem of reconstructing residual stresses is solved using an appropriate form of the airy stress function. This function is chosen to satisfy the stress equilibrium equations together with the boundary conditions for a domain within a convex polygon. The analytical solution is demonstrated by developing a reference solution from which selected "measurement" points are used. An artificial error is then randomly added to "measurement" points for studying the stability of the reconstruction method utilizing Tikhonov- Morozov regularization technique. It is found that there...

#### Metrical theory for small linear forms and applications to interference alignment

, Article Jonathan Borwein Commemorative Conference, JBCC 2017, 25 September 2017 through 29 September 2017 ; Volume 313 , 2020 , Pages 377-393 ; Mahboubi, S. H ; Seyed Motahari, A ; Sharif University of Technology
Springer
2020

Abstract

In this paper, the metric theory of Diophantine approximation associated with mixed type small linear forms is investigated. We prove Khintchine–Groshev type theorems for both the real and complex number systems. The motivation for these metrical results comes from their applications in signal processing. One such application is discussed explicitly. © Springer Nature Switzerland AG 2020

#### Assisted passive snake-like robots: Conception and dynamic modeling using Gibbs-Appell method

, Article Robotica ; Volume 26, Issue 3 , 2008 , Pages 267-276 ; 02635747 (ISSN) ; Pendar, H ; Heidari, Z ; Mohammadi, S ; Sharif University of Technology
2008

Abstract

In this paper, we present a novel structure of a snake-like robot. This structure enables passive locomotion in snake-like robots. Dynamic equations are obtained for motion in a horizontal plane, using Gibbs-Appell method. Kinematic model of the robot include numerous nonholonomic constraints, which can be omitted at the beginning by choosing proper coordinates to describe the model in Gibbs-Appell framework. In such a case, dynamic equations will be significantly simplified, resulting in considerable reduction of simulation time. Simulation results show that, by proper selection of initial conditions, joint angles operate in a limit cycle and robot can locomote steadily on a passive...

#### Unreliability of frequency-domain approximation in recognising chaos in fractional-order systems

, Article IET Signal Processing ; Volume 1, Issue 4 , December , 2007 , Pages 171-181 ; 17519675 (ISSN) ; Haeri, M ; Sharif University of Technology
2007

Abstract

The effects of using frequency-domain approximation in numerical simulation of fractional-order systems are analytically studied. The main aim in the study is to determine the number, location and stability property of the equilibriums in a fractional-order system and its frequency-based approximating counterpart. The comparison shows that the original fractional-order system and its frequency-based approximation may differ from each other in some or all issues considered in the study. Unfortunately, these differences can lead to wrong consequences in some special cases such as detecting chaos in the fractional-order systems. It is shown that using the frequency-domain approximation methods...

#### Spin dynamics characterization in magnetic dots

, Article Physica B: Condensed Matter ; Volume 399, Issue 2 , 2007 , Pages 81-93 ; 09214526 (ISSN) ; Esfarjani, K ; Sharif University of Technology
2007

Abstract

The spin structure in a magnetic dot is studied as a function of the exchange coupling strength and dot size within the semiclassical approximation on a discrete lattice. As the exchange coupling is decreased or the size is increased, the ground state undergoes a phase change from a homogeneous single-domain ferromagnet (HSDF) to a spin vortex. The line separating these two phases has been calculated numerically for small system sizes. The dipolar interaction has been fully included in our calculations. Magnon frequencies in such a dot have also been calculated in both phases by the linearized equation of motion method. These results have also been reproduced from the Fourier transform of...

#### The best approximation of some rational functions in uniform norm

, Article Applied Numerical Mathematics ; Volume 55, Issue 2 , 2005 , Pages 204-214 ; 01689274 (ISSN) ; Mehri, B ; Sharif University of Technology
2005

Abstract

Here we are concerned with the best approximation by polynomials to rational functions in the uniform norm. We give some new theorems about the best approximation of 1/(1+x) and 1/(x-a) where a>1. Finally we extend this problem to that of computing the best approximation of the Chebyshev expansion in uniform norm and give some results and conjectures about this. © 2005 IMACS. Published by Elsevier B.V. All rights reserved

#### Biased random satisfiability problems: From easy to hard instances

, Article Physical Review E - Statistical, Nonlinear, and Soft Matter Physics ; Volume 71, Issue 6 , 2005 ; 15393755 (ISSN) ; Moghimi Araghi, S ; Sharif University of Technology
2005

Abstract

In this paper we study biased random K-satisfiability (K-SAT) problems in which each logical variable is negated with probability p. This generalization provides us a crossover from easy to hard problems and would help us in a better understanding of the typical complexity of random K-SAT problems. The exact solution of 1-SAT case is given. The critical point of K-SAT problems and results of replica method are derived in the replica symmetry framework. It is found that in this approximation αcp-(K-1) for p→0. Solving numerically the survey propagation equations for K=3 we find that for p

