245 research outputs found
Convex Passivity Enforcement of Linear Macromodels via Alternate Subgradient Iterations
This paper introduces a new algorithm for passivity enforcement of linear lumped macromodels in scattering form. As typical in most state of the art passivity enforcement methods, we start with an initial non-passive macromodel obtained by a Vector Fitting process, and we perturb its parameters to make it passive. The proposed scheme is based on a convex formulation of both passivity constraints and objective function for accuracy preservation, thus allowing a formal proof of convergence to the unique optimal passive macromodel. This is a distinctive feature that differentiates the new scheme with respect to most state of the art methods, which either do not guarantee convergence or are not able to provide the most accurate solution. The presented algorithm can thus be safely used for those cases for which existing techniques fail. We illustrate the advantages of proposed method on a few benchmarks
Inhomogeneous String Cosmology Solutions with Regular Spacetime Curvature
In this work cosmological models are considered for the low energy string
cosmological effective action (tree level) in the absence of dilaton potential.
A two parametric non-diagonal family of analytic solutions is found. The
curvature is non singular, however the string coupling diverges exponentially.Comment: LaTeX, 6 pge
Magnetic Surfaces in Stationary Axisymmetric General Relativity
In this paper a new method is derived for constructing electromagnetic
surface sources for stationary axisymmetric electrovac spacetimes endowed with
non-smooth or even discontinuous
Ernst potentials. This can be viewed as a generalization of some classical
potential theory results, since lack of continuity of the potential is related
to dipole density and lack of smoothness, to monopole density. In particular
this approach is useful for constructing the dipole source for the magnetic
field. This formalism involves solving a linear elliptic differential equation
with boundary conditions at infinity. As an example, two different models of
surface densities for the Kerr-Newman electrovac spacetime are derived.Comment: 15 page
An iterative reweighting process for macromodel extraction of power distribution networks
This paper introduces a new algorithm for the generation of optimal time-domain macromodels of power distribution networks, starting from a set of tabulated scattering responses and given a nominal termination scheme for active blocks, decoupling capacitors, and voltage regulator module. The new concept being introduced is a modified metric to characterize and optimize the accuracy of the macromodel, which takes into account the operation conditions that will be applied to run transient simulations for power integrity assessment. This metric is applied through an iterative frequency-dependent reweighting scheme in a fully automated flow. Two examples illustrate the performance of the proposed algorith
The odd side of torsion geometry
We introduce and study a notion of `Sasaki with torsion structure' (ST) as an
odd-dimensional analogue of K\"ahler with torsion geometry (KT). These are
normal almost contact metric manifolds that admit a unique compatible
connection with 3-form torsion. Any odd-dimensional compact Lie group is shown
to admit such a structure; in this case the structure is left-invariant and has
closed torsion form.
We illustrate the relation between ST structures and other generalizations of
Sasaki geometry, and explain how some standard constructions in Sasaki geometry
can be adapted to this setting. In particular, we relate the ST structure to a
KT structure on the space of leaves, and show that both the cylinder and the
cone over an ST manifold are KT, although only the cylinder behaves well with
respect to closedness of the torsion form. Finally, we introduce a notion of
`G-moment map'. We provide criteria based on equivariant cohomology ensuring
the existence of these maps, and then apply them as a tool for reducing ST
structures.Comment: 34 pages; v2: added a small generalization (Proposition 3.6) of the
cone construction; two references added. To appear on Ann. Mat. Pura App
Tuning the accuracy of rational macromodels to nominal load conditions
We address the generation of broadband macromodels of complex linear systems via rational curve fitting. We show that standard approaches may not ensure that the macromodel accuracy is preserved in system-level simulations, under loading conditions that are different from the adopted identification settings. Our main contribution is an automated procedure for the definition of a frequency-dependent norm weighting strategy that tunes the macromodel accuracy for a specific nominal termination network, thus improving model robustness under realistic operation
Exterior Differential System for Cosmological G2 Perfect Fluids and Geodesic Completeness
In this paper a new formalism based on exterior differential systems is
derived for perfect-fluid spacetimes endowed with an abelian orthogonally
transitive G2 group of motions acting on spacelike surfaces. This formulation
allows simplifications of Einstein equations and it can be applied for
different purposes. As an example a singularity-free metric is rederived in
this framework. A sufficient condition for a diagonal metric to be geodesically
complete is also provided.Comment: 27 pages, 0 figures, LaTeX2e, to be published in Classical and
Quantum Gravit
New Techniques for Analysing Axisymmetric Gravitational Systems. 1. Vacuum Fields
A new framework for analysing the gravitational fields in a stationary,
axisymmetric configuration is introduced. The method is used to construct a
complete set of field equations for the vacuum region outside a rotating
source. These equations are under-determined. Restricting the Weyl tensor to
type D produces a set of equations which can be solved, and a range of new
techniques are introduced to simplify the problem. Imposing the further
condition that the solution is asymptotically flat yields the Kerr solution
uniquely. The implications of this result for the no-hair theorem are
discussed. The techniques developed here have many other applications, which
are described in the conclusions.Comment: 30 pages, no figure
New Non-Diagonal Singularity-Free Cosmological Perfect-Fluid Solution
We present a new non-diagonal G2 inhomogeneous perfect-fluid solution with
barotropic equation of state p=rho and positive density everywhere. It
satisfies the global hyperbolicity condition and has no curvature singularity
anywhere. This solution is very simple in form and has two arbitrary constants.Comment: Latex, no figure
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