781 research outputs found
Spin waves in quasi-equilibrium spin systems
Using the Landau Fermi liquid theory we have discovered a new regime for the
propagation of spin waves in a quasi-equilibrium spin systems. We have
determined the dispersion relation for the transverse spin waves and found that
one of the modes is gapless. The gapless mode corresponds to the precessional
mode of the magnetization in a paramagnetic system in the absence of an
external magnetic field. One of the other modes is gapped which is associated
with the precession of the spin current around the internal field. The gapless
mode has a quadratic dispersion leading to some interesting thermodynamic
properties including a contribution to the specific heat. We also
show that these modes make significant contributions to the dynamic structure
function.Comment: 4 pages, 3 figure
Asymptotic behaviour of the Rayleigh--Taylor instability
We investigate long time numerical simulations of the inviscid
Rayleigh-Taylor instability at Atwood number one using a boundary integral
method. We are able to attain the asymptotic behavior for the spikes predicted
by Clavin & Williams\cite{clavin} for which we give a simplified demonstration.
In particular we observe that the spike's curvature evolves like while
the overshoot in acceleration shows a good agreement with the suggested
law. Moreover, we obtain consistent results for the prefactor coefficients of
the asymptotic laws. Eventually we exhibit the self-similar behavior of the
interface profile near the spike.Comment: 4 pages, 6 figure
Lagrangian temperature, velocity and local heat flux measurement in Rayleigh-Benard convection
We have developed a small, neutrally buoyant, wireless temperature sensor.
Using a camera for optical tracking, we obtain simultaneous measurements of
position and temperature of the sensor as it is carried along by the flow in
Rayleigh-B\'enard convection, at . We report on statistics of
temperature, velocity, and heat transport in turbulent thermal convection. The
motion of the sensor particle exhibits dynamics close to that of Lagrangian
tracers in hydrodynamic turbulence. We also quantify heat transport in plumes,
revealing self-similarity and extreme variations from plume to plume.Comment: 4 page
Extraction of Plumes in Turbulent Thermal Convection
We present a scheme to extract information about plumes, a prominent coherent
structure in turbulent thermal convection, from simultaneous local velocity and
temperature measurements. Using this scheme, we study the temperature
dependence of the plume velocity and understand the results using the equations
of motion. We further obtain the average local heat flux in the vertical
direction at the cell center. Our result shows that heat is not mainly
transported through the central region but instead through the regions near the
sidewalls of the convection cell.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Effects of electromagnetic waves on the electrical properties of contacts between grains
A DC electrical current is injected through a chain of metallic beads. The
electrical resistances of each bead-bead contacts are measured. At low current,
the distribution of these resistances is large and log-normal. At high enough
current, the resistance distribution becomes sharp and Gaussian due to the
creation of microweldings between some beads. The action of nearby
electromagnetic waves (sparks) on the electrical conductivity of the chain is
also studied. The spark effect is to lower the resistance values of the more
resistive contacts, the best conductive ones remaining unaffected by the spark
production. The spark is able to induce through the chain a current enough to
create microweldings between some beads. This explains why the electrical
resistance of a granular medium is so sensitive to the electromagnetic waves
produced in its vicinity.Comment: 4 pages, 5 figure
Generalized scaling in fully developed turbulence
In this paper we report numerical and experimental results on the scaling
properties of the velocity turbulent fields in several flows. The limits of a
new form of scaling, named Extended Self Similarity(ESS), are discussed. We
show that, when a mean shear is absent, the self scaling exponents are
universal and they do not depend on the specific flow (3D homogeneous
turbulence, thermal convection , MHD). In contrast, ESS is not observed when a
strong shear is present. We propose a generalized version of self scaling which
extends down to the smallest resolvable scales even in cases where ESS is not
present. This new scaling is checked in several laboratory and numerical
experiment. A possible theoretical interpretation is also proposed. A synthetic
turbulent signal having most of the properties of a real one has been
generated.Comment: 25 pages, plain Latex, figures are available upon request to the
authors ([email protected], [email protected]
