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"Equations of motion"
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Explicit equations of motion for constrained mechanical systems with singular mass matrices and applications to multi-body dynamics
We present the new, general, explicit form of the equations of motion for constrained mechanical systems applicable to systems with singular mass matrices. The systems may have holonomic and/or non-holonomic constraints, which may or may not satisfy D'Alembert's principle at each instant of time. The equation provides new insights into the behaviour of constrained motion and opens up new ways of modelling complex multi-body systems. Examples are provided and applications of the equation to such systems are illustrated.
Journal Article
Periodic Travelling Wave Solutions of the Modified Short Pulse Equation
By introducing a change of variables, Grimshaw et al. (Stud.Appl.Math.129:414-436, 2012) and Hakkaev et al. (Stud.Appl.Math.139:405-433, 2017) successfully transformed the short pulse (SP) model into a standard Schrödinger equation, based on which the properties of the periodic travelling wave solutions of SP equation were investigated. However, the transformation is no longer applicable to the modified short pulse (mSP) equation. In this paper, we refine the previously introduced transformation and propose two novel changes of variables, thereby establishing a direct connection between the mSP equation and the simple harmonic motion equation. Using these transformations, we explicitly construct the periodic travelling wave solutions of the mSP equation for both the focusing and defocusing cases. Furthermore, by employing the bifurcation theory of dynamical systems, we analyze the phase portraits of the traveling wave system corresponding to the mSP equation.
Journal Article
On the Computational Methods for Solving the Differential-Algebraic Equations of Motion of Multibody Systems
In this investigation, different computational methods for the analytical development and the computer implementation of the differential-algebraic dynamic equations of rigid multibody systems are examined. The analytical formulations considered in this paper are the Reference Point Coordinate Formulation based on Euler Parameters (RPCF-EP) and the Natural Absolute Coordinate Formulation (NACF). Moreover, the solution approaches of interest for this study are the Augmented Formulation (AF) and the Udwadia–Kalaba Equations (UKE). As shown in this paper, the combination of all the methodologies analyzed in this work leads to general, effective, and efficient multibody algorithms that can be readily implemented in a general-purpose computer code for analyzing the time evolution of mechanical systems constrained by kinematic joints. This study demonstrates that multibody algorithm based on the combination of the NACF with the UKE turned out to be the most effective and efficient computational method. The conclusions drawn in this paper are based on the numerical results obtained for a benchmark multibody system analyzed by means of dynamical simulations.
Journal Article
Complex Dynamics of Glass-Forming Liquids
The book presents a self-contained exposition of the mode-coupling theory for the evolution of glassy dynamics in liquids. This theory is based on polynomial expressions for the correlations of force fluctuations in terms of those of density fluctua-tions. These mode-coupling polynomials are motivated as descriptions of the cage-effect-induced transient localization of particles in condensed matter. It is proven that the implied regular mode-coupling equations of motion determine uniquely models for a correlation-function description of the dynamics. This holds for all choices of the polynomial coefficients, which serve as coupling constants. The arrested parts of the correlations are solutions of fixed-point equations. They exhibit spontaneous singularities, which are equivalent to the bifurcation singularities of the real roots of real polynomials. They deal with idealized liquid-glass and glass-glass transitions. Driving the coupling constants towards their critical values, the correlation functions exhibit the evolution of complex dynamics. Its subtleties are due to the interplay of nonlinearities and divergent retardation effects. The book discusses that the relaxation features are similar to those observed in experimental and molecular-dynamics-simulation studies of con-ventional liquids and colloids. Asymptotic expansions are derived for the mode-coupling-theory functions for small frequencies and small separations of the coupling constants from the transition values. The leading-order asymptotic contributions provide an understanding of the essential facets of the scenarios. The leading-asymptotic corrections are deduced and applied to quantify the evolution of the leading-order description.
eBook
Second-Order Approximate Equations of the Large-Scale Atmospheric Motion Equations and Symmetry Analysis for the Basic Equations of Atmospheric Motion
In this paper, symmetry properties of the basic equations of atmospheric motion are proposed. The results on symmetries show that the basic equations of atmospheric motion are invariant under space-time translation transformation, Galilean translation transformations and scaling transformations. Eight one-parameter invariant subgroups and eight one-parameter group invariant solutions are demonstrated. Three types of nontrivial similarity solutions and group invariants are proposed. With the help of perturbation method, we derive the second-order approximate equations for the large-scale atmospheric motion equations, including the non-dimensional equations and the dimensional equations. The second-order approximate equations of the large-scale atmospheric motion equations not only show the characteristics of physical quantities changing with time, but also describe the characteristics of large-scale atmospheric vertical motion.
