Heat flow and calculus on metric measure spaces with Ricci curvature bounded below - the compact case

We provide a quick overview of various calculus tools and of the main results concerning the heat flow on compact metric measure spaces, with applications to spaces with lower Ricci curvature bounds. Topics include the Hopf-Lax semigroup and the Hamilton-Jacobi equation in metric spaces, a new approach to differentiation and to the theory of Sobolev spaces over metric measure spaces, the equivalence of the L^2-gradient flow of a suitably defined "Dirichlet energy" and the Wasserstein gradient flow of the relative entropy functional, a metric version of Brenier's Theorem, and a new (stronger) definition of Ricci curvature bound from below for metric measure spaces. This new notion is stable w.r.t. measured Gromov-Hausdorff convergence and it is strictly connected with the linearity of the heat flow.Comment: To the memory of Enrico Magenes, whose exemplar life, research and teaching shaped generations of mathematician

МОДЕЛЮВАННЯ ДИНАМІЧНИХ РЕЖИМІВ АСИНХРОННОГО ЕЛЕКТРОПРИВОДУ ПРИ ПЕРІОДИЧНОМУ НАВАНТАЖЕННІ

Goal. Development of methods and mathematical models, based on them, for the calculation of transients and steady-state modes of induction electric drives operating in periodic load mode. Methodology. The developed algorithms are based on a mathematical model of an induction motor, which takes into account the saturation of the magnetic core and the displacement of current in the rotor bars. The processes are described by a system of nonlinear differential equations in the orthogonal axes x, y, which enables the results to be obtained with the smallest amount of calculations. The magnetization characteristics by the main magnetic flux and the leakage fluxes are used to calculate the electromagnetic parameters of the motor. To account for the current displacement in the rotor bars, the short-circuited winding is considered as a multilayer structure formed by dividing the bars in height by several elements. Results. Due to the variable load on the motor shaft, electromagnetic processes in both transient and steady state modes of the electric drive in any coordinate system are described by a system of nonlinear differential equations. The result of the calculation of the transients is obtained as a result of their integration time dependencies of coordinates (currents, electromagnetic torque, etc.) at a given law of change of the moment of loading. The proposed method of calculating steady-state mode is based on algebraization of differential equations on the mesh of nodes of the process cyclicity period and allows to obtain periodic dependencies in the time domain. Originality. The problem of calculating a steady-state periodic mode is solved as a boundary problem for a system of first-order differential equations with periodic boundary conditions, which allows to obtain instantaneous dependences during the period of currents, electromagnetic torque, capacities and other coordinates. Practical significance. Using the developed algorithm, it is possible to calculate the static characteristics of periodic processes as dependencies on different parameters of the cycle of periodic load or other coordinates, which is the basis for the choice of the motor for overload, power, heating, etc., as well as to detect the possibility of resonance. Розроблено математичні моделі і алгоритми, з використанням яких складені програми розрахунку перехідних процесів і усталених режимів асинхронних електроприводів, які працюють в режимі періодичної зміни навантаження. В їх основу покладено математичну модель асинхронного двигуна, розроблену на основі теорії кіл і зображувальних векторів електричних координат, в якій враховується насичення магнітопроводу і витіснення струму в стержнях короткозамкненого ротора. Внаслідок змінного навантаження на валу двигуна електромагнітні процеси як в перехідних, так і усталених режимах в будь-якій системі координат описуються системою нелінійних диференціальних рівнянь. В роботі використано систему ортогональних координатних осей x, y, яка обертається з довільною швидкістю. Для обчислення електромагнітних параметрів двигуна використовуються характеристики намагнічування основним магнітним потоком, а також потоками розсіювання статора і ротора. Для урахування витіснення струму в стержнях ротора короткозамкнена обмотка подається у вигляді багатошарової структури, утвореної розбиттям стержнів по висоті на кілька елементів. Усталений періодичний режим розраховується методом розв’язування крайової задачі, розробленим на основі апроксимації координат кубічними сплайнами, що дає змогу отримати їх періодичні залежності в позачасовій області і розраховувати статичні характеристики як залежності від параметрів циклу періодично-змінного навантаження або інших координат

