298 research outputs found
On the mesh nonsingularity of the moving mesh PDE method
The moving mesh PDE (MMPDE) method for variational mesh generation and
adaptation is studied theoretically at the discrete level, in particular the
nonsingularity of the obtained meshes. Meshing functionals are discretized
geometrically and the MMPDE is formulated as a modified gradient system of the
corresponding discrete functionals for the location of mesh vertices. It is
shown that if the meshing functional satisfies a coercivity condition, then the
mesh of the semi-discrete MMPDE is nonsingular for all time if it is
nonsingular initially. Moreover, the altitudes and volumes of its elements are
bounded below by positive numbers depending only on the number of elements, the
metric tensor, and the initial mesh. Furthermore, the value of the discrete
meshing functional is convergent as time increases, which can be used as a
stopping criterion in computation. Finally, the mesh trajectory has limiting
meshes which are critical points of the discrete functional. The convergence of
the mesh trajectory can be guaranteed when a stronger condition is placed on
the meshing functional. Two meshing functionals based on alignment and
equidistribution are known to satisfy the coercivity condition. The results
also hold for fully discrete systems of the MMPDE provided that the time step
is sufficiently small and a numerical scheme preserving the property of
monotonically decreasing energy is used for the temporal discretization of the
semi-discrete MMPDE. Numerical examples are presented.Comment: Revised and improved version of the WIAS preprin
How a nonconvergent recovered Hessian works in mesh adaptation
Hessian recovery has been commonly used in mesh adaptation for obtaining the
required magnitude and direction information of the solution error.
Unfortunately, a recovered Hessian from a linear finite element approximation
is nonconvergent in general as the mesh is refined. It has been observed
numerically that adaptive meshes based on such a nonconvergent recovered
Hessian can nevertheless lead to an optimal error in the finite element
approximation. This also explains why Hessian recovery is still widely used
despite its nonconvergence. In this paper we develop an error bound for the
linear finite element solution of a general boundary value problem under a mild
assumption on the closeness of the recovered Hessian to the exact one.
Numerical results show that this closeness assumption is satisfied by the
recovered Hessian obtained with commonly used Hessian recovery methods.
Moreover, it is shown that the finite element error changes gradually with the
closeness of the recovered Hessian. This provides an explanation on how a
nonconvergent recovered Hessian works in mesh adaptation.Comment: Revised (improved proofs and a better example
Stability of explicit one-step methods for P1-finite element approximation of linear diffusion equations on anisotropic meshes
We study the stability of explicit one-step integration schemes for the
linear finite element approximation of linear parabolic equations. The derived
bound on the largest permissible time step is tight for any mesh and any
diffusion matrix within a factor of , where is the spatial
dimension. Both full mass matrix and mass lumping are considered. The bound
reveals that the stability condition is affected by two factors. The first one
depends on the number of mesh elements and corresponds to the classic bound for
the Laplace operator on a uniform mesh. The other factor reflects the effects
of the interplay of the mesh geometry and the diffusion matrix. It is shown
that it is not the mesh geometry itself but the mesh geometry in relation to
the diffusion matrix that is crucial to the stability of explicit methods. When
the mesh is uniform in the metric specified by the inverse of the diffusion
matrix, the stability condition is comparable to the situation with the Laplace
operator on a uniform mesh. Numerical results are presented to verify the
theoretical findings.Comment: Revised WIAS Preprin
On the mesh nonsingularity of the moving mesh PDE method
The moving mesh PDE (MMPDE) method for variational mesh generation and adaptation is studied theoretically at the discrete level, in particular the nonsingularity of the obtained meshes. Meshing functionals are discretized geometrically and the MMPDE is formulated as a modified gradient system of the corresponding discrete functionals for the location of mesh vertices. It is shown that if the meshing functional satisfies a coercivity condition, then the mesh of the semi-discrete MMPDE is nonsingular for all time if it is nonsingular initially. Moreover, the altitudes and volumes of its elements are bounded below by positive numbers depending only on the number of elements, the metric tensor, and the initial mesh. Furthermore, the value