5,752 research outputs found
On the unsteady behavior of turbulence models
Periodically forced turbulence is used as a test case to evaluate the
predictions of two-equation and multiple-scale turbulence models in unsteady
flows. The limitations of the two-equation model are shown to originate in the
basic assumption of spectral equilibrium. A multiple-scale model based on a
picture of stepwise energy cascade overcomes some of these limitations, but the
absence of nonlocal interactions proves to lead to poor predictions of the time
variation of the dissipation rate. A new multiple-scale model that includes
nonlocal interactions is proposed and shown to reproduce the main features of
the frequency response correctly
Origin of Lagrangian Intermittency in Drift-Wave Turbulence
The Lagrangian velocity statistics of dissipative drift-wave turbulence are
investigated. For large values of the adiabaticity (or small collisionality),
the probability density function of the Lagrangian acceleration shows
exponential tails, as opposed to the stretched exponential or algebraic tails,
generally observed for the highly intermittent acceleration of Navier-Stokes
turbulence. This exponential distribution is shown to be a robust feature
independent of the Reynolds number. For small adiabaticity, algebraic tails are
observed, suggesting the strong influence of point-vortex-like dynamics on the
acceleration. A causal connection is found between the shape of the probability
density function and the autocorrelation of the norm of the acceleration
Spectral imbalance and the normalized dissipation rate of turbulence
The normalized turbulent dissipation rate is studied in decaying
and forced turbulence by direct numerical simulations, large-eddy simulations,
and closure calculations. A large difference in the values of is
observed for the two types of turbulence. This difference is found at moderate
Reynolds number, and it is shown that it persists at high Reynolds number,
where the value of becomes independent of the Reynolds number, but
is still not unique. This difference can be explained by the influence of the
nonlinear cascade time that introduces a spectral disequilibrium for
statistically nonstationary turbulence. Phenomenological analysis yields simple
analytical models that satisfactorily reproduce the numerical results. These
simple spectral models also reproduce and explain the increase of
at low Reynolds number that is observed in the simulations
Magnetism and structure of LixCoO2 and comparison to NaxCoO2
The magnetic properties and structure of LixCoO2 for x between 0.5 and 1.0
are reported. Co4+ is found to be high-spin in LixCoO2 for x between 0.94 and
1.0 and low-spin for x between 0.50 and 0.78. Weak antiferromagnetic coupling
is observed, increasing in strength as more Co4+ is introduced. At an x value
of about 0.65, the temperature-independent contribution to the magnetic
susceptibility and the electronic contribution to the specific heat are
largest. Neutron diffraction analysis reveals that the lithium oxide layer
expands perpendicular to the basal plane and the Li ions displace from their
ideal octahedral sites with decreasing x. A comparison of the structures of the
NaxCoO2 and LixCoO2 systems reveals that the CoO2 layer changes substantially
with alkali content in the former but is relatively rigid in the latter.
Further, the CoO6 octahedra in LixCoO2 are less distorted than those in
NaxCoO2. We postulate that these structural differences strongly influence the
physical properties in the two systems
Direct evidence for the magnetic ordering of Nd ions in NdFeAsO by high resolution inelastic neutron scattering
We investigated the low energy excitations in the parent compound NdFeAsO of
the Fe-pnictide superconductor in the eV range by a back scattering
neutron spectrometer. The energy scans on a powder NdFeAsO sample revealed
inelastic peaks at E = 1.600 eV at T = 0.055 K on both energy
gain and energy loss sides. The inelastic peaks move gradually towards lower
energy with increasing temperature and finally merge with the elastic peak at
about 6 K. We interpret the inelastic peaks to be due to the transition between
hyperfine-split nuclear level of the Nd and Nd isotopes with
spin . The hyperfine field is produced by the ordering of the
electronic magnetic moment of Nd at low temperature and thus the present
investigation gives direct evidence of the ordering of the Nd magnetic
sublattice of NdFeAsO at low temperature
Iron spin-reorientation transition in NdFeAsO
The low-temperature magnetic structure of NdFeAsO has been revisited using
neutron powder diffraction and symmetry analysis using the Sarah
representational analysis program. Four magnetic models with one magnetic
variable for each of the Nd and Fe sublattices were tested. The best fit was
obtained using a model with Fe moments pointing along the c-direction, and Nd
moments along the a-direction. This signals a significant interplay between
rare-earth and transition metal magnetism, which results in a
spin-reorientation of the Fe sublattice upon ordering of the Nd moments. All
models that fit the data well, including collinear models with more than one
magnetic variable per sublattice, were found to have an Fe moment of 0.5 BM and
a Nd moment of 0.9 BM, demonstrating that the low-temperature Fe moment is not
substantially enhanced compared to the spin-density wave (SDW) state.Comment: accepted to J. Phys.: Cond. Ma
Comparing phenomenological recipes with a microscopic model for the electric amplitude in strangeness photoproduction
Corrections to the Born approximation in photo-induced strangeness production
off a proton are calculated in a semi-realistic microscopic model. The vertex
corrections and internal contributions to the amplitude of the reaction are included on the one-loop level. Different
gauge-invariant phenomenological prescriptions for the modification of the Born
contribution via the introduction of form factors and contact terms are
discussed. In particular, it is shown that the popular minimal-substitution
method of Ohta corresponds to a special limit of the more realistic approach.Comment: 10 pages, 6 figures in the tex
Inertial range scaling of the scalar flux spectrum in two-dimensional turbulence
Two-dimensional statistically stationary isotropic turbulence with an imposed
uniform scalar gradient is investigated. Dimensional arguments are presented to
predict the inertial range scaling of the turbulent scalar flux spectrum in
both the inverse cascade range and the enstrophy cascade range for small and
unity Schmidt numbers. The scaling predictions are checked by direct numerical
simulations and good agreement is observed
Octet Baryon Magnetic Moments in the Chiral Quark Model with Configuration Mixing
The Coleman-Glashow sum-rule for magnetic moments is always fulfilled in the
chiral quark model, independently of SU(3) symmetry breaking. This is due to
the structure of the wave functions, coming from the non-relativistic quark
model. Experimentally, the Coleman-Glashow sum-rule is violated by about ten
standard deviations. To overcome this problem, two models of wave functions
with configuration mixing are studied. One of these models violates the
Coleman-Glashow sum-rule to the right degree and also reproduces the octet
baryon magnetic moments rather accurately.Comment: 22 pages, RevTe
Nd induced Mn spin-reorientation transition in NdMnAsO
A combination of synchrotron X-ray, neutron powder diffraction,
magnetization, heat capacity and electrical resistivity measurements reveals
that NdMnAsO is an antiferromagnetic semiconductor with large Neel temperature
(TN = 359(2) K). At room temperature the magnetic propagation vector k = 0 and
the Mn moments are directed along the crystallographic c-axis (mMn = 2.41(6)
BM). Upon cooling a spin reorientation (SR) transition of the Mn moments into
the ab-plane occurs (TSR = 23 K). This coincides with the long range ordering
of the Nd moments, which are restricted to the basal plane. The magnetic
propagation vector remains k = 0. At base temperature (1.6 K) the fitted
moments are mab,Mn = 3.72(1) BM and mab,Nd = 1.94(1) BM. The electrical
resistivity is characterized by a broad maximum at 250 K, below which it has a
metallic temperature dependence but semiconducting magnitude (rho250K = 50 Ohm
cm, residual resistivity ratio = 2), and a slight upturn at the SR transition
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