Homogenization of a model for the propagation of sound in the lungs

International audienceIn this paper, we are interested in the mathematical modeling of the propagation of sound waves in the lung parenchyma, which is a foam-like elastic material containing millions of air-filled alveoli. In this study, the parenchyma is governed by the linearized elasticity equations, and the air by the acoustic wave equations. The geometric arrangement of the alveoli is assumed to be periodic with a small period ε > 0. We consider the time-harmonic regime forced by vibrations induced by volumic forces. We use the two-scale convergence theory to study the asymptotic behavior as ε goes to zero and prove the convergence of the solutions of the coupled fluid-structure problem to the solution of a linear-elasticity boundary value problem

Certified Reduced Basis Methods and Output Bounds for the Harmonic Maxwell's Equations

We propose certified reduced basis methods for the efficient and reliable evaluation of a general output that is implicitly connected to a given parameterized input through the harmonic Maxwell's equations. The truth approximation and the development of the reduced basis through a greedy approach is based on a discontinuous Galerkin approximation of the linear partial differential equation. The formulation allows the use of different approximation spaces for solving the primal and the dual truth approximation problems to respect the characteristics of both problem types, leading to an overall reduction in the off-line computational effort. The main features of the method are the following: (i) rapid convergence on the entire representative set of parameters, (ii) rigorous a posteriori error estimators for the output, and (iii) a parameter independent off-line phase and a computationally very efficient on-line phase to enable the rapid solution of many-query problems arising in control, optimization, and design. The versatility and performance of this approach is shown through a numerical experiment, illustrating the modeling of material variations and problems with resonant behavior

Driving forces for Ag-induced periodic faceting of vicinal Cu(111)

Adsorption of submonolayer amounts of Ag on vicinal Cu(111) induces periodic faceting. The equilibrium structure is characterized by Ag-covered facets that alternate with clean Cu stripes. In the atomic scale, the driving force is the matching of Ag(111)-like packed rows with Cu(111) terraces underneath. This determines the preference for the facet orientation and the evolution of different phases as a function of coverage. Both Cu and Ag stripe widths can be varied smoothly in the 3-30 nm range by tuning Ag coverage, allowing to test theoretical predictions of elastic theories.Comment: 1 text, 4 figure

Mathematical modelling of starting modes of induction motors with squirrel-cage rotor

Purpose. Development of methods and algorithms for calculation of starting modes of the induction motors with the squirrel-cage rotor. Methodology. Mathematical modelling of starting modes of asynchronous electric drives in various coordinate systems with the use of numerical methods for solving boundary problems for systems of differential equations and nonlinear systems of final equations and calculation with their use of static characteristics. Results. Methods and algorithms for numerical analysis of the starting modes of asynchronous electric drive have been developed, which make it possible to calculate the static characteristics and transients in fixed three-phase and orthogonal two-phase coordinate systems. Academic novelty. Known in the literature methods of calculation of starting modes have in their basis classical equivalent circuits with different approximate methods of calculating the parameters taking into account the saturation of a magnetic core and current displacement in a rotor winding. This approach requires a special solution of the adequacy problem for each problem. The methods described in the article for the saturation accounting use real magnetization characteristics of the main magnetic flux and leakage fluxes separately, and the skin effect phenomenon is taken into account by representing the squirrel-cage rotor winding as multilayer. Such mathematical model of the motor is universal and makes it possible to take into account the saturation and current displacement in any dynamic mode, including the start-up process. Mathematical models developed on the basis of the proposed methods provide the adequacy of results with a minimum amount of calculations and make it possible to perform optimization calculations. Practical value. Created on the basis of developed algorithms calculation programs allow to carry out with high reliability calculation of starting modes both in three-phase and two-phase axes of coordinates that gives the chance to analyze not only symmetric, but also asymmetric modes and to predict special features of functioning of system of the electric drive in the set technological conditions of operation and to form requirements to development and adjustment of starting systems with the purpose of maintenance of power effective and reliable operation of system of asynchronous electric drives

