32 research outputs found
(1+1)-dimensional turbulence
A class of dynamical models of turbulence living on a one-dimensional
dyadic-tree structure is introduced and studied. The models are obtained as a
natural generalization of the popular GOY shell model of turbulence. These
models are found to be chaotic and intermittent. They represent the first
example of (1+1)-dimensional dynamical systems possessing non trivial
multifractal properties. The dyadic structure allows to study spatial and
temporal fluctuations. Energy dissipation statistics and its scaling properties
are studied. Refined Kolmogorov Hypothesis is found to hold.Comment: 18 pages, 9 figures, submitted to Phys.of Fluid
Kolmogorov Similarity Hypotheses for Scalar Fields: Sampling Intermittent Turbulent Mixing in the Ocean and Galaxy
Kolmogorov's three universal similarity hypotheses are extrapolated to
describe scalar fields like temperature mixed by turbulence. By the analogous
Kolmogorov third hypothesis for scalars, temperature dissipation rates chi
averaged over lengths r > L_K should be lognormally distributed with
intermittency factors I that increase with increasing turbulence energy length
scales L_O as I_chi-r = m_T ln(L_O/r). Tests of Kolmogorovian velocity and
scalar universal similarity hypotheses for very large ranges of turbulence
length and time scales are provided by data from the ocean and the Galactic
interstellar medium. The universal constant for turbulent mixing intermittency
m_T is estimated from oceanic data to be 0.44+-0.01, which is remarkably close
to estimates for Kolmogorov's turbulence intermittency constant m_u of
0.45+-0.05 from Galactic as well as atmospheric data. Extreme intermittency
complicates the oceanic sampling problem, and may lead to quantitative and
qualitative undersampling errors in estimates of mean oceanic dissipation rates
and fluxes. Intermittency of turbulence and mixing in the interstellar medium
may be a factor in the formation of stars.Comment: 23 pages original of Proc. Roy. Soc. article, 8 figures; in
"Turbulence and Stochastic Processes: Kolmogorov's ideas 50 years on", London
The Royal Society, 1991, J.C.R. Hunt, O.M. Phillips, D. Williams Eds., pages
1-240, vol. 434 (no. 1890) Proc. Roy. Soc. Lond. A, PDF fil
On the universality of small scale turbulence
The proposed universality of small scale turbulence is investigated for a set
of measurements in a cryogenic free jet with a variation of the Reynolds number
(Re) from 8500 to 10^6. The traditional analysis of the statistics of velocity
increments by means of structure functions or probability density functions is
replaced by a new method which is based on the theory of stochastic Markovian
processes. It gives access to a more complete characterization by means of
joint probabilities of finding velocity increments at several scales. Based on
this more precise method our results call in question the concept of
universality.Comment: 4 pages, 4 figure
On an alternative explanation of anomalous scaling and how well-defined is the concept of inertial range
The main point of this communication is that there is a small non-negligible
amount of eddies-outliers/very strong events (comprising a significant subset
of the tails of the PDF of velocity increments in the nominally-defined
inertial range) for which viscosity/dissipation is of utmost importance at
whatever high Reynolds number. These events contribute significantly to the
values of higher-order structure functions and their anomalous scaling. Thus
the anomalous scaling is not an attribute of the conventionally-defined
inertial range, and the latter is not a well-defined concept. The claim above
is supported by an analysis of high-Reynolds-number flows in which among other
things it was possible to evaluate the instantaneous rate of energy
dissipation.Comment: 7 pages, 6 figure
The effect of turbulent intermittency on the deflagration to detonation transition in SN Ia explosions
We examine the effects of turbulent intermittency on the deflagration to
detonation transition (DDT) in Type Ia supernovae. The Zel'dovich mechanism for
DDT requires the formation of a nearly isothermal region of mixed ash and fuel
that is larger than a critical size. We primarily consider the hypothesis by
Khokhlov et al. and Niemeyer and Woosley that the nearly isothermal, mixed
region is produced when the flame makes the transition to the distributed
regime. We use two models for the distribution of the turbulent velocity
fluctuations to estimate the probability as a function of the density in the
exploding white dwarf that a given region of critical size is in the
distributed regime due to strong local turbulent stretching of the flame
structure. We also estimate lower limits on the number of such regions as a
function of density. We find that the distributed regime, and hence perhaps
DDT, occurs in a local region of critical size at a density at least a factor
of 2-3 larger than predicted for mean conditions that neglect intermittency.
This factor brings the transition density to be much larger than the empirical
value from observations in most situations. We also consider the intermittency
effect on the more stringent conditions for DDT by Lisewski et al. and Woosley.
We find that a turbulent velocity of cm/s in a region of size cm,
required by Lisewski et al., is rare. We expect that intermittency gives a
weaker effect on the Woosley model with stronger criterion. The predicted
transition density from this criterion remains below g/cm after
accounting for intermittency using our intermittency models.Comment: 31 pages, accepted for publication in Ap
Analysis of Velocity Fluctuation in Turbulence based on Generalized Statistics
The numerical experiments of turbulence conducted by Gotoh et al. are
analyzed precisely with the help of the formulae for the scaling exponents of
velocity structure function and for the probability density function (PDF) of
velocity fluctuations. These formulae are derived by the present authors with
the multifractal aspect based on the statistics that are constructed on the
generalized measures of entropy, i.e., the extensive R\'{e}nyi's or the
non-extensive Tsallis' entropy. It is revealed that there exist two scaling
regions separated by a crossover length, i.e., a definite length approximately
of the order of the Taylor microscale. It indicates that the multifractal
distribution of singularities in velocity gradient in turbulent flow is robust
enough to produce scaling behaviors even for the phenomena out side the
inertial range.Comment: 10 Pages, 5 figure