Pauli spin susceptibility of a strongly correlated two-dimensional electron liquid
Thermodynamic measurements reveal that the Pauli spin susceptibility of
strongly correlated two-dimensional electrons in silicon grows critically at
low electron densities - behavior that is characteristic of the existence of a
phase transition.Comment: As publishe
Fine-scale statistics of temperature and its derivatives in convective turbulence
We study the fine-scale statistics of temperature and its derivatives in
turbulent Rayleigh-Benard convection. Direct numerical simulations are carried
out in a cylindrical cell with unit aspect ratio filled with a fluid with
Prandtl number equal to 0.7 for Rayleigh numbers between 10^7 and 10^9. The
probability density function of the temperature or its fluctuations is found to
be always non-Gaussian. The asymmetry and strength of deviations from the
Gaussian distribution are quantified as a function of the cell height. The
deviations of the temperature fluctuations from the local isotropy, as measured
by the skewness of the vertical derivative of the temperature fluctuations,
decrease in the bulk, but increase in the thermal boundary layer for growing
Rayleigh number, respectively. Similar to the passive scalar mixing, the
probability density function of the thermal dissipation rate deviates
significantly from a log-normal distribution. The distribution is fitted well
by a stretched exponential form. The tails become more extended with increasing
Rayleigh number which displays an increasing degree of small-scale
intermittency of the thermal dissipation field for both the bulk and the
thermal boundary layer. We find that the thermal dissipation rate due to the
temperature fluctuations is not only dominant in the bulk of the convection
cell, but also yields a significant contribution to the total thermal
dissipation in the thermal boundary layer. This is in contrast to the ansatz
used in scaling theories and can explain the differences in the scaling of the
total thermal dissipation rate with respect to the Rayleigh number.Comment: 22 pages and 15 figure
A nonextensive entropy approach to solar wind intermittency
The probability distributions (PDFs) of the differences of any physical
variable in the intermittent, turbulent interplanetary medium are scale
dependent. Strong non-Gaussianity of solar wind fluctuations applies for short
time-lag spacecraft observations, corresponding to small-scale spatial
separations, whereas for large scales the differences turn into a Gaussian
normal distribution. These characteristics were hitherto described in the
context of the log-normal, the Castaing distribution or the shell model. On the
other hand, a possible explanation for nonlocality in turbulence is offered
within the context of nonextensive entropy generalization by a recently
introduced bi-kappa distribution, generating through a convolution of a
negative-kappa core and positive-kappa halo pronounced non-Gaussian structures.
The PDFs of solar wind scalar field differences are computed from WIND and ACE
data for different time lags and compared with the characteristics of the
theoretical bi-kappa functional, well representing the overall scale dependence
of the spatial solar wind intermittency. The observed PDF characteristics for
increased spatial scales are manifest in the theoretical distribution
functional by enhancing the only tuning parameter , measuring the
degree of nonextensivity where the large-scale Gaussian is approached for
. The nonextensive approach assures for experimental studies
of solar wind intermittency independence from influence of a priori model
assumptions. It is argued that the intermittency of the turbulent fluctuations
should be related physically to the nonextensive character of the
interplanetary medium counting for nonlocal interactions via the entropy
generalization.Comment: 17 pages, 7 figures, accepted for publication in Astrophys.
Universality in fully developed turbulence
We extend the numerical simulations of She et al. [Phys.\ Rev.\ Lett.\ 70,
3251 (1993)] of highly turbulent flow with Taylor-Reynolds number
up to , employing a reduced wave
vector set method (introduced earlier) to approximately solve the Navier-Stokes
equation. First, also for these extremely high Reynolds numbers ,
the energy spectra as well as the higher moments -- when scaled by the spectral
intensity at the wave number of peak dissipation -- can be described by
{\it one universal} function of for all . Second, the ISR
scaling exponents of this universal function are in agreement with
the 1941 Kolmogorov theory (the better, the large is), as is the
dependence of . Only around viscous damping leads to
slight energy pileup in the spectra, as in the experimental data (bottleneck
phenomenon).Comment: 14 pages, Latex, 5 figures (on request), 3 tables, submitted to Phys.
Rev.
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