#### A modified pressure-based algorithm to solve flow fields with shock and expansion waves

, Article Numerical Heat Transfer, Part B: Fundamentals ; Volume 46, Issue 5 , 2004 , Pages 497-504 ; 10407790 (ISSN) ; Mokarizadeh, V ; Sharif University of Technology
2004

Abstract

The use of collocated grid schemes in control-volume-based methods has resulted in developing different strategies for coupling mass and momentum governing equations. Defining two different velocity components at cell faces is one remedy to suppress the possible checkerboard problem in the solution domain. These velocity components are widely known as convected and convecting velocities which are normally used in the momentum and mass governing equations, respectively. However, the linearization of the nonlinear governing equations generates a number of lagged velocity components which must be carefully treated in a manner to preserve the conceptual definition of the convected and convecting...

#### Effects of bulk viscosity in non-linear bubble dynamics

, Article Journal of Physics Condensed Matter ; Volume 16, Issue 10 , 2004 , Pages 1687-1694 ; 09538984 (ISSN) ; Sadighi Bonabi, R ; Taeibi Rahni, M ; Sharif University of Technology
2004

Abstract

The non-linear bubble dynamics equations in a compressible liquid have been modified by considering the effects of compressibility of both the liquid and the gas at the bubble interface. A new bubble boundary equation has been derived, which includes a new term resulting from the liquid bulk viscosity effect. The influence of this term has been numerically investigated by considering the effects of water vapour and chemical reactions inside the bubble. The results clearly indicate that the new term has an important damping role at the collapse, so that its consideration dramatically decreases the amplitude of the bubble rebounds after the collapse. This damping feature is more remarkable for...

#### Saturation and its application in the vibration control of nonlinear systems

, Article Proceedings of the 7th Biennial Conference on Engineering Systems Design and Analysis - 2004, Manchester, 19 July 2004 through 22 July 2004 ; Volume 2 , 2004 , Pages 93-100 ; 0791841731 (ISBN); 9780791841730 (ISBN) ; Ghorashi, M ; Sharif University of Technology
American Society of Mechanical Engineers
2004

Abstract

An investigation on the nonlinear vibrations of a system with two degrees of freedom when subjected to saturation is presented. The method is especially applied to a system that consists of a DC motor with a nonlinear controller and subjected to a harmonic forcing voltage. Approximate solutions are sought using the method of multiple scales. Also, singular point analysis is used to study stability of solutions. These findings are then compared with the corresponding numerical results. Good agreement between the two is observed

#### A modified upwind-biased strategy to calculate flow on structured- unstructured grid topologies

, Article 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 5 January 2004 through 8 January 2004 ; 2004 , Pages 685-694 ; Schneider, G. E ; Vakilipour, S ; Sharif University of Technology
American Institute of Aeronautics and Astronautics Inc
2004

Abstract

A numerical upwind-biased procedure which respects the essence of upwinding is suitably extended in order to reduce the false diffusion induced by a first-order approximation. In this regard, some arbitrarily first and second order gradient terms are added to the primary upwind approximation. The additional terms are then discretized using second-order schemes which essentially produce dispersive errors. The suitable choices for the weights of the new added terms result in lowering the dissipative role of the original upwind scheme. Additionally, the implicit appearance of the third-order terms, which are the consequences of second-order discretizations, helps to reduce the dissipative...

#### A new model in correlating the activity coefficients of aqueous electrolyte solutions with ion pair formation

, Article Fluid Phase Equilibria ; Volume 261, Issue 1-2 , December , 2007 , Pages 313-319 ; 03783812 (ISSN) ; Taghikhani, V ; Ghotbi, C ; Sharif University of Technology
2007

Abstract

In this work, the Ion Pair Modified Ghotbi-Vera Mean Spherical Approximation (IP-MGV-MSA) model was proposed to correlate the mean ionic activity coefficients (MIAC) for a number of symmetric and asymmetric aqueous electrolyte solutions at 25 °C. The new model is based on the recently proposed MGV-MSA model by Mortazavi-Manesh et al. In the IP-MGV-MSA model, the effects arising from the ion pair formation in the electrolyte solution was taken into account. Also, in the proposed model, while the cation diameter as well as the relative permittivity of water was considered to be dependent on electrolyte concentration, the anion diameter was independent of electrolyte concentration. The results...

#### Numerical simulation of thermobuoyant flow with large temperature variation

, Article Journal of Thermophysics and Heat Transfer ; Volume 20, Issue 2 , 2006 , Pages 285-296 ; 08878722 (ISSN) ; Hosseinizadeh, S. F ; Sharif University of Technology
American Institute of Aeronautics and Astronautics Inc
2006

Abstract

The use of the classical Boussinesq approximation is a straightforward strategy for taking into account the buoyancy effect in incompressible solvers. This strategy is highly effective if density variation is low. However, ignoring the importance of density variation in highly thermobuoyant flow fields can cause considerable deviation from the correct prediction of fluid flow behavior and the accurate estimation of heat transfer rate. In this study, an incompressible algorithm is suitably extended to solve high-density-variation fields caused by strong natural-convection influence. The key point in this research is the way that an ordinary incompressible algorithm is extended to...