of the discrete meshing functional is convergent as time increases, which can be used as a stopping criterion in computation. Finally, the mesh trajectory has limiting meshes which are critical points of the discrete functional. The convergence of the mesh trajectory can be guaranteed when a stronger condition is placed on the meshing functional. Two meshing functionals based on alignment and equidistribution are known to satisfy the coercivity condition. The results also hold for fully discrete systems of the MMPDE provided that the time step is sufficiently small and a numerical scheme preserving the property of monotonically decreasing energy is used for the temporal discretization of the semi-discrete MMPDE. Numerical examples are presente
A comparative numerical study of meshing functionals for variational mesh adaptation
We present a comparative numerical study for three functionals used for
variational mesh adaptation. One of them is a generalisation of Winslow's
variable diffusion functional while the others are based on equidistribution
and alignment. These functionals are known to have nice theoretical properties
and work well for most mesh adaptation problems either as a stand-alone
variational method or combined within the moving mesh framework. Their
performance is investigated numerically in terms of equidistribution and
alignment mesh quality measures. Numerical results in 2D and 3D are presented.Comment: Additional example (H1), journal referenc
Measurements and Code Comparison of Wave Dispersion and Antenna Radiation Resisitance for Helicon Waves in a High Density Cylindrical Plasma Source
Helicon wave dispersion and radiation resistance measurements in a high density (ne ≈ 1019 - 1020m-3) and magnetic field (B < 0.2 T) cylindrical plasma source are compared to the results of a recently developed numerical plasma wave code [I. V. Kamensk
Tetrahedral mesh improvement using moving mesh smoothing, lazy searching flips, and RBF surface reconstruction
Given a tetrahedral mesh and objective functionals measuring the mesh quality
which take into account the shape, size, and orientation of the mesh elements,
our aim is to improve the mesh quality as much as possible. In this paper, we
combine the moving mesh smoothing, based on the integration of an ordinary
differential equation coming from a given functional, with the lazy flip
technique, a reversible edge removal algorithm to modify the mesh connectivity.
Moreover, we utilize radial basis function (RBF) surface reconstruction to
improve tetrahedral meshes with curved boundary surfaces. Numerical tests show
that the combination of these techniques into a mesh improvement framework
achieves results which are comparable and even better than the previously
reported ones.Comment: Revised and improved versio
Structure and mechanism of the RNA polymerase II CTD phosphatase Scp1 and large-scale preparation of the RNA polymerase II-TFIIF complex
TFIIF is the only general transcription factor that has been implicated in the
preinitiation complex assembly, open complex formation, initiation and transcription
elongation. In addition, TFIIF stimulates Fcp1, a central phosphatase needed for
recycling of RNA polymerase II (Pol II) after transcription by dephosphorylation of the
Pol II C-terminal domain (CTD). This thesis reports the X-ray structure of the small
CTD phosphatase Scp1, which is homologous to the Fcp1 catalytic domain. The
structure shows a core fold and an active center similar to phosphotransferases and
–hydrolases that solely share a DXDX(V/T) signature motif with Fcp1/Scp1. It was
further demonstrated that the first aspartate in the signature motif undergoes metalassisted
phosphorylation during catalysis, resulting in a phosphoaspartate
intermediate that was structurally mimicked with the inhibitor beryllofluoride.
Specificity may result from CTD binding to a conserved hydrophobic pocket between
the active site and an insertion domain that is unique to Fcp1/Scp1. Fcp1 specificity
may additionally arise from phosphatase recruitment near the CTD via the Pol II
subcomplex Rpb4/7, which is shown to be required for Fcp1 binding to the
polymerase in vitro. Until now, the main impediment in the high resolution
crystallographic studies of TFIIF in complex with Pol II and other members of
transcription machinery was unavailability of soluble, stoichiometric TFIIF complex in
sufficient amounts. This thesis reports on the development of the overexpression
system in yeast and a purification protocol that enabled for the first time to isolate
milligram amounts of a pure and soluble, 15-subunit (~0,7 MDa) stoichiometric Pol IITFIIF
complex. Such complex together with the promoter DNA, RNA, TBP and TFIIB
assembles in vitro into the yeast initially transcribing complex, which can now be
studied structurally
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