316,743 research outputs found
Radiative instabilities in simulations of spherically symmetric supernova blast waves
High-resolution simulations of the cooling regions of spherically symmetric
supernova remnants demonstrate a strong radiative instability. This
instability, whose presence is dependent on the shock velocity, causes
large-amplitude fluctuations in the shock velocity. The fluctuations begin
almost immediately after the radiative phase begins (upon shell formation) if
the shock velocity lies in the unstable range; they last until the shock slows
to speeds less than approximately 130 km/s. We find that shock-velocity
fluctuations from the reverberations of waves within the remnant are small
compared to those due to the instability. Further, we find (in plane-parallel
simulations) that advected inhomogeneities from the external medium do not
interfere with the qualitative nature of the instability-driven fluctuations.
Large-amplitude inhomogeneities may alter the phases of shock-velocity
fluctuations, but do not substantially reduce their amplitudes.Comment: 18 pages text, LaTeX/AASTeX (aaspp4); 10 figures; accepted by Ap
Superdiffusion of massive particles induced by multi-scale velocity fields
We study drag-induced diffusion of massive particles in scale-free velocity
fields, where superdiffusive behavior emerges due to the scale-free size
distribution of the vortices of the underlying velocity field. The results show
qualitative resemblance to what is observed in fluid systems, namely the
diffusive exponent for the mean square separation of pairs of particles and the
preferential concentration of the particles, both as a function of the response
time.Comment: 5 pages, 5 figures. Accepted for publication in EP
Thermally-Assisted Current-Driven Domain Wall Motion
Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive
Langevin equations that describe the nonzero-temperature dynamics of a rigid
domain wall. We derive an expression for the average drift velocity of the
domain wall as a function of the applied current, and find qualitative
agreement with recent magnetic semiconductor experiments. Our model implies
that at any nonzero temperature the average domain-wall velocity initially
varies linearly with current, even in the absence of non-adiabatic spin
torques.Comment: 4 pages, 2 figure
Anisotropic velocity distributions in 3D dissipative optical lattices
We present a direct measurement of velocity distributions in two dimensions
by using an absorption imaging technique in a 3D near resonant optical lattice.
The results show a clear difference in the velocity distributions for the
different directions. The experimental results are compared with a numerical 3D
semi-classical Monte-Carlo simulation. The numerical simulations are in good
qualitative agreement with the experimental results.Comment: Accepted for publication in Eur. Phys. J., "Special issue: Quantum
fluctuations and coherence in optical and atomic structures" (2003
Velocity profiles in strongly turbulent Taylor-Couette flow
We derive the velocity profiles in strongly turbulent Taylor-Couette flow for
the general case of independently rotating cylinders. The theory is based on
the Navier-Stokes equations in the appropriate (cylinder) geometry. In
particular, we derive the axial and the angular velocity profiles as functions
of distance from the cylinder walls and find that both follow a logarithmic
profile, with downwards-bending curvature corrections, which are more
pronounced for the angular velocity profile as compared to the axial velocity
profile, and which strongly increase with decreasing ratio between inner
and outer cylinder radius. In contrast, the azimuthal velocity does not follow
a log-law. We then compare the angular and azimuthal velocity profiles with the
recently measured profiles in the ultimate state of (very) large Taylor
numbers. Though the {\em qualitative} trends are the same -- down-bending for
large wall distances and (properly shifted and non-dimensionalized) angular
velocity profile being closer to a log-law than (properly shifted
and non-dimensionalized) azimuthal velocity profile -- {\em
quantitative} deviations are found for large wall distances. We attribute these
differences to the Taylor rolls and the height dependence of the profiles,
neither of which are considered in the theoretical approach
Universal Model of Finite-Reynolds Number Turbulent Flow in Channels and Pipes
In this Letter we suggest a simple and physically transparent analytical
model of the pressure driven turbulent wall-bounded flows at high but finite
Reynolds numbers Re. The model gives accurate qualitative description of the
profiles of the mean-velocity and Reynolds-stresses (second order correlations
of velocity fluctuations) throughout the entire channel or pipe in the wide
range of Re, using only three Re-independent parameters. The model sheds light
on the long-standing controversy between supporters of the century-old log-law
theory of von-K\`arm\`an and Prandtl and proposers of a newer theory promoting
power laws to describe the intermediate region of the mean velocity profile.Comment: 4 pages, 6 figs, re-submitted PRL according to referees comment
A 2-D asymmetric exclusion model for granular flows
A 2-D version of the asymmetric exclusion model for granular sheared flows is
presented. The velocity profile exhibits two qualitatively different behaviors,
dependent on control parameters. For low friction, the velocity profile follows
an exponential decay while for large friction the profile is more accurately
represented by a Gaussian law. The phase transition occurring between these two
behavior is identified by the appearance of correlations in the cluster size
distribution. Finally, a mean--field theory gives qualitative and quantitative
good agreement with the numerical results.Comment: 13 pages, 5 figures; typos added, one definition change
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