4,258 research outputs found
Russia's Post-Electoral Landscape
Dubois Patrick. PELLISSIER. In: , . Le dictionnaire de pédagogie et d'instruction primaire de Ferdinand Buisson : répertoire biographique des auteurs. Paris : Institut national de recherche pédagogique, 2002. p. 114. (BibliothÚque de l'Histoire de l'Education, 17
Asymptotic self-similar solutions with a characteristic time-scale
For a wide variety of initial and boundary conditions, adiabatic one
dimensional flows of an ideal gas approach self-similar behavior when the
characteristic length scale over which the flow takes place, , diverges or
tends to zero. It is commonly assumed that self-similarity is approached since
in the limit the flow becomes independent of any characteristic
length or time scales. In this case the flow fields must be of the
form with . We show that
requiring the asymptotic flow to be independent only of characteristic length
scales imply a more general form of self-similar solutions,
with , which includes the
exponential () solutions, . We demonstrate that
the latter, less restrictive, requirement is the physically relevant one by
showing that the asymptotic behavior of accelerating blast-waves, driven by the
release of energy at the center of a cold gas sphere of initial density
, changes its character at large : The flow is
described by , , solutions for
solutions with
diverging at finite time () for , and by exponential
solutions for ( depends on the adiabatic index of
the gas, for ). The properties of the new
solutions obtained here for are analyzed, and self-similar
solutions describing the behavior for are also derived.Comment: Minor corrections, Accepted to Ap
Closing the gap in the solutions of the strong explosion problem: An expansion of the family of second-type self-similar solutions
Shock waves driven by the release of energy at the center of a cold ideal gas
sphere of initial density rho\propto r^{-omega} approach a self-similar (SLS)
behavior, with velocity \dot{R}\propto R^delta, as R->\infty. For omega>3 the
solutions are of the second-type, i.e., delta is determined by the requirement
that the flow should include a sonic point. No solution satisfying this
requirement exists, however, in the 3\leq omega\leq omega_{g}(gamma) ``gap''
(\omega_{g}=3.26 for adiabatic index gamma=5/3). We argue that second-type
solutions should not be required in general to include a sonic point. Rather,
it is sufficient to require the existence of a characteristic line r_c(t), such
that the energy in the region r_c(t)\infty,
and an asymptotic solution given by the SLS solution at r_c(t)<r<R and
deviating from it at r<r_c may be constructed. The two requirements coincide
for omega>omega_g and the latter identifies delta=0 solutions as the asymptotic
solutions for 3\leq omega\leq omega_{g} (as suggested by Gruzinov03). In these
solutions, r_c is a C_0 characteristic. It is difficult to check, using
numerical solutions of the hydrodynamic equations, whether the flow indeed
approaches a delta=0 SLS behavior as R->\infty, due to the slow convergence to
SLS for omega~3. We show that in this case the flow may be described by a
modified SLS solution, d\ln\dot{R}/d\ln R=delta with slowly varying delta(R),
eta\equiv d delta/d\ln R<<1, and spatial profiles given by a sum of the SLS
solution corresponding to the instantaneous value of delta and a SLS correction
linear in eta. The modified SLS solutions provide an excellent approximation to
numerical solutions obtained for omega~3 at large R, with delta->0 (and
eta\neq0) for 3\leq omega\leq omega_{g}. (abridged)Comment: 10 pages, 11 figures, somewhat revised, version accepted to Ap
Stability of an Ultra-Relativistic Blast Wave in an External Medium with a Steep Power-Law Density Profile
We examine the stability of self-similar solutions for an accelerating
relativistic blast wave which is generated by a point explosion in an external
medium with a steep radial density profile of a power-law index > 4.134. These
accelerating solutions apply, for example, to the breakout of a gamma-ray burst
outflow from the boundary of a massive star, as assumed in the popular
collapsar model. We show that short wavelength perturbations may grow but only
by a modest factor <~ 10.Comment: 12 pages, 3 figures, submitted to Physical Review
Bosonic String in Affine-Metric Curved Space
The sigma model approach to the closed bosonic string on the affine-metric
manifold is considered. The two-loop metric counterterms for the nonlinear
two-dimensional sigma model with affine-metric target manifold are calculated.
The correlation of the metric and affine connection is considered as the result
of the ultraviolet finiteness (or beta-function vanishing) condition for the
nonlinear sigma model. The examples of the nonflat nonRiemannian manifolds
resulting in the trivial metric beta-function are suggested.Comment: 15 pages, LaTe
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