8,355 research outputs found
Multi-Harnack smoothings of real plane branches
We introduce a new method for the construction of smoothings of a real plane
branch by using Viro Patchworking method. Since real plane branches
are Newton degenerated in general, we cannot apply Viro Patchworking method
directly. Instead we apply the Patchworking method for certain Newton non
degenerate curve singularities with several branches. These singularities
appear as a result of iterating deformations of the strict transforms of the
branch at certain infinitely near points of the toric embedded resolution of
singularities of . We characterize the -smoothings obtained by this
method by the local data. In particular, we analyze the class of multi-Harnack
smoothings, those smoothings arising in a sequence -smoothings of the strict
transforms of (C,0) which are in maximal position with respect to the
coordinate lines. We prove that there is a unique the topological type of
multi-Harnack smoothings, which is determined by the complex equisingularity
type of the branch. This result is a local version of a recent Theorem of
Mikhalkin
On weak and strong magnetohydrodynamic turbulence
Recent numerical and observational studies contain conflicting reports on the
spectrum of magnetohydrodynamic turbulence. In an attempt to clarify the issue
we investigate anisotropic incompressible magnetohydrodynamic turbulence with a
strong guide field . We perform numerical simulations of the reduced MHD
equations in a special setting that allows us to elucidate the transition
between weak and strong turbulent regimes. Denote ,
characteristic field-parallel and field-perpendicular wavenumbers of the
fluctuations, and the fluctuating field at the scale . We find that when the critical balance condition, , is satisfied, the turbulence is strong, and the energy
spectrum is . As the width of
the spectrum increases, the turbulence rapidly becomes weaker, and in the limit
, the spectrum approaches
. The observed sensitivity of the spectrum
to the balance of linear and nonlinear interactions may explain the conflicting
numerical and observational findings where this balance condition is not well
controlled.Comment: 4 pages, 2 figure
On the energy spectrum of strong magnetohydrodynamic turbulence
The energy spectrum of magnetohydrodynamic turbulence attracts interest due
to its fundamental importance and its relevance for interpreting astrophysical
data. Here we present measurements of the energy spectra from a series of
high-resolution direct numerical simulations of MHD turbulence with a strong
guide field and for increasing Reynolds number. The presented simulations, with
numerical resolutions up to 2048^3 mesh points and statistics accumulated over
30 to 150 eddy turnover times, constitute, to the best of our knowledge, the
largest statistical sample of steady state MHD turbulence to date. We study
both the balanced case, where the energies associated with Alfv\'en modes
propagating in opposite directions along the guide field, E^+ and $E^-, are
equal, and the imbalanced case where the energies are different. In the
balanced case, we find that the energy spectrum converges to a power law with
exponent -3/2 as the Reynolds number is increased, consistent with
phenomenological models that include scale-dependent dynamic alignment. For the
imbalanced case, with E^+>E^-, the simulations show that E^- ~ k_{\perp}^{-3/2}
for all Reynolds numbers considered, while E^+ has a slightly steeper spectrum
at small Re. As the Reynolds number increases, E^+ flattens. Since both E^+ and
E^- are pinned at the dissipation scale and anchored at the driving scales, we
postulate that at sufficiently high Re the spectra will become parallel in the
inertial range and scale as E^+ ~ E^- ~ k_{\perp}^{-3/2}. Questions regarding
the universality of the spectrum and the value of the "Kolmogorov constant" are
discussed.Comment: 13 pages, 10 figures, accepted for publication in Physical Review X
(PRX
- …