804 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
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
Comment on "Turbulent heat transport near critical points: Non-Boussinesq effects" (cond-mat/0601398)
In a recent preprint (cond-mat/0601398), D. Funfschilling and G. Ahlers
describe a new effect, that they interpret as non-Boussinesq, in a convection
cell working with ethane, near its critical point. They argue that such an
effect could have spoiled the Chavanne {\it et al.} (Phys. Rev. Lett. {\bf 79}
3648, 1997) results, and not the Niemela {\it et al.} (Nature, {\bf 404}, 837,
2000) ones, which would explain the differences between these two experiments.
We show that:-i)Restricting the Chavanne's data to situations as far from the
critical point than the Niemela's one, the same discrepancy remains.-ii)The
helium data of Chavanne show no indication of the effect observed by D.
Funfschilling and G. Ahlers.Comment: comment on cond-mat/060139
La sélection générative du caféier Robusta au Cameroun : analyse des résultats d'un essai d'hybrides diallÚle partiel implanté en 1973
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
Multi-Scale Statistical Approach of the Elastic and Thermal Behavior of a Thermoplastic Polyamid-Glass Fiber Composite
The strong heterogeneity and the anisotropy of composite materials require a rigorous and precise analysis as a result of their impact on local properties. First, mechanical tests are performed to determine the macroscopical behavior of a polyamid glass fiber composite. Then we focus on the influence of the heterogeneities of the microstructure on thermal and mechanical properties from finite element calculations on the real microstructure, after plane strain assumptions. 100 images in 10 different sizes (50, 100, 150, 200, 250, 300, 350, 400, 450, 600 pixels) are analysed. The influence of the area fraction and the spatial arrangement of fibers is then established. For the thermal conductivity and the bulk modulus the fiber area fraction is the most important factor. These properties are improved by increasing the area fraction. On the other hand, for the shear modulus, the fibers spatial arrangement plays the paramount role if the size of the microstructure is smaller than the RVE. Therefore, to make a good prediction from a multi-scale approach the knowledge of the RVE is fundamental. By a statistical approach and a numerical homogenization method, we determine the RVE of the composite for the elastic behavior (shear and bulk moduli), the thermal behavior (thermal conductivity), and for the area fraction. There is a relatively good agreement between the effective properties of this RVE and the experimental macroscopical behavior. These effective properties are estimated by the Hashin-Shtrikman lower bound
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
Medium and Small Scale Analysis of Financial Data
A stochastic analysis of financial data is presented. In particular we
investigate how the statistics of log returns change with different time delays
. The scale dependent behaviour of financial data can be divided into two
regions. The first time-range, the small-timescale region (in the range of
seconds) seems to be characterized by universal features. The second
time-range, the medium-timescale range from several minutes upwards and can be
characterized by a cascade process, which is given by a stochastic Markov
process in the scale . A corresponding Fokker-Planck equation can be
extracted from given data and provides a non equilibrium thermodynamical
description of the complexity of financial data.Comment: 4 pages, 5 figure
Universal scattering behavior of co-assembled nanoparticle-polymer clusters
Water-soluble clusters made from 7 nm inorganic nanoparticles have been
investigated by small-angle neutron scattering. The internal structure factor
of the clusters was derived and exhibited a universal behavior as evidenced by
a correlation hole at intermediate wave-vectors. Reverse Monte-Carlo
calculations were performed to adjust the data and provided an accurate
description of the clusters in terms of interparticle distance and volume
fraction. Additional parameters influencing the microstructure were also
investigated, including the nature and thickness of the nanoparticle adlayer.Comment: 5 pages, 4 figures, paper published in Physical Review
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.
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