A degenerate Cahn‐Hilliard model as constrained Wasserstein gradient flow

Space–time adaptive splitting scheme for the numerical simulation of polycrystallization

In this paper we deal with the numerical solution of moving boundary problems of two-phase Stefan type. Based on an implicit discretization in time and the use of continuous, piecewise linear finite elements in the space variables with respect to the weak formulation of the problem, a globally convergent multi-grid algorithm is developed. That algorithm strongly relies on the variational characterization of the fully discretized problem as the unconstrained minimization of a subdifferentiable convex objective functional. Numerical results indicate a significant improvement in efficiency compared with previous multi-grid approaches

Stability and dispersion analysis of improved time discretization for simply supported prestressed Timoshenko systems. Application to the stiff piano string

Thanks to a finite element method, we solve numerically parabolic partial differential equations on complex domains by avoiding the mesh generation, using a regular background mesh, not fitting the domain and its real boundary exactly. Our technique follows the ϕ-FEM paradigm, which supposes that the domain is given by a level-set function. In this paper, we prove a priori error estimates in l 2 (H 1 ) and l ∞ (L 2 ) norms for an implicit Euler discretization in time. We give numerical illustrations to highlight the performances of ϕ-FEM, which combines optimal convergence accuracy, easy implementation process and fastness.

WENO schemes applied to the quasi-relativistic Vlasov–Maxwell model for laser–plasma interaction

This paper establishes a theory of nonlinear spectral decompositions by considering the eigenvalue problem related to an absolutely one-homogeneous functional in an infinite-dimensional Hilbert space. This approach is both motivated by works for the total variation, where interesting results on the eigenvalue problem and the relation to the total variation flow have been proven previously, and by recent results on finite-dimensional polyhedral semi-norms, where gradient flows can yield spectral decompositions into eigenvectors. We provide a geometric characterization of eigenvectors via a dual unit ball and prove them to be subgradients of minimal norm. This establishes the connection to gradient flows, whose time evolution is a decomposition of the initial condition into subgradients of minimal norm. If these are eigenvectors, this implies an interesting orthogonality relation and the equivalence of the gradient flow to a variational regularization method and an inverse scale space flow. Indeed we verify that all scenarios where these equivalences were known before by other arguments - such as one-dimensional total variation, multidimensional generalizations to vector fields, or certain polyhedral semi-norms - yield spectral decompositions, and we provide further examples. We also investigate extinction times and extinction profiles, which we characterize as eigenvectors in a very general setting, generalizing several results from literature.

Gas foil bearings (GFBs) have been successfully introduced in the field of high speed turbo machineries. A combination of low power loss, high speed operation and the omission of an oil system heighten the importance for small and medium sized turbo machineries, e.g. turbochargers or range extenders. However, experimental and numerical investigations have shown subsynchronous vibrations, which affect the rotor dynamic behaviour. Structural damping generated by friction contacts inside the compliant structure may reduce vibrations up to a certain level. In addition, several proved methods and devices, e.g. side feed pressurerisation, pre-loading due to shims and viscoelastic foil bearings are common techniques to decrease non synchronous vibrations. However, far too little attention has been paid to the causes of these non-linear effects. Understanding the causes may results in a higher knowledge of the overall GFB dynamic behaviour. Thus, the aim of this paper is to analyse the causes of these non-linear vibrations. A hypothesis is stated, that the non-linear vibrations are influenced by a self excitation and a forced non-linearity. The non-linear compressible transient Reynolds equation is discretised by a hybrid finite difference scheme with an implicit time discretisation while the pressure field is coupled with a 2D plate model. This plate model is linked to a spring-damper configuration. The time domain analysis shows, that the subsynchronous frequencies may excite the system eigenfrequency. In addition, good correlations between the onset speed of sub synchronous vibrations of the time domain simulations and the linearised frequency domain analysis are shown. In the second part of this paper, the effects of different bump foil configurations (bump-type GFB, shimmed GFB and a lobed GFB) on the dynamic performance are considered. It is shown, that an effective reduction of sub synchronous vibrations due to a non-uniform circumferential stiffness distribution and the use of shims is possible. Especially, the low loaded case (5 N) has an increase of onset speed of subsynchronous vibration of ≈173 %, compared to the same bearing setup without shims.KeywordsFoil bearingsTime domainNon-linear vibrationsShims

Toward robust linear implicit schemes for steady state hypersonic flows

In this paper, we present a numerical scheme for a non linear Fokker–Planck equation of one‐dimensional granular medium. We consider a kinetic description of a system of particles undergoing nearly elastic particles and interacting with a thermal bath. We construct a numerical method which preserve all the properties of the continuous model, conservation laws, and decay of the entropy. Moreover, the discretization is such that, on a fixed grid, we deal with arbitrary small temperatures for the bath. Explicit and implicit time discretization are analyzed.