Improved successive constraint method based a posteriori error estimate for reduced basis approximation of 2D Maxwell's problem

In a posteriori error analysis of reduced basis approximations to affinely parametrized partial differential equations, the construction of lower bounds for the coercivity and inf-sup stability constants is essential. In [Huynh et al., C. R. Acad. Sci. Paris Ser. I Math. 345 ( 2007) 473-478], the authors presented an efficient method, compatible with an off-line/on-line strategy, where the online computation is reduced to minimizing a linear functional under a few linear constraints. These constraints depend on nested sets of parameters obtained iteratively using a greedy algorithm. We improve here this method so that it becomes more efficient and robust due to two related properties: (i) the lower bound is obtained by a monotonic process with respect to the size of the nested sets; (ii) less eigen-problems need to be solved. This improved evaluation of the inf-sup constant is then used to consider a reduced basis approximation of a parameter dependent electromagnetic cavity problem both for the greedy construction of the elements of the basis and the subsequent validation of the reduced basis approximation. The problem we consider has resonance features for some choices of the parameters that are well captured by the methodology

Математичне моделювання реостатно-реакторного пуску асинхронних двигунів з фазним ротором

Introduction. Wound-rotor induction motors are less common compared squirrel-cage induction motors. However, they occupy a significant share among electric drives with difficult starting conditions. Their advantage is obtaining a high starting electromagnetic torque at lower values of starting currents. Problem. Due to the possibility of including different devices in the rotor circuit, it is possible to shape the starting characteristics according to the needs of the technological process. Due to a narrower range of applications of electric drives based on wound-rotor induction motors, they are less investigated. Selection of parameters of starting and regulating devices, included in the rotor circuit, is carried out by simplified methods, which do not satisfy modern requirements to regulated electric drives. Goal. The paper aims to develop mathematical models and methods for calculating the dynamic modes and static characteristics of the wound-rotor induction motor with a reactor in the rotor circuit. Methodology. In the developed algorithms, the mathematical model of the motor is presented by the differential equations made for electric circuits in a system of orthogonal coordinates that allows excluding angular coordinate from equations of electric equilibrium. The elements of the Jacobi matrix of equilibrium equations of motor circuits are eigenvalues, and mutual is the differential inductances of electrical circuits, which are determined based on the magnetization characteristics of the main magnetic flux and leakage fluxes of the rotor and stator circuits. Results. Mathematical models for the study of starting modes of wound rotor induction motor allow to calculate transients and static characteristics and, on their basis, to carry out design synthesis of starting reactors, which provide the law of change of electromagnetic torque during start-up operating conditions. Originality. The mathematical basis of the developed algorithms is the method of solving nonlinear systems of equations by Newton method in combination with the method of continuation by parameter. The developed mathematical models and software made on their basis have high speed that allows to carry out high-reliability calculation of starting modes taking into account saturation of a magnetic circuit of the motor. Practical value. The developed algorithms do not require significant computing resources, have high speed, and can be used both for the design synthesis of start-control devices and control of the electric drive in real time and to predict its course.Розроблено математичні моделі, методи і алгоритми аналізу пускових режимів і статичних характеристик асинхронних двигунів з фазним ротором. В розроблених алгоритмах математична модель двигуна подана диференціальними рівняннями, складеними для електричних контурів в системі ортогональних координат. Математичною основою розроблених алгоритмів розрахунку статичних характеристик є розв’язування нелінійних систем скінченних рівнянь електричної рівноваги методом Ньютона в поєднанні з методом продовження по параметру, а пускових режимів – числове інтегрування нелінійних систем диференціальних рівнянь електромеханічної рівноваги. Елементами матриці Якобі в розроблених алгоритмах є власні і взаємні диференціальні індуктивності електричних контурів, які визначаються на основі характеристик намагнічування основним магнітним потоком, а також потоками розсіювання контурів ротора і статора, що дає змогу здійснювати розрахунок з урахуванням насичення магнітопроводу двигуна. Розроблені програми і алгоритми мають високу швидкодію і дають змогу здійснювати проектний синтез пускових активних і реактивних опорів в колі ротора з метою забезпечення закону зміни електромагнітного моменту під час пуску, який відповідає роботі системи електроприводу в заданих технологічних умовах, а також здійснювати мікропроцесорне керування в динамічних режимах

