29,977 research outputs found
Three-dimensional elliptic grid generation technique with application to turbomachinery cascades
Described is a numerical method for generating 3-D grids for turbomachinery computational fluid dynamic codes. The basic method is general and involves the solution of a quasi-linear elliptic partial differential equation via pointwise relaxation with a local relaxation factor. It allows specification of the grid point distribution on the boundary surfaces, the grid spacing off the boundary surfaces, and the grid orthogonality at the boundary surfaces. A geometry preprocessor constructs the grid point distributions on the boundary surfaces for general turbomachinery cascades. Representative results are shown for a C-grid and an H-grid for a turbine rotor. Two appendices serve as user's manuals for the basic solver and the geometry preprocessor
Nonlinear force-free modeling of the solar coronal magnetic field
The coronal magnetic field is an important quantity because the magnetic
field dominates the structure of the solar corona. Unfortunately direct
measurements of coronal magnetic fields are usually not available. The
photospheric magnetic field is measured routinely with vector magnetographs.
These photospheric measurements are extrapolated into the solar corona. The
extrapolated coronal magnetic field depends on assumptions regarding the
coronal plasma, e.g. force-freeness. Force-free means that all non-magnetic
forces like pressure gradients and gravity are neglected. This approach is well
justified in the solar corona due to the low plasma beta. One has to take care,
however, about ambiguities, noise and non-magnetic forces in the photosphere,
where the magnetic field vector is measured. Here we review different numerical
methods for a nonlinear force-free coronal magnetic field extrapolation:
Grad-Rubin codes, upward integration method, MHD-relaxation, optimization and
the boundary element approach. We briefly discuss the main features of the
different methods and concentrate mainly on recently developed new codes.Comment: 33 pages, 3 figures, Review articl
Characterization of the austenite recrystallization by comparing double deformation and stress relaxation tests
A high amount of deformation below the non-recrystallization temperature (T-nr) is a common industrial practice to achieve a good combination of toughness and strength in microalloyed steels. To combine the industrially relevant optimum combination of high productivity and product quality, an accurate knowledge of T-nr and the recrystallization kinetics is required. Although a lot of literature data is available on the recrystallization behaviour of microalloyed steels, correlations are often difficult to be made due to the effect of different experimental setups, types of analysis and test schedules that are used to obtain this data. Although this would significantly improve the knowledge about these steels, so far, no systematic comparison has been presented in literature to correlate the different techniques and methods. In this study, different hot rolling simulation techniques, testing schedules and types of analysis were used to determine the recrystallization kinetics of a microalloyed steel. On the one hand, good agreement was found between the results from different test equipment for the double deformations tests. On the other hand, stress relaxation tests showed accelerated kinetics and appeared to be less effective
Adaptive Mesh Refinement for Coupled Elliptic-Hyperbolic Systems
We present a modification to the Berger and Oliger adaptive mesh refinement
algorithm designed to solve systems of coupled, non-linear, hyperbolic and
elliptic partial differential equations. Such systems typically arise during
constrained evolution of the field equations of general relativity. The novel
aspect of this algorithm is a technique of "extrapolation and delayed solution"
used to deal with the non-local nature of the solution of the elliptic
equations, driven by dynamical sources, within the usual Berger and Oliger
time-stepping framework. We show empirical results demonstrating the
effectiveness of this technique in axisymmetric gravitational collapse
simulations. We also describe several other details of the code, including
truncation error estimation using a self-shadow hierarchy, and the
refinement-boundary interpolation operators that are used to help suppress
spurious high-frequency solution components ("noise").Comment: 31 pages, 15 figures; replaced with published versio
Configurational entropy measurements in extremely supercooled liquids that break the glass ceiling
Liquids relax extremely slowly upon approaching the glass state. One
explanation is that an entropy crisis, due to the rarefaction of available
states, makes it increasingly arduous to reach equilibrium in that regime.
Validating this scenario is challenging, because experiments offer limited
resolution, while numerical studies lag more than eight orders of magnitude
behind experimentally-relevant timescales. In this work we not only close the
colossal gap between experiments and simulations but manage to create in-silico
configurations that have no experimental analog yet. Deploying a range of
computational tools, we obtain four estimates of their configurational entropy.
These measurements consistently confirm that the steep entropy decrease
observed in experiments is also found in simulations, even beyond the
experimental glass transition. Our numerical results thus extend the new
observational window into the physics of glasses and reinforce the relevance of
an entropy crisis for understanding their formation.Comment: 4+23 pages, 3+12 figures; v2: final version, with various changes
made. Data relevant to this work can be accessed at
http://dx.doi.org/10.7924/G8ZG6Q9
Solar Force-free Magnetic Fields
The structure and dynamics of the solar corona is dominated by the magnetic
field. In most areas in the corona magnetic forces are so dominant that all
non-magnetic forces like plasma pressure gradient and gravity can be neglected
in the lowest order. This model assumption is called the force-free field
assumption, as the Lorentz force vanishes. This can be obtained by either
vanishing electric currents (leading to potential fields) or the currents are
co-aligned with the magnetic field lines. First we discuss a mathematically
simpler approach that the magnetic field and currents are proportional with one
global constant, the so-called linear force-free field approximation. In the
generic case, however, the relation between magnetic fields and electric
currents is nonlinear and analytic solutions have been only found for special
cases, like 1D or 2D configurations. For constructing realistic nonlinear
force-free coronal magnetic field models in 3D, sophisticated numerical
computations are required and boundary conditions must be obtained from
measurements of the magnetic field vector in the solar photosphere. This
approach is currently of large interests, as accurate measurements of the
photospheric field become available from ground-based (for example SOLIS) and
space-born (for example Hinode and SDO) instruments. If we can obtain accurate
force-free coronal magnetic field models we can calculate the free magnetic
energy in the corona, a quantity which is important for the prediction of
flares and coronal mass ejections. Knowledge of the 3D structure of magnetic
field lines also help us to interpret other coronal observations, e.g.,
EUV-images of the radiating coronal plasma.Comment: 49 pages, 11 figures, Living Reviews in Solar Physics, accepte
Alternatives to Kronig-Kramers Transformation and Testing, and Estimation of Distributions
Two alternatives to Kronig-Kramers analysis of small-signal ac immittance data are discussed and illustrated using both synthetic and experimental data. The first, a derivative method of approximating imaginary-part response from real-part data, is found to be too approximate in regions where the imaginary-part varies appreciably with frequency. The second, a distribution of relaxation-times fitting method, is shown to be valuable for testing whether a data set satisfies the Kronig-Kramers relations and so is associated with a system whose properties are time-invariant. It also is valuable for estimating real- or imaginary-part response from the other part, usually with small error. Unlike Kronig-Kramers analysis, the second method usually requires no extrapolation outside the range of the measured data. Finally, this discrete-function method also allows one to estimate the distribution of relaxation times or activation energies associated with a given set of frequency-response data. This application is described and illustrated for both synthetic and experimental data and is shown to yield good but somewhat approximate results for the estimation of continuous distributions. It is particularly valuable for identifying response regions arising from a continuous distribution and distinguishing them from those associated with discrete time-constant response
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