An unfactored implicit time-marching method for the solution of the unsteady two-dimensional Euler equations on deforming grids is described. The present work is placed into a multiblock framework and e ts into the development of a generally applicable parallel multiblock e ow solver. The convective terms are discretized using an upwind total variation diminishing scheme, whereas the unsteady governing equations are discretized using an implicit dual-time approach. The large sparse linear system arising from the implicit time discretization at each pseudotime step is solved efe ciently by using a conjugate-gradient-type method with a preconditioning based on a block incomplete lower-upper factorization. Results are shown for a series of pitching airfoil test cases selected from the AGARD aeroelastic cone gurations for the NACA 0012 airfoil. Comparisons with experimental data and previous published results are presented. The efe ciency of the method is demonstrated by looking at the effect of a number of numerical parameters, such as the conjugate gradient tolerance and the size of the global time step and by carrying out a grid ree nement study. Finally, a demonstration test case forthe Williamsairfoil (Williams, B. R., “ An Exact Test Case for the Plane Potential Flow About Two Adjacent Lifting Aerofoils,” National Physical Lab., Aeronautical Research Council, Research Memorandum 3717, London, 1973 )with an oscillating e ap is presented, highlighting the capability of the grid deformation technique.

A finite element method is presented, which allows to determine power frequency two dimensional field distributions including arbitrary hysteresis processes. The method employs an implicit time discretisation that helps to avoid stability restrictions and therefore permits a high resolution in space. Numerical tests were carried out, which proved the method to be very efficient.

Parameter identification is an important research topic with a variety of applications in industrial and environmental problems. Usually, a functional has to be minimized in conjunction with parameter identification; thus, there is a certain similarity between the parameter identification and optimization. A number of rigorous and efficient algorithms for optimization problems were developed in recent decades for the case of a convex functional. In the case of a non-convex functional, the metaheuristic algorithms dominate. This paper discusses an optimization method called modified bee colony algorithm (MBC), which is a modification of the standard bees algorithm (SBA). The SBA is inspired by a particular intelligent behavior of honeybee swarms. The algorithm is adapted for the parameter identification of reaction-dominated pore-scale transport when a non-convex functional has to be minimized. The algorithm is first checked by solving a few benchmark problems, namely finding the minima for Shekel, Rosenbrock, Himmelblau and Rastrigin functions. A statistical analysis was carried out to compare the performance of MBC with the SBA and the artificial bee colony (ABC) algorithm. Next, MBC is applied to identify the three parameters in the Langmuir isotherm, which is used to describe the considered reaction. Here, 2D periodic porous media were considered. The simulation results show that the MBC algorithm can be successfully used for identifying admissible sets for the reaction parameters in reaction-dominated transport characterized by low Pecklet and high Damkholer numbers. Finite element approximation in space and implicit time discretization are exploited to solve the direct problem.

We present a variational multiscale strategy in the spirit of FE $$^2$$ method for simulating the real time evolution of resonant elastic metamaterials with space/time modulation. The implicit time discretization used guarantees the stability of the numerical solution, while the accuracy is quantified by direct comparison with the response obtained via direct numerical simulation. The results showcase a remarkable accuracy of the method, even when the dynamic material properties drastically change over length scales that are equal to the macroscopic mesh size and time scales that are of the same order of magnitude than the external excitations. We further propose a strategy to identify the band gap structure of these metamaterials based on their real time response. The methodology is verified for spatially periodic time invariant systems via classical unit cell analyses in the frequency domain.

In this paper we give an energy formulation of the binary alloy solidification problem. We assume the change in solidification temperature is only the result of the phase diagram; supercooling, surface tension and curvature effects are ignored. Both the mass and heat diffusions are described by partial differential equations which are similar to the enthalpy (energy) formulation for the pure heat diffusion or Stefan problem. For a one space variable problem uniqueness (Theorem 4) and existence (Theorem 5) are established. Also, an implicit time discretization, which has no stability conditions on the time increment, of this formulation is given. This numerical method is shown to converge, as the mesh goes to zero, to the solution of the continuum problem. Numerical examples are given.

Strain gradient plasticity with nonlinear evolutionary energetic higher order stresses