Journal Article
Dynamics of spin relaxation in nonequilibrium magnetic nanojunctions
We investigate nonequilibrium phenomena in magnetic nano-junctions using a numerical approach that combines classical spin dynamics with the hierarchical equations of motion technique for quantum dynamics of conduction electrons. Our focus lies on the spin dynamics, where we observe non-monotonic behavior in the spin relaxation rates as a function of the coupling strength between the localized spin and conduction electrons. Notably, we identify a distinct maximum at intermediate coupling strength, which we attribute to a competition that involves the increasing influence of the coupling between the classical spin and electrons, as well as the influence of decreasing local density of states at the Fermi level. Furthermore, we demonstrate that the spin dynamics of a large open system can be accurately simulated by a short chain coupled to semi-infinite metallic leads. In the case of a magnetic junction subjected to an external DC voltage, we observe resonant features in the spin relaxation, reflecting the electronic spectrum of the system. The precession of classical spin gives rise to additional side energies in the electronic spectrum, which in turn leads to a broadened range of enhanced damping in the voltage.
Journal Article
Survey of the hierarchical equations of motion in tensor-train format for non-Markovian quantum dynamics
This work is a pedagogical survey about the hierarchical equations of motion and their implementation with the tensor-train format. These equations are a great standard in non-perturbative non-Markovian open quantum systems. They are exact for harmonic baths in the limit of relevant truncation of the hierarchy. We recall the link with the perturbative second-order time convolution equations also known as the Bloch–Redfield equations. Some theoretical tools characterizing non-Markovian dynamics such as the non-Markovianity measures or the dynamical map are also briefly discussed in the context of HEOM simulations. The main points of the tensor-train expansion are illustrated in an example with a qubit interacting with a bath described by a Lorentzian spectral density. Finally, we give three illustrative applications in which the system–bath coupling operator is similar to that of the analytical treatment. The first example revisits a model in which population-to-coherence transfer via the bath creates a long-lasting coherence between two states. The second one is devoted to the computation of stationary absorption and emission spectra. We illustrate the link between the spectral density and the Stokes shift in situations with and without nonadiabatic interaction. Finally, we simulate an excitation transfer when the spectral density is discretized by undamped modes to illustrate a situation in which the TT formulation is more efficient than the standard one.
Journal Article
1D Radiative Fluid and Liquid Crystal Equations
This book presents recent results on nonlinear evolutionary fluid equations,in particular the global well-posedness and asymptotic behavior of solutions to1D radiative fluid equations, as well as liquid crystal equations. It provides complete elements related to the one-dimensional compressibleliquid crystal fluid system.
eBook
Comparison of Kane’s and Lagrange’s Methods in Analysis of Constrained Dynamical Systems
Dynamic modeling is a fundamental step in analyzing the movement of any mechanical system. Methods for dynamical modeling of constrained systems have been widely developed to improve the accuracy and minimize computational cost during simulations. The necessity to satisfy constraint equations as well as the equations of motion makes it more critical to use numerical techniques that are successful in decreasing the number of computational operations and numerical errors for complex dynamical systems. In this study, performance of a variant of Kane’s method compared to six different techniques based on the Lagrange’s equations is shown. To evaluate the performance of the mentioned methods, snake-like robot dynamics is considered and different aspects such as the number of the most time-consuming computational operations, constraint error, energy error, and CPU time assigned to each method are compared. The simulation results demonstrate the superiority of the variant of Kane’s method concerning the other ones.
Journal Article