703 research outputs found
Heat Transfer in Turbulent Rayleigh-Benard Convection below the Ultimate Regime
A Rayleigh-B\'enard cell has been designed to explore the Prandtl (Pr)
dependence of turbulent convection in the cross-over range and for
the full range of soft and hard turbulences, up to Rayleigh number . The set-up benefits from the favourable characteristics of cryogenic
helium-4 in fluid mechanics, in-situ fluid property measurements, and special
care on thermometry and calorimetric instrumentation. The cell is cylindrical
with . The effective heat transfer has been
measured with unprecedented accuracy for cryogenic turbulent convection
experiments in this range of Rayleigh numbers. Spin-off of this study include
improved fits of helium thermodynamics and viscosity properties. Three main
results were found. First the dependence exhibits a bimodality of the
flow with difference in for given and . Second, a
systematic study of the side-wall influence reveals a measurable effect on the
heat transfer. Third, the dependence is very small or null : the
absolute value of the average logarithmic slope is smaller
than 0.03 in our range of , which allows to disciminate between
contradictory experiments [Ashkenazi \textit{et al.}, Phys. Rev.Lett. 83:3641
(1999)][Ahlers \textit{et al.}, Phys.Rev.Lett. 86:3320 (2001)].Comment: submitted for publication to JLTP (august 2003
Fermi liquid theory of ultra-cold trapped Fermi gases: Implications for Pseudogap Physics and Other Strongly Correlated Phases
We show how Fermi liquid theory can be applied to ultra-cold Fermi gases,
thereby expanding their "simulation" capabilities to a class of problems of
interest to multiple physics sub-disciplines. We introduce procedures for
measuring and calculating position dependent Landau parameters. This lays the
ground work for addressing important controversial issues: (i) the suggestion
that thermodynamically, the normal state of a unitary gas is indistinguishable
from a Fermi liquid (ii) that a fermionic system with strong repulsive contact
interactions is associated with either ferromagnetism or localization; this
relates as well to He and its p-wave superfluidity.Comment: 4 pages, 2 figures, revised versio
Unified Multifractal Description of Velocity Increments Statistics in Turbulence: Intermittency and Skewness
The phenomenology of velocity statistics in turbulent flows, up to now,
relates to different models dealing with either signed or unsigned longitudinal
velocity increments, with either inertial or dissipative fluctuations. In this
paper, we are concerned with the complete probability density function (PDF) of
signed longitudinal increments at all scales. First, we focus on the symmetric
part of the PDFs, taking into account the observed departure from scale
invariance induced by dissipation effects. The analysis is then extended to the
asymmetric part of the PDFs, with the specific goal to predict the skewness of
the velocity derivatives. It opens the route to the complete description of all
measurable quantities, for any Reynolds number, and various experimental
conditions. This description is based on a single universal parameter function
D(h) and a universal constant R*.Comment: 13 pages, 3 figures, Extended version, Publishe
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
From turbulence to financial time series
We develop a framework especially suited to the autocorrelation properties
observed in financial times series, by borrowing from the physical picture of
turbulence. The success of our approach as applied to high frequency foreign
exchange data is demonstrated by the overlap of the curves in Figure (1), since
we are able to provide an analytical derivation of the relative sizes of the
quantities depicted. These quantities include departures from Gaussian
probability density functions and various two and three-point autocorrelation
functions.Comment: 10 pages, 1 figure, LaTeX, version to appear in Physica
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
Lognormal scale invariant random measures
In this article, we consider the continuous analog of the celebrated
Mandelbrot star equation with lognormal weights. Mandelbrot introduced this
equation to characterize the law of multiplicative cascades. We show existence
and uniqueness of measures satisfying the aforementioned continuous equation;
these measures fall under the scope of the Gaussian multiplicative chaos theory
developed by J.P. Kahane in 1985 (or possibly extensions of this theory). As a
by product, we also obtain an explicit characterization of the covariance
structure of these measures. We also prove that qualitative properties such as
long-range independence or isotropy can be read off the equation.Comment: 31 pages; Probability Theory and Related Fields (2012) electronic
versio
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
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
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