932 research outputs found
Dissipation Scale Fluctuations and Chemical Reaction Rates in Turbulent Flows
Small separation between reactants, not exceeding , is the
necessary condition for various chemical reactions. It is shown that random
advection and stretching by turbulence leads to formation of scalar-enriched
sheets of {\it strongly fluctuating thickness} . The molecular-level
mixing is achieved by diffusion across these sheets (interfaces) separating the
reactants. Since diffusion time scale is , the
knowledge of probability density is crucial for evaluation of
chemical reaction rates. In this paper we derive the probability density
and predict a transition in the reaction rate behavior from
() to the high-Re asymptotics . The theory leads to an approximate universality of
transitional Reynolds number . It is also shown that if
chemical reaction involves short-lived reactants, very strong anomalous
fluctuations of the length-scale may lead to non-negligibly small
reaction rates
Probability Densities in Strong Turbulence
According to modern developments in turbulence theory, the "dissipation"
scales (u.v. cut-offs) form a random field related to velocity
increments . In this work we, using Mellin's transform combined
with the Gaussain large -scale boundary condition, calculate probability
densities (PDFs) of velocity increments and the PDF of the
dissipation scales , where is the large-scale Reynolds
number. The resulting expressions strongly deviate from the Log-normal PDF
often quoted in the literature. It is shown that the
probability density of the small-scale velocity fluctuations includes
information about the large (integral) scale dynamics which is responsible for
deviation of from . A framework for
evaluation of the PDFs of various turbulence characteristics involving spatial
derivatives is developed. The exact relation, free of spurious Logarithms
recently discussed in Frisch et al (J. Fluid Mech. {\bf 542}, 97 (2005)), for
the multifractal probability density of velocity increments, not based on the
steepest descent evaluation of the integrals is obtained and the calculated
function is close to experimental data. A novel derivation (Polyakov,
2005), of a well-known result of the multi-fractal theory [Frisch, "Turbulence.
{\it Legacy of A.N.Kolmogorov}", Cambridge University Press, 1995)), based on
the concepts described in this paper, is also presented.Comment: 25 pages and 9 figure
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