961 research outputs found
High-resolution simulations and modeling of reshocked single-mode Richtmyer-Meshkov instability: Comparison to experimental data and to amplitude growth model predictions
The reshocked single-mode Richtmyer-Meshkov instability is simulated in two spatial dimensions using the fifth- and ninth-order weighted essentially nonoscillatory shock-capturing method with uniform spatial resolution of 256 points per initial perturbation wavelength. The initial conditions and computational domain are modeled after the single-mode, Mach 1.21 air(acetone)/SF6 shock tube experiment of Collins and Jacobs [J. Fluid Mech. 464, 113 (2002)]. The simulation densities are shown to be in very good agreement with the corrected experimental planar laser-induced fluorescence images at selected times before reshock of the evolving interface. Analytical, semianalytical, and phenomenological linear and nonlinear, impulsive, perturbation, and potential flow models for single-mode Richtmyer-Meshkov unstable perturbation growth are summarized. The simulation amplitudes are shown to be in very good agreement with the experimental data and with the predictions of linear amplitude growth models for small times, and with those of nonlinear amplitude growth models at later times up to the time at which the driver-based expansion in the experiment (but not present in the simulations or models) expands the layer before reshock. The qualitative and quantitative differences between the fifth- and ninth-order simulation results are discussed. Using a local and global quantitative metric, the prediction of the Zhang and Sohn [Phys. Fluids 9, 1106 (1997)] nonlinear Padé model is shown to be in best overall agreement with the simulation amplitudes before reshock. The sensitivity of the amplitude growth model predictions to the initial growth rate from linear instability theory, the post-shock Atwood number and amplitude, and the velocity jump due to the passage of the shock through the interface is also investigated numerically
Parton densities and structure functions at next-to-next-to-leading order and beyond
We summarize recent results on the evolution of unpolarized parton densities
and deep-inelastic structure functions in massless perturbative QCD. Due to
last year's extension of the integer-moment calculations of the three-loop
splitting functions, the NNLO evolution of the parton distributions can now be
performed reliably at momentum fractions x >= 10^-4, facilitating a
considerably improved theoretical accuracy of their extraction from data on
deep-inelastic scattering. The NNLO corrections are not dominated, at relevant
values of x, by their leading small-x terms. At large x the splitting-function
series converges very rapidly, hence, employing results on the three-loop
coefficient functions, the structure functions can be analysed at N^3LO for x >
10^-2. The resulting values for alpha_s do not significantly change beyond
NNLO, their renormalization scale dependence reaches about +-1% at N^3LO.Comment: 10 pages, LaTeX, 6 figures. Talk presented at the workshops `New
Trends in HERA Physics 2001', Ringberg Castle (Germany), June 2001 and `DIS
2001', Bologna (Italy), April 2001. To appear, slightly shortened in the
latter case, in the proceeding
Unitarization Technics in Hadron Physics with Historical Remarks
We review a series of unitarization techniques that have been used during the
last decades, many of them in connection with the advent and development of
current algebra and later of Chiral Perturbation Theory. Several methods are
discussed like the generalized effective-range expansion, K-matrix approach,
Inverse Amplitude Method, Pad\'e approximants and the N/D method. More details
are given for the latter though. We also consider how to implement them in
order to correct by final-state interactions. In connection with this some
other methods are also introduced like the expansion of the inverse of the form
factor, the Omn\'es solution, generalization to coupled channels and the
Khuri-Treiman formalism, among others.Comment: 45 pages, 2 figures. Invited contribution to a special issue on
"Effective Field Theories - Chiral Perturbation Theory and Non-relativistic
QFT". Updated to match the published versio
On the nature of the lightest scalar resonances
We briefly review the recent progresses in the new unitarization approach
being developed by us. Especially we discuss the large
scatterings by making use of the partial wave matrix parametrization form.
We find that the pole may move to the negative real axis on the second
sheet of the complex plane, therefore it raises the interesting question
that this `' pole may be related to the in the linear
model.Comment: Talk presented by Zheng at ``Quark Confinement and Hadron
Spectroscopy VI'', 21--25 Sept. 2004, Cagliari, Italy. 3 pages with 2 figure
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