3,218 research outputs found
A reduced basis localized orthogonal decomposition
In this work we combine the framework of the Reduced Basis method (RB) with
the framework of the Localized Orthogonal Decomposition (LOD) in order to solve
parametrized elliptic multiscale problems. The idea of the LOD is to split a
high dimensional Finite Element space into a low dimensional space with
comparably good approximation properties and a remainder space with negligible
information. The low dimensional space is spanned by locally supported basis
functions associated with the node of a coarse mesh obtained by solving
decoupled local problems. However, for parameter dependent multiscale problems,
the local basis has to be computed repeatedly for each choice of the parameter.
To overcome this issue, we propose an RB approach to compute in an "offline"
stage LOD for suitable representative parameters. The online solution of the
multiscale problems can then be obtained in a coarse space (thanks to the LOD
decomposition) and for an arbitrary value of the parameters (thanks to a
suitable "interpolation" of the selected RB). The online RB-LOD has a basis
with local support and leads to sparse systems. Applications of the strategy to
both linear and nonlinear problems are given
Localized orthogonal decomposition method for the wave equation with a continuum of scales
This paper is devoted to numerical approximations for the wave equation with
a multiscale character. Our approach is formulated in the framework of the
Localized Orthogonal Decomposition (LOD) interpreted as a numerical
homogenization with an -projection. We derive explicit convergence rates
of the method in the -, - and
-norms without any assumptions on higher order space
regularity or scale-separation. The order of the convergence rates depends on
further graded assumptions on the initial data. We also prove the convergence
of the method in the framework of G-convergence without any structural
assumptions on the initial data, i.e. without assuming that it is
well-prepared. This rigorously justifies the method. Finally, the performance
of the method is demonstrated in numerical experiments
The Finite Element Method for the time-dependent Gross-Pitaevskii equation with angular momentum rotation
We consider the time-dependent Gross-Pitaevskii equation describing the
dynamics of rotating Bose-Einstein condensates and its discretization with the
finite element method. We analyze a mass conserving Crank-Nicolson-type
discretization and prove corresponding a priori error estimates with respect to
the maximum norm in time and the - and energy-norm in space. The estimates
show that we obtain optimal convergence rates under the assumption of
additional regularity for the solution to the Gross-Pitaevskii equation. We
demonstrate the performance of the method in numerical experiments
Quantitative Anderson localization of Schr\"odinger eigenstates under disorder potentials
This paper concerns spectral properties of linear Schr\"odinger operators
under oscillatory high-amplitude potentials on bounded domains. Depending on
the degree of disorder, we prove the existence of spectral gaps amongst the
lowermost eigenvalues and the emergence of exponentially localized states. We
quantify the rate of decay in terms of geometric parameters that characterize
the potential. The proofs are based on the convergence theory of iterative
solvers for eigenvalue problems and their optimal local preconditioning by
domain decomposition.Comment: accepted for publication in M3A
A localized orthogonal decomposition method for semi-linear elliptic problems
In this paper we propose and analyze a new Multiscale Method for solving
semi-linear elliptic problems with heterogeneous and highly variable
coefficient functions. For this purpose we construct a generalized finite
element basis that spans a low dimensional multiscale space. The basis is
assembled by performing localized linear fine-scale computations in small
patches that have a diameter of order H |log H| where H is the coarse mesh
size. Without any assumptions on the type of the oscillations in the
coefficients, we give a rigorous proof for a linear convergence of the H1-error
with respect to the coarse mesh size. To solve the arising equations, we
propose an algorithm that is based on a damped Newton scheme in the multiscale
space
Two-Level discretization techniques for ground state computations of Bose-Einstein condensates
This work presents a new methodology for computing ground states of
Bose-Einstein condensates based on finite element discretizations on two
different scales of numerical resolution. In a pre-processing step, a
low-dimensional (coarse) generalized finite element space is constructed. It is
based on a local orthogonal decomposition and exhibits high approximation
properties. The non-linear eigenvalue problem that characterizes the ground
state is solved by some suitable iterative solver exclusively in this
low-dimensional space, without loss of accuracy when compared with the solution
of the full fine scale problem. The pre-processing step is independent of the
types and numbers of bosons. A post-processing step further improves the
accuracy of the method. We present rigorous a priori error estimates that
predict convergence rates H^3 for the ground state eigenfunction and H^4 for
the corresponding eigenvalue without pre-asymptotic effects; H being the coarse
scale discretization parameter. Numerical experiments indicate that these high
rates may still be pessimistic.Comment: Accepted for publication in SIAM J. Numer. Anal., 201
Collapse of massive magnetized dense cores using radiation-magneto-hydrodynamics: early fragmentation inhibition
We report the results of radiation-magneto-hydrodynamics calculations in the
context of high mass star formation, using for the first time a self-consistent
model for photon emission (i.e. via thermal emission and in radiative shocks)
and with the high resolution necessary to resolve properly magnetic braking
effects and radiative shocks on scales <100 AU. We investigate the combined
effects of magnetic field, turbulence, and radiative transfer on the early
phases of the collapse and the fragmentation of massive dense cores. We
identify a new mechanism that inhibits initial fragmentation of massive dense
cores, where magnetic field and radiative transfer interplay. We show that this
interplay becomes stronger as the magnetic field strength increases. Magnetic
braking is transporting angular momentum outwards and is lowering the
rotational support and is thus increasing the infall velocity. This enhances
the radiative feedback owing to the accretion shock on the first core. We
speculate that highly magnetized massive dense cores are good candidates for
isolated massive star formation, while moderately magnetized massive dense
cores are more appropriate to form OB associations or small star clusters.Comment: Accepted for publication in ApJL, 19 pages, 4 figure
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