2,246 research outputs found
Universal statistics of non-linear energy transfer in turbulent models
A class of shell models for turbulent energy transfer at varying the
inter-shell separation, , is investigated. Intermittent corrections in
the continuous limit of infinitely close shells () have
been measured. Although the model becomes, in this limit, non-intermittent, we
found universal aspects of the velocity statistics which can be interpreted in
the framework of log-poisson distributions, as proposed by She and Waymire
(1995, Phys. Rev. Lett. 74, 262). We suggest that non-universal aspects of
intermittency can be adsorbed in the parameters describing statistics and
properties of the most singular structure. On the other hand, universal aspects
can be found by looking at corrections to the monofractal scaling of the most
singular structure. Connections with similar results reported in other shell
models investigations and in real turbulent flows are discussed.Comment: 4 pages, 2 figures available upon request to [email protected]
Emergence of a filamentary structure in the fireball from GRB spectra
It is shown that the concept of a fireball with a definite filamentary
structure naturally emerges from the analysis of the spectra of Gamma-Ray
Bursts (GRBs). These results, made possible by the recently obtained analytic
expressions of the equitemporal surfaces in the GRB afterglow, depend crucially
on the single parameter R describing the effective area of the fireball
emitting the X- and gamma ray radiation. The X- and gamma ray components of the
afterglow radiation are shown to have a thermal spectrum in the co-moving frame
of the fireball and originate from a stable shock front described
self-consistently by the Rankine-Hugoniot equations. Precise predictions are
presented on a correlations between spectral changes and intensity variations
in the prompt radiation verifiable, e.g., by the Swift and future missions. The
highly variable optical and radio emission depends instead on the parameters of
the surrounding medium. The GRB 991216 is used as a prototype for this model.Comment: 9 pages, 3 figures, to appear on International Journal of Modern
Physics
Non equilibrium in statistical and fluid mechanics. Ensembles and their equivalence. Entropy driven intermittency
We present a review of the chaotic hypothesis and discuss its applications to
intermittency in statistical mechanics and fluid mechanics proposing a
quantitative definition. Entropy creation rate is interpreted in terms of
certain intermittency phenomena. An attempt to a theory of the experiment of
Ciliberto-Laroche on the fluctuation law is presented.Comment: 22 page
Precipitation of high Mg-calcite and protodolomite using dead biomass of aerobic halophilic bacteria
The microbial dolomite model has been used to interpret the origin of sedimentary dolomite. In this model, the formation of low-temperature protodolomite, an important precursor to sedimentary dolomite, can be facilitated either by actively metabolizing cells of anaerobic microbes and aerobic halophilicarchaea or by their inactive biomass. Aerobic halophilic bacteria are widely distributed in (proto-)dolomite-depositing evaporitic environments and their biomass might serve as a template for the crystallization of protodolomite. To test this hypothesis, carbonation experiments were conducted using dead biomass of an aerobic halophilic bacterium (Exiguobacterium sp. strain JBHLT-3). Our results show that dead biomass of JBHLT-3 can accelerate Mg2+ uptake in carbonate mineral precipitates. In addition, the amount of Mg incorporated into Ca-Mg carbonates is proportional to the concentration of biomass. High Mg-calcite is produced with 0.25 or 0.5 g/L biomass, whereasprotodolomite forms with 1 g/L biomass. This is confirmed by the main Raman peak of Ca-Mg carbonates, which shifts towards higher wavenumbers with increased Mg substitution. Microbial cells and their imprints are preserved on the surface of high Mg-calcite and protodolomite. Hence, this study furthers our understanding of the dolomitization within buried and dead microbial mats, which provides useful insights into the origin of ancient dolomite
Nonperiodic oscillation of bright solitons in the condensates with a periodically oscillating harmonic potential
Considering a periodically oscillating harmonic potential, we explore the
dynamics properties of bright solitons in a Bose-Einstein condensate. It is
found that under a slower oscillating potential, soliton movement exhibits a
nonperiodic oscillation while it is hardly affected under a fast oscillating