Solitary coherent structures in viscoelastic shear flow: computation and mechanism

Starting from stationary bifurcations in Couette-Dean flow, we compute nontrivial stationary solutions in inertialess viscoelastic circular Couette flow. These solutions are strongly localized vortex pairs, exist at arbitrarily large wavelengths, and show hysteresis in the Weissenberg number, similar to experimentally observed ``diwhirl'' patterns. Based on the computed velocity and stress fields, we elucidate a heuristic, fully nonlinear mechanism for these flows. We propose that these localized, fully nonlinear structures comprise fundamental building blocks for complex spatiotemporal dynamics in the flow of elastic liquids.Comment: 5 pages text and 4 figures. Submitted to Physical Review Letter

Математичне моделювання пускових режимів асинхронних двигунів з короткозамкненим ротором

Purpose. Development of methods and algorithms for calculation of starting modes of the induction motors with the squirrel-cage rotor. Methodology. Mathematical modelling of starting modes of asynchronous electric drives in various coordinate systems with the use of numerical methods for solving boundary problems for systems of differential equations and nonlinear systems of final equations and calculation with their use of static characteristics. Results. Methods and algorithms for numerical analysis of the starting modes of asynchronous electric drive have been developed, which make it possible to calculate the static characteristics and transients in fixed three-phase and orthogonal two-phase coordinate systems. Academic novelty. Known in the literature methods of calculation of starting modes have in their basis classical equivalent circuits with different approximate methods of calculating the parameters taking into account the saturation of a magnetic core and current displacement in a rotor winding. This approach requires a special solution of the adequacy problem for each problem. The methods described in the article for the saturation accounting use real magnetization characteristics of the main magnetic flux and leakage fluxes separately, and the skin effect phenomenon is taken into account by representing the squirrel-cage rotor winding as multilayer. Such mathematical model of the motor is universal and makes it possible to take into account the saturation and current displacement in any dynamic mode, including the start-up process. Mathematical models developed on the basis of the proposed methods provide the adequacy of results with a minimum amount of calculations and make it possible to perform optimization calculations. Practical value. Created on the basis of developed algorithms calculation programs allow to carry out with high reliability calculation of starting modes both in three-phase and two-phase axes of coordinates that gives the chance to analyze not only symmetric, but also asymmetric modes and to predict special features of functioning of system of the electric drive in the set technological conditions of operation and to form requirements to development and adjustment of starting systems with the purpose of maintenance of power effective and reliable operation of system of asynchronous electric drives.Розроблено методи і алгоритми числового аналізу пускових режимів асинхронного електроприводу, які дають змогу розраховувати статичні характеристики і перехідні процеси з урахуванням насичення магнітопроводу і явища скін-ефекту в стержнях ротора. Математичною основою розроблених алгоритмів є проекційний метод розв’язування нелінійних крайових задач, метод продовження по параметру, метод Ньютона розв’язування нелінійних систем рівнянь, числового інтегрування нелінійних систем диференціальних рівнянь. Розроблені математичні моделі дають змогу здійснювати з високою достовірністю розрахунок пускових режимів у трифазних і двофазних координатних осях, що дає змогу аналізувати не тільки симетричні, але й несиметричні режими і прогнозувати особливості функціонування системи асинхронного електроприводу в заданих технологічних умовах експлуатації