5,220 research outputs found
Ablation sensor Patent
Ablation sensor for measuring char layer recession rate using electric wire
Extreme objects with arbitrary large mass, or density, and arbitrary size
We consider a generalization of the interior Schwarzschild solution that we
match to the exterior one to build global C^1 models that can have arbitrary
large mass, or density, with arbitrary size. This is possible because of a new
insight into the problem of localizing the center of symmetry of the models and
the use of principal transformations to understand the structure of space.Comment: 20 pages, 6 figures. Fixed one reference. Added a new equatio
Comparing metrics at large: harmonic vs quo-harmonic coordinates
To compare two space-times on large domains, and in particular the global
structure of their manifolds, requires using identical frames of reference and
associated coordinate conditions. In this paper we use and compare two classes
of time-like congruences and corresponding adapted coordinates: the harmonic
and quo-harmonic classes. Besides the intrinsic definition and some of their
intrinsic properties and differences we consider with some detail their
differences at the level of the linearized approximation of the field
equations. The hard part of this paper is an explicit and general determination
of the harmonic and quo-harmonic coordinates adapted to the stationary
character of three well-know metrics, Schwarzschild's, Curzon's and Kerr's, to
order five of their asymptotic expansions. It also contains some relevant
remarks on such problems as defining the multipoles of vacuum solutions or
matching interior and exterior solutions.Comment: 27 pages, no figure
Frame dragging and super-energy
We show that the vorticity appearing in stationary vacuum spacetimes is
always related to the existence of a flow of super-energy on the plane
orthogonal to the vorticity vector. This result, toghether with the previously
established link between vorticity and super--energy in radiative (Bondi-Sachs)
spacetimes strength further the case for this latter quantity as the cause of
frame dragging.Comment: 12 pages Latex. To appear in Phys.Rev. D. Typos correcte
Improving the modelling of redshift-space distortions - II. A pairwise velocity model covering large and small scales
We develop a model for the redshift-space correlation function, valid for
both dark matter particles and halos on scales Mpc. In its simplest
formulation, the model requires the knowledge of the first three moments of the
line-of-sight pairwise velocity distribution plus two well-defined
dimensionless parameters. The model is obtained by extending the
Gaussian-Gaussianity prescription for the velocity distribution, developed in a
previous paper, to a more general concept allowing for local skewness, which is
required to match simulations. We compare the model with the well known
Gaussian streaming model and the more recent Edgeworth streaming model. Using
N-body simulations as a reference, we show that our model gives a precise
description of the redshift-space clustering over a wider range of scales. We
do not discuss the theoretical prescription for the evaluation of the velocity
moments, leaving this topic to further investigation.Comment: 18 pages, 10 figures, published in MNRA
Myopic PPPs: Risk allocation and hidden liabilities for taxpayers and users
Drawing on evidence from three case studies, we show how the State's Financial Liability has worked in assigning risk in large PPP contracts in Spain. Project failure and the concessionaires' bankruptcy have resulted in the government having to assume heavy financial obligations, which have ultimately been absorbed by taxpayers and users. In contrast, Spain's leading construction companies, which were also major investors in the concessionaires, have been able to minimize their risk. Myopic PPPs have been entered into based on the transference of liabilities to taxpayers and users, and the, consequent, minimization of risks for the main private investors
Simplicity of Completion Time Distributions for Common Complex Biochemical Processes
Biochemical processes typically involve huge numbers of individual reversible
steps, each with its own dynamical rate constants. For example, kinetic
proofreading processes rely upon numerous sequential reactions in order to
guarantee the precise construction of specific macromolecules. In this work, we
study the transient properties of such systems and fully characterize their
first passage (completion) time distributions. In particular, we provide
explicit expressions for the mean and the variance of the completion time for a
kinetic proofreading process and computational analyses for more complicated
biochemical systems. We find that, for a wide range of parameters, as the
system size grows, the completion time behavior simplifies: it becomes either
deterministic or exponentially distributed, with a very narrow transition
between the two regimes. In both regimes, the dynamical complexity of the full
system is trivial compared to its apparent structural complexity. Similar
simplicity is likely to arise in the dynamics of many complex multi-step
biochemical processes. In particular, these findings suggest not only that one
may not be able to understand individual elementary reactions from macroscopic
observations, but also that such understanding may be unnecessary
Why does gravitational radiation produce vorticity?
We calculate the vorticity of world--lines of observers at rest in a
Bondi--Sachs frame, produced by gravitational radiation, in a general Sachs
metric. We claim that such an effect is related to the super--Poynting vector,
in a similar way as the existence of the electromagnetic Poynting vector is
related to the vorticity in stationary electrovacum spacetimes.Comment: 9 pages; to appear in Classical and Quantum Gravit
Grains charges in interstellar clouds
The charge of cosmic grains could play an important role in many astrophysical phenomena. It probably has an influence on the coagulation of grains and more generally on grain-grain collisions, and on interaction between charged particles and grains which could lead to the formation of large grains or large molecules. The electrostatic charge of grains depends mainly on the nature of constitutive material of the grain and on the physical properties of its environment: it results from a delicate balance between the plasma particle collection and the photoelectron emission, both of them depending on each other. The charge of the grain is obtained in two steps: (1) using the numerical model the characteristics of the environment of the grain are computed; (2) the charge of a grain which is embedded in this environment is determined. The profile of the equilibrium charge of some typical grains through different types of interstellar clouds is obtained as a function of the depth of the cloud. It is shown that the grain charge can reach high values not only in hot diffuse clouds, but also in clouds with higher densities. The results are very sensitive to the mean UV interstellar radiation field. Three parameters appear to be essential but with different levels of sensitivity of the charge: the gas density, the temperature, and the total thickness of the cloud
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