potential. Furthermore, the head-on and/or "chase" collisions of two solitons
have been obtained, which can be controlled by the oscillating frequency of
potential.Comment: 4 pages, 2 figure
Developed turbulence: From full simulations to full mode reductions
Developed Navier-Stokes turbulence is simulated with varying wavevector mode
reductions. The flatness and the skewness of the velocity derivative depend on
the degree of mode reduction. They show a crossover towards the value of the
full numerical simulation when the viscous subrange starts to be resolved. The
intermittency corrections of the scaling exponents of the pth order velocity
structure functions seem to depend mainly on the proper resolution of the
inertial subrange. Universal scaling properties (i.e., independent of the
degree of mode reduction) are found for the relative scaling exponents rho
which were recently defined by Benzi et al.Comment: 4 pages, 5 eps-figures, replaces version from August 5th, 199
Toward the End of Time
The null-brane space-time provides a simple model of a big crunch/big bang
singularity. A non-perturbative definition of M-theory on this space-time was
recently provided using matrix theory. We derive the fermion couplings for this
matrix model and study the leading quantum effects. These effects include
particle production and a time-dependent potential. Our results suggest that as
the null-brane develops a big crunch singularity, the usual notion of
space-time is replaced by an interacting gluon phase. This gluon phase appears
to constitute the end of our conventional picture of space and time.Comment: 31 pages, reference adde
The Termination and Aftermath of the Lomagundi-Jatuli Carbon Isotope Excursions in the Paleoproterozoic Hutuo Group, North China
The Lomagundi-Jatuli Event (LJE) is one of the largest and earliest positive carbon isotope excursions preserving δ13Ccarb values between +5 and +16‰ in Paleoproterozoic carbonates worldwide. However, the duration, amplitude and patterns of these excursions remain poorly constrained. The 2.14–1.83 Ga Hutuo Group in theNorth ChinaCraton is a >10 kmthick volcano-sedimentary sequence, including >5 kmthick well-preserved carbonates that were deposited in supra-tidal to sub-tidal environments. C-O isotopic and elemental analyses of 152 least altered samples of the carbonates revealed a three-stage δ13Cevolution. It began with an exclusively positive δ13Ccarb (+1.3 to + 3.4‰) stage in the ~2.1 Ga carbonate in the Dashiling and Qingshicun Formations, followed by a transition from positive values to oscillating positive and negative values in ~3000 mthick carbonates of the Wenshan, Hebiancun, Jianancun, and Daguandong Formations, and end with exclusively negative δ13Ccarb values preserved in >500 mthick dolostones of the Huaiyincun and Beidaxing Formations. It appears that much of the LJE, particularly those extremely positive δ13Ccarb signals, was not recorded in the Hutuo carbonates. The exclusively positive δ13Ccarb values (+1.3 to + 3.4‰) preserved in the lower formations likely correspond to the end of the LJE, whereas the subsequent two stages reflect the aftermath of the LJE and the onset of Shunga-Francevillian event (SFE). The present data point to an increased influence of oxygen on the carbon cycle from the Doucun to the Dongye Subgroups and demonstrate that the termination of the LJE in the North China Craton is nearly simultaneous with those in Fennoscandia and South Africa
Spurious diffusion in particle simulations of the Kolmogorov flow
Particle simulations of the Kolmogorov flow are analyzed by the
Landau-Lifshitz fluctuating hydrodynamics. It is shown that a spurious
diffusion of the center of mass corrupts the statistical properties of the
flow. The analytical expression for the corresponding diffusion coefficient is
derived.Comment: 10 pages, no figure
Turbulence and Multiscaling in the Randomly Forced Navier Stokes Equation
We present an extensive pseudospectral study of the randomly forced
Navier-Stokes equation (RFNSE) stirred by a stochastic force with zero mean and
a variance , where is the wavevector and the dimension . We present the first evidence for multiscaling of velocity structure
functions in this model for . We extract the multiscaling exponent
ratios by using extended self similarity (ESS), examine their
dependence on , and show that, if , they are in agreement with those
obtained for the deterministically forced Navier-Stokes equation (NSE). We
also show that well-defined vortex filaments, which appear clearly in studies
of the NSE, are absent in the RFNSE.Comment: 4 pages (revtex), 6 figures (postscript
- …