#### Nonlinear free vibration of conservative oscillators with inertia and static type cubic nonlinearities using homotopy analysis method

, Article Journal of Sound and Vibration ; Volume 316, Issue 1-5 , 2008 , Pages 263-273 ; 0022460X (ISSN) ; Pirbodaghi, T ; Asghari, M ; Farrahi, G. H ; Ahmadian, M. T ; Sharif University of Technology
2008

Abstract

In this study, an accurate analytical solution for the nonlinear free vibration of a conservative oscillator with inertia and static type cubic nonlinearities is derived. This solution has been obtained using homotopy analysis method (HAM). Then, homotopy Pade technique is applied to accelerate the convergence rate of the series solution. This study shows that the HAM leads to an accurate analytical solution, which is valid for a wide range of considered system parameters. Unlike the other analytical methods, HAM can control and adjust the convergence region and rate of the approximation series solution. The excellent accuracy of the current results is demonstrated by comparing with the...

#### Dynamic behavior and modal control of beams under moving mass

, Article Journal of Sound and Vibration ; Volume 306, Issue 3-5 , 2007 , Pages 712-724 ; 0022460X (ISSN) ; Rahimzadeh Rofooei, F ; Shadnam, M. R ; Sharif University of Technology
Academic Press
2007

Abstract

The constitutive equation of an Euler-Bernoulli beam under the excitation of moving mass is considered. The dynamics of the uncontrolled system is governed by a linear, self-adjoint partial differential equation. A Dirac-delta function is used to describe the position of the moving mass along the beam and its inertial effects. An approximate formulation to the problem is obtained by limiting the inertial effect of the moving mass merely to the vertical component of acceleration. Having defined a "critical velocity" in terms of the fundamental period and span of the beam, it is shown that for smaller velocities, the approximate and exact approaches to the problem almost coincide. Since, the...

#### Analysis of time-based random waypoint mobility model for wireless mobile networks

, Article 4th International Conference on Information Technology-New Generations, ITNG 2007, Las Vegas, NV, 2 April 2007 through 4 April 2007 ; 2007 , Pages 42-47 ; 0769527760 (ISBN); 9780769527765 (ISBN) ; Rahimi, M. R ; Sarbazi Azad, H ; Sharif University of Technology
2007

Abstract

In this paper Time-Based Random Waypoint (TBRWP) model is analyzed in detail. Random waypoint model has been studied in mobility literature extensively. Traditional RWP model is based on the independency of the velocity and trajectory; TBRWP, instead, focuses on the independency of motion time and trajectory. Basic model is introduced and general method to evaluate the asymptotical probability density function (PDF) is presented. The model for the one-dimensional and two-dimensional cases with uniform destination distribution in unit square area is solved and exact closed form formulas is obtained. Moreover, normal distribution of nodes in one- and two- dimensional models is investigated and...

#### An enriched finite element algorithm for numerical computation of contact friction problems

, Article International Journal of Mechanical Sciences ; Volume 49, Issue 2 , 2007 , Pages 183-199 ; 00207403 (ISSN) ; Nikbakht, M ; Sharif University of Technology
2007

Abstract

In this paper, the extended finite element method (XFEM) is employed to model the presence of discontinuities caused by frictional contact. The method is used in modeling strong discontinuity within a standard finite element framework. In extended finite element method (XFEM) technique, the special functions are included in standard FEM to simulate discontinuity without considering the boundary conditions in meshing the domain. In this study, the classical finite element approximation is enriched by applying additional terms to simulate the frictional behavior of contact between two bodies. These terms, which are included for enrichment of nodal displacements, depend on the contact condition...

#### Numerical study of natural convection in vertical enclosures using a novel non-Boussinesq algorithm

, Article Numerical Heat Transfer; Part A: Applications ; Volume 52, Issue 9 , 2007 , Pages 849-873 ; 10407782 (ISSN) ; Hosseinizadeh, S. F ; Sharif University of Technology
2007

Abstract

This article applies a novel non-Boussinesq numerical algorithm to solve the free-convection problem in a wide range of thin to thick vertical cavities subject to different side-wall temperatures. In this regard, the compressible flow equations are solved using a primitive incompressible method. No Boussinesq approximation and low Mach number consideration are included in the formulation. To implement the compressibility effect, the density field is calculated via the equation of state for gas. The temperature gradient is suitably varied to generate different low to high thermobuoyant fields, where the Boussinesq approximation may or may not be valid. Contrary to published works on the thin...