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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 properties of the ENO method
We review the currently available stability properties of the ENO
reconstruction procedure, such as its monotonicity and non-oscillatory
properties, the sign property, upper bounds on cell interface jumps and a total
variation-type bound. We also outline how these properties can be applied to
derive stability and convergence of high-order accurate schemes for
conservation laws.Comment: To appear in Handbook of Numerical Methods for Hyperbolic Problem
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